The

Faculty

of

Language

What

Is

It

Who

Has

It

and

How

Did

It

Evolve

Marc

D.

Hauser

et

al.

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SCIENCE

'S

COMPASS

REVIEW

NEUROSCIENCE

REVIEW

The

Faculty

of

Language

What

Is

It

Who

Has

It

and

How

Did

It

Evolve

Marc

D.

Hauser

Noam

Chomsky

W.

Tecumseh

question

of

language

evolution

and

of

how

humans

acquired

the

faculty

of

language

In

exploring

the

problem

of

language

evolution

it

is

important

to

distinguish

between

questions

concerning

language

as

a

communicative

system

and

questions

concerning

the

computations

underlying

this

system

such

as

those

underlying

recursion

As

we

argue

below

many

acrimonious

debates

in

this

field

have

been

launched

by

a

failure

to

distinguish

between

these

problems

According

to

one

view

-LRB-

-RRB-

questions

concerning

abstract

computational

mechanisms

are

distinct

from

those

concerning

communication

the

latter

targeted

at

problems

at

the

interface

between

abstract

computation

and

both

sensory-motor

and

conceptual-intentional

interfaces

This

view

should

not

of

course

be

taken

as

a

claim

against

a

relationship

between

compuWe

argue

that

an

understanding

of

the

faculty

of

language

requires

substantial

interdisciplinary

cooperation

We

suggest

how

current

developments

in

linguistics

can

be

profitably

wedded

to

work

in

evolutionary

biology

anthropology

psychology

and

neuroscience

We

submit

that

a

distinction

should

be

made

between

the

faculty

of

language

in

the

broad

sense

-LRB-

FLB

-RRB-

and

in

the

narrow

sense

-LRB-

FLN

-RRB-

FLB

includes

a

sensory-motor

system

a

conceptual-intentional

system

and

the

computational

mechanisms

for

recursion

providing

the

capacity

to

generate

an

infinite

range

of

expressions

from

a

finite

set

of

elements

We

hypothesize

that

FLN

only

includes

recursion

and

is

the

only

uniquely

human

component

of

the

faculty

of

language

We

further

argue

that

FLN

may

have

evolved

for

reasons

other

than

language

hence

comparative

studies

might

look

for

evidence

of

such

computations

outside

of

the

domain

of

communication

-LRB-

for

example

number

navigation

and

social

relations

-RRB-

I

f

a

martian

graced

our

planet

it

would

be

tures

it

might

further

note

that

the

human

struck

by

one

remarkable

similarity

among

faculty

of

language

appears

to

be

organized

Earth

's

living

creatures

and

a

key

difference

like

the

genetic

code

hierarchical

generaConcerning

similarity

it

would

note

that

all

tive

recursive

and

virtually

limitless

with

living

things

are

designed

on

the

basis

of

highly

conserved

developmental

systems

that

read

an

-LRB-

almost

-RRB-

universal

language

encoded

in

DNA

base

pairs

As

such

life

is

arranged

hierarchically

with

a

foundation

of

discrete

unblendable

units

-LRB-

codons

and

for

the

most

part

genes

-RRB-

capable

of

combining

to

create

increasingly

complex

and

virtually

limitless

varieties

of

both

species

and

individual

organisms

In

contrast

it

would

notice

the

absence

of

a

universal

code

of

communication

-LRB-

Fig.

-RRB-

If

our

martian

nat

Fig.

The

animal

kingdom

has

been

designed

on

the

basis

of

highly

conserved

developmental

systems

that

read

an

almost

universal

language

coded

in

DNA

base

pairs

This

system

is

shown

on

the

left

in

terms

of

a

phylogenetic

tree

In

contrast

animals

uralist

were

meticu

lack

a

common

universal

code

of

communication

indicated

on

the

right

by

unconnected

animal

groups

-LSB-

Illustration

John

Yanson

-RSB-

lous

it

might

note

that

the

faculty

mediating

human

communication

appears

remarkrespect

to

its

scope

of

expression

With

these

tation

and

communication

It

is

possible

as

ably

different

from

that

of

other

living

creapieces

in

hand

this

martian

might

begin

to

we

discuss

below

that

key

computational

wonder

how

the

genetic

code

changed

in

such

capacities

evolved

for

reasons

other

than

a

way

as

to

generate

a

vast

number

of

mutucommunication

but

after

they

proved

to

have

Department

of

Psychology

Harvard

University

ally

incomprehensible

communication

sysutility

in

communication

were

altered

Cambridge

MA

USA

Department

of

Linguistems

across

species

while

maintaining

clarity

cause

of

constraints

imposed

at

both

the

petics

and

Philosophy

Massachusetts

Institute

of

Techof

comprehension

within

a

given

species

The

riphery

-LRB-

e.g.

what

we

can

hear

and

say

or

see

nology

Cambridge

MA

USA

martian

would

have

stumbled

onto

some

of

and

sign

the

rapidity

with

which

the

auditory

To

whom

correspondence

should

be

addressed

Email

mdhauser@wjh.harvard.edu

the

essential

problems

surrounding

the

cortex

can

process

rapid

temporal

and

specwww.sciencemag.org

SCIENCE

VOL

NOVEMBER

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SCIENCE

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faculty

with

a

special

focus

on

comparative

work

with

nonhuman

animals

and

conclude

with

a

discussion

of

how

inquiry

might

profitably

advance

highlighting

some

outstanding

problems

We

make

no

attempt

to

be

comprehensive

in

our

coverage

of

relevant

or

interesting

topics

and

problems

Nor

is

it

our

goal

to

review

the

history

of

the

field

Rather

we

focus

on

topics

that

make

important

contact

between

empirical

data

and

theoretical

positions

about

the

nature

of

the

language

faculty

We

believe

that

if

explorations

into

the

problem

of

language

evolution

are

to

progress

we

need

a

clear

explication

of

the

computational

requirements

for

language

the

role

of

evolutionary

theory

in

testing

hypotheses

of

character

evolution

and

a

research

program

that

will

enable

a

productive

interchange

between

linguists

and

biologists

Fig.

A

schematic

representation

of

organism-external

and

internal

factors

related

to

the

faculty

of

language

FLB

includes

sensory-motor

conceptual-intentional

and

other

possible

systems

-LRB-

which

we

leave

open

-RRB-

FLN

includes

the

core

grammatical

computations

that

we

suggest

are

limited

to

recursion

See

text

for

more

complete

discussion

Defining

the

Target

Two

Senses

of

the

Faculty

of

Language

tral

changes

-RRB-

and

more

central

levels

-LRB-

e.g.

conceptual

and

cognitive

structures

pragmatics

memory

limitations

-RRB-

At

least

three

theoretical

issues

cross-cut

the

debate

on

language

evolution

One

of

the

oldest

problems

among

theorists

is

the

shared

versus

unique

distinction

Most

current

commentators

agree

that

although

bees

dance

birds

sing

and

chimpanzees

grunt

these

systems

of

communication

differ

qualitatively

from

human

language

In

particular

animal

communication

systems

lack

the

rich

expressive

and

open-ended

power

of

human

language

-LRB-

based

on

humans

capacity

for

recursion

-RRB-

The

evolutionary

puzzle

therefore

lies

in

working

out

how

we

got

from

there

to

here

given

this

apparent

discontinuity

A

second

issue

revolves

around

whether

the

evolution

of

language

was

gradual

versus

saltational

this

differs

from

the

first

issue

because

a

qualitative

discontinuity

between

extant

species

could

have

evolved

gradually

involving

no

discontinuities

during

human

evolution

Finally

the

continuity

versus

exaptation

issue

revolves

around

the

problem

of

whether

human

language

evolved

by

gradual

extension

of

preexisting

communication

systems

or

whether

important

aspects

of

language

have

been

exapted

away

from

their

previous

adaptive

function

-LRB-

e.g.

spatial

or

numerical

reasoning

Machiavellian

social

scheming

tool-making

-RRB-

Researchers

have

adopted

extreme

or

intermediate

positions

regarding

these

basically

independent

questions

leading

to

a

wide

variety

of

divergent

viewpoints

on

the

evolution

of

language

in

the

current

literature

There

is

however

an

emerging

consensus

that

although

humans

and

animals

share

a

diversity

of

important

computational

and

perceptual

resources

there

has

been

substantial

evolutionary

remodeling

since

we

diverged

from

a

common

ancestor

some

million

years

ago

The

empirical

challenge

is

to

determine

what

was

inherited

unchanged

from

this

common

ancestor

what

has

been

subjected

to

minor

modifications

and

what

-LRB-

if

anything

-RRB-

is

qualitatively

new

The

additional

evolutionary

challenge

is

to

determine

what

selectional

pressures

led

to

adaptive

changes

over

time

and

to

understand

the

various

constraints

that

channeled

this

evolutionary

process

Answering

these

questions

requires

a

collaborative

effort

among

linguists

biologists

psychologists

and

anthropologists

One

aim

of

this

essay

is

to

promote

a

stronger

connection

between

biology

and

linguistics

by

identifying

points

of

contact

and

agreement

between

the

fields

Although

this

interdisciplinary

marriage

was

inaugurated

more

than

years

ago

it

has

not

yet

been

fully

consummated

We

hope

to

further

this

goal

by

first

helping

to

clarify

the

biolinguistic

perspective

on

language

and

its

evolution

-LRB-

-RRB-

We

then

review

some

promising

empirical

approaches

to

the

evolution

of

the

language

The

word

language

has

highly

divergent

meanings

in

different

contexts

and

disciplines

In

informal

usage

a

language

is

understood

as

a

culturally

specific

communication

system

-LRB-

English

Navajo

etc.

-RRB-

In

the

varieties

of

modern

linguistics

that

concern

us

here

the

term

language

is

used

quite

differently

to

refer

to

an

internal

component

of

the

mind/brain

-LRB-

sometimes

called

internal

language

or

I-language

-RRB-

We

assume

that

this

is

the

primary

object

of

interest

for

the

study

of

the

evolution

and

function

of

the

language

faculty

However

this

biologically

and

individually

grounded

usage

still

leaves

much

open

to

interpretation

-LRB-

and

misunderstanding

-RRB-

For

example

a

neuroscientist

might

ask

What

components

of

the

human

nervous

system

are

recruited

in

the

use

of

language

in

its

broadest

sense

Because

any

aspect

of

cognition

appears

to

be

at

least

in

principle

accessible

to

language

the

broadest

answer

to

this

question

is

probably

most

of

it

Even

aspects

of

emotion

or

cognition

not

readily

verbalized

may

be

influenced

by

linguistically

based

thought

processes

Thus

this

conception

is

too

broad

to

be

of

much

use

We

therefore

delineate

two

more

restricted

conceptions

of

the

faculty

of

language

one

broader

and

more

inclusive

the

other

more

restricted

and

narrow

-LRB-

Fig.

-RRB-

Faculty

of

language

broad

sense

-LRB-

FLB

-RRB-

FLB

includes

an

internal

computational

system

-LRB-

FLN

below

-RRB-

combined

with

at

least

two

other

organism-internal

sys

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tems

which

we

call

sensory-motor

and

conceptual-intentional

Despite

debate

on

the

precise

nature

of

these

systems

and

about

whether

they

are

substantially

shared

with

other

vertebrates

or

uniquely

adapted

to

the

exigencies

of

language

we

take

as

uncontroversial

the

existence

of

some

biological

capacity

of

humans

that

allows

us

-LRB-

and

not

for

example

chimpanzees

-RRB-

to

readily

master

any

human

language

without

explicit

instruction

FLB

includes

this

capacity

but

excludes

other

organisminternal

systems

that

are

necessary

but

not

sufficient

for

language

-LRB-

e.g.

memory

respiration

digestion

circulation

etc.

-RRB-

Faculty

of

language

narrow

sense

-LRB-

FLN

-RRB-

FLN

is

the

abstract

linguistic

computational

system

alone

independent

of

the

other

systems

with

which

it

interacts

and

interfaces

FLN

is

a

component

of

FLB

and

the

mechanisms

underlying

it

are

some

subset

of

those

underlying

FLB

Others

have

agreed

on

the

need

for

a

restricted

sense

of

language

but

have

suggested

different

delineations

For

example

Liberman

and

his

associates

-LRB-

-RRB-

have

argued

that

the

sensory-motor

systems

were

specifically

adapted

for

language

and

hence

should

be

considered

part

of

FLN

There

is

also

a

long

tradition

holding

that

the

conceptualintentional

systems

are

an

intrinsic

part

of

language

in

a

narrow

sense

In

this

article

we

leave

these

questions

open

restricting

attention

to

FLN

as

just

defined

but

leaving

the

possibility

of

a

more

inclusive

definition

open

to

further

empirical

research

The

internal

architecture

of

FLN

so

conceived

is

a

topic

of

much

current

research

and

debate

-LRB-

-RRB-

Without

prejudging

the

issues

we

will

for

concreteness

adopt

a

particular

conception

of

this

architecture

We

assume

putting

aside

the

precise

mechanisms

that

a

key

component

of

FLN

is

a

computational

system

-LRB-

narrow

syntax

-RRB-

that

generates

internal

representations

and

maps

them

into

the

sensory-motor

interface

by

the

phonological

system

and

into

the

conceptual-intentional

interface

by

the

-LRB-

formal

-RRB-

semantic

system

adopting

alternatives

that

have

been

proposed

would

not

materially

modify

the

ensuing

discussion

All

approaches

agree

that

a

core

property

of

FLN

is

recursion

attributed

to

narrow

syntax

in

the

conception

just

outlined

FLN

takes

a

finite

set

of

elements

and

yields

a

potentially

infinite

array

of

discrete

expressions

This

capacity

of

FLN

yields

discrete

infinity

-LRB-

a

property

that

also

characterizes

the

natural

numbers

-RRB-

Each

of

these

discrete

expressions

is

then

passed

to

the

sensory-motor

and

conceptual-intentional

systems

which

process

and

elaborate

this

information

in

the

use

of

language

Each

expression

is

in

this

sense

a

pairing

of

sound

and

meaning

It

has

been

recognized

for

thousands

of

years

that

language

is

fundamentally

a

system

of

sound-meaning

connections

the

potential

infiniteness

of

this

system

has

been

explicitly

recognized

by

Galileo

Descartes

and

the

philosophical

grammarians

and

their

successors

notably

von

Humboldt

One

goal

of

the

study

of

FLN

and

more

broadly

FLB

is

to

discover

just

how

the

faculty

of

language

satisfies

these

basic

and

essential

conditions

The

core

property

of

discrete

infinity

is

intuitively

familiar

to

every

language

user

Sentences

are

built

up

of

discrete

units

There

are

sentences

and

sentences

but

no

sentences

There

is

no

longest

sentence

-LRB-

any

candidate

sentence

can

be

trumped

by

for

example

embedding

it

in

Mary

thinks

that

-RRB-

and

there

is

no

nonarbitrary

upper

bound

to

sentence

length

In

these

respects

language

is

directly

analogous

to

the

natural

numbers

-LRB-

see

below

-RRB-

At

a

minimum

then

FLN

includes

the

capacity

of

recursion

There

are

many

organisminternal

factors

outside

FLN

or

FLB

that

impose

practical

limits

on

the

usage

of

the

system

For

example

lung

capacity

imposes

limits

on

the

length

of

actual

spoken

sentences

whereas

working

memory

imposes

limits

on

the

complexity

of

sentences

if

they

are

to

be

understandable

Other

limitations

for

example

on

concept

formation

or

motor

output

speed

represent

aspects

of

FLB

which

have

their

own

evolutionary

histories

and

may

have

played

a

role

in

the

evolution

of

the

capacities

of

FLN

Nonetheless

one

can

profitably

inquire

into

the

evolution

of

FLN

without

an

immediate

concern

for

these

limiting

aspects

of

FLB

This

is

made

clear

by

the

observation

that

although

many

aspects

of

FLB

are

shared

with

other

vertebrates

the

core

recursive

aspect

of

FLN

currently

appears

to

lack

any

analog

in

animal

communication

and

possibly

other

domains

as

well

This

point

therefore

represents

the

deepest

challenge

for

a

comparative

evolutionary

approach

to

language

We

believe

that

investigations

of

this

capacity

should

include

domains

other

than

communication

-LRB-

e.g.

number

social

relationships

navigation

-RRB-

Given

the

distinctions

between

FLB

and

FLN

and

the

theoretical

distinctions

raised

above

we

can

define

a

research

space

as

sketched

in

Fig.

This

research

space

identifies

as

viable

problems

concerning

the

evolution

of

sensory-motor

systems

of

conceptual-intentional

systems

and

of

FLN

The

comparative

approach

to

which

we

turn

next

provides

a

framework

for

addressing

questions

about

each

of

these

components

of

the

faculty

of

language

Fig.

Investigations

into

the

evolution

of

the

faculty

of

language

are

confronted

with

a

three-dimensional

research

space

that

includes

three

comparative-evolutionary

problems

cross-cut

by

the

core

components

of

the

faculty

of

language

Thus

for

each

problem

researchers

can

investigate

details

of

the

sensory-motor

system

the

conceptual-intentional

system

FLN

and

the

interfaces

among

these

systems

The

Comparative

Approach

to

Language

Evolution

The

empirical

study

of

the

evolution

of

language

is

beset

with

difficulties

Linguistic

behavior

does

not

fossilize

and

a

long

tradi

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tion

of

analysis

of

fossil

skull

shape

and

cranial

endocasts

has

led

to

little

consensus

about

the

evolution

of

language

-LRB-

-RRB-

A

more

tractable

and

we

think

powerful

approach

to

problems

of

language

evolution

is

provided

by

the

comparative

method

which

uses

empirical

data

from

living

species

to

draw

detailed

inferences

about

extinct

ancestors

-LRB-

-RRB-

The

comparative

method

was

the

primary

tool

used

by

Darwin

-LRB-

-RRB-

to

analyze

evolutionary

phenomena

and

continues

to

play

a

central

role

throughout

modern

evolutionary

biology

Although

scholars

interested

in

language

evolution

have

often

ignored

comparative

data

altogether

or

focused

narrowly

on

data

from

nonhuman

primates

current

thinking

in

neuroscience

molecular

biology

and

developmental

biology

indicates

that

many

aspects

of

neural

and

developmental

function

are

highly

conserved

encouraging

the

extension

of

the

comparative

method

to

all

vertebrates

-LRB-

and

perhaps

beyond

-RRB-

For

several

reasons

detailed

below

we

believe

that

the

comparative

method

should

play

a

more

central

role

in

future

discussions

of

language

evolution

An

overarching

concern

in

studies

of

language

evolution

is

with

whether

particular

components

of

the

faculty

of

language

evolved

specifically

for

human

language

and

therefore

-LRB-

by

extension

-RRB-

are

unique

to

humans

Logically

the

human

uniqueness

claim

must

be

based

on

data

indicating

an

absence

of

the

trait

in

nonhuman

animals

and

to

be

taken

seriously

requires

a

substantial

body

of

relevant

comparative

data

More

concretely

if

the

language

evolution

researcher

wishes

to

make

the

claim

that

a

trait

evolved

uniquely

in

humans

for

the

function

of

language

processing

data

indicating

that

no

other

animal

has

this

particular

trait

are

required

Although

this

line

of

reasoning

may

appear

obvious

it

is

surprisingly

common

for

a

trait

to

be

held

up

as

uniquely

human

before

any

appropriate

comparative

data

are

available

A

famous

example

is

categorical

perception

which

when

discovered

seemed

so

finely

tuned

to

the

details

of

human

speech

as

to

constitute

a

unique

human

adaptation

-LRB-

-RRB-

It

was

some

time

before

the

same

underlying

perceptual

discontinuities

were

discovered

in

chinchillas

and

macaques

-LRB-

-RRB-

and

even

birds

-LRB-

-RRB-

leading

to

the

opposite

conclusion

that

the

perceptual

basis

for

categorical

perception

is

a

primitive

vertebrate

characteristic

that

evolved

for

general

auditory

processing

as

opposed

to

specific

speech

processing

Thus

a

basic

and

logically

ineliminable

role

for

comparative

research

on

language

evolution

is

this

simple

and

essentially

negative

one

A

trait

present

in

nonhuman

animals

did

not

evolve

specifically

for

human

language

although

it

may

be

part

of

the

language

faculty

and

play

an

intimate

role

in

language

processing

It

is

possible

of

course

that

a

trait

evolved

in

nonhuman

animals

and

humans

independently

as

analogs

rather

than

homologs

This

would

preserve

the

possibility

that

the

trait

evolved

for

language

in

humans

but

evolved

for

some

other

reason

in

the

comparative

animal

group

In

cases

where

the

comparative

group

is

a

nonhuman

primate

and

perhaps

especially

chimpanzees

the

plausibility

of

this

evolutionary

scenario

is

weaker

In

any

case

comparative

data

are

critical

to

this

judgment

Despite

the

crucial

role

of

homology

in

comparative

biology

homologous

traits

are

not

the

only

relevant

source

of

evolutionary

data

The

convergent

evolution

of

similar

characters

in

two

independent

clades

termed

analogies

or

homoplasies

can

be

equally

revealing

-LRB-

-RRB-

The

remarkably

similar

-LRB-

but

nonhomologous

-RRB-

structures

of

human

and

octopus

eyes

reveal

the

stringent

constraints

placed

by

the

laws

of

optics

and

the

contingencies

of

development

on

an

organ

capable

of

focusing

a

sharp

image

onto

a

sheet

of

receptors

Detailed

analogies

between

the

parts

of

the

vertebrate

and

cephalopod

eye

also

provide

independent

evidence

that

each

component

is

an

adaptation

for

image

formation

shaped

by

natural

selection

Furthermore

the

discovery

that

remarkably

conservative

genetic

cascades

underlie

the

development

of

such

analogous

structures

provides

important

insights

into

the

ways

in

which

developmental

mechanisms

can

channel

evolution

-LRB-

-RRB-

Thus

although

potentially

misleading

for

taxonomists

analogies

provide

critical

data

about

adaptation

under

physical

and

developmental

constraints

Casting

the

comparative

net

more

broadly

therefore

will

most

likely

reveal

larger

regularities

in

evolution

helping

to

address

the

role

of

such

constraints

in

the

evolution

of

language

An

analogy

recognized

as

particularly

relevant

to

language

is

the

acquisition

of

song

by

birds

-LRB-

-RRB-

In

contrast

to

nonhuman

primates

where

the

production

of

species-typical

vocalizations

is

largely

innate

-LRB-

-RRB-

most

songbirds

learn

their

species-specific

song

by

listening

to

conspecifics

and

they

develop

highly

aberrant

song

if

deprived

of

such

experience

Current

investigation

of

birdsong

reveals

detailed

and

intriguing

parallels

with

speech

-LRB-

-RRB-

For

instance

many

songbirds

pass

through

a

critical

period

in

development

beyond

which

they

produce

defective

songs

that

no

amount

of

acoustic

input

can

remedy

reminiscent

of

the

difficulty

adult

humans

have

in

fully

mastering

new

languages

Further

and

in

parallel

with

the

babbling

phase

of

vocalizing

or

signing

human

infants

-LRB-

-RRB-

young

birds

pass

through

a

phase

of

song

development

in

which

they

spontaneously

produce

amorphous

versions

of

adult

song

termed

subsong

or

babbling

Although

the

mechanisms

underlying

the

acquisition

of

birdsong

and

human

language

are

clearly

analogs

and

not

homologs

their

core

components

share

a

deeply

conserved

neural

and

developmental

foundation

Most

aspects

of

neurophysiology

and

development

including

regulatory

and

structural

genes

as

well

as

neuron

types

and

neurotransmitters

are

shared

among

vertebrates

That

such

close

parallels

have

evolved

suggests

the

existence

of

important

constraints

on

how

vertebrate

brains

can

acquire

large

vocabularies

of

complex

learned

sounds

Such

constraints

may

essentially

force

natural

selection

to

come

up

with

the

same

solution

repeatedly

when

confronted

with

similar

problems

Testing

Hypotheses

About

the

Evolution

of

the

Faculty

of

Language

Given

the

definitions

of

the

faculty

of

language

together

with

the

comparative

framework

we

can

distinguish

several

plausible

hypotheses

about

the

evolution

of

its

various

components

Here

we

suggest

two

hypotheses

that

span

the

diversity

of

opinion

among

current

scholars

plus

a

third

of

our

own

Hypothesis

FLB

is

strictly

homologous

to

animal

communication

This

hypothesis

holds

that

homologs

of

FLB

including

FLN

exist

-LRB-

perhaps

in

less

developed

or

otherwise

modified

form

-RRB-

in

nonhuman

animals

-LRB-

-RRB-

This

has

historically

been

a

popular

hypothesis

outside

of

linguistics

and

closely

allied

fields

and

has

been

defended

by

some

in

the

speech

sciences

According

to

this

hypothesis

human

FLB

is

composed

of

the

same

functional

components

that

underlie

communication

in

other

species

Hypothesis

FLB

is

a

derived

uniquely

human

adaptation

for

language

According

to

this

hypothesis

FLB

is

a

highly

complex

adaptation

for

language

on

a

par

with

the

vertebrate

eye

and

many

of

its

core

components

can

be

viewed

as

individual

traits

that

have

been

subjected

to

selection

and

perfected

in

recent

human

evolutionary

history

This

appears

to

represent

the

hypothesis

for

many

scholars

who

take

the

complexity

of

language

seriously

-LRB-

-RRB-

The

argument

starts

with

the

assumption

that

FLB

as

a

whole

is

highly

complex

serves

the

function

of

communication

with

admirable

effectiveness

and

has

an

ineliminable

genetic

component

Because

natural

selection

is

the

only

known

biological

mechanism

capable

of

generating

such

functional

complexes

-LSB-

the

argument

from

design

-LRB-

-RRB-

-RSB-

proponents

of

this

view

conclude

that

natural

selection

has

played

a

powerful

role

in

shaping

many

aspects

of

FLB

including

FLN

and

further

that

many

of

these

are

without

parallel

in

nonhuman

animals

Although

homologous

mechanisms

may

exist

in

other

animals

the

human

versions

have

been

modified

by

natural

selection

to

the

extent

that

they

can

be

reasonably

seen

as

constituting

novel

traits

perhaps

exapted

from

other

contexts

-LSB-

e.g.

social

intelligence

tool-making

-LRB-

-RRB-

-RSB-

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Hypothesis

Only

FLN

is

uniquely

human

On

the

basis

of

data

reviewed

below

we

hypothesize

that

most

if

not

all

of

FLB

is

based

on

mechanisms

shared

with

nonhuman

animals

-LRB-

as

held

by

hypothesis

-RRB-

In

contrast

we

suggest

that

FLN

the

computational

mechanism

of

recursion

is

recently

evolved

and

unique

to

our

species

-LRB-

-RRB-

According

to

this

hypothesis

much

of

the

complexity

manifested

in

language

derives

from

complexity

in

the

peripheral

components

of

FLB

especially

those

underlying

the

sensory-motor

-LRB-

speech

or

sign

-RRB-

and

conceptual-intentional

interfaces

combined

with

sociocultural

and

communicative

contingencies

FLB

as

a

whole

thus

has

an

ancient

evolutionary

history

long

predating

the

emergence

of

language

and

a

comparative

analysis

is

necessary

to

understand

this

complex

system

By

contrast

according

to

recent

linguistic

theory

the

computations

underlying

FLN

may

be

quite

limited

In

fact

we

propose

in

this

hypothesis

that

FLN

comprises

only

the

core

computational

mechanisms

of

recursion

as

they

appear

in

narrow

syntax

and

the

mappings

to

the

interfaces

If

FLN

is

indeed

this

restricted

this

hypothesis

has

the

interesting

effect

of

nullifying

the

argument

from

design

and

thus

rendering

the

status

of

FLN

as

an

adaptation

open

to

question

Proponents

of

the

idea

that

FLN

is

an

adaptation

would

thus

need

to

supply

additional

data

or

arguments

to

support

this

viewpoint

The

available

comparative

data

on

animal

communication

systems

suggest

that

the

faculty

of

language

as

a

whole

relies

on

some

uniquely

human

capacities

that

have

evolved

recently

in

the

approximately

million

years

since

our

divergence

from

a

chimpanzee-like

common

ancestor

-LRB-

-RRB-

Hypothesis

in

its

strongest

form

suggests

that

only

FLN

falls

into

this

category

-LRB-

-RRB-

By

this

hypothesis

FLB

contains

a

wide

variety

of

cognitive

and

perceptual

mechanisms

shared

with

other

species

but

only

those

mechanisms

underlying

FLN

particularly

its

capacity

for

discrete

infinity

are

uniquely

human

This

hypothesis

suggests

that

all

peripheral

components

of

FLB

are

shared

with

other

animals

in

more

or

less

the

same

form

as

they

exist

in

humans

with

differences

of

quantity

rather

than

kind

-LRB-

-RRB-

What

is

unique

to

our

species

is

quite

specific

to

FLN

and

includes

its

internal

operations

as

well

as

its

interface

with

the

other

organism-internal

systems

of

FLB

Each

of

these

hypotheses

is

plausible

to

some

degree

Ultimately

they

can

be

distinguished

only

by

empirical

data

much

of

which

is

currently

unavailable

Before

reviewing

some

of

the

relevant

data

we

briefly

consider

some

key

distinctions

between

them

From

a

comparative

evolutionary

viewpoint

an

important

question

is

whether

linguistic

precursors

were

involved

in

communication

or

in

something

else

Proponents

of

both

hypotheses

and

posit

a

direct

correspondence

by

descent

with

modification

between

some

trait

involved

in

FLB

in

humans

and

a

similar

trait

in

another

species

these

hypotheses

differ

in

whether

the

precursors

functioned

in

communication

Table

A

sampler

of

empirical

approaches

to

understanding

the

evolution

of

the

faculty

of

language

including

both

broad

-LRB-

FLB

-RRB-

and

narrow

-LRB-

FLN

-RRB-

components

Empirical

problem

Examples

References

Vocal

imitation

and

invention

Neurophysiology

of

action-perception

systems

Discriminating

the

sound

patterns

of

language

Constraints

imposed

by

vocal

tract

anatomy

Biomechanics

of

sound

production

Modalities

of

language

production

and

perception

Theory

of

mind

attribution

of

mental

states

Capacity

to

acquire

nonlinguistic

conceptual

representations

Referential

vocal

signals

Imitation

as

a

rational

intentional

system

Voluntary

control

over

signal

production

as

evidence

of

intentional

communication

Spontaneous

and

training

methods

designed

to

uncover

constraints

on

rule

learning

Sign

or

artificial

language

in

trained

apes

and

dolphins

Models

of

the

faculty

of

language

that

attempt

to

uncover

the

necessary

and

sufficient

mechanisms

Experiments

with

animals

that

explore

the

nature

and

content

of

number

representation

Shared

mechanisms

across

different

cognitive

domains

FLB

sensory-motor

system

Tutoring

studies

of

songbirds

analyses

of

vocal

dialects

in

whales

spontaneous

imitation

of

artificially

created

sounds

in

dolphins

Studies

assessing

whether

mirror

neurons

which

provide

a

core

substrate

for

the

action-perception

system

may

subserve

gestural

and

-LRB-

possibly

-RRB-

vocal

imitation

Operant

conditioning

studies

of

the

prototype

magnet

effect

in

macaques

and

starlings

Studies

of

vocal

tract

length

and

formant

dispersion

in

birds

and

primates

Studies

of

primate

vocal

production

including

the

role

of

mandibular

oscillations

Cross-modal

perception

and

sign

language

in

humans

versus

unimodal

communication

in

animals

FLB

conceptual-intentional

system

Studies

of

the

seeing/knowing

distinction

in

chimpanzees

Studies

of

rhesus

monkeys

and

the

object/kind

concept

Studies

of

primate

vocalizations

used

to

designate

predators

food

and

social

relationships

Comparative

studies

of

chimpanzees

and

human

infants

suggesting

that

only

the

latter

read

intentionality

into

action

and

thus

extract

unobserved

rational

intent

Comparative

studies

that

explore

the

relationship

between

signal

production

and

the

composition

of

a

social

audience

FLN

recursion

Studies

of

serial

order

learning

and

finite-state

grammars

in

tamarins

and

macaques

Studies

exploring

symbol

sequencing

and

open-ended

combinatorial

manipulation

Game

theory

models

of

language

acquisition

reference

and

universal

grammar

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

-LRB-

-RRB-

Operant

conditioning

studies

to

determine

whether

nonhuman

primates

can

represent

number

including

properties

such

as

ordinality

and

cardinality

using

such

representations

in

conjunction

with

mathematical

operands

-LRB-

e.g.

add

divide

-RRB-

Evolution

of

musical

processing

and

structure

including

analyses

of

brain

function

and

comparative

studies

of

music

perception

-LRB-

-RRB-

-LRB-

-RRB-

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Although

many

aspects

of

FLB

very

likely

arose

in

this

manner

the

important

issue

for

these

hypotheses

is

whether

a

series

of

gradual

modifications

could

lead

eventually

to

the

capacity

of

language

for

infinite

generativity

Despite

the

inarguable

existence

of

a

broadly

shared

base

of

homologous

mechanisms

involved

in

FLB

minor

modifications

to

this

foundational

system

alone

seem

inadequate

to

generate

the

fundamental

difference

discrete

infinity

between

language

and

all

known

forms

of

animal

communication

This

claim

is

one

of

several

reasons

why

we

suspect

that

hypothesis

may

be

a

productive

way

to

characterize

the

problem

of

language

evolution

A

primary

issue

separating

hypotheses

and

is

whether

the

uniquely

human

capacities

of

FLN

constitute

an

adaptation

The

viewpoint

stated

in

hypothesis

especially

the

notion

that

FLN

in

particular

is

a

highly

evolved

adaptation

has

generated

much

enthusiasm

recently

-LSB-

e.g.

-LRB-

-RRB-

-RSB-

especially

among

evolutionary

psychologists

-LRB-

-RRB-

At

present

however

we

see

little

reason

to

believe

either

that

FLN

can

be

anatomized

into

many

independent

but

interacting

traits

each

with

its

own

independent

evolutionary

history

or

that

each

of

these

traits

could

have

been

strongly

shaped

by

natural

selection

given

their

tenuous

connection

to

communicative

efficacy

-LRB-

the

surface

or

phenotypic

function

upon

which

selection

presumably

acted

-RRB-

We

consider

the

possibility

that

certain

specific

aspects

of

the

faculty

of

language

are

spandrels

by-products

of

preexisting

constraints

rather

than

end

products

of

a

history

of

natural

selection

-LRB-

-RRB-

This

possibility

which

opens

the

door

to

other

empirical

lines

of

inquiry

is

perfectly

compatible

with

our

firm

support

of

the

adaptationist

program

Indeed

it

follows

directly

from

the

foundational

notion

that

adaptation

is

an

onerous

concept

to

be

invoked

only

when

alternative

explanations

fail

-LRB-

-RRB-

The

question

is

not

whether

FLN

in

toto

is

adaptive

By

allowing

us

to

communicate

an

endless

variety

of

thoughts

recursion

is

clearly

an

adaptive

computation

The

question

is

whether

particular

components

of

the

functioning

of

FLN

are

adaptations

for

language

specifically

acted

upon

by

natural

selection

or

even

more

broadly

whether

FLN

evolved

for

reasons

other

than

communication

An

analogy

may

make

this

distinction

clear

The

trunk

and

branches

of

trees

are

near-optimal

solutions

for

providing

an

individual

tree

's

leaves

with

access

to

sunlight

For

shrubs

and

small

trees

a

wide

variety

of

forms

-LRB-

spreading

spherical

multistalked

etc.

-RRB-

provide

good

solutions

to

this

problem

For

a

towering

rainforest

canopy

tree

however

most

of

these

forms

are

rendered

impossible

by

the

various

constraints

imposed

by

the

properties

of

cellulose

and

the

problems

of

sucking

water

and

nutrients

up

to

the

leaves

high

in

the

air

Some

aspects

of

such

trees

are

clearly

adaptations

channeled

by

these

constraints

others

-LRB-

e.g.

the

popping

of

xylem

tubes

on

hot

days

the

propensity

to

be

toppled

in

hurricanes

-RRB-

are

presumably

unavoidable

by-products

of

such

constraints

Recent

work

on

FLN

-LRB-

-RRB-

suggests

the

possibility

that

at

least

the

narrow-syntactic

component

satisfies

conditions

of

highly

efficient

computation

to

an

extent

previously

unsuspected

Thus

FLN

may

approximate

a

kind

of

optimal

solution

to

the

problem

of

linking

the

sensory-motor

and

conceptual-intentional

systems

In

other

words

the

generative

processes

of

the

language

system

may

provide

a

near-optimal

solution

that

satisfies

the

interface

conditions

to

FLB

Many

of

the

details

of

language

that

are

the

traditional

focus

of

linguistic

study

-LSB-

e.g.

subjacency

Wh

movement

the

existence

of

garden-path

sentences

-LRB-

-RRB-

-RSB-

may

represent

by-products

of

this

solution

generated

automatically

by

neural/computational

constraints

and

the

structure

of

FLB

components

that

lie

outside

of

FLN

Even

novel

capacities

such

as

recursion

are

implemented

in

the

same

type

of

neural

tissue

as

the

rest

of

the

brain

and

are

thus

constrained

by

biophysical

developmental

and

computational

factors

shared

with

other

vertebrates

Hypothesis

raises

the

possibility

that

structural

details

of

FLN

may

result

from

such

preexisting

constraints

rather

than

from

direct

shaping

by

natural

selection

targeted

specifically

at

communication

Insofar

as

this

proves

to

be

true

such

structural

details

are

not

strictly

speaking

adaptations

at

all

This

hypothesis

and

the

alternative

selectionist

account

are

both

viable

and

can

eventually

be

tested

with

comparative

data

human

mechanisms

special

to

speech

despite

a

persistent

tendency

to

assume

uniqueness

even

in

the

absence

of

relevant

animal

data

On

the

side

of

perception

for

example

many

species

show

an

impressive

ability

to

both

discriminate

between

and

generalize

over

human

speech

sounds

using

formants

as

the

critical

discriminative

cue

-LRB-

-RRB-

These

data

provide

evidence

not

only

of

categorical

perception

but

also

of

the

ability

to

discriminate

among

prototypical

exemplars

of

different

phonemes

-LRB-

-RRB-

Further

in

the

absence

of

training

nonhuman

primates

can

discriminate

sentences

from

two

different

languages

on

the

basis

of

rhythmic

differences

between

them

-LRB-

-RRB-

On

the

side

of

production

birds

and

nonhuman

primates

naturally

produce

and

perceive

formants

in

their

own

species-typical

vocalizations

-LRB-

-RRB-

The

results

also

shed

light

on

discussions

of

the

uniquely

human

structure

of

the

vocal

tract

and

the

unusual

descended

larynx

of

our

species

-LRB-

-RRB-

because

new

evidence

shows

that

several

other

mammalian

species

also

have

a

descended

larynx

-LRB-

-RRB-

Because

these

nonhuman

species

lack

speech

a

descended

larynx

clearly

has

nonphonetic

functions

one

possibility

is

exaggerating

apparent

size

Although

this

particular

anatomical

modification

undoubtedly

plays

an

important

role

in

speech

production

in

modern

humans

it

need

not

have

first

evolved

for

this

function

The

descended

larynx

may

thus

be

an

example

of

classic

Darwinian

preadaptation

Many

phenomena

in

human

speech

perception

have

not

yet

been

investigated

in

animals

-LSB-

e.g.

the

McGurk

effect

an

illusion

in

which

the

syllable

perceived

from

a

talking

head

represents

the

interaction

between

an

articulatory

gesture

seen

and

a

different

syllable

heard

see

-LRB-

-RRB-

-RSB-

However

the

available

data

suggest

a

much

stronger

continuity

between

animals

and

humans

with

respect

to

speech

than

previously

believed

We

argue

that

the

continuity

hypothesis

thus

deserves

the

status

of

a

hypothesis

which

must

be

rejected

by

comparative

work

before

any

claims

of

uniqueness

can

be

validated

For

now

this

hypothesis

of

no

truly

novel

traits

in

the

speech

domain

appears

to

stand

There

is

however

a

striking

ability

tied

to

speech

that

has

received

insufficient

attention

the

human

capacity

for

vocal

imitation

-LRB-

-RRB-

Imitation

is

obviously

a

necessary

component

of

the

human

capacity

to

acquire

a

shared

and

arbitrary

lexicon

which

is

itself

central

to

the

language

capacity

Thus

the

capacity

to

imitate

was

a

crucial

prerequisite

of

FLB

as

a

communicative

system

Vocal

imitation

and

learning

are

not

uniquely

human

Rich

multimodal

imitative

capacities

are

seen

in

other

mammals

-LRB-

dolphins

-RRB-

and

some

birds

-LRB-

parrots

-RRB-

with

most

songbirds

exhibiting

a

well-developed

vocal

imitative

Comparative

Evidence

for

the

Faculty

of

Language

Study

of

the

evolution

of

language

has

accelerated

in

the

past

decade

-LRB-

-RRB-

Here

we

offer

a

highly

selective

review

of

some

of

these

studies

emphasizing

animal

work

that

seems

particularly

relevant

to

the

hypotheses

advanced

above

many

omissions

were

necessary

for

reasons

of

space

and

we

firmly

believe

that

a

broad

diversity

of

methods

and

perspectives

will

ultimately

provide

the

richest

answers

to

the

problem

of

language

evolution

For

this

reason

we

present

a

broader

sampler

of

the

field

's

offerings

in

Table

How

special

is

speech

Comparative

study

of

the

sensory-motor

system

Starting

with

early

work

on

speech

perception

there

has

been

a

tradition

of

considering

speech

special

and

thus

based

on

uniquely

human

mechanisms

adapted

for

speech

perception

and/or

production

-LSB-

e.g.

-LRB-

-RRB-

-RSB-

This

perspective

has

stimulated

a

vigorous

research

program

studying

animal

speech

perception

and

more

recently

speech

production

Surprisingly

this

research

has

turned

up

little

evidence

for

uniquely

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capacity

-LRB-

-RRB-

What

is

surprising

is

that

monparticular

way

and

when

the

same

individual

have

rich

conceptual

representations

-LRB-

-RRB-

keys

show

almost

no

evidence

of

visually

sees

someone

else

act

in

this

same

way

-LRB-

Surprisingly

however

there

is

a

mismatch

bemediated

imitation

with

chimpanzees

-RRB-

these

neurons

are

not

sufficient

for

imitween

the

conceptual

capacities

of

animals

and

ing

only

slightly

better

capacities

-LRB-

-RRB-

Even

tation

in

macaques

as

many

have

presumed

the

communicative

content

of

their

vocal

and

more

striking

is

the

virtual

absence

of

eviAs

mentioned

there

is

no

convincing

evivisual

signals

-LRB-

-RRB-

For

example

although

a

dence

for

vocal

imitation

in

either

monkeys

dence

of

vocal

or

visual

imitation

in

monwide

variety

of

nonhuman

primates

have

access

or

apes

-LRB-

-RRB-

For

example

intensively

trained

keys

Consequently

as

neuroimaging

studies

to

rich

knowledge

of

who

is

related

to

whom

as

chimpanzees

are

incapable

of

acquiring

anycontinue

to

explore

the

neural

basis

of

imitawell

as

who

is

dominant

and

who

is

subordithing

but

a

few

poorly

articulated

spoken

tion

in

humans

-LRB-

-RRB-

it

will

be

important

nate

their

vocalizations

only

coarsely

express

words

whereas

parrots

can

readily

acquire

a

to

distinguish

between

the

necessary

and

sufsuch

complexities

large

vocal

repertoire

With

respect

to

their

ficient

neural

correlates

of

imitation

This

is

Studies

using

classical

training

approachown

vocalizations

there

are

few

convincing

especially

important

given

that

some

recent

es

as

well

as

methods

that

tap

spontaneous

studies

of

vocal

dialects

in

primates

thereby

attempts

to

model

the

evolution

of

language

abilities

reveal

that

animals

acquire

and

use

a

suggesting

that

they

lack

a

vocal

imitative

begin

with

a

hypothetical

organism

that

is

wide

range

of

abstract

concepts

including

capacity

-LRB-

-RRB-

Evidence

for

spontaneous

equipped

with

the

capacity

for

imitation

and

tool

color

geometric

relationships

food

and

visuomanual

imitation

in

chimpanzees

is

not

intentionality

as

opposed

to

working

out

how

number

-LRB-

-RRB-

More

controversially

much

stronger

although

with

persistent

trainthese

mechanisms

evolved

in

the

first

place

but

of

considerable

relevance

to

intentional

ing

they

can

learn

several

hundred

hand

-LSB-

see

below

-LRB-

-RRB-

-RSB-

If

a

deeper

evolutionaspects

of

language

and

conditions

of

felicisigns

Further

even

in

cases

where

nonhuman

animals

are

capable

of

imitating

in

one

modality

-LRB-

e.g.

song

copying

in

songbirds

-RRB-

only

dolphins

and

humans

appear

capable

of

imitation

in

multiple

modalities

The

detachment

from

modality-specific

inputs

may

represent

a

substantial

change

in

neural

organization

one

that

affects

not

only

imitation

but

also

communication

only

humans

can

lose

one

modality

-LRB-

e.g.

hearing

-RRB-

and

make

up

for

this

deficit

by

communicating

with

complete

competence

in

a

different

modality

-LRB-

i.e.

signing

-RRB-

Our

discussion

of

limitations

is

not

meant

to

diminish

the

impressive

achievements

of

monkeys

and

apes

but

to

highlight

how

different

the

mechanisms

underlying

the

production

of

human

and

nonhuman

primate

gestures

either

vocally

expressed

or

signed

must

be

After

all

the

average

high

school

graduate

knows

up

to

words

a

vocabulary

achieved

with

little

effort

especially

when

contrasted

with

the

herculean

efforts

devoted

to

training

animals

In

sum

the

impressive

ability

of

any

normal

human

child

for

vocal

imitation

may

represent

a

novel

capacity

that

evolved

in

our

recent

evolutionary

history

some

time

after

the

divergence

from

our

chimpanzee-like

ancestors

The

existence

of

analogs

in

Fig.

The

distribution

of

imitation

in

the

animal

kingdom

is

patchy

Some

animals

such

as

songbirds

dolphins

and

humans

have

evolved

exceptional

abilities

to

imitate

other

animals

such

as

apes

and

distantly

related

species

such

as

monkeys

either

lack

such

abilities

or

have

them

in

a

relatively

impoverished

form

-LSB-

Illustration

John

Yanson

-RSB-

birds

and

cetaceans

suggests

considerable

potential

for

the

detailed

comparative

study

of

vocal

imitation

There

are

ary

exploration

is

desired

one

dating

back

to

tous

use

some

studies

claim

that

animals

however

potential

traps

that

must

be

avoida

chimpanzee-like

ancestor

then

we

need

to

have

a

theory

of

mind

-LRB-

-RRB-

including

a

ed

especially

with

respect

to

explorations

of

explain

how

and

why

such

capacities

sense

of

self

and

the

ability

to

represent

the

the

neurobiological

substrates

of

imitation

emerged

from

an

ancestral

node

that

lacked

beliefs

and

desires

of

other

group

members

For

example

although

macaque

monkeys

such

abilities

-LRB-

-RRB-

-LRB-

Fig.

-RRB-

On

the

side

of

positive

support

recent

studies

and

humans

are

equipped

with

so-called

The

conceptual-intentional

systems

of

nonof

chimpanzees

suggest

that

they

recognize

mirror

neurons

in

the

premotor

cortex

that

linguistic

animals

A

wide

variety

of

studies

the

perceptual

act

of

seeing

as

a

proxy

for

the

respond

both

when

an

individual

acts

in

a

indicate

that

nonhuman

mammals

and

birds

mental

state

of

knowing

-LRB-

-RRB-

These

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studies

suggest

that

at

least

chimpanzees

but

perhaps

no

other

nonhuman

animals

have

a

rudimentary

theory

of

mind

On

the

side

of

negative

support

other

studies

suggest

that

even

chimpanzees

lack

a

theory

of

mind

failing

for

example

to

differentiate

between

ignorant

and

knowledgeable

individuals

with

respect

to

intentional

communication

-LRB-

-RRB-

Because

these

experiments

make

use

of

different

methods

and

are

based

on

small

sample

sizes

it

is

not

possible

at

present

to

derive

any

firm

conclusions

about

the

presence

or

absence

of

mental

state

attribution

in

animals

Independently

of

how

this

controversy

is

resolved

however

the

best

evidence

of

referential

communication

in

animals

comes

not

from

chimpanzees

but

from

a

vaular

human

language

as

consisting

of

words

and

computational

procedures

-LRB-

rules

-RRB-

for

constructing

expressions

from

them

The

computational

system

has

the

recursive

property

briefly

outlined

earlier

which

may

be

a

distinct

human

property

However

key

aspects

of

words

may

also

be

distinctively

human

There

are

first

of

all

qualitative

differences

in

scale

and

mode

of

acquisition

which

suggest

that

quite

different

mechanisms

are

involved

as

pointed

out

above

there

is

no

evidence

for

vocal

imitation

in

nonhuman

primates

and

although

human

children

may

use

domain-general

mechanisms

to

acquire

and

recall

words

-LRB-

-RRB-

the

rate

at

which

children

build

the

lexicon

is

so

massively

different

from

nonhuman

primates

that

one

must

entertain

the

possibility

of

an

independently

evolved

mechanism

Furthermore

unlike

the

best

animal

examples

of

putatively

referential

signals

most

of

the

words

of

human

language

are

not

associated

with

specific

functions

-LRB-

e.g.

warning

cries

food

announcements

-RRB-

but

can

be

linked

to

virtually

any

concept

that

humans

can

entertain

Such

usages

are

often

highly

intricate

and

detached

from

the

here

and

now

Even

for

the

simplest

words

there

is

typically

no

straightforward

word-thing

relationship

if

thing

is

to

be

understood

in

mind-independent

terms

Without

pursuing

the

matter

here

it

appears

that

many

of

the

elementary

properties

of

words

including

those

that

enter

into

referentiality

have

only

weak

analogs

or

homologs

in

natural

animal

communication

systems

with

only

slightly

better

evidence

from

the

training

studies

with

apes

and

dolphins

Future

research

must

therefore

provide

stronger

support

for

the

preFig

Human

and

nonhuman

animals

exhibit

the

capacity

to

compute

numerosities

including

small

precise

cursor

position

or

it

must

instead

number

quantification

and

large

approximate

number

estimation

Humans

may

be

unique

however

in

the

ability

to

show

open-ended

precise

quantificational

skills

with

large

numbers

including

the

integer

count

list

In

parallel

abandon

this

hypothesis

arguing

that

with

the

faculty

of

language

our

capacity

for

number

relies

on

a

recursive

computation

-LSB-

Illustration

John

Yanson

-RSB-

this

component

of

FLB

-LRB-

conceptualintentional

-RRB-

is

also

uniquely

human

Discrete

infinity

and

constraints

riety

of

monkeys

and

birds

species

for

which

tionally

referring

to

the

objects

and

events

on

learning

The

data

summarized

thus

far

there

is

no

convincing

evidence

for

a

theory

that

they

have

encountered

As

such

vervet

although

far

from

complete

provide

overall

of

mind

alarm

calls

and

rhesus

monkey

food

calls

to

support

for

the

position

of

continuity

between

The

classic

studies

of

vervet

monkey

take

two

examples

were

interpreted

as

wordhumans

and

other

animals

in

terms

of

FLB

alarm

calls

-LRB-

-RRB-

have

now

been

joined

by

like

with

callers

referring

to

different

kinds

However

we

have

not

yet

addressed

one

several

others

each

using

comparable

methof

predators

or

different

kinds

of

food

More

issue

that

many

regard

as

lying

at

the

heart

of

ods

with

extensions

to

different

species

-LRB-

marecent

discussions

have

considerably

weaklanguage

its

capacity

for

limitless

expressive

caques

Diana

monkeys

meerkats

prairie

ened

this

interpretation

suggesting

that

if

the

power

captured

by

the

notion

of

discrete

dogs

chickens

-RRB-

and

different

communicative

signal

is

referential

at

all

it

is

in

the

mind

of

infinity

It

seems

relatively

clear

after

nearly

contexts

-LRB-

social

relationships

food

interthe

listener

who

can

extract

information

a

century

of

intensive

research

on

animal

group

aggression

-RRB-

-LRB-

-RRB-

From

these

studabout

the

signaler

's

current

context

from

the

communication

that

no

species

other

than

ies

we

can

derive

five

key

points

relevant

to

acoustic

structure

of

the

call

alone

-LRB-

-RRB-

humans

has

a

comparable

capacity

to

recomour

analysis

of

the

faculty

of

language

First

Despite

this

evidence

that

animals

can

extract

bine

meaningful

units

into

an

unlimited

variindividuals

produce

acoustically

distinctive

information

from

the

signal

there

are

several

ety

of

larger

structures

each

differing

syscalls

in

response

to

functionally

important

reasons

why

additional

evidence

is

required

tematically

in

meaning

However

little

contexts

including

the

detection

of

predators

before

such

signals

can

be

considered

as

preprogress

has

been

made

in

identifying

the

and

the

discovery

of

food

Second

the

acouscursors

for

or

homologs

of

human

words

specific

capabilities

that

are

lacking

in

other

tic

morphology

of

the

signal

although

arbiRoughly

speaking

we

can

think

of

a

particanimals

trary

in

terms

of

its

association

with

a

particular

context

is

sufficient

to

enable

listeners

to

respond

appropriately

without

requiring

any

other

contextual

information

Third

the

number

of

such

signals

in

the

repertoire

is

small

restricted

to

objects

and

events

experienced

in

the

present

with

no

evidence

of

creative

production

of

new

sounds

for

new

situations

Fourth

the

acoustic

morphology

of

the

calls

is

fixed

appearing

early

in

development

with

experience

only

playing

a

role

in

refining

the

range

of

objects

or

events

that

elicit

such

calls

Fifth

there

is

no

evidence

that

calling

is

intentional

in

the

sense

of

taking

into

account

what

other

individuals

believe

or

want

Early

interpretations

of

this

work

suggested

that

when

animals

vocalize

they

are

func

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The

astronomical

variety

of

sentences

any

natural

language

user

can

produce

and

understand

has

an

important

implication

for

language

acquisition

long

a

core

issue

in

developmental

psychology

A

child

is

exposed

to

only

a

small

proportion

of

the

possible

sentences

in

its

language

thus

limiting

its

database

for

constructing

a

more

general

version

of

that

language

in

its

own

mind/brain

This

point

has

logical

implications

for

any

system

that

attempts

to

acquire

a

natural

language

on

the

basis

of

limited

data

It

is

immediately

obvious

that

given

a

finite

array

of

data

there

are

infinitely

many

theories

consistent

with

it

but

inconsistent

with

one

another

In

the

present

case

there

are

in

principle

infinitely

many

target

systems

-LRB-

potential

I-languages

-RRB-

consistent

with

the

data

of

experience

and

unless

the

search

space

and

acquisition

mechanisms

are

constrained

selection

among

them

is

impossible

A

version

of

the

problem

has

been

formalized

by

Gold

-LRB-

-RRB-

and

more

recently

and

rigorously

explored

by

Nowak

and

colleagues

-LRB-

-RRB-

No

known

general

learning

mechanism

can

acquire

a

natural

language

solely

on

the

basis

of

positive

or

negative

evidence

and

the

prospects

for

finding

any

such

domain-independent

device

seem

rather

dim

The

difficulty

of

this

problem

leads

to

the

hypothesis

that

whatever

system

is

responsible

must

be

biased

or

constrained

in

certain

ways

Such

constraints

have

historically

been

termed

innate

dispositions

with

those

underlying

language

referred

to

as

universal

grammar

Although

these

particular

terms

have

been

forcibly

rejected

by

many

researchers

and

the

nature

of

the

particular

constraints

on

human

-LRB-

or

animal

-RRB-

learning

mechanisms

is

currently

unresolved

the

existence

of

some

such

constraints

can

not

be

seriously

doubted

On

the

other

hand

other

constraints

in

animals

must

have

been

overcome

at

some

point

in

human

evolution

to

account

for

our

ability

to

acquire

the

unlimited

class

of

generative

systems

that

includes

all

natural

languages

The

nature

of

these

latter

constraints

has

recently

become

the

target

of

empirical

work

We

focus

here

on

the

nature

of

number

representation

and

rule

learning

in

nonhuman

animals

and

human

infants

both

of

which

can

be

investigated

independently

of

communication

and

provide

hints

as

to

the

nature

of

the

constraints

on

FLN

More

than

years

of

research

using

classical

training

studies

demonstrates

that

animals

can

represent

number

with

careful

controls

for

various

important

confounds

-LRB-

-RRB-

In

the

typical

experiment

a

rat

or

pigeon

is

trained

to

press

a

lever

x

number

of

times

to

obtain

a

food

reward

Results

show

that

animals

can

hit

the

target

number

to

within

a

closely

matched

mean

with

a

standard

deviation

that

increases

with

magnitude

As

the

target

number

increases

so

does

variation

around

the

mean

These

results

have

led

to

the

idea

that

animals

including

human

infants

and

adults

can

represent

number

approximately

as

a

magnitude

with

scalar

variability

-LRB-

-RRB-

Number

discrimination

is

limited

in

this

system

by

Weber

's

law

with

greater

discriminability

among

small

numbers

than

among

large

numbers

-LRB-

keeping

distances

between

pairs

constant

-RRB-

and

between

numbers

that

are

farther

apart

-LRB-

e.g.

versus

is

harder

than

versus

-RRB-

The

approximate

number

sense

is

accompanied

by

a

second

precise

mechanism

that

is

limited

to

values

less

than

but

accurately

distinguishes

from

from

and

from

this

second

system

appears

to

be

recruited

in

the

context

of

object

tracking

and

is

limited

by

working

memory

constraints

-LRB-

-RRB-

Of

direct

relevance

to

the

current

discussion

animals

can

be

trained

to

understand

the

meaning

of

number

words

or

Arabic

numeral

symbols

However

these

studies

reveal

striking

differences

in

how

animals

and

human

children

acquire

the

integer

list

and

provide

further

evidence

that

animals

lack

the

capacity

to

create

openended

generative

systems

Boysen

and

Matsuzawa

have

trained

chimpanzees

to

map

the

number

of

objects

onto

a

single

Arabic

numeral

to

correctly

order

such

numerals

in

either

an

ascending

or

descending

list

and

to

indicate

the

sums

of

two

numerals

-LRB-

-RRB-

For

example

Boysen

shows

that

a

chimpanzee

seeing

two

oranges

placed

in

one

box

and

another

two

oranges

placed

in

a

second

box

will

pick

the

correct

sum

of

four

out

of

a

lineup

of

three

cards

each

with

a

different

Arabic

numeral

The

chimpanzees

performance

might

suggest

that

their

representation

of

number

is

like

ours

Closer

inspection

of

how

these

chimpanzees

acquired

such

competences

however

indicates

that

the

format

and

content

of

their

number

representations

differ

fundamentally

from

those

of

human

children

In

particular

these

chimpanzees

required

thousands

of

training

trials

and

often

years

to

acquire

the

integer

list

up

to

nine

with

no

evidence

of

the

kind

of

aha

experience

that

all

human

children

of

approximately

years

acquire

-LRB-

-RRB-

A

human

child

who

has

acquired

the

numbers

and

-LRB-

and

sometimes

-RRB-

goes

on

to

acquire

all

the

others

he

or

she

grasps

the

idea

that

the

integer

list

is

constructed

on

the

basis

of

the

successor

function

For

the

chimpanzees

in

contrast

each

number

on

the

integer

list

required

the

same

amount

of

time

to

learn

In

essence

although

the

chimpanzees

understanding

of

Arabic

numerals

is

impressive

it

parallels

their

understanding

of

other

symbols

and

their

referential

properties

The

system

apparently

never

takes

on

the

open-ended

generative

property

of

human

language

This

limitation

may

however

reveal

an

interesting

quirk

of

the

child

's

learning

environment

and

a

difference

from

the

training

regime

of

animals

Children

typically

first

learn

an

arbitrary

ordered

list

of

symbols

-LRB-

-RRB-

and

later

learn

the

precise

meaning

of

such

words

apes

and

parrots

in

contrast

were

taught

the

meanings

one

by

one

without

learning

the

list

As

Carey

-LRB-

-RRB-

has

argued

this

may

represent

a

fundamental

difference

in

experience

a

hypothesis

that

could

be

tested

by

first

training

animals

with

an

arbitrary

ordered

list

A

second

possible

limitation

on

the

class

of

learnable

structures

concerns

the

kinds

of

statistical

inferences

that

animals

can

compute

Early

work

in

computational

linguistics

-LRB-

-RRB-

suggested

that

we

can

profitably

think

about

language

as

a

system

of

rules

placed

within

a

hierarchy

of

increasing

complexity

At

the

lowest

level

of

the

hierarchy

are

rule

systems

that

are

limited

to

local

dependencies

a

subcategory

of

so-called

finite-state

grammars

Despite

their

attractive

simplicity

such

rule

systems

are

inadequate

to

capture

any

human

language

Natural

languages

go

beyond

purely

local

structure

by

including

a

capacity

for

recursive

embedding

of

phrases

within

phrases

which

can

lead

to

statistical

regularities

that

are

separated

by

an

arbitrary

number

of

words

or

phrases

Such

long-distance

hierarchical

relationships

are

found

in

all

natural

languages

for

which

at

a

minimum

a

phrase-structure

grammar

is

necessary

It

is

a

foundational

observation

of

modern

generative

linguistics

that

to

capture

a

natural

language

a

grammar

must

include

such

capabilities

-LRB-

Fig.

-RRB-

Recent

studies

suggest

that

the

capacity

to

compute

transitional

probabilities

an

example

of

a

rule

at

the

lowest

level

of

the

hierarchy

might

be

available

to

human

infants

and

provide

a

mechanism

for

segmenting

words

from

a

continuous

acoustic

stream

-LRB-

-RRB-

Specifically

after

familiarization

to

a

continuous

sequence

of

consonant-vowel

-LRB-

CV

-RRB-

syllables

where

particular

trigrams

-LRB-

three

CVs

in

sequence

considered

to

be

words

in

this

context

-RRB-

have

a

high

probability

of

appearing

within

the

corpus

infants

are

readily

able

to

discriminate

these

trigrams

from

others

that

are

uncommon

Although

this

ability

may

provide

a

mechanism

for

word

segmentation

it

is

apparently

not

a

mechanism

that

evolved

uniquely

in

humans

or

for

language

The

same

computation

is

spontaneously

available

to

human

infants

for

visual

sequences

and

tonal

melodies

-LRB-

-RRB-

as

well

as

to

nonhuman

primates

-LRB-

cotton-top

tamarins

-RRB-

tested

with

the

same

methods

and

stimuli

-LRB-

-RRB-

Similarly

in

the

same

way

that

human

infants

appear

capable

of

computing

algebraic

rules

that

operate

over

particular

CV

sequences

-LRB-

-RRB-

so

too

can

cotton-top

tamarins

-LRB-

-RRB-

again

demonstrating

that

the

capacity

to

discover

abstract

rules

at

a

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local

level

is

not

unique

to

humans

and

almost

certainly

did

not

evolve

specifically

for

language

Fitch

and

Hauser

-LRB-

-RRB-

recently

completed

a

study

comparing

finite-state

and

phrasestructure

grammar

acquisition

in

human

adults

and

tamarins

using

the

same

subjects

and

methods

as

the

studies

above

The

phrase-structure

rule

tested

was

AnBn

where

A

and

B

were

each

represented

by

one

of

a

set

of

eight

different

CVs

The

rule

therefore

specified

both

a

set

of

consistent

strings

-LRB-

n

A

's

must

precede

n

B

's

-RRB-

and

a

set

of

inconsistent

strings

the

latter

consisted

of

violations

of

order

-LRB-

B

tokens

precede

A

tokens

-RRB-

or

of

patterning

-LRB-

alternations

of

A

's

and

B

's

such

as

ABAB

-RRB-

Results

showed

that

human

adults

rapidly

learned

this

rule

implicitly

distinguishing

consistent

and

inconsistent

strings

Tamarins

in

contrast

failed

in

three

separate

experiments

testing

their

ability

to

acquire

this

grammar

but

they

readily

mastered

a

finite-state

variant

-LRB-

ABn

-RRB-

implemented

with

the

same

stimuli

and

testing

conditions

This

suggests

that

tamarins

have

a

limited

capacity

to

learn

the

type

of

longdistance

hierarchical

dependencies

necessary

to

achieve

the

class

of

phrase-structure

grammars

If

true

this

limitation

would

place

severe

restrictions

on

their

capacity

to

learn

any

natural

human

language

It

is

currently

unclear

whether

this

limitation

generalizes

to

other

animals

and

whether

it

is

similarly

imposed

on

humans

at

different

stages

of

development

Nonetheless

such

experiments

provide

an

empirical

approach

to

exploring

key

differences

between

humans

and

animals

relevant

to

FLN

Our

review

has

stressed

the

usefulness

of

animal

data

for

theories

about

humans

but

this

exchange

need

not

be

one-way

As

the

research

program

we

have

sketched

progresses

more

general

principles

about

cognitive

evolution

may

emerge

For

example

suppose

we

adopt

the

conception

of

hypothesis

oversimplifying

radically

that

the

interface

systems

sensory-motor

and

conceptual-intentional

are

given

and

the

innovation

that

yielded

the

faculty

of

language

was

the

evolution

of

the

computational

system

that

links

them

The

computational

system

must

-LRB-

i

-RRB-

construct

an

infinite

array

of

internal

expressions

from

the

finite

resources

of

the

conceptual-intentional

system

and

-LRB-

ii

-RRB-

provide

the

means

to

externalize

and

interpret

them

at

the

sensory-motor

end

We

may

now

ask

to

what

extent

the

computational

system

is

optimal

meeting

natural

conditions

of

efficient

computation

such

as

minimal

search

and

no

backtracking

To

the

extent

that

this

can

be

established

we

will

be

able

to

go

beyond

the

-LRB-

extremely

difficult

and

still

distant

-RRB-

accomplishment

of

finding

the

principles

of

the

faculty

of

language

to

an

understanding

of

why

the

faculty

follows

these

particular

principles

and

not

others

We

would

then

understand

why

languages

of

a

certain

class

are

attainable

whereas

other

imaginable

languages

are

impossible

to

learn

and

sustain

Such

progress

would

not

only

open

the

door

to

a

greatly

simplified

and

empirically

more

tractable

evolutionary

approach

to

the

faculty

of

language

but

might

also

be

more

generally

applicable

to

domains

beyond

language

in

a

wide

range

of

species

perhaps

especially

in

the

domain

of

spatial

navigation

and

foraging

where

problems

of

optimal

search

are

relevant

For

example

elegant

studies

of

insects

birds

and

primates

reveal

that

individuals

often

search

for

food

by

an

optimal

strategy

one

involving

minimal

distances

recall

of

locations

searched

and

kinds

of

objects

retrieved

-LRB-

-RRB-

Only

after

a

concerted

multidisciplinary

attack

on

the

problems

of

language

evolution

paralleling

years

of

optimal

foraging

research

will

we

learn

whether

such

similarities

are

more

than

superficial

mals

but

in

a

noncommunicative

domain

then

we

are

more

likely

to

pinpoint

the

mechanisms

underlying

this

ability

and

the

selective

pressures

that

led

to

it

This

discovery

in

turn

would

open

the

door

to

another

suite

of

puzzles

Why

did

humans

but

no

other

animal

take

the

power

of

recursion

to

create

an

open-ended

and

limitless

system

of

communication

Why

does

our

system

of

recursion

operate

over

a

broader

range

of

elements

or

inputs

-LRB-

e.g.

numbers

words

-RRB-

than

other

animals

One

possibility

consistent

with

current

thinking

in

the

cognitive

sciences

is

that

recursion

in

animals

represents

a

modular

system

designed

for

a

particular

function

-LRB-

e.g.

navigation

-RRB-

and

impenetrable

with

respect

to

other

systems

During

evolution

the

modular

and

highly

domain-specific

system

of

recursion

may

have

become

penetrable

and

domain-general

This

opened

the

way

for

humans

perhaps

uniquely

to

apply

the

power

of

recursion

to

other

problems

This

change

from

domain-specific

to

domain-general

may

have

been

guided

by

particular

selective

pressures

unique

to

our

evolutionary

past

or

as

a

consequence

-LRB-

by-product

-RRB-

of

other

kinds

of

neural

reorganization

Either

way

these

are

testable

hypotheses

a

refrain

that

highlights

the

importance

of

comparative

approaches

to

the

faculty

of

language

N.

Chomsky

Aspects

of

the

Theory

of

Syntax

-LRB-

MIT

Press

Cambridge

MA

-RRB-

Reflections

on

Language

-LRB-

Pantheon

New

York

-RRB-

M.

D.

Hauser

The

Evolution

of

Communication

-LRB-

MIT

Press

Cambridge

MA

-RRB-

R.

Jackendoff

Foundations

of

Language

-LRB-

Oxford

Univ.

Press

New

York

-RRB-

L.

Jenkins

Biolinguistics

-LRB-

Cambridge

Univ.

Press

Cambridge

-RRB-

E.

H.

Lenneberg

Biological

Foundations

of

Language

-LRB-

Wiley

New

York

-RRB-

P.

Lieberman

The

Biology

and

Evolution

of

Language

-LRB-

Harvard

Univ.

Press

Cambridge

MA

-RRB-

A.

Liberman

Speech

A

Special

Code

-LRB-

MIT

Press

Cambridge

MA

-RRB-

W.

T.

Fitch

Trends

Cognit

Sci

-LRB-

-RRB-

D.

L.

Cheney

R.

M.

Seyfarth

How

Monkeys

See

the

World

Inside

the

Mind

of

Another

Species

-LRB-

Univ.

of

Chicago

Press

Chicago

-RRB-

A.

Doupe

P.

Kuhl

Annu

Rev.

Neurosci

-LRB-

-RRB-

P.

Marler

Am

Sci

-LRB-

-RRB-

C.

Darwin

On

the

Origin

of

Species

-LRB-

John

Murray

London

-RRB-

The

Descent

of

Man

and

Selection

in

tion

to

Sex

-LRB-

John

Murray

London

-RRB-

A.

M.

Liberman

K.

S.

Harris

H.

S.

Hoffman

B.

C.

Griffith

J.

Exp

Psychol

-LRB-

-RRB-

A.

M.

Liberman

F.

S.

Cooper

D.

P.

Shankweiler

M.

Studdert-Kennedy

Psychol

Rev.

-LRB-

-RRB-

P.

K.

Kuhl

J.

D.

Miller

Science

-LRB-

-RRB-

P.

K.

Kuhl

D.

M.

Padden

Percept

Psychophys

-LRB-

-RRB-

K.

R.

Kluender

R.

Diehl

P.

R.

Killeen

Science

-LRB-

-RRB-

S.

J.

Gould

in

Evolution

Brain

and

Behavior

Persistent

Problems

R.

B.

Masterton

W.

Hodos

H.

Jerison

Eds

-LRB-

Wiley

New

York

-RRB-

pp.

W.

J.

Gehring

Master

Control

Genes

in

Development

and

Evolution

The

Homeobox

Story

-LRB-

Yale

Univ.

Press

New

Haven

CT

-RRB-

R.

M.

Seyfarth

D.

L.

Cheney

in

The

Design

of

Animal

Conclusions

We

conclude

by

making

three

points

First

a

practical

matter

Linguists

and

biologists

along

with

researchers

in

the

relevant

branches

of

psychology

and

anthropology

can

move

beyond

unproductive

theoretical

debate

to

a

more

collaborative

empirically

focused

and

comparative

research

program

aimed

at

uncovering

both

shared

-LRB-

homologous

or

analogous

-RRB-

and

unique

components

of

the

faculty

of

language

Second

although

we

have

argued

that

most

if

not

all

of

FLB

is

shared

with

other

species

whereas

FLN

may

be

unique

to

humans

this

represents

a

tentative

testable

hypothesis

in

need

of

further

empirical

investigation

Finally

we

believe

that

a

comparative

approach

is

most

likely

to

lead

to

new

insights

about

both

shared

and

derived

features

thereby

generating

new

hypotheses

concerning

the

evolutionary

forces

that

led

to

the

design

of

the

faculty

of

language

Specifically

although

we

have

said

relatively

little

about

the

role

of

natural

selection

in

shaping

the

design

features

of

FLN

we

suggest

that

by

considering

the

possibility

that

FLN

evolved

for

reasons

other

than

language

the

comparative

door

has

been

opened

in

a

new

and

-LRB-

we

think

-RRB-

exciting

way

Comparative

work

has

generally

focused

on

animal

communication

or

the

capacity

to

acquire

a

human-created

language

If

however

one

entertains

the

hypothesis

that

recursion

evolved

to

solve

other

computational

problems

such

as

navigation

number

quantification

or

social

relationships

then

it

is

possible

that

other

animals

have

such

abilities

but

our

research

efforts

have

been

targeted

at

an

overly

narrow

search

space

-LRB-

Fig.

-RRB-

If

we

find

evidence

for

recursion

in

ani

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