How do scientists figure out relationships between living
things? Some scientists compare every single feature of every
single organism to try to work out their evolutionary
relationships. But, another group of scientists has come up with
a more efficient, more reliable approach. With this method,
called cladistic analysis, you only compare a few carefully
selected traits. For example, how would a scientist figure out
evolutionary relationships between sharks, dolphins and wolves?
Two fishy-looking animals and a four-legged, furry one. First we
need to find the ancestral line they all came from. For this, we
can go to the fossil record. Fish appear in the fossil record
before four-legged animals (tetrapods), so the ancestor of this
group must be at least as primitive as a fish. The most ancient
fish were jawless, and since our whole group has jaws, we can
safely choose a jawless fish as a fairly recent ancestor.
Unfortunately, this ancestor is long dead. To compare anything
besides bones, we need a living ani