Structure and development of early multicellular organisms
Until the late 1950s, the Precambrian was not believed to have hosted multicellular organisms. However, with radiometric dating techniques, it has been found that fossils initially found in the Ediacara Hills in Southern Australia date back to the late Precambrian. These fossils are body impressions of organisms shaped like disks, fronds and some with ribbon patterns that were most likely tentacles.
These are the earliest multicellular organisms in Earth's history, despite the fact that unicellularity had been around for a long time before that. The requirements for multicellularity were embedded in the genes of some of these cells, specifically choanoflagellates. These are thought to be the precursors for all animals. They are highly related to sponges (Porifera), which are the simplest multicellular animals.
In order to understand the transition to multicellularity during the Precambrian, it is important to look at the requirements for multicellularity—both biological and environmental.
Precambrian.
The Precambrian dates from the beginning of Earth's formation (4.6 billion years ago) to the beginning of the Cambrian Period, 539 million years ago. The Precambrian consists of the Hadean, Archaean and Proterozoic eons. Specifically, this article examines the Ediacaran, when the first multicellular bodies are believed to have arisen, as well as what caused the rise of multicellularity. This time period arose after the Snowball Earth of the mid Neoproterozoic. The "Snowball Earth" was a period of worldwide glaciation, which is believed to have served as a population bottleneck for the subsequent evolution of multicellular organisms.
Precambrian bodies.
The Earth formed around 4.6 billion years ago, with unicellular life emerging somewhat later after the cessation of the Late Heavy Bombardment, a period of intense asteroid impacts possibly caused by migration of the gas giant planets to their current orbits, however multicellularity and bodies are a relatively recent event in Earth's history. Bodies first started appearing towards the end of the Precambrian Era, during the Ediacaran period. The fossils of the Ediacaran period were first found in Southern Australia in the Ediacara Hills, hence the name. However these fossils were initially thought to be part of the Cambrian and it wasn't until the late 1950s when Martin Glaessner identified the fossils as actually being from the Precambrian era. The fossils that were found date to about 600 million years ago and are found in a variety of morphologies.
Fossils of the Ediacaran.
"For more information, see Ediacaran biota".
The fossils found that date back to the Precambrian lack distinct structures since there were no skeletal forms during this period. Skeletons did not arise until the Cambrian Period when oxygen levels increased. This is because skeletons require collagen, which uses Vitamin C as a cofactor, which requires oxygen. For more information on the rise of oxygen see the section on oxygen. The majority of fossils from this Era come from either Mistaken Point on the East Coast of Canada or the Ediacara Hills in Southern Australia.
Most of the fossils are found as impressions of soft-bodied organisms in the shape of disks, ribbons or fronds. There are also trace fossils that provide evidence that some of these Precambrian organisms were most-likely worm-like creatures that were locomotive. Most of these fossils lack any recognizable heads, mouths or digestive organs, and are thought to have fed via absorptive mechanisms and symbiotic relationships with chemoautotrophs (Chemotroph), photoautotrophs (Phototroph) or osmoautotrophs. The ribbon-like fossils resemble tentacled organisms, and are thought to have fed by capturing prey. The frondose fossils resemble sea pens and other cnidarians. The trace fossils suggest that there were annelid type creatures, and the disk fossils resemble sponges. Despite these similarities, much of the identification is speculation since the fossils do not show very distinct structures. Other fossils do not resemble any known lineages.
Many of the organisms, such as "Charnia", found in Mistaken Point, were not like any organisms seen today. They had distinct bodies, however were lacking a head and digestive regions. Rather their body was organized in a very simple, fractal-like branching pattern. Every element of the body was finely branched and grew by repetitive branching. This allowed the organism to have a large surface area and maximize nutrient absorption without needing a mouth and digestive system. However, there was minimal genetic information and therefore did not have the requirements that would have allowed them to evolve more efficient feeding techniques. This means they were probably outcompeted by other organisms, and thus became extinct.
The organisms found in the Ediacaran Hills in Southern Australia displayed either radially symmetric body plans or, one organism, "Spriggina," displayed the first bilateral symmetry. The Ediacaran Hills are thought to have once had a shallow reef where more light could penetrate the bottom of the ocean floor. This allowed for more diversity of organisms. The organisms found here resemble relatives of the cnidarians, mollusks or annelids.
"Charnia".
"Charnia" fossils were originally found in the Charnwood Forest in England, hence named "Charnia". These fossils are from marine organisms that lived on the bottom of the ocean floor. The fossils have a fractal body plan and were frond shaped, meaning they resembled broad-leafed plants such as ferns. However they could not have been plants since they resided in the dark depths of the ocean floor. In Charnwood Forest, "Charnia" was found as an isolated species, however there were many more fossils found on the East Coast of Canada in Mistaken Point in Newfoundland. "Charnia" was attached to the bottom of the ocean floor, and was strongly current aligned. This is seen because there are disk-like shapes at the bottom of the "Charnia" fossil, which show where "Charnia" was tethered, and all the nearby fossils are facing the same direction. These fossils at Mistaken Point were preserved well under volcanic ash and layers of soft mud. It has been determined via radiometric dating of the fossils that "Charnia" must have lived around 565 million years ago.
"Dickinsonia".
"Dickinsonia" fossils are another notable fossil from the Ediacaran period, found in Southern Australia and Russia. It remains unknown what type of organism "Dickinsonia" was; however, it has been considered a polychaete, turbellarian/annelid worm, jellyfish, polyp, protist, lichen or mushroom. They were preserved in quartz sandstones, and date back to around 550 million years ago. "Dickinsonia" were soft-bodied organisms, that show some evidence of very slow movement. There are faint, circular imprints in the rock which follow a path, and then following the same path there is a more definite circular imprint of the same size. This indicates that the organism probably moved slowly from one feeding area to the next and absorbed nutrients. It is speculated that the organism probably had very small appendages that allowed it to move much like starfish do today.
"Spriggina".
"Spriggina" fossils represent the first known organisms with a bilaterally symmetric body plan. They had a head, tail and almost identical halves. They probably had sensory organs in the head and digestive organs in the tail which would have allowed them to find food more efficiently. They were capable of locomotion, which gave them an advantage over other organisms from that era that were either tethered to the bottom of the ocean floor or moved very slowly. "Spriggina" was soft bodied, which leave the fossils as faint imprints. It is most likely related to annelids, however there is some speculation that it could be related to arthropods since it somewhat resembles trilobite fossils.
Trace fossils.
The Ediacaran fossils of Southern Australia contain trace fossils, which indicate that there were motile benthic organisms. The organisms that produced the traces in the sediments were all worm-like sediment feeders or detritus feeders (Detritivore). There are a few trace fossils, which resemble arthropod trails. Evidence suggests that arthropod-like organisms existed during the Precambrian. This evidence is in the type of trails left behind; specifically one specimen that shows six pairs of symmetrically placed impressions, which resemble trilobite walking trails.
Transition from unicellularity to multicellularity.
For the majority of Earth’s history life has been unicellular. However, unicellular organisms had the ingredients in them for multicellularity to arise. Despite having the ingredients for multicellularity, organisms were restricted due to the lack of hospitable environmental conditions. The rise of oxygen (The Great Oxygenation Event) led organisms to be able to develop more complex body plans. In order for multicellularity to have occurred, organisms must have been capable of cellular communication, aggregation, and specialized functions. The transition to multicellularity that began the evolution of animals from protozoa is one of the most poorly understood of history’s life events. Understanding choanoflagellates and their relation to sponges is important when positing theories on the origins of multicellularity
Choanoflagellates.
Choanoflagellates, also called "collar-flagellates" are unicellular protists that exist in both freshwaters and oceans. Choanoflagellates have a spherical (or ovoid) cell body and a flagellum that is surrounded by a collar composed of actin microvilli. The flagellum is used to facilitate movement and food intake. As the flagellum beats, it takes in water through the microvilli attached to the collar, which helps filter out unwanted bacteria and other tiny food particles. Choanoflagellates are composed of approximately 150 species and reproduce by simple division.
Choanoflagellate Salpingoeca rosetta.
The choanoflagellate "Salpingoeca rosetta" is a rare freshwater eukaryote consisting of a number of cells embedded in a jelly-like matrix. This organism demonstrates a very primitive level of cell differentiation and specialization. This is seen with flagellated cells and their collar structures that move the cell colony through the water, while the amoeboid cells on the inside serve to divide into new cells to assist in colony growth.
 ;br ;Similar low level cellular differentiation and specification can also be seen in sponges. They also have collar cells (also called choanocytes due to their similarities to choanoflaggellates) and amoeboid cells arranged in a gelatinous matrix. Unlike choanoflagellate "Salpingoeca rosetta", sponges also have other cell-types that can perform different functions (see sponges). Also, the collar cells of sponges beat within canals in the sponge body, whereas "Salpingoeca rosetta"’s collar cells reside on the inside and it lacks internal canals. Despite these minor differences, there is strong evidence that Proterospongia and Metazoa are highly related.
Choanoflagellate Perplexa.
These choanoflagellates are able to attach to one another via the pairing of collar microvilli.
Choanoflagellate Codosiga Botrytis and Desmerella.
These choanoflagellates are capable of forming colonies via fine intercellular bridges that allow the individual cells to attach. These bridges resemble ring canals that link developing spermatogonia or oogonia in animals.
Sponges (Porifera).
Sponges are some of Earth’s oldest and most ubiquitous animals. The appearance of sponge spicule fossils date back to the Precambrian Era around 580 million years ago. An assemblage of these fossils were found in the Doushanto formation in Southern China. Some circular impressions from the Ediacaran Hills in Southern Australia are also reported to be sponges. They are one of the only lineages of metazoans from this era that continue to survive, and remain relatively unchanged.
Sponges are such successful organisms due to their simple, yet effective morphology. They do not possess mouths or any digestive, nervous or circulatory systems. Instead they are filter feeders, which means that they obtain food through nutrients in the water. They have pores, called ostia, that water travels through to a chamber called the spongocoel, and exits through a chamber called the osculum. Through this water filtration system, they obtain nutrients that are needed for their survival. Specifically, they intracellularly digest bacteria, micro-algae or colloids.
Sponge skeletons consist of either spongin or calcareous and siliceous spicules with some collagen molecules interspersed. The collagen holds the sponge cells together. Different lineages of sponges are distinguished based on the composition of their skeletons. The three main classes of sponges are Demospongiae, Hexactinellid, and Calcareous.
Demonsponges are the most well-known type of sponge since they are used by humans. They are distinguished by a siliceous skeleton of two and four rayed spicules and contain the protein spongin.
Hexactinellid are also called glass sponges, and are distinguished by a six-rayed glass skeleton. These sponges are also capable of carrying out action potentials.
Calcareous sponges are characterized by a calcium carbonate skeleton and comprise less than 5% of sponges.
Cells.
Sponges have around 6 different types of cells that can perform different functions. Sponges are a good model for studying the origin of multicellularity because the cells are capable of communicating with one another and re-aggregating. In an experiment conducted by Henry Van Peters Wilson in 1910, it was found that cells from dissociated sponges could send out signals and recognize each other to