The Evolutionary History of the Animal Kingdom

Pre-Cambrian Animal Life

The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time. Some protist species called choanoflagellates closely resemble the choanocyte cells in the simplest animals, sponges. In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

Table A describes eras in earth’s history. The earth’s history is divided into four eons, the Pre-Archean, Archaea, Proteozoic, Phanerozoic. The oldest eon, the Pre-Archean, spans the beginning of earth’s history to about 3.8 billion years ago. The Archean eon spans 2.5 to 3.8 billion years ago, and the Proterozoic spans 570 million to 2.5 billion years ago. The Pharenozoic eon, from 570 million years ago to present time, is sub-divided into the Paleozoic, Mesozoic and Cenozoic eras. The Paleozoic era, from 240 to 570 million years ago, is further divided into seven periods: the Cambrian from 500 to 570 million years ago, the Ordovician from 435 to 500 million years ago, the Silurian from 410 to 435 million years ago, the Devonian from 360 to 410 million years ago, the Missisippian from 330 to 360 million years ago, the Pennsylvanian from 290 to 330 million years ago, and the Permian from 240 to 290 million years ago. The Mesozoic era, from 66 to 240 million years ago, is divided into three periods, the Triassic from 205 to 240 million years ago, the Jurassic from 138 to 205 million years ago, and the Cretaceous, from 66 to 138 million years ago. The Cenozoic era, from 66 million years ago to modern times, is divided into two eras, the Tertiary and the Quaternary. The tertiary period spans 66 to 1.6 million years ago. The quaternary period spans 1.6 million years ago to modern times. Illustration B shows geological periods in a spiral starting with the beginning of earth’s history  at the bottom and ending with modern times at the top. The diversity and complexity of life increases toward the top of the spiral.
(a) Earth’s history is divided into eons, eras, and periods. Note that the Ediacaran period starts in the Proterozoic eon and ends in the Cambrian period of the Phanerozoic eon. (b) Stages on the geological time scale are represented as a spiral. (credit: modification of work by USGS)

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figureab.

Part a shows a fossil that resembles a wheel, with spokes radiating out from the center, imprinted on a rock. Part b shows a fossil that resembles a teardrop shaped leaf, with grooves radiating out from a central rib.
Fossils of (a) Cyclomedusa and (b) Dickinsonia date to 650 million years ago, during the Ediacaran period. (credit: modification of work by “Smith609”/Wikimedia Commons)

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.