The earliest known animal fossil, Vernanimalcula, dates back 600 million years, but evidence suggests animals may have existed over a billion years ago. This includes a decline in stromatolites, an increase in acritarchs, and trace fossils of worm-like metazoans. However, the absence of body fossils from this period makes it a controversial topic.
Most scientists agree that the first animals emerged about 600 million years ago, during the Ediacaran Period. A simple oval fossil, just 200 microns in diameter, called Vernanimalcula (“little vernal animal”) represents the earliest known body fossil of an animal (although some scientists believe the fossil is a relic created through inorganic processes, or a fossil of a giant bacterium).
Vernanimalcula was discovered in 2005 in a phosphate fossil deposit in Guizhou Province, China known as the Doushantuo Formation. The Doushantuo Formation also has some fossils interpreted as embryos dated to 630 million years ago, a few million years after the Varangian/Marinoan glaciation, one of the most severe glaciations in planetary history.
Although the earliest fossils of the body date back 600 million years, there is some intriguing evidence that animals may have existed before this date, up to a billion years ago or even a little more. Proponents of the idea point to the fact that, for over a hundred years, scientists believed the oldest animals came from the Cambrian period, but in 1957, after the discovery of the iconic Charnia fossil, it was realized that the animals had lived much like 50 million years before the beginning of the Cambrian, during the Ediacaran period.
There are several pieces of evidence suggesting the existence of animals up to a billion years ago. One is the sudden decline of stromatolites about a billion years ago. Today, stromatolites are isolated in areas hostile to other life forms, such as extremely salty lagoons, because they are otherwise eaten. The sharp decline in stromatolytic diversity over a billion years ago is evidence that grazing animals evolved during this period. This hypothesis is supported by another stromatolite decline during the Ordovician evolutionary radiation, when numerous marine animals evolved, and stromatolite flowering soon after the Late Ordovician and Late Permian extinctions.
A second piece of evidence for the possibility of animals over a billion years ago comes from microscopic fossils called acritarchs. Acritarchs represent a range of organisms, including cysts of resting algae and possible early animal embryos. Acritarchs appear in the fossil record about 2 billion years ago, but exploded in diversity and numbers about a billion years ago. Thorny acritarchs also appeared in this period, suggesting an adaptation to defend against predators. If there were no predators, why would acritarchs suddenly evolve spines?
A third evidence for metazoans over a billion years ago comes from trace fossils dated to this period. Trace fossils are small scratches on the ground made by animals in motion, but some can be created by physical processes and misinterpreted as biological in origin. Some trace fossils in India have been dated to 1 billion years ago. Scientists who have analyzed these tracks concluded that they were produced by a worm-like metazoan up to 5 mm wide, traveling over and under the microbial mats covering the sea floor at the time. Other scientists disagree with this interpretation.
Whether or not animals existed over a billion years ago is a controversial topic. Many researchers question the possibility due to the observed absence of body fossils from this period. Any paleontologist discovering a fossil of the pre-Ediacaran body was sure to become famous, but as of 2008, no such fossil has been found.
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