Earliest evidence of bilaterians

Article By:

Pecoits, Ernesto Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.

Konhauser, Kurt O. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.

Aubet, Natalie R. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.

Gingras, Murray K. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB140294

Life has existed on Earth for almost 4 billion years, but most major groups of animals only appear in the fossil record approximately 542 million years ago (MYA) in a relatively short period of time known as the Cambrian explosion of life. During much of the Precambrian, living organisms were small, unicellular, and comparably simple. In this context, the Ediacaran Period (630–542 MYA) is arguably one of the most critical periods in evolutionary history. Complex animals with bilateral symmetry (that is, with a front end and a back end, as well as an up side and a down side) probably emerged in the Ediacaran, but the timing of their appearance and diversification is still a matter of debate. A major factor that negatively affects our understanding of Ediacaran life is that the first animals lacked skeletons or any external hard parts; thus, they had a very low preservation potential. As such, fossilized tracks and trails (known as ichnofossils or trace fossils) provide important evidence of the first bilaterians.

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