Using a specific type of sedimentary rock as their guide, researchers are beginning to tackle the question of when animals first appeared on Earth.
Estimates of the arrival of Earth’s first animals—mostly small, soft-bodied marine creatures—differ by hundreds of millions of years. Some estimates suggest that the first animals must have appeared at least 800 million years ago, based on how long it would have taken for the genetic differences between today’s major animal groups to develop.
The oldest known animal fossils date back 574 million years. This discontinuity has puzzled scientists since the time of Charles Darwin, who became famous for the sudden appearance of animal fossils in Cambrian rocks (539 to 485 million years ago) but not in older rocks.
„One of the main challenges we face in studying the origin of animals is determining when they first appeared on Earth,” said Derek Briggs, a professor of earth and planetary sciences at Yale University and senior author of the new study published in the journal Nature. Trends in Ecology and Evolution.
„What are the chances of finding fossils of ancient, small, decomposing animals in rocks older than 600 million years? Is the absence of animals in those rocks real or simply a situation not favorable to preservation?”
For the study, Briggs and colleagues focused on sedimentary rocks such as the Burgess Shale in the Canadian Rockies region of British Columbia. The 508-million-year-old Burgess Shale deposit is known to preserve fossils of animals with soft biological tissues of varying composition, including animals without a shell or mineralized skeleton.
Several environmental factors contribute to better preservation in deposits such as the Burgess Shale. Muddy sediments contained low levels of oxygen and sulfate; A mixture of clay inhibits bacteria that promote the decomposition of organic residues; It has mineralization properties that mimic soft tissue in geologically long-lived ores; and the rapid cementation of sedimentary imprints on possible fossils at an early stage.
The researchers studied areas around the world with sediments closest to the conditions of the Burgess Shale, particularly older deposits with sediments with similar clay compositions. Those sites—including the Lakanda Group in Russia, the Winniat Formation in Canada, and the Svanbergfjellet Formation in Norway—are 789 million years old and older.
But none of those sites yield animal fossils, indicating that animals did not evolve at that time.
„Such systems are available for fossilization as in younger rocks, but yield no animals,” says Briggs. „Such considerations suggest that animals did not exist before 790 million years ago. Undoubtedly, this estimate will be tested and refined by future discoveries.
Ross Anderson of the University of Oxford, a former graduate student in the Briggs lab at Yale, is lead author of the study. Co-authors are University of California, Santa Barbara and University of Cambridge.
Source: Yale University