Scientists led by Orin Lole Durbin say animals may have appeared on Earth about 800 million years ago, a timeline that places the origin of life’s earliest animal branches deep in the Tonian era and far earlier than the unstable solar system implied by the fossil record alone. Durbin worked at the University of Oxford during the study and is now at Virginia Tech.
The team’s estimate pushes the possible origin of animals back more than 200 million years from the 574 million-year Ediacara fossil minimum. Durbin said, "The timing of the origin of animals on Earth has puzzled scientists for centuries."
Orin Lole Durbin study
Durbin’s team used molecular clock analysis to compare evolutionary timing against fossil deposits from China’s Weng'an Biota, dated to 590 million years ago, and Mongolia’s Kheseen Biota, dated to 550 million years ago. The study argues that those deposits help set a broader window for when Metazoa could have emerged, even though the earliest clearly preserved animals still come from the Ediacara period.
Durbin also said, "Unambiguous animal body fossils within the Ediacara Macrobiota provide a hard minimum calibration of 574 [million years] for crown Metazoa. Animals must have evolved before this date. Determining a maximum calibration for animals (a date at which they had not yet evolved) is a more complex task."
China and Mongolia fossils
The comparison leaves a split between a hard fossil floor and an older estimated origin. The fossil record gives a minimum at 574 million years, but the new analysis suggests animals may have originated about 800 million years ago, which would place early metazoan ancestors in the Tonian era and through Snowball Earth episodes.
That same result supports a simple reading of the evidence: simple marine sponges and comb jellies may have been part of the earliest animal branch, even if they left little direct trace in the rocks. The study says hypotheses linking animal diversification to Cryogenian Snowball Earth glaciations cannot be discounted.
Cryogenian Snowball Earth
Durbin said, "The possibility of a pre-Ediacaran origin and diversification of animals means we cannot discount hypotheses that link the timing of animal diversification to Cryogenian Snowball Earth glaciations." The open issue now is how molecular clock analysis turned deposits dated to 590 million and 550 million years ago into an estimate about 800 million years ago, and whether paleontological and geochemical data can narrow that gap.
