Chemical traces left behind by ancient sponges suggest that the first animals may have appeared more than 200 million years earlier than the oldest undisputed animal fossils indicate, according to a new analysis of the fossil and biomarker record. The finding, if confirmed, would push the origin of animal life to before 635 million years ago, well before the emergence of the Ediacaran biota that has long served as the earliest widely accepted evidence of animals.
The oldest undisputed animal fossils come from the Ediacaran biota and are roughly 574 million years old. These soft-bodied organisms, preserved in rocks around the world, represent the first large and complex life forms that scientists agree were animals. But lipid biomarkers — molecular traces left by sponges — have been detected in rock layers older than 635 million years, implying that animal life already existed in some form during the Cryogenian period, before the planet emerged from the last of its global glaciations.
Sponges are among the simplest animals alive today, lacking true tissues and organs. If they were present before 635 million years ago, they would have been small, soft-bodied, and easily overlooked in the fossil record. Their scarcity as fossils is not surprising; what survives instead are the chemical fingerprints of the molecules they produced. These lipid biomarkers are stable over hundreds of millions of years and can be extracted from ancient sedimentary rocks, offering a window into life that left no visible remains.
The discrepancy between the fossil and biomarker records has been a subject of debate among paleontologists for years. Some researchers have argued that the biomarkers could have been produced by other organisms or contaminated during later geological processes. Others maintain that the molecular evidence is robust and points to a deep, hidden history of animal life that predates the Ediacaran by a wide margin. The new study adds weight to the latter view, suggesting that the fossil record is incomplete rather than the biomarkers misleading.
If animals arose before 635 million years ago, the timeline of early evolution shifts dramatically. The Cryogenian period, lasting from about 720 to 635 million years ago, was marked by severe ice ages that may have covered much of the planet in ice. The presence of animals during or before this interval would mean that complex life endured extreme environmental conditions, or that animals evolved in refuges such as deep oceans or near hydrothermal vents. It would also imply that the evolutionary steps leading to multicellular animals began much earlier than the Cambrian explosion, the rapid diversification of life that occurred around 540 million years ago.
The implications extend beyond paleontology. Understanding when animals first appeared helps calibrate molecular clocks, which estimate evolutionary divergence times based on genetic differences among living species. If the biomarker evidence is correct, molecular clocks that place the origin of animals around 800 million to 900 million years ago gain support, while those that align more closely with the fossil record may need revision. The question of whether the first animals were sponges or something even simpler — perhaps colonial protists — also remains open.
For now, the oldest undisputed animal fossils remain those of the Ediacaran biota, and the biomarker evidence, while suggestive, is not universally accepted. The new analysis does not claim to have found fossilized animals older than 574 million years; instead, it highlights a persistent gap between two lines of evidence. Resolving that gap will require additional geochemical analyses, better-preserved sedimentary sequences, and perhaps new fossils from rocks that have so far yielded only chemical traces. Until then, the earliest chapters of animal evolution remain written largely in molecules rather than in stone.