A study published in Science Advances rethinks how the origin of animals is calculated and questions whether the absence of fossils allows us to rule out their existence.
“A fossil-free site proves they did not exist,” is the premise that has been challenged in a new investigation published in Science Advances and conducted by an international team with participants from Oxford and Yale. And what if the first animals emerged earlier than the fossils reveal? The debate is open.
Ross Anderson, an Oxford researcher and coauthor of the study, will explain why some sites with exceptional preservation may lack animal fossils from eras in which we know they existed. The key to understanding this puzzle lies in distinguishing between the appearance of an organism and the preservation of its traces.
In the lines to follow, we will see on what arguments they based the hypothesis, even though it has not been proven, that posits a much older animal history and why the start date of the animal kingdom remains open.
The Origin of Animals
Animals could have evolved up to 200 million years before their first appearance in the fossil record, in the Neoproterozoic — roughly 800 to 700 million years ago — or even earlier than some of the most extreme glaciations in Earth’s history.
This is the hypothesis proposed by an international study led by the University of Oxford (United Kingdom) and featuring members from the University of California, Berkeley and Yale (United States) and ETH Zurich (Switzerland), published this Friday in Science Advances.
The authors contend that the finding could transform our understanding of one of the biggest mysteries of evolution: why animals appear relatively suddenly in the fossil record just before the Cambrian (between 539 and 487 million years ago), even though some evidence suggests their evolutionary history began much earlier.
The study argues that, until now, scientists have used the fossil record to set a maximum age for the origin of animals, and this idea may be wrong.
The fossil site the Weng’an Biota, about 590 million years old and found in Ediacaran rocks in China, is one of the most important and widely used for studying the origin of complex multicellular life. This site preserves microscopic organisms with extraordinary detail, though none are true animals.
The scientific community has always believed that the first animals must have originated after Weng’an, or else this exceptional fossil site would have preserved them if they had existed at that time.
Casting Doubt on the Premise
The team analyzed more than a hundred samples from the Kheseen biota (Mongolia), a fossil assemblage more than 40 million years younger than Weng’an, and in doing so identified new species of microfossils, including acritarchs — microscopic spherical organisms with spines and ramifications — and embryo-like fossils with internal cells.
Despite their excellent preservation, Kheseen also reveals no fossils that can be confidently identified as animals, but the point is that, thanks to fossil records from places like Namibia and southern China, scientists have evidence that animals already existed at that time.
For Ross Anderson, a researcher at the Oxford Museum of Natural History and the study’s lead author, this fact is decisive: the absence of animals in an exceptionally preserved site does not allow us to conclude that they did not exist. Animals could have inhabited environments where, unlike Kheseen, chemical conditions prevented fossilization from preserving them.
The Origin of the Animal Kingdom
The authors believe that this idea requires reconsidering the dates used to calculate the origin of the animal kingdom.
Rather than treating Weng’an as a maximum limit, the researchers turned to much older fossil deposits, dating between 850 and 730 million years ago, located in Norway, Australia, and Arizona, where no animals have been found either, but whose characteristics indicate they could have preserved organisms of this kind.
With these new geological references, they applied the molecular clock technique, which compares genetic differences among living species and, combining them with known fossil ages and estimated rates of evolution, allows calculating when their common ancestors could have lived.
The results pushed back the estimated origin of animals by roughly 200 million years compared with calculations based on Weng’an.
The resulting interval, between 800 and 700 million years ago, places the possible birth of animals far earlier than the oldest unequivocal body fossils.
The hypothesis also aligns with other lines of evidence. Chemical biomarkers preserved in ancient rocks have been interpreted as possible hints of sponge life dating back at least 650 million years.
These early organisms were probably small and soft-bodied, so they had far fewer chances of fossilizing than later animals endowed with shells, bones, or other hard, durable structures.
The new scenario also raises an additional question: the first animals could have appeared before the known glaciations such as Snowball Earth, during the Cryogenian period, which began around 720 million years ago and was marked by episodes in which vast portions of the planet were covered in ice.
The authors believe that, if animals emerged between 800 and 700 million years ago, their early evolution could have coincided with these extreme conditions.
They emphasize, however, that their work does not prove that animals existed 800 million years ago, but that the absence of fossils in certain deposits is not enough to rule out that possibility and that molecular analyses allow for a much older origin.
