It may be time for the animal kingdom to reset its clock.
According to a new study by an international group of researchers, including a renowned Yale paleontologist, the beginning of animal life on Earth may predate the oldest specimens in the fossil record by a whopping 100 to 200 million years.
Not only would such a discovery change our understanding of evolutionary processes, it also would mean that animals existed prior to extreme climate events such as the ice ages collectively known as “Snowball Earth,” the researchers say.
What you need to know
When do animals first appear in the fossil record?
Currently, the oldest known animals in the fossil record date back to 574 million years ago. However, trace biological chemicals relating to ancient sponges suggest animals existed much earlier than that.
What is molecular clock analysis?
This methodology compares genetic differences between living species and, using confirmed dates from the fossil record as reference points, works backward to estimate when their ancestors lived.
The findings, published Oct. 2 in Science Advances, emerged from newly discovered fossils combined with a review of well-known sites worldwide where fossils were remarkably well-preserved, as well as recent data about chemical traces of animal life that predate the first identifiable fossils.
“The timing of animal origins, even in forms as ‘lowly’ as sponges, is of major interest,” said co-author Derek Briggs, the G. Evelyn Hutchinson Professor of Earth and Planetary Sciences in Yale’s Faculty of Arts and Sciences and former director of the Yale Peabody Museum. “It throws light on what we know about rates of evolution, the timing of biological innovations, and the processes leading up to animal diversification.”
Currently, the oldest known animals revealed in the fossil record date back to 574 million years ago. However, chemical traces in older rocks relating to ancient sponges suggests animals existed much earlier than that.
Setting a maximum age for the first animals has been highly challenging in recent years, the researchers said. Part of the process hinges on the lack of animal fossils at various fossil beds around the world, and making assumptions based on those absences.
For example, one recent theory centers on a 590-million-year-old fossil assemblage in South China, known as the Weng’an Biota. This site has yielded abundant, well-preserved microscopic organisms — but no definitive animal fossils. By this reasoning, the first animals must have appeared some time after 590 million years ago.
But such logic is flawed, the authors of the new study say. They cited other fossil beds around the world, including the 550-million-year-old Kheseen Biota in Mongolia, which formed at a time when animals were definitively known to exist — yet which contains no animal fossils.
The authors even found new, non-animal microfossil species at the Kheseen site, including spherical organisms with spines and branching projections (called acritarchs).
Asterocapsoides wenganensis, a spiny acritarch from the Kheseen Biota, Mongolia. Scale 100 µm (microns).
“The Kheseen Biota breaks the argument that the exceptional microfossils of Weng’an mean we would have seen animal fossils in the assemblage had they existed at the time,” said Ross Anderson, an associate professor at Oxford University, former student of Earth and planetary sciences at the Yale Graduate School of Arts and Sciences, and senior author of the study.
So how might scientists estimate animals’ first appearance on Earth?
To answer this question, the researchers used new scientific approaches, while focusing on much older sites where there are both well-preserved microfossils and chemical biomarkers to be found. Specifically, they focused on three sites where there are exceptionally preserved fossil assemblages dating to roughly 850 to 730 million years ago: Bitter Springs Group in Australia, the Svanbergfjellet Formation in Norway, and the Chuar Group in Arizona. All three sites were fossilized in conditions appropriate for preserving early animals, but no animal fossils have been found.
The researchers used the ages of these sites as different maximum ages of animal origins to calibrate “molecular clock” analyses. Such analyses calculate rates of evolution based on genetic differences between living species, working backwards to estimate when their ancestors first appeared.
Using this method, the researchers estimated that the origin of animal life on the planet may have occurred 800 to 700 million years ago, which would be 100 to 200 million years or more before the first recognizable fossils.
“Researching microscopic fossils in very ancient rocks — both finding them and determining what they are — is a demanding enterprise,” Briggs said. “Future discoveries are needed to refine our knowledge of the nature and timing of the earliest animal life on the planet.”
The finding also raises the possibility that animals predated a series of ice ages, known as “Snowball Earth,” which theoretically began about 720 million years ago, during which the planet’s surface was covered in ice for thousands or even millions of years. Paleontologists have debated whether environmental conditions related to Snowball Earth may have been a contributing factor in slowing rates of animal evolution.
The first author of the study is Orin Lole Durbin, a doctoral student at Virginia Tech. Additional co-authors are Eliel Anttila of ETH Zurich in Switzerland and Francis Macdonald of the University of California-Berkeley.
