Mongolian microfossils undercut the case that animals first appeared in the Ediacaran
An Oxford-led team found no animals in 140 samples from Mongolia's Kheseen Biota, a fossil bed formed after animals are known to have existed. That weakens the main argument for an Ediacaran start and leaves room for animals before or during Snowball Earth.
The Scientist · Science desk
What happened
- China's 590-million-year-old Weng'an Biota preserves microfossils but no definite animals, so animals were assumed to have arisen between it and the 574-million-year-old Ediacaran fossils.
- Kheseen was picked because it is exceptionally preserved and shares many species with Weng'an, so by the team's logic it should hold animals if such beds record them.
- The team also ran a molecular-clock analysis tied to three roughly 800-million-year-old deposits in Norway, Australia and Arizona.
- The results put the emergence of animal life at about 700 to 800 million years ago, before or during the Cryogenian Period of about 720 to 635 million years ago.
Why it matters
- What it sets up precedent Any exceptionally preserved bed cited as a maximum age for animals is now open to the same challenge: a younger bed of the same type, from a time animals existed, that also shows none.
- Who has to decide decision With an empty Weng'an discounted, researchers can weigh the 650-million-year sponge biomarkers as an animal signal without setting them against the fossil record.
- What it limits constraint A Snowball Earth origin is permitted by the new window without being favoured by it, so it stays a hypothesis that needs its own evidence.
Next story · Science · Oct 2
Mongolian microfossils undercut the 590-million-year ceiling on the age of animalsConfirmed2 publishers
Kheseen is a positive control, and a neat one. The bed formed about 550 to 526 million years ago [11], some 24 to 48 million years after the oldest fossils accepted as animals, the roughly 574-million-year-old Ediacaran Biota [9][19]. Animals were around. If Weng'an-style preservation reliably recorded them, Kheseen should show some. It did not. The scanning electron microscope turned up new species instead, including acritarchs, spherical forms with spines [13].
Taken as evidence of absence, Weng'an had confined animal origins to the roughly 16 million years between its 590-million-year age and the Ediacaran fossils [18]. "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, the senior author and an associate professor at Oxford [14][15]. The study was published in Science Advances [3].
The logic runs one way. A deposit that misses animals when they were present shows the method can fail; it does not place animals at 590 million years. The researchers make the matching point themselves: Weng'an cannot prove animals were absent when it formed, just as Kheseen does not prove they were absent 40 million years later [16]. "Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago," said first author Orin Lole Durbin, who did the work as an Oxford undergraduate and is now a PhD student at Virginia Tech [17][8]. "However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval," Lole Durbin said [7].
Chemical traces linked to ancient sponges point to animals by 650 million years ago, about 60 million years before the first animal body fossils, and Discover reports that the findings may help explain that mismatch [5].
The older limit comes from the second half of the paper, and Discover's description of it is brief. The three 800-million-year deposits are exceptionally preserved and hold no soft animal tissue. The team takes that to mean animals arose after they formed [2]. "Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend," Lole Durbin said [6].
The thing this doesn't tell you is why three empty deposits make a firmer maximum age than Weng'an turned out to be. Discover's account does not address it. In my view the 800-million-year limit rests on the same kind of absence that Kheseen has just shown can mislead [2][16].
The Snowball Earth framing needs its own check. Earth may have been covered in ice during the Cryogenian [4][1]. Of the roughly 100-million-year window the team proposes, only the youngest 20 million years fall inside that period; the other 80 million predate it [20]. A Cryogenian origin fits these numbers, though most of the window lies before the ice. The 650-million-year sponge biomarkers do sit inside the Cryogenian [21].
What to watch
- A pre-Ediacaran animal body fossil, which Lole Durbin says still eludes researchers, would turn this permitted window into a dated origin.
- A Kheseen-style test of the Svanbergfjellet, Bitter Springs and Chuar deposits would show whether their lack of animal tissue holds up as an 800-million-year ceiling.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
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- [1]
The findings raise the possibility that animal life emerged during a 'Snowball Earth' roughly 700 million years ago, when the entire planet is hypothesized to have been covered in thick ice.
- [2]
The team applied molecular clock analysis to three fossil assemblages each approximately 800 million years old: the Svanbergfjellet Formation (Norway), the Bitter Springs Group (Australia) and the Chuar Group (Arizona). These exceptionally preserved deposits would be expected to contain soft animal tissue if present; they do not, suggesting animals appeared after their formation.
- [3]
The results were published in Science Advances.
- [4]
The results suggest animal life may have emerged about 700 million to 800 million years ago, raising the question of whether animals arrived before or during the Cryogenian Period (circa 720 million to 635 million years ago), when Earth may have been entirely covered in ice.
- [5]
Biomarkers, specifically chemical traces linked to ancient sponges, hint at animal life as early as 650 million years ago, while the first fossilized animals do not appear for another 60 or so million years, during the Ediacaran; the results may help explain this inconsistency.
- [6]
"Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend,"
- [7]
"However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval."
- [8]
First author Orin Lole Durbin is a PhD student at Virginia Tech and was completing an undergraduate degree at the University of Oxford at the time of the study.
- [9]
Soft-bodied lifeforms of the Ediacaran Biota, approximately 574 million years old, are believed to be the oldest fossilized animals known.
- [10]
The 590-million-year-old Weng'an Biota in China contains an array of microscopic fossils but none that can be definitively described as animal life, so it has been assumed that animals emerged sometime between Weng'an and the Ediacaran Biota.
- [11]
The study, led by researchers at the University of Oxford, examined microfossils from the Kheseen Biota in Mongolia, dating to about 550 to 526 million years ago.
- [12]
Kheseen was chosen because it is extremely well preserved, contains many of the same species as the Weng'an Biota, and formed at a time when animal life was already known to exist, so by this logic it should contain evidence of animal life.
- [13]
The team took 140 samples and studied them with scanning electron microscopy, finding new species including acritarchs (spherical forms with spines) but none that could be described as an animal.
- [14]
"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,"
- [15]
Ross Anderson, the senior author, is an associate professor at Oxford University.
- [16]
The researchers say the Weng'an Biota cannot prove animals did not exist at the time of its formation, just as the Kheseen Biota does not prove that animals did not exist 40 million years later.
- [17]
"Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago,"
- [18]
Under the old assumption, animal origins were confined to a window of about 16 million years between Weng'an and the oldest Ediacaran animal fossils.
- [19]
Kheseen formed about 24 to 48 million years after the roughly 574-million-year-old Ediacaran animal fossils.
- [20]
Of the 100-million-year window of 700 to 800 million years ago, only the youngest 20 million years (700 to 720) fall inside the Cryogenian; the other 80 million years (720 to 800) predate it.
- [21]
The 650-million-year sponge biomarkers fall within the Cryogenian Period.
Sources
2 independent publishers whose own reporting we read for this story.
- discovermagazine.comThe First Animals May Have Arrived 200-Million Years Earlier Than Previously Thought
1 article · October 6, 2026
- sciencedaily.comAnimals may have evolved 200 million years earlier than we thought
1 article · October 6, 2026
Topics and entities
Topics
- Paleontology
- Molecular clock dating
- Origin of animals
Entities
- Virginia Tech
- Orin Lole Durbin
- University of Oxford
- Science Advances