Mongolian microfossils undercut the 590-million-year ceiling on the age of animals
Oxford-led researchers say the 590-million-year-old Weng'an Biota wrongly caps animal origins, pushing a clock estimate back about 200 million years. The resulting 800-to-700-million-year range depends on which older deposits replace Weng'an as the ceiling.
The Scientist · Science desk
What happened
- Researchers examined more than 140 Kheseen Biota samples from Mongolia by scanning electron microscopy and found finely preserved acritarchs and embryo-like fossils, none confidently an animal.
- The replacement maximum ages come from the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia and the Chuar Group in Arizona, dated about 850 to 730 million years ago.
- The resulting range would put the first animals deep in the Neoproterozoic Era, possibly before or during some of Earth's most extreme ice ages.
Why it matters
- Who has to decide decision Groups dating the animal tree now have to decide whether Weng'an stays in as a hard maximum, since that single choice moves the clock answer by about 200 million years.
- What's now possible capability Checking a deposit from a time when animals are known to have lived gives palaeontologists a direct test of whether a given preservation window could have caught them at all.
- Who's exposed exposure With the date now resting on three older deposits, a revised age or an animal find at Svanbergfjellet, Bitter Springs or Chuar would shift the estimate.
Next story · Science · Oct 6
Mongolian microfossils undercut the case that animals first appeared in the EdiacaranConfirmed2 publishers
Weng'an's role as a ceiling rests on one premise: rocks that keep microscopic organisms in such detail would have kept animals too, had any been alive [3]. The premise can be checked against a deposit from a time when animals certainly existed. Kheseen formed after about 550 million years ago [19], later than the oldest animal fossils at roughly 574 million years [15]. If a site that good can miss animals known to be alive, an equally good empty site 590 million years ago says little about whether animals were around.
"The Kheseen microfossils are just as well preserved, yet animals continue to be absent," said Ross Anderson, the senior author and an associate professor at Oxford's Museum of Natural History [8]. "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," he said [7]. The team's explanations are that animals lived in other environments, or that the local chemistry did not preserve them [14]. Those early animals would have been small and soft-bodied, according to the Oxford account [18].
The molecular clock is the next step. It compares genetic differences among living species and works backward, using fossil dates as reference points along with rates of evolution [10]. "So the game is basically how much does it predate our first fossil discovery by and what happened in that evolutionary black box between when they actually originated versus when we get our first fossil window," Karma Nanglu, an assistant professor of evolutionary paleobiology at the University of California, Riverside, who was not involved, told Live Science [17]. With the older deposits as maximum ages, the estimate moved back about 200 million years from Weng'an-calibrated runs, to 800 to 700 million years ago [1]. Against the oldest animal fossils, the gap is roughly 126 to 226 million years [13].
The thing this doesn't tell you is how much of the new date comes from the new ceiling. Given maximum ages of 850 to 730 million years [9], the clock returned 800 to 700 million years, just under those bounds [1]. The older ceilings are also arguments from absence: fossil-rich deposits with the potential to preserve animals, where none have turned up [9]. The published accounts do not explain why that reasoning holds at 850 million years when Kheseen undercuts it at 590.
I think the negative result is the firmer finding: a well-preserved deposit without animals is weak evidence that animals were absent, while the 800-million-year date depends on the bounds chosen. Biomarkers consistent with sponges at least 650 million years ago [11] reach about 76 million years past the oldest animal fossils [20], well short of 800 million. "Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain," Anderson said in a statement [16].
What to watch
- Independent clock studies that drop Weng'an as a maximum: if they reach 800 to 700 million years ago with different calibrations, the date firms up.
- Closer study of the Kheseen embryo-like fossils; if any is shown to be an animal, the Mongolian control loses its force.
- A sponge biomarker or animal fossil older than 650 million years would be the first direct test of the 800-to-700-million-year range.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence55
- Adoption
- Not scored
- Hype gap+15
- Incentives
- Not scored
- Confidence60
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Compared with analyses constrained by the Weng'an deposit, using the older constraints shifted the estimated origin of animals back by around 200 million years, to between 800 and 700 million years ago.
- [2]
A University of Oxford-led study published in Science Advances suggests the earliest animals may have evolved hundreds of millions of years before their first unmistakable fossils appear.
- [3]
The roughly 590-million-year-old Weng'an Biota in Ediacaran rocks in China preserves microscopic organisms in extraordinary detail, none definitively identified as animals; scientists inferred that the first animals must have originated after Weng'an, assuming the deposit would have preserved them if they existed.
- [4]
The Kheseen Biota in Mongolia is more than 40 million years younger than Weng'an, though they share some species.
- [5]
By the time of the Kheseen Biota, animal fossils are known from sites elsewhere, including Namibia and South China.
- [6]
The researchers studied over 140 Kheseen samples, some from previously undocumented localities, using scanning electron microscopy, finding new microfossil species including acritarchs and embryo-like fossils with internal cells; none can be confidently identified as an animal.
- [7]
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.
- [8]
The Kheseen microfossils are just as well preserved, yet animals continue to be absent
- [9]
The team used deposits dated roughly 850 to 730 million years ago, the Svanbergfjellet Formation (Norway), Bitter Springs Group (Australia) and Chuar Group (Arizona), as constraints; these are fossil-rich deposits with the potential to preserve animals, though animal fossils have not been found, and they have previously been used as maximum dates for animal origins.
- [10]
Molecular clock analysis compares genetic differences between living species and, using fossil dates as reference points combined with rates of evolution, works backward to estimate when ancestors lived.
- [11]
Biomarkers preserved in ancient rocks provide evidence consistent with sponges living at least 650 million years ago, tens of millions of years before the oldest definitive macroscopic animal fossils.
- [12]
An 800-to-700-million-year origin would place the emergence of animals deep within the Neoproterozoic era (1 billion to 539 million years ago), possibly before or during some of Earth's most extreme ice ages.
- [13]
The 800-to-700-million-year range sits roughly 126 to 226 million years before the oldest animal fossils at about 574 million years.
- [14]
Animals must have lived in different environments from those that preserved the Kheseen fossils, or the chemical conditions there failed to preserve animal remains.
- [15]
The earliest fossil evidence of animals dates to around 574 million years ago, during the Ediacaran period (635 million to 538 million years ago).
- [16]
Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain
- [17]
So the game is basically how much does it predate our first fossil discovery by and what happened in that evolutionary black box between when they actually originated versus when we get our first fossil window
- [18]
These early animals would have been small and soft-bodied.
- [19]
The Kheseen Biota is younger than about 550 million years.
- [20]
Sponge biomarkers of at least 650 million years sit about 76 million years before the oldest animal fossils at about 574 million years.
Sources
2 independent publishers whose own reporting we read for this story.
- livescience.comAnimals may have evolved 200 million years earlier than previously thought, new study suggests
1 article · October 2, 2026
- phys.orgAnimals may have evolved vastly earlier than fossil evidence suggests
1 article · October 2, 2026
Topics and entities
Topics
- Paleontology
- Ediacaran period
- Molecular clocks
- Origin of animals
Entities
- Orin Lole Durbin
- University of Oxford
- Science Advances