The animal kingdom may have started its journey on a much earlier Earth than scientists once thought.
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scientists → start → Earth
Long before recognisable creatures appeared in the fossil record, and perhaps before the planet was plunged into some of its most severe ice ages, primitive animals may already have been quietly evolving in ancient seas.
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animals → appear → seas
A new study led by the University of Oxford suggests that animal life could have originated between 700 million and 800 million years ago, potentially 200 million years earlier than the clearest fossil evidence indicates.
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evidence → lead → Oxford
Published in Science Advances, the research challenges one of the assumptions that has shaped recent estimates of when animals first emerged.
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animals → publish → estimates
The fossil record gives scientists an incomplete picture of early animal life.
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record → give → life
The oldest definitive animal fossils date to around 574 million years ago, just before the dramatic diversification associated with the Cambrian Period.
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fossils → date → Period
One important site in this debate is the Weng’an Biota in China, a roughly 590-million-year-old deposit famous for preserving microscopic organisms in remarkable detail.
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site → preserve → detail
Because none of the fossils found there can be confidently identified as animals, scientists have used the site as a possible upper limit for animal origins.
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scientists → find → origins
The reasoning seemed straightforward: if conditions were good enough to preserve delicate microscopic life, surely animals would have been preserved too if they had existed.
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they → seem → life
What Mongolia revealed
Researchers examined the Kheseen Biota in Mongolia, a fossil deposit more than 40 million years younger than Weng’an.
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Researchers → reveal → Weng’an
By the time these rocks formed, animals were already known to have existed elsewhere, including in Namibia and South China.
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animals → form → Namibia
The team examined more than 140 samples, including material from previously undocumented locations, using scanning electron microscopy.
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team → examine → microscopy
The exceptionally preserved fossils included previously unknown acritarchs, tiny spherical organisms with spines and branching projections, as well as embryo-like fossils containing internal cells.
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fossils → preserve → cells
Yet, despite the extraordinary preservation, there were still no fossils that could confidently be identified as animals.
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that → be → animals
That matters because it shows that even a fossil deposit formed during a period when animals definitely existed can contain no recognisable animal remains.
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animals → matter → remains
The absence of animal fossils at Weng’an, therefore, cannot by itself establish that animals had not yet evolved.
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animals → establish → itself
Pushing the molecular clock back
Instead of relying primarily on the younger Ediacaran deposits, the researchers turned to much older geological formations.
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researchers → push → formations
These included Norway’s Svanbergfjellet Formation, Australia’s Bitter Springs Group and Arizona’s Chuar Group, dating from roughly 850 million to 730 million years ago.
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These → include → Formation
All three contain abundant and exceptionally preserved fossils and have geological characteristics capable of preserving early life.
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three → contain → life
Yet no definitive animal fossils have been found there.
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fossils → find → ?
The researchers used these formations as older constraints in molecular clock analyses.
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researchers → use → analyses
The technique compares genetic differences among living organisms and combines them with known fossil dates and estimated evolutionary rates to calculate when their ancient ancestors may have existed.
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ancestors → compare → dates
With the older geological evidence incorporated into the calculations, the estimated origin of animals shifted backwards by around 200 million years, placing it between 800 million and 700 million years ago.
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origin → incorporate → it
Chemical biomarkers preserved in ancient rocks have been interpreted as evidence consistent with sponges living at least 650 million years ago, well before the oldest definitive large animal fossils.
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biomarkers → preserve → fossils
If animals really did emerge between 700 million and 800 million years ago, they would have lived through a radically different planet.
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they → emerge → planet
The Cryogenian Period began around 720 million years ago and included episodes of extreme global glaciation commonly known as Snowball Earth.
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Period → begin → Earth
That raises a provocative possibility: animal life may have existed before these frozen episodes, or perhaps even endured them.
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life → raise → them
An evolutionary mystery remains
The study does not provide an exact birth date for the animal kingdom.
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study → remain → kingdom
Instead, it highlights the limitations of using fossil absence as evidence of evolutionary absence, particularly when dealing with tiny, soft-bodied organisms that were unlikely to fossilise easily.
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that → highlight → organisms
Researchers will need to consider not only conventional body fossils but also traces of animal activity and chemical biomarkers.
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Researchers → need → activity
The Aporia
