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About 890 million years ago (disputed)Peer-reviewed · 2 sourcesDebated

Possibly the first animals: a disputed sponge

On the timeline · around About 890 million years ago (disputed) · Toward the Rise of AnimalsThe Boring BillionToward the Rise of AnimalsPossibly the first animals: a disputed sponge1 Ga975 Ma950 Ma925 Ma900 Ma875 Ma850 Ma825 Ma800 Ma775 Ma

What happened

In weathered reef rock in Canada's Northwest Territories, geologist Elizabeth Turner identified microscopic branching structures she interprets as the fossilized skeletal fibers of keratosan demosponges, a group of simple sponges alive today. Published in Nature in 2021, the claim would push the animal fossil record back to about 890 million years ago, roughly 350 million years earlier than the next-oldest confirmed animal fossils, with the sponges living as small patches tucked into the nooks of ancient microbial reefs. The claim has not been accepted. Paleobiologist Nicholas Butterfield, the same researcher who described Bangiomorpha, argued that several past claims of sponge fossils in similarly aged rock were later disproven, and that an animal turning up this early runs against the expectation that a genuinely new, successful body plan spreads and diversifies quickly rather than staying rare and easy to miss for hundreds of millions of years.

Why it matters

If confirmed, the sponge would close much of the long gap between when genetic evidence suggests animals evolved and when their fossils actually show up. Until then, it is a real, published, and seriously contested claim, which is exactly why it belongs on this timeline as an open question rather than a settled fact.

How we know

Turner's evidence is the three-dimensional branching network preserved in thin rock slices, a shape she argues matches the internal skeletal mesh of modern keratosan sponges better than any known non-biological process. Critics counter that similar-looking structures have been proposed as sponge fossils before and then reinterpreted as products of ordinary mineral growth, so the case is not yet closed either way.

Sources

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Part of a timelineEarly Life on Earth9 events · How a single microbial ancestor became a planet run by oxygen, complex cells, and eventually animals, from LUCA to the doorstep of the Cambrian explosion.View all →