Animal ancestor probably survived ancient ice age: chemical fossils date back to at least 635 million years ago.

A new analysis of ancient chemical fossils has rocked the cradle of early animal evolution, bumping back compelling evidence of animal life to at least 635 million years ago.

The findings, published in the Feb. 5 Nature, suggest that the ancient ancestor of fully formed animals survived a massive glaciation that enshrouded the Earth in ice at the end of the Cryogenian period. Debate continues over how much of the planet was frozen during two ice ages, each possibly a "snowball Earth" event, that flanked this period, which extended from about 790 million to 630 million years ago. The new results suggest that even if glaciers reached the equator during the second ice age, warm pockets may have persisted and harbored life.

The find is "really something," says Jochen Brocks of the Australian National University in Canberra, who coauthored a Nature commentary on the work.

The Cambrian explosion is often cited as the inaugural ball of animal evolution, a period of roughly 20 million years that began about 520 million years ago, and a time when representatives of many of today's major animal groups became established. But there's evidence that some animals evolved before the Cambrian, including the Ediacaran fauna, a bizarre assemblage that flourished between the Cryogenian and Cambrian. Many scientists believe these multicellular animals were an early experiment in animal evolution that ended badly.

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Sponges, however, may have come on the scene before the Ediacaran period and lived through it. The new analysis, led by organic geochemist Gordon Love of the University of California, Riverside, documents the molecular remains of an animal steroid in ancient Ediacaran strata and in the layers beneath.

The steroid fossil, 24-isopropylcholestane, or 24-ipc, is a form of a steroid known today from the cellular membranes of a class of sponges, the Demospongiae. Some algae also make the molecule, but the ratios of 24-ipc to other compounds rules them out as a source.

Working with cores from a salt basin in south Oman, Love and his team document the presence of 24-ipc in rock deposited 150 meters below the end of the Cryogenian. The findings can't say when multicellular animals first appeared but can say that they had to be around by at least 635 million years ago and, the researchers report, maybe as early as 751 million years ago. The creatures may have evolved in the warming period between the Cryogenian's two snowball Earth events.

Putative fossil embryos dating to about 580 million years ago were previously the oldest fossils considered ancestors of today's animals.

Even if the glaciations during the second snowball Earth event didn't freeze the seas, they would have forever altered ocean chemistry, says Love. The environmental upheaval could have opened new niches and presented an opportunity for multicellular sponges to spread.

These early sponges might have helped bring about the oxygenation of the deep oceans, paving the way for more life, says molecular paleobiologist Kevin Peterson of Dartmouth College.