Fossil traces found in an oil field on the Arabian Peninsula are the oldest evidence yet of animals, pushing back the known origins of higher life to more than 635 million years ago.

The animals' remains don't look like traditional fossils. They're more like fossil echoes: chemical traces of a compound only produced — at least in modern times — by demosponges, descendants of what some scientists consider to be the last common ancestor of all animals.

"It is, definitively, the earliest evidence for animals," said geochemist Gordon Love of the University of California, Riverside, lead author of the study published Wednesday in Nature.

Love's team identified the fossils while analyzing sedimentary deposits mined by Oman's national oil company. The sediments date to the last stages of the the aptly-named Cryogenian period after a deep freeze referred to by scientists as Snowball Earth.

Until now, the oldest animal fossils dated to Earth's next geological period, called the Ediacaran. Scientists had been unsure whether they reflected the actual birth of animal life, or merely the beginning of the fossil record.

The new findings show that animals indeed evolved before the Ediacaran, giving these humble sponges at least 100 million years to develop the kaleidoscopic physiologies that bloomed during the early Cambrian period.

"Biologists might argue about which animals diverged first," said Love, "but regardless of that, we're certainly looking at very basal animals."

The telltale sign discovered by Love's team is a fatty chemical called 24-isopropylcholestane, which scientists have found only in the skeletal structures of demosponges, the most common member of the sponge family. Until recently, sponges were believed to be modern descendants of the first animals.

Other recent research suggests that an ancestor of placozoa — an amoebalike creature whose genome was sequenced in 2008, providing genetic clues of an ancient lineage — was the first animal. Regardless of this taxonomical controversy, however, Love's fossils are clearly old. Both uranium dating and the fossils' sedimentary position confirm a late-Cryogenian origin.

In a commentary accompanying the findings paleobiologists Jochen Brocks and Nicholas Butterfield raise the possibility that some other organism than a sponge may have left the 24-isopropylcholestane behind. Fossil descendents of sponges, they note, have not been found in the Ediacaran or Cambrian periods.

Love, however, called the chemical a paleobiological smoking gun.

"Screening has been done on modern organisms, and there's only one that produces these in abundance: demosponges," he said. "One day, we might come across a microbe, but it hasn't happened so far."

Love next plans to further excavate Cryogenian sediments in order to determine exactly where and when his proto-sponges developed.

"Was it the conditions of the first glaciation that caused a change in biology?" he said. "Was it the aftermath, during a change in ocean chemistry? We're trying to understand the context of the first appearance of animals."

Citations: "Fossil steroids record the appearance of Demospongiae during the Cryogenian period." By Gordon D. Love, Emmanuelle Grosjean, Charlotte Stalvies, David A. Fike, John P. Grotzinger, Alexander S. Bradley, Amy E. Kelly, Maya Bhatia, William Meredith, Colin E. Snape, Samuel A. Bowring, Daniel J. Condon & Roger E. Summons. Nature, Vol. 457 No. 7229, Feb. 4, 2009.

"Early animals out in the cold." By Jochen J. Brocks and Nicholas J. Butterfield. Nature, Vol. 457 No. 7229, Feb. 4, 2009.

Image: 1. Demosponge / Gordon Love 2. Sediment deposits dating from 635 million years ago / David Fike 3. Diorama of Ediacaran life / WikiMedia Commons

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