Oldest Animal Fossil
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.