Where did all the animals come from? The fossil record is virtually animal-free up until the Cambrian Explosion 540 million years ago, and then--boom--thousands of critters of all shapes and sizes show up. The mystery has plagued scientists for more than a century and a half, beginning with Charles Darwin. Now, with a brilliant bit of detective work, researchers have located our missing ancestors.
The problem with the earliest animals, from a paleontologist's perspective, is that they lacked hard parts. Without bones, beaks, claws, and shells to fossilize, these squishy creatures never became part of the geological record. That's made life difficult for evolutionary biologists. They know that natural selection had to be operating for at least tens of millions of years to give rise to all of the Cambrian critters. Yet where was the evidence that animals had actually been around that long?
The answer lies in the unique molecules they left behind. A team led by organic geochemist Gordon Love of the University of California, Riverside, hit pay dirt with one such molecule. Called 24-IPC, it is only produced by Demospongiae, a class of animals that includes most modern sponges and is thought to constitute the roots of the animal family tree. The researchers acquired 30 pristine drill cores removed from underneath the southern Arabian Peninsula by the oil company Petroleum Development Oman. The cores ran through sedimentary layers and extended back beyond the Cambrian Explosion into the Cryogenian Period, about 635 million years ago near the end of a long, global ice age.
After treating the core samples with chemicals that react with and reveal the presence of 24-IPC, the researchers found "anomalously high amounts of 24-IPC" in even the oldest parts of the core, they report in tomorrow's issue of Nature. That pegs the origins of animals at least 100 million years before the Cambrian explosion. As a result, animals appeared on Earth slowly, as Darwin suspected, and not suddenly and spectacularly, as the fossil record seems to show.
The findings seem to solve one mystery, but they intensify another. Pushing back animal origins by 100 million years places their beginnings during a time called Snowball Earth, when our planet was almost completely encased in ice. If true, it means Demospongiae arose in an extremely harsh environment, not the relatively warm seas of today. It's possible, Love says, that sponges could have emerged before Snowball Earth and inhabited a haven just large enough to allow them to survive.
It's an exciting find, says paleobiologist Kevin Peterson of Dartmouth College. And it confirms, he says, that we animals can all trace our origins back to sponges.