A paper in the 5 February Nature reports chemical evidence of the earliest complex animalssponges from rocks in Oman that are as old as about 750 million years, much older than the Cambrian Period, older even than the Ediacaran-age animal fossils, in fact older than the Marinoan "snowball Earth" episode during the Cryogenian Period. The chemical signs are abundant molecules of 24-isopropylcholestane, which occurs in the cell walls of sponges. This work bolsters what researchers have long suspected: the first multicelled animals were sponges. Moreover, the first reef environments like those of today must have been based on sponges, setting a fertile stage for the evolution of many more animal types during the Ediacaran. If the Nature paper is hard to follow, triangulate on its contents from press releases from MIT, University of California Riverside, and the National Science Foundation.
Cryogenian rocks from Oman Photo courtesy David Fike, Caltech