Complex animals first appeared 850 million years ago but didn't evolve into a great variety of forms until the Cambrian period, 540 million years ago. Why the delay? Answering this is difficult, in part because rocks that formed immediately before the Cambrian appear to have vanished. Cambrian rocks often sit on much older layers, creating the Great Unconformity - a geological formation seen around the world, including in the Grand Canyon.
Shanan Peters of the University of Wisconsin-Madison and Robert Gaines of Pomona College in Claremont, California, now suggest this crucial rock record may have dissolved into the ocean. The pair examined Cambrian rocks and found chemical evidence that the oceans at the time were rich in minerals from weathered rock that would have turned them alkaline. What triggered this erosion, which lasted tens of millions of years, is not known.