About 530 million years ago, Earth experienced a renaissance of animal evolution, as the ancestors of all modern-day creatures began to experiment with skeletons and shells. Scientists have long puzzled over what prompted the sudden explosion of diversity in the Early Cambrian period. Now, geochemists have found the first direct evidence that a surge in calcium in the oceans may have played a role.
Why evolve shells? The most obvious reason would be to avoid being eaten. Another idea, which might explain why hard parts first appeared in the Early Cambrian, is that rising levels of atmospheric oxygen enabled animals to build more complicated bodies. And other scientists suggested in the 1970s that marine creatures may have started building shells to purge their bodies of a sudden pulse of calcium in the environment in the Early Cambrian. But direct evidence of a change in ocean chemistry was lacking.
Now, a trio of geochemists has measured calcium levels in tiny pockets of Early Cambrian seawater trapped inside rocks. Sean Brennan of the U.S. Geological Survey in Reston, Virginia, Tim Lowenstein of the State University of New York, Binghamton, and Juske Horita of Oak Ridge National Laboratory in Tennessee examined 24 salt crystals that likely formed on the sea bottom about 544 million years ago, just before the Cambrian, and 14 Early Cambrian samples from around 515 million years ago. They broke open the crystals and analyzed the composition of the tiny water droplets within. Calcium levels in the Early Cambrian were at least three times higher than calcium levels in the Proterozoic, they report in the June issue of Geology.
All that calcium may have come from hot seawater spewed out of volcanically active midocean ridges, the researchers say. They argue that the timing of this heightened tectonic activity, the calcium surge, and the "Cambrian explosion" seem to support the idea that early life began innovating with shells and skeletons to deal with a potentially toxic rise in calcium in the oceans.
The idea that the chemistry of seawater may have had an impact on the evolution of life is exciting, says Robert Goldstein, a geologist at the University of Kansas in Lawrence. It looks like calcium in the ocean may have been an important player, he says, although more work is needed to show whether it is a cause or effect of the explosion of life. Other environmental changes may also have been important.