Paleontologists, developmental biologists, geochemists, and many other sci- entists are testing hypotheses that may explain this remarkable radiation. Some researchers observe that the radiation of animals came at a time when the Earth was going through some dramatic physical changes. It was emerging from a cli- mate so cold that the entire planet was covered in glaciers. The ocean’s chem- istry was also changing drastically. For most of Earth’s history, the oceans were almost devoid of free oxygen. While the atmosphere contained some free oxy- gen, the molecule could not survive for long in the ocean before bonding with other molecules. But Paul Hoffman of Harvard University and.
