The fossil record is virtually animal-free up until about 540 million years ago, and then–boom–thousands of critters of all shapes and sizes show up.
Now scientists have found the fossils of some sponge animals that lived at least 100 million years earlier, at least 640 million years ago. The find proves that animals appeared on Earth slowly, and not suddenly and spectacularly, as the fossil record used 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 animals arose in an extremely harsh environment, not the relatively warm seas of today. Although it is also possible that sponges could have emerged before Snowball Earth and inhabited a haven just large enough to allow them to survive. Read more here or here.