Brooke Borel writes: Battles fought 542 million years before today helped fuel a blast that brought humans and most animals into existence. The great Cambrian Explosion was a period of unprecedented one-upmanship. Beastly claws crushed through thin skin, and soft-bodied creatures evolved shells shaped like scythes, sickles, and shields.

For about a billion years prior, the cells and genes that would later create animals were evolving in microscopic organisms who inhabited the oceans of Earth. These essential molecular changes may only be inferred today because they’re not preserved in fossils. The earliest traces of animals, about 580 million years old, appear soft, with no sign of claws, teeth, limbs, or brains. Then, within 54 million years (a relative blink but still, 270 times the duration of humans’ existence thus far), most of the main animal groups around today originated. This rapid rate of increase in animal architectures has never since been repeated.

A simple species count does not do justice to the power of the Cambrian Explosion. Species have continuously formed over time. A new type of moth may have antennae that are furrier than its sisters; a new species of dinosaur may be distinguished by clawed wings and vicious front fangs. But a new phylum — a major branch on the tree of life, the upper-level ranking that separates an insect from a pterodactyl — is rarely born.

Most of today’s 30 to 40 animal phyla originated in the Cambrian, and have persisted through time with hundreds of variations on a theme. [Continue reading…]