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.
DEEP TIME: If the timeline of Earth were mapped onto the human arm, it would begin around the shoulder where the earth formed about 4.6 billion years ago. Animals originated within the palm, but the myriad forms alive today exploded onto the scene around the first knuckle, in the Cambrian period. Blocks along the fingers represent the periods that followed, such as the Jurassic (dinosaurs!) and the Cenozoic (in which humans evolved, a microscopic sliver at the tip of a fingernail).
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 (see Explosion). Where the Cambrian Explosion saw a proliferation of architectures (picture igloos, cabins, skyscrapers, suburban houses, and grass huts), the rest of time has mainly been about remodeling existing forms (add a Jacuzzi, a deck, or a tin roof). The explosion of animal phyla in the Cambrian includes the category, the chordates, to which humans, reptiles, sloths, and fish belong. Chordates are united by a central bundle of nerve fibers running down our backs, supported by a stiff rod.