Animals

Animals show up in the fossil record in the late Precambrian and then explode in variety in the Cambrian, a bit over 500 million years ago. An animal is a eukaryote that eats other organisms, moves, and is built from many cells with jobs.

Movement is the contribution. A plant stays. An animal has to sense and act. Over hundreds of millions of years that produces nerves, muscles, skeletons, and eventually limbs. Fish, then animals on land, then the line that becomes mammals and primates.

The timeline's '500 million years' is the Cambrian window, when hard parts make fossils common. Soft-bodied animals are older. None of them are human. Humans are one twig that arrives only in the last few hundred thousand years.

What animals hand forward is a body that can manipulate the world. Stone tools need a hand and a grip. Those come from this branch, not from the invention of metal.