About 600 million years ago, the first animals formed - creatures with more than one cell. The earliest animals were sponges. As soon as cells begin to join together, they encounter the big problem of all animals - how do the inside cells get water, food, and oxygen? How do they get rid of their wastes? And, how do they decide what to do? Sponges let sea water flow through them, reaching all the cells. Hydras formed into a u-shaped tube, so that water could flow in and out again.
By about 550 million years ago, flatworms evolved eyespots and long neuron cells that could carry directions from one end of the worm to the other. The end of the tube with the eyespots became the head and mouth, and the other end was the butt. Soon after that, roundworms developed branched tubes, so they could be thicker than two cells. Then segmented worms developed stomachs in the middle of their tube, to digest food better, and they evolved to produce their own blood, instead of just letting sea water wash in and out of them. So segmented worms didn't have to live in the ocean.
Very soon some segmented worms evolved into arthropods and mollusks and fish. Arthropods and mollusks had legs, and arthropods had hard shells to protect them. By 530 million years ago, some arthropods - small insects and spiders - moved on to dry land. Fish are still basically tubes, but with spinal cords and vertebrae to protect their spinal cords, and fins so they can move quickly in the water. But about 420 million years ago, some fish developed jaws and teeth. Then 400 million years ago, lungfish evolved ways to breathe air instead of water, and animals moved on to the land. Lungfish evolved into frogs, which had legs. By about 300 million years ago, frogs evolved into reptiles. Reptiles laid eggs with hard shells, so they didn't have to go back to the water to have babies. Some reptiles grew to be huge dinosaurs; others began to evolve feathers and wings and turned into birds. About 200 million years ago, some reptiles evolved into small mammals like mice. Mammals were warm-blooded and had hair so they could control their own body temperature. They took better care of their babies, carrying the babies inside their own bodies until they were ready to be born. And they made milk to feed babies even after they were born. About six million years ago, the first humans evolved from these earlier mammals.
But like other animals, humans still are fundamentally a tube with a mouth at one end and a butt at the other, with branches to get food and oxygen to all the cells and carry away waste. And even today, there are more of the earlier types of animals and fewer of the more recent ones: there are about five million trillion trillion one-celled creatures on Earth, about ten quintillion insects, at least several trillion fish, and only about six billion humans.
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