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When groups of cells started joining together on Earth some 650 million years ago, they began forming creatures we now know as sponges and tissues. These specialized cells became increasingly advanced when nerves and muscles first started appearing in what we know as jellyfish. These animal didn't have brains, so they couldn't think. But they could respond to the touch automatically. After all, nerves are the main characteristic that separates animals from plants.
From jellyfish, other animals formed such as corals, hydras, men-of-war, and sea anemones. Then, about 450 million years ago, fish became even more developed, as jawless fish with brains evolved. These fish could actually think and make decisions about where to live and swim. Scientists estimate these time periods based on when various types of fossils were found.
Jawless fish evolved into fish that are much closer to what we know as fish today: lobe finned fish. This happened about 390 million years ago. These fish were longer than they were wide, and they had fins across their body, which better enabled them to swim and go where they wanted to go. These fish developed bladders, then air sacs from which they could breathe. These fish eventually developed into amphibians and all types of vertebrates, according to many scientists. Some even insist they are ancestors of man!
Some scientists say that skeletons in fish evolved much earlier than many people think. They evolved along with many of the other animals that existed millions of years ago.