FOR ALMOST A THOUSAND MILLION YEARS after its formation, there was no known life on earth. The first simple , sea-dwelling organic structures appeared about 3,400 years ago ; they may have form when certain chemical molecules jointed together. Prokaryotoe, single celled micro-organisms such as blue-green algae, were able to photosynthesize, and thus produce oxygen. A thousand millions years later , sufficient oxygen had built in the earth's atmosphere to allow multicellular organisms to proliferate in the Precambrian seas. Soft-bodied jelly fish, corals and sea worms flourished about 700 million years ago. Trilobites, the first animals with hard body frames , developed during the Cambrian period (570-510 million years ago). How ever it was not until the beginning of the Devonian period (409-363 million years ago) that early land plants, such as Asteroxylon, formed water-retaining cuticle, which ended their dependence on an aquatic environment. About 363 million years ago, the first amphibians crawled onto the land, although they still returned to the water to lay their soft eggs. Not until the emergence of the first reptiles would animals appear that were independent of water in this way.
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