Early signs of life: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 3.5 billion years ago; they may have formed when certain chemical molecules joined together. Prokaryotes, single-celled microorganisms such as blue-green algae, were able to photosynthesize, and thus produce oxygen. A thousand million years later, sufficient oxygen had built up in the Earth’s atmosphere to allow
multicellular organisms to proliferate in the Precambrian seas (before 570 million years ago). Soft-bodied jellyfish, corals, and seaworms flourished about 700 million years ago. Trilobites, the first animals with hard body frames, developed during the Cambrian period (570–510 million years ago). However, it was not until the beginning of the Devonian period (409–363 million years ago) that early land plants, such as Asteroxylon, formed a waterretaining cuticle, which ended their dependence on an aquatic environment. About 360 million years ago, the first amphibians crawled onto the land, although they probably still returned to the water to lay their soft eggs. By the time the first reptiles and synapsids appeared late in the Carboniferous, animals with backbones had become fully independent of water.
