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 3,500 million years ago; they may have formed when certain chemical molecules joined together. Prokaryotes, single celled micro organisms 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 (409-565 million years ago) that early land plants, such as Asteroxylon, formed a water-retaining cuticle, which ended their dependence on an aqu...
