Time Line
Life on Earth is presumed to be very old. Carbon 12 - which constitutes possible evidence of photosynthesis and therefore, perhaps, plant life - has been found in rocks 3.8 billion years old.
About 1.5 billion years ago, the sea contained a large number of unicellular algae which significantly increased the quantity of oxygen in the atmosphere. Around the same time, the cells developed a nucleus and began exchanging genes. This was the appearance of sexuality. Multicellular organisms - including stromatoporoids and receptaculites - appeared between 700 and 530 million years ago; original forms of animal life were also present, as evidenced by the Ediacara fauna. Corals and nautiloids appeared around 480 million years ago. Fish appeared around 400 million years ago; the fossils at the Miguasha site, in Québec, abundantly illustrate this new life form. Around 360 million years ago the first amphibian, known as the labyrinthodont, ventured forth on land, and 70 million years later, the reptiles had come to stay.
The Ediacara fauna
The Ediacara fauna and the Burgess Shale include many fossils of soft-bodied animals (animals without a shell) who apparently left no descendants. These fossils are evidence that ancient animal life was much richer than previously recognized.