The current view of science is that Earth is around 4.6 billion years old, and the first 4 billion years of its development are known as the Precambrian period. For the first billion years or so, there was no life in Earth. Then the first single-celled life-forms, early bacteria and algae, began to emerge. It's unclear where they came from or even if they originated on this planet at all, but this gradual development continued until around four billion years ago when suddenly (in geological terms) more complex forms of life began to emerge.
Scientists call this time of an explosion of new forms of life the Paleozoic Era, and it stretched from around 541 to 250 million years ago. In the oceans and then on land, new creatures and plants began to appear in bewildering variety, and by the end of this period, life on Earth had diversified into a myriad of complex forms that filled virtually every habitat and niche available in the seas and on the planet's only continent, Pangea.
The Precambrian Period is divided into three eons: the Hadean, the Archaean, and the Proterozoic. One layer of carbon in rocks that date to the Archaean Eon, found on islands west of Greenland, seems to have fossilized tracks that may have been left by some form of organic life, but scientists simply are not certain. The oldest confirmed remains of life date from the same eon and were found in rocks in Western Australia, which were found to contain fossilized bacteria estimated to be 3.46 billion years old. It seems likely that there were thousands, perhaps even millions of forms of life in these early periods, and most seem to have been microscopic in size and very simple in developmental terms.
