The oldest fossils are far smaller than these animal bones; the oldest fossils on Earth are microscopic.
The oldest fossils on Earth are microscopic fossils of blue-green algae that lived 3.5 billion years ago. These fossils are found in the oldest rocks and are representative of life on the planet at the time that they lived.
The oldest fossils are found in rocks that were formed during the Archaean eon, which lasted from 3.6 to 2.7 billion years ago. Before this time, during the Hadean eon that covered the time from 4.6 billion to 3.6 billion years ago, the surface of the Earth was covered in molten rock on which no life could survive. Once the rock cooled and hardened, the geological history of the Earth could begin. During the Archaean period, life began on Earth, and some of this life is preserved in the fossil record.
Blue-green algae, also known as cyanobacteria, are single-celled photosynthetic organisms that lived in the seas. All life on Earth during the Archaean eon consisted of single-celled organisms, such as bacteria and archaeans, that first evolved in the seas. Blue-green algae differ from the other single-celled organisms of the period because they were protected by a thick cell wall. As well, blue-green algae sometimes formed large, layered dome-shaped structures known as stromatolites or round structures known as oncolites. Stromatolites and oncolites, when fossilized, protect the inner cells so that thinly sliced sections of these fossilized structures reveal well preserved details of cyanobacteria.
Australia and South Africa
Only two regions of the world have rocks formed in the Archaean that still exist undisturbed by geological processes such as erosion or plate tectonics. These pristine Archaean crusts are found in western Australia and South Africa. Rocks from these areas are renowned as sources of the earliest fossils.
For the first 1 billion years after life evolved on our planet, all living organisms were single-celled. The end of the Archaean period was marked by the evolution of multicellular organisms during the Proterozoic eon, which lasted from 2.5 billion years ago to 540 million years ago. Soft-bodied invertebrates and colonial algae were the first multi-cellular organisms to appear. By the middle of the Proterozoic era, enough oxygen had been produced by the algae of the oceans to facilitate the evolution of larger and more complicated organisms. An August 2010 article from Discovery News announces the discovery of what might be the oldest animal fossil found to date. The 1 cm tall fossils are those of a sponge believed to be 650 million years old which lived on stromatolite reefs made by bacteria. This find is 70 million years older than any known animal fossils and could mean that multi-cellular animals were present on our planet 70 million years earlier than previously believed.