The oldest known fossil is a stromatolite (meaning "mattress rock" in Greek) dating to 2.74 billion years ago. A stromatolite is a layered accretionary structure created when bacteria, especially photosynthetic cyanobacteria, trap layers of sediment between the biofilms they create.
Stromalites come in variety of different shapes and sizes, including cones, columns, and branching types. For a long time, it was thought that stromatolites didn't exist in the present (too many grazing organisms), until they were discovered in isolated, highly saline ecosystems such as Shark Bay and Lake Thetis in Australia and Cuatro Cienegas in Mexico. Like horseshoe crabs, because of their age, stromatolites are considered "living fossils."
The oldest known fossil of stromatolites is almost five times older than the oldest fossil of a multicellular organism, which dates back to about 600 million years ago. There are some even older stromalite-like formations dating even further back than 2.74 billion years, also candidates for the oldest fossil, but none are confirmed by the scientific community.
Stromatolites are signs of the organisms that may have had a role in changing the Earth more significant than any other organism: the cyanobacteria. These photosynthetic bacteria converted carbon dioxide into oxygen on a massive scale, changing the composition of the atmosphere and causing a so-called "Oxygen Catastrophe" that would have killed off many of the other organisms alive at the time, for which oxygen was poisonous.
There are non-biological stromatolite-like structures which may be confused for the oldest known fossil, but signify nothing more than the precipitation of tiny crystals. These have not been observed today, but it is thought that conditions in the oceans of the past may have been ripe for their creation.
The oldest known fossils of animals do not appear until the Middle Ediacaran period, about 600 million years ago. These include a variety of simple sponges, cnidarians (like modern hydras), and even simple bilaterians (organisms with bilateral symmetry) that resemble tiny balloons. These animals emerged shortly after the greatest Ice Age in the history of the planet, which caused glaciers to form at sea level on the equator.