What is a fossil?

A fossil is the preserved remains, impression or trace of a living thing from a past geological age. Most fossils form when an organism is buried quickly by sediment, and over thousands to millions of years minerals gradually replace its hard parts like bones, teeth or shells, turning them to stone in a process called permineralization. Trace fossils, such as footprints, burrows and even fossilised droppings, record behaviour rather than bodies. Fossils are the primary evidence for the history of life on Earth, allowing scientists to reconstruct extinct creatures and track how species evolved. The oldest known fossils are of microbes around 3.5 billion years old.