Fossils are the hardened remains of life forms from thousands or millions of years ago, and include impressions of skeletons or leaves, or even footprints. The oldest fossils found are approximately 3.8-billion years old. Animals with hard body parts are most likely to be preserved by fossilisation, although the right conditions will create fossils from plant material. Completely soft-bodied creatures like jellyfish are unlikely to leave fossils. They’re all the evidence we have of what was happening on Earth before humans evolved to tell stories and record information, and they’re particularly rare because only a few things are able to be leave a fossil when they die, as most decay naturally. Fossils are created when living organisms are changed by outside forces such as heat and pressure, which triggers an organic process that leaves a carbon impression – also known as a fossil. This is how we know about dinosaurs; their fossils were created through a process called permineralization or petrification.