Fossils: bodies of evidence from the depths of time
The Earth's crust is made up of rock layers, formed over time by various geological processes such as sedimentation or volcanism.
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Soft bodies and posterity don't mix well. Jellyfish, worms, algae, fungi, or sponges are rarely fossilized because their tissues decompose quickly. Fortunately, exceptional sites called Lagerstätten offer us a unique insight on these life forms, often invisible in the fossil record. Their secret? Very specific preservation conditions: oxygen-free (anoxic) water, volcanic ash, fine calcareous mud, etc. These environments prevent decomposition by blocking the action of bacteria and scavengers, thus allowing the preservation of soft tissues. Sometimes, sudden events such as underwater landslides, storms, or flash floods can also quickly bury organisms, freezing life in place.
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To decipher our planet's past, paleontologists and geologists rely on various clues, of which fossils are an essential part. But what is exactly a fossil?
Fossilization is the process by which remains are gradually replaced by minerals, solidifying them into stone. This process takes thousands or even millions of years. A fossil comes either from a living organism (a shell fragment, a piece of bone, a tooth, a leaf, etc.), or from its activity (footprints or excrement, for example). Ideally, to fossilize, animals must be buried quickly (to be protected from predators and decomposition) and have hard parts: we mostly find marine animals with shells or skeletons.
The oldest evidence of life on Earth is evidence of activity rather than the remains of organisms themselves (see panel 16). The oldest life forms identified to date were discovered in South Africa and date back to 3.45 billion years.