What Are Fossils?

A fossil is the preserved trace of a plant or animal that lived long ago. Fossils are how we know what life on Earth was like before humans existed. Almost everything we know about dinosaurs, ancient sea creatures, the first plants on land, and our own human ancestors comes from fossils dug out of rocks. The study of fossils is called palaeontology. New fossils are discovered every year, and they keep changing what we know about the long history of life.

• Oldest known fossilsApprox. 3.5 billion yearsMicrobial stromatolites
• Oldest animal fossilsApprox. 600 million yearsSoft-bodied Ediacaran creatures
• Famous fossil sitesBurgess Shale, Solnhofen, Hell Creek
• % of species fossilisedTiny fraction of 1%Most life leaves no trace
• Most common fossilsShells and bonesHard parts survive best
• Largest dinosaur fossilPatagotitanApprox. 37 m long, 69 tonnes

What counts as a fossil?

Almost any preserved evidence of a past living thing counts as a fossil. The main types are:

• Body fossils: preserved hard parts of the body. Bones, teeth, shells, plant leaves, occasionally even soft tissue.
• Trace fossils: signs the animal made while alive. Footprints, burrows, nests, droppings (preserved poo is called a coprolite).
• Casts and moulds: the original material has rotted away, leaving a shape in the rock. Often filled with later mineral, creating a perfect copy.
• Preserved soft tissue: extremely rare. Insects in amber, mammoths in permafrost, leaves in coal.
• Chemical fossils: tiny molecules from ancient organisms preserved in rocks.

Where fossils form

Most fossils form in sedimentary rocks: rocks made from layers of sand, mud, ash or sea-bottom ooze. Only sedimentary rocks gently bury dead things; igneous rock (cooled from lava) and metamorphic rock (cooked under heat and pressure) are too violent to preserve them.

Famous fossil regions tend to be old sea beds (like the chalk cliffs of southern England) or river floodplains (like the bone beds of western North America). Many fossils are easiest to find on rocky beaches, in quarries and in road cuts, where erosion has exposed old sedimentary rocks.

Why fossils matter

Fossils have revolutionised what we know about Earth and life.

• Evidence for evolution: fossils show life slowly changing over time, exactly as evolutionary theory predicts.
• Showing extinction: fossils prove that many species (like dinosaurs and trilobites) once existed and then disappeared.
• Tracing past climates: tropical fossils in Arctic rocks show that the climate has changed dramatically. Fossil plants and pollen are some of the best climate records.
• Confirming continental drift: identical fossils on continents now separated by oceans show that those continents were once joined.
• Telling deep time: matching fossils between rock layers lets geologists date rocks all over the world.

Fact Becoming a fossil is incredibly rare. Best estimates suggest that of all the billions of creatures that have ever lived, only a tiny fraction of 1% ever became fossils. Out of those, only a tiny fraction are ever discovered. Every dinosaur fossil in every museum represents an extraordinarily lucky chain of events: quick burial, undisturbed preservation for millions of years, eventual exposure by erosion, and someone walking past at exactly the right time.

The oldest fossils on Earth

The oldest fossils ever found are tiny layered mineral mats called stromatolites, built by colonies of bacteria in shallow seas approximately 3.5 billion years ago. They are not the bacteria themselves, but the layered rocks the bacteria built up, much like coral reefs today. Stromatolites still grow in a few places in the world (like Shark Bay in Western Australia), looking almost identical to their 3.5-billion-year-old ancestors.

The first complex animals with bodies appear in the fossil record approximately 600 million years ago, the first fish approximately 530 million years ago, the first land plants approximately 470 million years ago, and the first dinosaurs approximately 230 million years ago.

Did you know? Some of the most extraordinary fossils preserve colour, not just shape. Microscopic structures called melanosomes (the cells that produce pigments) sometimes survive in fossils of feathered dinosaurs. Scientists have used them to work out the actual colours of dinosaurs like Microraptor, which had iridescent black feathers like a modern crow. The first colour reconstruction of a dinosaur was published only in 2010.

Deeper dive: dinosaurs are the most famous fossils but not the most useful

Dinosaurs get all the attention, but the most useful fossils for science are often the tiniest. Many of the most important discoveries in palaeontology come from microscopic fossils of single-celled creatures.

The two big winners are foraminifera (tiny shelled marine protozoa) and diatoms (single-celled algae with intricate glass shells). Both lived in vast numbers in ancient oceans and left fossils so abundant that they can be found in almost any marine sedimentary rock.

Because there are so many of them, and because they evolved fast and lived all over the world, micro-fossils are perfect for dating rock layers anywhere. An oil company drilling a borehole can take small samples and use the micro-fossils to know exactly which rock layer they are drilling through. This is one of the main practical reasons palaeontology has earned its keep over the last 150 years: oil and gas companies have spent billions paying micro-palaeontologists to identify rocks.

Micro-fossils are also some of the best records of past climate. By measuring the chemistry of foraminifera shells, scientists can work out the ocean temperatures hundreds of millions of years ago. This is how we know about the long history of ice ages, warm periods and ocean acidification on Earth. Tiny fossils, huge insights.

For more, see how fossils form, dinosaurs and famous fossil sites.