Georges Cuvier, a French anatomist, firmly established that extinction is real. Georges Cuvier used comparative anatomy, comparing living and fossil animals part by part, to test whether extinction was real. He held that an animal could be rebuilt from fragments. Mary Anning found the first complete skeleton of a Plesiosaurus, an ancient sea reptile. Cuvier doubted Mary Anning's Plesiosaurus when he first saw a drawing of it, and disputed the find. Mary Anning's fossil discoveries helped scientists rebuild the history of life, and an ichthyosaur skeleton she found made her famous. William Smith used fossils to identify rock layers, and became the first person to map the geology of an entire nation, England and Wales. Fossils tell us which plants and animals lived long ago.

Fossils can show how animals lived, not only what they looked like; trace fossils record what an animal did, such as walking, burrowing or nesting, rather than what its body looked like. Tracks tell how long a dinosaur's stride was, and the spacing of the prints shows it; footprint speed is estimated by dividing stride length by hip height, and hip height of a two-legged dinosaur is roughly four times its footprint length. This is an estimate, and it can be wrong.

Pakicetus was an early whale that lived on land, and Ambulocetus, an early whale, had short legs, paddle-like feet and a long muscular tail, which suggest it lived a more watery life than Pakicetus. Chemical clues in fossil bones help show where extinct whales lived; salt water and fresh water carry different oxygen isotopes, so a bone records what the animal drank. Ambulocetus is placed in estuaries.

Fossils are what is left of plants and animals from long ago; a common rule counts preserved remains as fossils when they are older than 10,000 years. Remains that are only partly fossilised, often from the Ice Age or younger, are called subfossils. As of November 2026, the oldest known fossils are about 3.5 billion years old and come from tiny living things in the sea in Australia. Stromatolites are fossils made by mats of tiny living things called cyanobacteria. As of November 2026, the oldest stromatolites, layered rock made by ancient cyanobacteria, date to the Early Archaean. As of November 2026, they later became widespread and were among the first reefs. Making a fossil takes a very long time, often thousands to millions of years.

Most animals and plants that die rot away and do not become fossils; remains must be buried deeply and quickly to last. Burial slows decay because there is little oxygen, and otherwise bacteria that use oxygen break the remains down to rubble; in permineralization, minerals carried by water fill the tiny pores in wood, bone or shell. Petrified wood is a standard example, and the fossil ends up heavier than the original; the fossil record is skewed toward animals with hard parts, because soft tissues and organs do not usually fossilise, and many soft-bodied species are missing from it. As of November 2026, more than 99 percent of all species that have ever lived are thought to be extinct, and this is an estimate, not a count.

Index fossils are used to work out the age of sedimentary rocks, and when the same index fossil turns up in rocks in different places, the rocks formed around the same time. Because fossils occur over wide regions of the Earth, they let geologists match up rock layers far apart; volcanic ash layers can be dated by radiometric methods. A fossil in a layer below a dated ash is older than the ash, and a fossil above it is younger; the radioactive atoms in some rocks break down at a steady, constant rate, so they work like a clock that starts when the rock forms. Counting what is left tells scientists how long ago the rock formed.

Tiktaalik was a fish with a flat head like a crocodile. Tiktaalik fossils are about 375 million years old. Tiktaalik is better called a transitional form than a missing link; it shows a mix of fish and four-legged traits. As of November 2026, Archaeopteryx has long been called a transitional form between reptiles and birds. As of November 2026, scientists still do not know if it could fly or only glide.

Tiny sea creatures build their shells from elements in the ocean water, and their fossil shells help scientists learn about past climates. Fossils can show what the climate was like long ago. Fossils help show how living things changed over time. Fossils are our best evidence for the history of life on Earth.

Words to know

• anatomy: body structure
• isotopes: element forms
• estuaries: river mouths
• stromatolites: layered reef rock
• cyanobacteria: tiny living things