The Fossil Record

The fossil record is the long story of life on Earth preserved in the rocks. Over billions of years, the remains of dead plants and animals have been buried in sediment, slowly turned to stone and stored in the Earth's crust. By digging them up and carefully studying which fossils are found in which rock layers, scientists called palaeontologists can reconstruct the history of life. The fossil record is one of the most powerful pieces of evidence we have for evolution: it shows life slowly changing over time in exactly the way the theory predicts.

  • Oldest known fossilsapprox. 3.5 billion yearsStromatolites built by bacteria
  • Oldest animal fossilsapprox. 600 million yearsSoft-bodied Ediacaran creatures
  • Famous "missing link"Tiktaalik375 million years old, fish-to-amphibian transition
  • Famous transitional fossilArchaeopteryx150 million years old, dinosaur-to-bird transition
  • % of species fossilisedTiny fraction of 1%Most life leaves no trace
  • Largest dinosaur knownArgentinosaurusapprox. 35 metres long, approx. 75 tonnes

What is a fossil?

A fossil is any preserved evidence of a once-living thing. Fossils can include:

  • Body fossils: bones, teeth, shells, leaves and other hard parts that have been preserved (often turned to stone by minerals seeping in).
  • Trace fossils: signs left behind by living animals, including footprints, burrows, nests, droppings (preserved poo, called coprolites) and even fossilised stomach contents.
  • Casts and moulds: where the original tissue has rotted away but left a hollow shape in the rock.
  • Preserved soft tissue: extremely rare, but some exceptional fossils preserve skin, feathers, hair or even internal organs.

How fossils form

For something to become a fossil, several lucky things have to happen in sequence.

  1. The dead body needs to be quickly buried in sediment (mud, sand, ash), usually under water. Otherwise scavengers and rot will destroy it.
  2. More sediment must build up on top, compressing the layers and turning them slowly to rock.
  3. Minerals in the surrounding water seep into the buried remains, gradually replacing the original tissue with stone in a process called permineralisation.
  4. The fossil must survive millions of years without being broken by earthquakes, melted by heat or eroded away.
  5. Finally, the rock must be brought back to the surface by uplift or erosion, and someone needs to find it.

This is why fossilisation is rare. Of every billion creatures that have lived on Earth, only a tiny fraction have left any trace in the rocks.

Reading the rocks

Rocks form in layers (called strata), with new layers on top and older layers underneath. By looking at which fossils are found in which layers, scientists can build up a timeline of life. The basic pattern is dramatically clear:

  • The very oldest rocks (over 3 billion years old) contain only single-celled life.
  • The first simple multi-cellular life appears around 600 million years ago.
  • The first fish appear about 530 million years ago.
  • The first land plants appear about 470 million years ago.
  • The first amphibians appear about 370 million years ago.
  • The first reptiles appear about 320 million years ago.
  • The first mammals appear about 225 million years ago.
  • The first birds appear about 150 million years ago.
  • The first apes appear about 25 million years ago.
  • Modern humans appear about 300,000 years ago.

The order is always the same. You never find a rabbit fossil in rocks older than mammals, or a dinosaur in rocks older than reptiles. Evolution predicts exactly this pattern; alternative explanations cannot.

Famous transitional fossils

Some of the most exciting fossils are transitional fossils: ones that show features halfway between two major groups, exactly what evolution predicts.

  • Tiktaalik (375 million years old): a fish-like creature with primitive limbs and a neck, halfway between fish and amphibians. Found in Arctic Canada in 2004.
  • Archaeopteryx (150 million years old): a dinosaur-bird halfway. It had teeth, claws on its wings and a long bony tail like a dinosaur, but also full feathers and the ability to fly like a bird.
  • Ambulocetus (49 million years old): an early whale with four legs, halfway between land mammals and modern whales. Its name means "walking whale".
  • Australopithecus (4 million years old): a small upright ape that could walk on two legs but had a small brain. A key transitional fossil for human ancestry.

How we date fossils

Scientists use several methods to work out how old fossils are.

  • Relative dating: simply looking at the order of rock layers. Older rocks are below younger rocks (usually).
  • Index fossils: certain widespread fossils that only existed for a short period can be used to roughly date rocks anywhere in the world.
  • Radioactive dating: measuring the decay of certain radioactive elements (like uranium or potassium) in rocks. Different decay rates allow accurate dating from a few hundred years (carbon-14) to billions of years (uranium-lead).
Fact The biggest dinosaur skeleton ever found belonged to a long-necked plant-eater called Patagotitan mayorum, discovered in Argentina in 2014. It was around 37 metres long from nose to tail and weighed an estimated 69 tonnes, about the weight of 12 African elephants. Cast skeletons of Patagotitan are now on display at several major museums around the world, and they are so big that some museums had to build extensions just to house them.

What the fossil record cannot show

The fossil record is amazing but incomplete. Many groups of living things rarely fossilise:

  • Soft-bodied creatures (jellyfish, worms, fungi) rarely leave behind anything.
  • Land-living creatures fossilise less often than sea creatures.
  • Small creatures fossilise less often than large ones.
  • Hot, wet environments destroy fossils faster than cool, dry ones.

So palaeontologists know much more about ancient seas than about ancient forests, and much more about big animals than about microbes. There are doubtless many groups of creatures that lived and died and left no trace at all.

Did you know? Some of the most extraordinary fossils preserve colour as well as shape. Scientists studying microscopic structures called melanosomes (the cells that produce pigments in modern animals) have worked out the actual colours of some dinosaur feathers. We now know, for example, that Microraptor (a small four-winged dinosaur from China) had iridescent black feathers like a modern crow.
Deeper dive: the Cambrian explosion

One of the most exciting episodes in the fossil record is the Cambrian explosion, around 538 million years ago. In a relatively short geological time (perhaps 20 to 25 million years), almost every major group of animals alive today suddenly appeared in the fossil record. Before the Cambrian, animals were mostly simple soft-bodied creatures of the Ediacaran period. After the Cambrian, the seas were full of recognisable shapes: arthropods, molluscs, brachiopods, primitive fish, and many other groups, all in their first recognisable forms.

For decades the Cambrian explosion looked like a problem for evolution. How could so many complex new body plans appear in such a short time? More recent discoveries have eased the puzzle. The "explosion" was probably not as sudden as it first looks: the older Ediacaran fossils were soft-bodied and rarely preserved. Many Cambrian "new" groups likely had Precambrian ancestors that simply did not fossilise. Genetics also suggests that some of the basic "Hox genes" that build animal body plans evolved hundreds of millions of years before the Cambrian, and were already in place when conditions allowed bigger and harder-bodied animals to evolve.

Some special fossil sites give us amazing windows into the Cambrian world. The Burgess Shale in British Columbia, Canada, preserves Cambrian sea creatures with soft tissues intact: bristly worms, weird five-eyed predators, alien-looking spike-bodied things. Similar sites in China (the Chengjiang fossils) preserve even older Cambrian fauna. Together they show that the Cambrian seas were home to some of the strangest creatures that ever lived, many of which left no modern descendants at all.

For more on evolution generally see what is evolution. For ancient catastrophes that reshaped life, see extinction. For how new species form, see species and speciation.