The Fossil Record

The fossil record is the long history 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 carefully studying which fossils are found in which rock layers, palaeontologists have reconstructed the entire story of life: from the first microscopic bacteria 3.5 billion years ago to the modern world. The fossil record is incomplete (most species left no trace), but it is the single most important window into deep time.

  • Total span3.5 billion yearsOf preserved life
  • Three main erasPalaeozoic, Mesozoic, CenozoicPlus the long Precambrian before them
  • First multicellular lifeApprox. 600 million yearsEdiacaran fauna
  • First dinosaursApprox. 230 million yearsLate Triassic
  • Mass extinctions5 big onesPlus a 6th happening now
  • % of species fossilised<1%Almost everything is missing

How the fossil record is read

Rocks form in layers over time, with older layers underneath and younger layers on top. By looking at which fossils appear in which layers, scientists can trace how life changed over time. The basic pattern is dramatically clear and consistent:

  • The oldest rocks contain only single-celled life.
  • The first complex animals appear approximately 600 million years ago.
  • The first fish appear approximately 530 million years ago.
  • The first land plants appear approximately 470 million years ago.
  • The first amphibians crawl onto land approximately 370 million years ago.
  • The first reptiles appear approximately 320 million years ago.
  • The first mammals appear approximately 225 million years ago.
  • The first birds appear approximately 150 million years ago.
  • Modern humans appear approximately 300,000 years ago.

This order is always the same. You never find a rabbit fossil in rocks older than mammals, or a dinosaur in rocks older than reptiles. This pattern is exactly what evolution predicts, and no alternative explanation accounts for it.

The three main eras of life

Geologists divide the last 540 million years (the time since complex animals appeared) into three big eras based on the kinds of life that lived in each.

  • Palaeozoic Era ("old life"), 540 to 252 million years ago. The age of fish, trilobites, early amphibians and giant insects. Ended in the most catastrophic mass extinction in Earth's history.
  • Mesozoic Era ("middle life"), 252 to 66 million years ago. The age of dinosaurs. Includes the Triassic, Jurassic and Cretaceous periods. Ended with the asteroid impact that killed the dinosaurs.
  • Cenozoic Era ("recent life"), 66 million years ago to today. The age of mammals and (later) humans. Still ongoing.

The Five Great Mass Extinctions

The fossil record shows five enormous mass extinctions, each wiping out a huge percentage of all species in a relatively short time.

  1. End-Ordovician (445 million years ago): approx. 85% of species lost.
  2. Late Devonian (375 to 360 million years ago): approx. 75% lost.
  3. End-Permian (252 million years ago): the worst. Approx. 95% of all species died, including the trilobites which had ruled the seas for 300 million years.
  4. End-Triassic (201 million years ago): approx. 80% lost. Cleared the way for dinosaurs.
  5. End-Cretaceous (66 million years ago): approx. 75% lost, including all non-bird dinosaurs.

Many scientists believe a sixth mass extinction is happening now, this time driven by human activity rather than asteroids or volcanoes.

Transitional fossils

Some of the most exciting fossils show creatures with features halfway between two major groups: exactly what evolution predicts.

  • Tiktaalik (375 million years old): a fish with primitive limbs, halfway between fish and amphibians. Found in Arctic Canada in 2004.
  • Archaeopteryx (150 million years): half dinosaur, half bird. Teeth, claws on wings and a long bony tail, but also feathers.
  • Ambulocetus (49 million years): an early whale with four legs. The name means "walking whale".
  • Australopithecus (4 million years): an upright ape that could walk on two legs but had a small brain. Key transitional fossil for human ancestry.
Fact Becoming a fossil is incredibly rare. Best estimates suggest that of all the billions of creatures that have ever lived, less than 1% became fossils, and an even smaller fraction of those have been found. The fossil record we have is therefore a tiny snapshot, mostly skewed towards big hard-bodied things in marine environments. Whole groups of creatures must have existed and disappeared without leaving a single trace.

Why some chapters are clearer than others

The fossil record is wildly uneven. Some kinds of organism are over-represented:

  • Sea creatures with hard shells: ammonites, brachiopods, clams.
  • Vertebrates with bones: fish, dinosaurs, mammals.
  • Plants with hard parts: woody trees, especially in coal-forming swamps.

Others are barely represented at all:

  • Soft-bodied creatures: jellyfish, worms, octopuses, fungi.
  • Tropical rainforest life: the wet rotting conditions destroy bodies fast.
  • Most insects: their tough exoskeletons sometimes preserve, but only in special conditions like amber.
  • Tiny terrestrial life: mites, springtails, microbes.
Did you know? The fossil record gets more incomplete the further back you go. We have detailed fossils of many ice age mammals from the last 100,000 years. We have good (though incomplete) fossils of dinosaurs from millions of years ago. But the first 3 billion years of Earth's life left almost no fossils beyond layered microbial mats called stromatolites. Almost everything we know about that long early stretch comes from chemistry and a few tiny preserved microfossils.
Deeper dive: how scientists date fossils

How do palaeontologists know that a particular fossil is 200 million years old or 200,000 years old? They use several techniques.

Relative dating. The simplest method is just looking at the rock layers. In undisturbed rock, lower layers are older than upper layers (called the law of superposition). If fossil A is found in a layer below fossil B, fossil A is older. This works well within one rock outcrop but does not give absolute ages.

Index fossils. Certain widespread short-lived species can be used to identify rock layers anywhere in the world. If a particular ammonite species lived only between 165 and 160 million years ago, then any rock layer containing it must be from that time. Index fossils let geologists match rock layers across continents.

Radioactive dating. The most powerful method. Some elements in rocks (like uranium, potassium, carbon) are radioactive: their atoms slowly break down into other elements at a precisely known rate. By measuring the ratio of original element to decay product in a sample, scientists can calculate exactly how long it has been since the rock formed. Different methods work over different time ranges:

  • Carbon-14: useful for organic material up to about 50,000 years old.
  • Potassium-argon: useful for rocks 100,000 to billions of years old.
  • Uranium-lead: useful for very old rocks, up to over 4 billion years.

Modern fossil ages are usually pinned down to within a few percent of their age. The methods can be cross-checked against each other and have been refined for over a century. The picture of deep time built up by all these methods together is one of the most thoroughly tested ideas in all of science.

For more, see what are fossils, dinosaurs and the fossil record in the evolution section.