Living Fossils

A living fossil is a species (or small group) that has hardly changed for tens or hundreds of millions of years, while everything else around it has evolved dramatically. The term was first used by Charles Darwin in 1859. Living fossils are usually creatures whose close relatives died out long ago but who, somehow, survived almost unchanged. Each one is a window into deep time: a chance to see what extinct creatures actually looked like and how they lived. Famous examples include the coelacanth, the horseshoe crab, the tuatara, the ginkgo tree and the nautilus.

Term coined byCharles Darwin1859, in "On the Origin of Species"
Famous fishCoelacanthThought extinct for 66 million years until 1938
Oldest unchanged animalHorseshoe crabApprox. 450 million years
Living dinosaur cousinTuataraNew Zealand reptile, last of its order
Oldest unchanged treeGinkgoApprox. 270 million years
Most famous fossil cousinAmmoniteCousins (nautiluses) still alive today

What is a "living fossil"?

The term is informal and a bit misleading. No species can be truly "unchanged" over millions of years; evolution never stops. But some species look so similar to their distant ancestors in the fossil record that the term feels deserved. A few criteria are usually used:

The species (or its very close relatives) is recognisably similar to fossils from millions of years ago.
The wider group it belongs to is mostly extinct.
It lives in a stable, often isolated environment.
It has changed slowly compared to other animals or plants.

The coelacanth

The most famous living fossil is the coelacanth, a deep-sea fish first described from fossils in the early 1800s. Coelacanths were a major group of lobe-finned fish related to the ancestors of all four-legged land animals. They were thought to have gone extinct around 66 million years ago, in the same disaster that killed the dinosaurs.

Then on 22 December 1938, a young curator named Marjorie Courtenay-Latimer at a small South African museum was sorting through a fisherman's catch when she spotted a strange blue fish with stiff fleshy fins. She had no idea what it was, but she preserved it and sent a sketch to a friend. Within weeks scientists realised she had found a living coelacanth. It became one of the most famous biological discoveries of the 20th century: a fish from the age of dinosaurs, alive in the modern ocean. A second species of coelacanth was discovered in Indonesia in 1997. Both live in deep water off rocky tropical coasts.

The horseshoe crab

Horseshoe crabs are not actually crabs but ancient relatives of spiders. They have been crawling around shallow tropical seas for over 450 million years, almost unchanged from their oldest fossils. They survived four mass extinctions, including the big one 252 million years ago that killed 95% of all species. Modern horseshoe crabs still look almost identical to their Devonian ancestors. They are still incredibly numerous on certain Atlantic and Pacific beaches.

Their blue blood is also extraordinarily valuable. Horseshoe crab blood contains a chemical that detects bacterial contamination. The pharmaceutical industry uses it to test vaccines and medical equipment for safety: an estimated 500,000 horseshoe crabs are caught and bled (then released) each year to make the test possible.

The tuatara

The tuatara looks like an ordinary lizard but is actually the only survivor of an entire reptile order called the Rhynchocephalia, which was widespread in the age of the dinosaurs. Tuataras live only on small islands off New Zealand. They have several unique features: a primitive third "eye" on top of the head (visible only in babies), unusual jaws and teeth, and a slow metabolism that can let them live to over 100 years old. Tuataras can lay eggs at age 70 and stay active in temperatures that would put any normal lizard to sleep.

The ginkgo tree

The ginkgo tree (Ginkgo biloba) is the last survivor of an entire group of plants that flourished during the dinosaur age. Modern ginkgos are essentially identical to their fossils from approximately 270 million years ago. They almost went extinct: by the time humans noticed them, ginkgos survived only in a few remote temple gardens in China. Buddhist and Taoist monks had been planting and caring for them for centuries, unknowingly saving a creature from before the dinosaurs.

Modern ginkgo trees are now widely planted in cities around the world. They are unusually tough: they survived the Hiroshima atomic bomb (six trees within 1 to 2 km of the blast site sprouted again the next spring), they shrug off air pollution, and they live for over 1,000 years. Most autumn parks in temperate cities have at least one beautiful golden-leaved ginkgo.

Fact The nautilus is a living cousin of the famous extinct ammonites (spiral-shelled sea creatures whose fossils are found all over the world). Nautiluses have been swimming in tropical Pacific oceans for over 500 million years. They live in beautiful chambered spiral shells, jet-propelling themselves through the deep ocean with tentacles dangling out the open end. They are the closest you can get to seeing a real live ammonite, although they are increasingly threatened by humans collecting them for their shells.

Other living fossils

Lampreys and hagfish: primitive jawless fish, hardly changed for over 300 million years.
Crocodiles: essentially unchanged for 200 million years.
Sharks (especially frilled shark and goblin shark): ancient body plans still in use today.
Sturgeon: large bony-plated fish, unchanged for 200 million years.
Wollemi pine: an Australian tree thought extinct for 90 million years, rediscovered alive in 1994 in a remote Sydney canyon.
Sea spiders: not real spiders, but a separate ancient marine group. Unchanged for over 400 million years.
Velvet worms: rare worms with stubby legs, thought to look almost identical to the ancestors of arthropods.

Did you know? The Wollemi pine was discovered alive in 1994 by a National Parks Service officer named David Noble, who was canyoning in a remote sandstone gorge in the Blue Mountains near Sydney. The tree was known from 90-million-year-old fossils but was thought to be long extinct. Only about 100 wild Wollemi pines exist today, in a secret location protected against fire and disease. They are now grown in arboretums around the world to ensure the species survives.

Deeper dive: why some species change so slowly

Why have some species barely changed for hundreds of millions of years, while others have evolved dramatically in just a few?

The simple answer is that evolution speeds up when the environment changes. If the environment stays stable for a very long time, and an organism is well suited to it, natural selection has no reason to favour any change. The species keeps producing copies of itself, generation after generation, with only minor tweaks.

Living fossils tend to be found in stable environments where conditions have not changed much. Coelacanths live in deep ocean caves where temperatures, pressure and food sources have been constant for a very long time. Horseshoe crabs live on shallow tropical beaches that have existed for hundreds of millions of years. Tuataras live on isolated cold New Zealand islands with very few predators. Ginkgos lived in protected Chinese temple gardens. Wollemi pines hid in a sheltered Australian canyon.

The opposite happens too. Species in rapidly changing environments evolve quickly. Cichlid fish in African lakes have evolved hundreds of new species in just thousands of years, because the lakes themselves change rapidly. Humans evolved relatively quickly during repeated ice ages. Modern bacteria evolve antibiotic resistance in years.

So living fossils are not "stuck" or "behind". They are perfectly suited to their stable corner of the world, and natural selection has no reason to disturb a winning combination. As long as their environment lasts, they will continue almost unchanged. The only problem is that humans now disturb almost every environment on Earth, which means even living fossils may not be safe for much longer.

For more, see what are fossils and the fossil record.