Extinction

Extinction is when an entire species dies out completely, with no individuals left anywhere. Extinction is a normal part of life on Earth: over 99% of all species that have ever lived are now extinct. Most species die out slowly, usually because their environment changes or because they cannot compete with newer rival species. But the planet has also been hit by at least five great mass extinctions, when huge numbers of species died out in a short time, usually because of some catastrophic event. Many scientists think we are in the early stages of a sixth mass extinction right now, this time driven by humans.

• Species ever livedapprox. 5 billionBest estimate
• % now extinctapprox. 99%Extinction is the normal fate of every species
• Mass extinctions5 big onesPlus a 6th happening now
• Worst everEnd-Permianapprox. 95% of all species wiped out, 252 million years ago
• Most famousEnd-CretaceousKilled the non-bird dinosaurs, 66 million years ago
• Average species lifespanapprox. 1 to 10 million yearsThen either evolves or dies out

The five great mass extinctions

Over the last 540 million years (the time since complex animals first appeared), Earth has been hit by five mass extinction events. In each one, huge numbers of species died out in geologically short timescales.

• End-Ordovician (445 million years ago): about 85% of species lost. Caused by a sudden ice age that locked up sea water and dropped sea levels.
• Late Devonian (375-360 million years ago): about 75% lost. Probably caused by major changes in ocean chemistry, possibly linked to the spread of land plants.
• End-Permian (252 million years ago): the worst. About 95% of all species on Earth died out, including the trilobites which had ruled the seas for 300 million years. Caused by massive volcanic eruptions in Siberia, which released enormous amounts of carbon dioxide and sulphur.
• End-Triassic (201 million years ago): about 80% lost. Cleared the way for dinosaurs to dominate.
• End-Cretaceous (66 million years ago): about 75% of species lost, including all the non-bird dinosaurs. Caused by a 10-km asteroid hitting what is now Mexico.

The dinosaurs and the asteroid

The most famous mass extinction is the End-Cretaceous event 66 million years ago, which killed off all the dinosaurs (except for birds). For decades scientists puzzled over what caused it. The answer became clear in 1980, when American physicist Luis Alvarez and his son Walter found a thin layer of the rare element iridium in rocks from exactly that age all around the world. Iridium is rare on Earth's surface but common in asteroids. They proposed that a giant asteroid had hit the Earth.

The crater was eventually found in 1991, buried beneath the seabed off the Yucatan Peninsula in Mexico. Called Chicxulub, the crater is 180 km across, made by an asteroid roughly 10 km in diameter (slightly bigger than Mount Everest is tall). The impact blasted enormous amounts of rock and dust into the upper atmosphere, blocking out the sunlight for years. Plants died, then herbivores starved, then carnivores starved. Within months to a few years, an estimated 75% of all species on Earth were gone, including every species of non-bird dinosaur.

The sixth mass extinction

Many scientists believe we are now in the early stages of a sixth mass extinction. The current rate of species loss is estimated at 100 to 1,000 times faster than the natural background rate. Around 1 million species are currently at risk of extinction. The major causes are all human:

• Habitat loss: forests, wetlands and grasslands being cleared for farms, cities and roads.
• Climate change: rising temperatures, shifting rainfall, melting ice and changing oceans are pushing species beyond their tolerance.
• Pollution: pesticides, plastics, oil spills and chemical runoff are damaging ecosystems.
• Over-hunting and over-fishing: many large fish stocks have collapsed; rhinos, elephants and other large mammals are heavily targeted by poachers.
• Invasive species: humans accidentally or intentionally moving species to new places, where they out-compete or eat native species.

The good news is that, unlike the previous five mass extinctions, this one is happening for reasons we understand and can change.

Recent extinctions

Famous species lost in recent centuries include:

• Dodo (last seen 1681): a large flightless pigeon from the island of Mauritius, killed off by sailors and the rats and cats they brought.
• Steller's sea cow (1768): a huge 9-metre-long marine mammal hunted to extinction within 27 years of being discovered.
• Passenger pigeon (1914): once the most numerous bird on Earth with flocks of billions, hunted to extinction in a few decades.
• Thylacine or "Tasmanian tiger" (1936): a marsupial predator hunted to extinction in Australia. The last one died in a zoo.
• Northern white rhino (functionally extinct since 2018): only two females remain, both unable to breed naturally.

Fact The dodo became extinct in 1681, less than 100 years after the first humans set foot on Mauritius. It was a peaceful 1-metre-tall flightless pigeon that had evolved on an island with no predators, so it had no fear of people and was easy to catch. The pigs, rats and monkeys that sailors brought to the island also ate dodo eggs. The dodo is now the symbol of human-caused extinction; the phrase "dead as a dodo" has been in English since the 19th century.

Conservation: stopping the losses

Despite the scale of the problem, conservation efforts can and do work. Many species have been brought back from the brink of extinction.

• Giant pandas: numbers have grown from under 1,000 in the 1970s to nearly 2,000 wild pandas today, thanks to habitat protection in China.
• Bald eagle: down to 417 nesting pairs in the United States in the 1960s, now over 300,000 thanks to a ban on the pesticide DDT.
• Humpback whale: hunted to near-extinction in the 20th century, now recovering well thanks to whaling bans.
• California condor: down to just 22 individual birds in 1987. A captive-breeding programme has brought the population back to over 500 today.

Did you know? Some scientists are working on de-extinction: bringing back species that have already died out, using DNA recovered from museum specimens or frozen remains. Several teams are trying to bring back the woolly mammoth using elephant cells edited with mammoth genes. The Australian gastric-brooding frog (which incubated its young in its stomach) is another target. Most of these projects are still in early research, and many scientists question whether the resources would be better spent saving species not yet extinct.

Deeper dive: what happens after a mass extinction

Mass extinctions are catastrophic, but they are also evolutionary opportunities. Each of the five great extinctions cleared the world of dominant species, creating empty ecological niches that surviving groups then expanded into. The result has been some of the most dramatic bursts of evolution in the history of life.

After the End-Permian extinction (252 million years ago), the dominant land animals at the time (a group called the synapsids) were almost wiped out. The survivors included a few small reptiles, who then rapidly diversified to fill the empty niches. Some of those reptiles became the first dinosaurs about 20 million years later. The Permian extinction set the stage for the entire age of dinosaurs.

Then 66 million years ago, the End-Cretaceous extinction killed off all the non-bird dinosaurs. The surviving mammals (which had been small, mostly nocturnal creatures throughout the dinosaur age) suddenly had empty habitats to fill. Within 10 million years they had diversified into the ancestors of bats, whales, primates, hoofed animals, carnivores and many other modern groups. Without the asteroid 66 million years ago, mammals (including us) might still be small, hiding in the undergrowth, and the dinosaurs might still be the dominant land animals.

Recovery from a mass extinction takes a long time. After the End-Permian event, it took the world about 10 million years to return to its previous level of biodiversity. The current sixth mass extinction may have similar long-term consequences: it could take millions of years for biodiversity to recover, no matter what we do now. The species we lose today are gone for geological time.

For Earth's rich fossil record, see the fossil record. For the rate of evolution that produces new species, see species and speciation.