Who Were the Neanderthals? Our Closest Relatives and What Became of Them

Neanderthals and modern humans share an ancestor who lived in Africa somewhere between 500,000 and 700,000 years ago. One population of that ancestor spread into Europe and, isolated by ice ages, evolved over the next few hundred thousand years into Homo neanderthalensis; the population that stayed in Africa became Homo sapiens. By around 400,000 years ago the European line was recognisably Neanderthal, and at its greatest extent it ranged from Wales to Siberia and south to Israel.

They were built for cold. Shorter and much more heavily muscled than us, with a broad chest, a large nose to warm the air, and a long, low skull holding a brain slightly bigger than ours. The heavy brow and the absence of a chin are the features by which a skull is recognised, and an adult male stood about 165 centimetres and weighed around 80 kilograms, more than a modern man of that height.

They were skilled hunters of big game, mammoth, bison, horse and reindeer, killed at close quarters with thrusting spears, and the healed fractures on many skeletons match the injuries of rodeo riders, the marks of a life spent close to large animals. They made a characteristic stone toolkit, the Mousterian, by striking flakes from a prepared core, and they manufactured a birch-bark tar to glue stone points to wooden shafts, a process that needs controlled heating and is the oldest known synthetic material. They controlled fire and cooked, and the tartar on their teeth records plants, mushrooms and, in one Spanish group, a diet with almost no meat.

What they did beyond survival is the contested part. The evidence that has accumulated over the last twenty years:

• •Burials: several bodies laid in dug pits, one at Shanidar in Iraq possibly with flowers, though that is disputed
• •Ornament: eagle talons and shells pierced or coloured with pigment, at sites older than any modern human presence in Europe
• •Cave structures: rings of broken stalagmites built 176,000 years ago deep inside Bruniquel cave in France, for no purpose anyone can identify
• •Art: red pigment marks on cave walls in Spain dated to 65,000 years ago, before modern humans arrived, though the dating is argued over
• •Care: individuals who survived for years after injuries that left them unable to hunt, including a man at Shanidar with a withered arm and a crushed eye socket

Modern humans reached the Middle East by 100,000 years ago and Europe around 45,000 years ago, and within five thousand years of that the Neanderthals were gone. For a century the explanation was simple superiority: our ancestors were cleverer, better armed, better organised, and either killed the natives or out-competed them. The genome complicated it. In 2010 the Neanderthal genome was sequenced from bones in a Croatian cave, and it showed that every living person whose ancestry is not entirely sub-Saharan African carries about two percent Neanderthal DNA. The two species interbred, more than once, and the children were raised.

The inherited genes are not neutral. Neanderthal variants affect skin and hair, the immune system, blood clotting, sleep patterns and, as was found in 2020, the severity of COVID-19. Some were useful to a species arriving in a cold, unfamiliar continent; others have been slowly weeded out since.

No single cause is now thought to be enough. Neanderthals lived in small, scattered groups with low genetic diversity, and they had been through population crashes in earlier cold spells; a species living close to the margin can be tipped over by a small disadvantage. Modern humans arrived in larger numbers with wider social networks, projectile weapons, sewn clothing and possibly dogs, and the climate lurched through a series of sharp cold swings at exactly that time. Interbreeding itself may have absorbed the smaller population into the larger. In the end the difference between extinction and absorption is not sharp, because a Neanderthal ancestry runs through billions of people alive today.

Their reputation has changed with the evidence. The stooping brute of 1908 turned out to be an old man with arthritis. The current picture is of a human species with a culture, a capacity for symbols and a long record of surviving a hard continent, which met a close cousin, mixed with it, and did not outlast the meeting.

The methods that decoded the Neanderthal genome revealed a group nobody had predicted. In 2010 researchers sequenced DNA from a tiny finger bone found in Denisova Cave in the Altai Mountains of Siberia and discovered that it belonged to neither Neanderthals nor modern humans, but to a related population now called the Denisovans. Their fossils are still extremely scarce, a few teeth, bone fragments and a jaw found in Tibet, so most of what is known about them comes from their genes rather than their skeletons. Svante Pääbo, who led much of this work, received the Nobel Prize in 2022.

The DNA shows a tangled family tree. Denisovans interbred with modern humans too, and people in Papua New Guinea and Aboriginal Australians carry a notable share of Denisovan ancestry. A gene variant that helps Tibetans live at high altitude appears to have come from Denisovans. Most striking, a bone fragment from the same Siberian cave came from a girl whose mother was a Neanderthal and whose father was a Denisovan, a first-generation child of two groups. Instead of a neat line leading to us, the evidence shows several human populations meeting and mixing whenever their ranges overlapped.

Ancient DNA is hard to work with. After death it breaks into short fragments, often only a few dozen letters long, and most of the DNA extracted from a fossil bone belongs to bacteria and fungi that colonised it, not to the animal itself. Worse, everyone who has handled the bone has left their own DNA on it, and a Neanderthal sequence differs from ours so slightly that contamination is a real danger. Laboratories therefore work in sealed clean rooms, with staff in full protective suits, and they sequence millions of fragments at once and sort them by computer.

The key test of authenticity is damage. Over thousands of years one of DNA's chemical letters, cytosine, slowly changes so that sequencing machines read it as thymine, and this happens most at the frayed ends of the fragments. Genuinely old DNA shows that pattern; fresh contamination does not. The same methods now reach beyond bones. In 2017 researchers recovered Neanderthal and Denisovan DNA from cave sediments, the dirt of the floor, including at sites where no bones had been found. Where DNA has decayed completely, proteins in tooth enamel and bone, which last far longer, can still show which kind of human a fossil came from.

Whether Neanderthals created symbolic art has been hotly debated, and several finds have pushed the argument forward. In 2014 researchers described a deliberate cross-hatched pattern cut into the rock floor of Gorham's Cave in Gibraltar, beneath deposits linked to Neanderthals, and argued that it was made on purpose rather than by accident or butchery.

In 2018 a team dated mineral crusts over red paintings in three Spanish caves to more than 64,000 years ago, before modern humans are thought to have reached Europe, implying that Neanderthals made them. Those dates have been challenged by other researchers, so the question is not settled. The debate matters because symbolic art is often treated as a sign of a fully modern mind.

Neanderthals were a human species that evolved in Europe from a common ancestor with us, lived there for around 350,000 years as skilled big-game hunters with tools, fire, adhesives, ornament and possibly art, and disappeared about 40,000 years ago within a few thousand years of modern humans arriving. They interbred with us, and roughly two percent of the genome of most people outside sub-Saharan Africa is theirs.