Just 300,000 years ago – a blink in evolutionary time – at least nine species of humans wandered the planet. Today, only our own, Homo sapiens, remains. And this raises one of the biggest questions in the story of human evolution: where did everyone else go?
“It’s not a coincidence that several of them disappeared around the time that Homo sapiens started to spread out of Africa and around the rest of the world,” says Prof Chris Stringer, head of human origins at the Natural History Museum in London. “What we don’t know is if that was a direct connection.”
There are many theories around the disappearance of our human cousins, and limited evidence to decipher exactly what happened. But recent studies are providing tantalising clues.
What we do know is that from about 40,000 years ago, H sapiens was the last human standing out of a large and diverse group of bipedal hominins. Hypotheses range from benign, such as H sapiens having better infant survival rates than other hominins, or climate changes pushing other species to the brink. Others suggest a more active role, such as H sapiens hunting other humans or interbreeding with them and assimilating their genetics.
About 300,000 years ago, the first H sapiens populations were springing up in Africa. They didn’t look like modern humans, but they are more similar to us than other Homo species. They had tall, rounded skulls with an almost vertical forehead. They didn’t have the glowering brows of Neanderthals (Homo neanderthalensis) or the protruding jaw of archaic-looking species such as Homo naledi. They also had chins; something that no other Homo species has had (although we don’t know why only H sapiens has the protuberance).
A study published in Nature this year exploded the idea that H sapiens originated from a single place in Africa in one great evolutionary leap. By analysing the genomes of 290 people, the researchers showed that H sapiens descended from at least two populations that lived in Africa for 1m years, before merging in several interactions.
Palaeoanthropologists continue to argue (quite vociferously) over who the last ancestor of H Sapiens was, but so far there is no conclusive evidence. Also, there is no single origin for H sapiens. There are ancient remains of early H sapiens in Jebel Irhoud in Morocco, Omo Kibish in Ethiopia and Florisbad in South Africa, suggesting that our species arose from multiple sites.
When H sapiens moved out of Africa is also the subject of debate. Genetic evidence suggests there was a big foray out of the continent between 80,000 and 60,000 years ago. But it was not the first expedition. A perplexing H sapiens skull in Apidima in Greece has been dated to being at least 210,000 years old.
We know of several other Homo groups that existed alongside H sapiens between 300,000 and 100,000 years ago. Some were quite similar to H sapiens. Stocky Neanderthals endured Europe’s chilly weather and the mysterious Denisovans eked out an existence in the rarefied air of what is now Siberia and Tibet, and possibly further afield.
Homo erectus, the long-legged “cosmopolitan” species – so called because of the impressive geographical range it spanned – still wandered through parts of Indonesia; Homo longi (also known as the “Dragon man”) lived in China. Homo rhodesiensis (also known as Homo bodoensis or Homo heidelbergensis – scientists continue to debate its name and membership) was alive in central and southern Africa.
Other species were rather distinct from us: H naledi, with its ape-size brain, rambled through the woody grasslands of South Africa, and the diminutive Homo floresiensis and Homo luzonensis lived, breathed and died on the islands of Flores and Luzon in Indonesia and the Philippines respectively.
“Hominin species were likely dying out all the time,” says Prof Eleanor Scerri, head of the human palaeosystems group at the Max Planck Institute of Geoanthropology in Jena, Germany. “It’s probably unusual that we are still around.”
For most ancient human species, the fossil record is sparse. H naledi individuals, for example, are only found at a single site in South Africa. Some of the other species are known only by a handful of individuals. In Africa, where H sapiens first emerged, there are surprisingly few Homo fossils. “We don’t have a very good understanding of what other hominins were on the landscape in Africa with H sapiens yet,” says Scerri.
However, there is a plethora of data about Neanderthals, including full genomes extracted from bones. These close relatives roamed Eurasia until about 40,000 years ago, living in small groups. Scientists know significantly less about the Denisovans, but what they do know has reshuffled our understanding of human origins. In 2008, in the Denisova cave in Siberia, Russian archaeologists found several hominin bone fragments, including a finger bone and part of a toe. The cold weather had preserved some of the DNA in the finger bone, which yielded the full genome of this previously unknown species.
From Neanderthal and Denisovan genomes, researchers have inferred that they lived in small groups and frequently interbred. Some population estimates, based on mitochondrial DNA (inherited maternally), suggest that at their most abundant there were about 52,000 Neanderthals in Eurasia before they began to decline. Others suspect that there could have been between 20,000 and 50,000 individuals.
An important advantage that our direct ancestors appear to have had was population size. “Because of those small population sizes [among Neanderthals and Denisovans], there was a lot more interbreeding and the genetics reflects that,” says Scerri. The lack of genetic diversity would have rendered these populations more susceptible to diseases and thus less likely to survive.
H sapiens, by comparison, had larger groups and greater genetic diversity. The consequences of this extend beyond fitness against disease. “In H sapiens, we see larger social networks extending across the wider landscape,” says Stringer. “Having wide networks gives you an insurance policy because if you’re related to people a bit further away, if there is an environmental crisis – you’re running out of food or water – you can move into their environments and they’re not enemies, they’re your kin.” Such networks also allow for the exchange of ideas and innovation, Stringer adds.
This social resilience could have helped H sapiens survive climatic changes that would have killed off less adaptable individuals and species. A 2022 study in Nature modelled the ancient climates and ecosystems in which H erectus, H heidelbergensis and Neanderthals lived and found that they lost significant portions of their environmental niches before disappearing.
A larger 2023 simulation, which included six Homo species and the climate and vegetation over the past 3m years, found that later Homo species were able to live in a wider range of habitats, particularly H sapiens.
Prof Axel Timmermann, a co-author of this study and director of the IBS Centre for Climate Physics in Busan, South Korea, believes that H sapiens outcompeted Neanderthals, ultimately leading to the latter’s demise.
He built a numerical model, outlined in a 2020 paper, that simulated H sapiens’s spread out of Africa and combined it with available food sources. Using this, he tested three hypotheses for the extinction of Neanderthals: that they were assimilated into H sapiens; that there was a huge climate catastrophe; or that H sapiens outcompeted them. “It’s only the last one [competitive exclusion] that is able to contribute to a realistic extinction of Neanderthals,” Timmermann says.
The model didn’t investigate what the specific competitive advantage may have been, although it could have included better tools, better offspring survival rates, or maybe even social hunting, he says.
Interbreeding human species
Stringer believes a number of small advantages allowed H sapiens to outcompete its cousins. “We know now that Neanderthals were very capable, but maybe H sapiens was just slightly more capable,” he says. Seemingly small innovations, such as weaving or sewing needles (both were known in the H sapiens fossil record from 35,000 and 30,000 years ago respectively), could have tipped the scales in H sapiens’s favour, he says.
“Once you weave, you can make baskets or snare nets… A sewing needle gives you a better seal [on materials], so you have better-insulated tents and you can keep your babies warm, which is of course critical for infant survival.” Larger social networks would also have allowed H sapiens to share such innovations, he adds.
Another possibility is that H sapiens assimilated its cousins into the gene pool – and there is genetic evidence that this did happen, although whether it is responsible for the disappearance of the other species is still contentious. Some people currently living in Eurasia have up to 2% Neanderthal DNA. In fact, some geneticists claim they can assemble about 40% of the Neanderthal genome from the sequences of living people.
Meanwhile, populations in Oceania, which comprises Australasia, Melanesia, Micronesia and Polynesia, have between 2% and 4% Denisovan DNA. Some groups have an even higher percentage. There’s also the tantalising mystery of an unknown human ancestor, who contributed between 2% and 19% of their genetic ancestry to people living in west Africa today.
In 2020, two researchers from the University of California, Los Angeles obtained the genomes of more than 400 people living in Nigeria, Sierra Leone and the Gambia. They estimated that the ancient humans interbred with H sapiens in the region at some point in the last 124,000 years. “This raises an important philosophical argument,” says Scerri. “Did they really die out, or are they still with us in some way?”
Prof Rebecca Ackermann, co-director of the Human Evolution Research Institute at the University of Cape Town in South Africa, says that it depends on how you define species. This is the source of much debate among palaeoanthropologists: some recognise many species, while others acknowledge only a handful. “My take is that they probably weren’t distinct species,” she says, with the exception of obvious outliers such as small-brained H naledi. “We should really be talking about them as regional variants.”
But some groups – whether a different species or not – definitely fared better than others, with our own direct forebears surviving. This is in large part because of luck and their behaviour, agree the experts I spoke to – and is something people living today need to recognise to overcome the challenges on the horizon.
“Networking is important, the ability to adapt to change is important, and that’s certainly something we’re all going to face with climate change,” says Stringer. “Humanity will be faced with either cooperating in the face of those crises or competing. And what we see from Neanderthals and H sapiens is that the groups that cooperated better were the ones that got through.”