Animal life on Earth existed for over half a billion years before hominins hit the scene – a complex combination of environmental changes, innovations in technology and competition may have led to us.
why hominins like us evolved. This is one of the bigger questions we can ask about human evolution. How come we’re like this? Animal life had been trucking along on Earth for over half a billion years, without throwing up anything quite like Homo sapiens: a bunch of two-footed apes with small teeth and uniquely chunky brains that enable us to invent new tools and cooperate across huge social groups.
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| Reconstruction of Australopithecus afarensis Frank Nowikowski/Alamy |
Our origin is an enormously important milestone in evolution. At least, it seems so to us, and who’s going to tell us otherwise? So not only do we want an explanation for our existence, we want that explanation to be suitably grand. Something with sweep. Something almost mythological.
There’s no getting away from this impulse. As Terry Pratchett and others have pointed out, we are the storytelling ape: not so much Homo sapiens as Homo narrativium. Even when our stories about human evolution are buttressed by reams of data, they’re still stories and so we tell them, and hear them, in certain ways.
This hand-waving is all prompted by a recent study that identifies something unexpected about human evolution. In a sentence, competition between Homo species seems to have driven the emergence of new species. This is peculiar and hadn’t been previously recognised. It also complicates, but doesn’t disprove, some of the existing stories about our origins.
In short, this is a story about how evolution works. There are fossils and data and testable hypotheses and all that good scientific stuff. But just bear in mind that it’s also a narrative about how we came to be, and our minds love a narrative.
Drivers of evolution
The study I’m drawing on was performed by Laura van Holstein and Robert Foley at the University of Cambridge and published in Nature Ecology & Evolution in April. They tried to find out why new hominin species evolved.
In other groups of animals, evolution tends to follow recognisable patterns. One such pattern is “negative diversity-dependent speciation”. This is a bit of a mouthful, but it just means that new species are less likely to form in a given group if there are already lots of species in that group.
Suppose there are a lot of different species of sparrow living in an area. There are only so many ways to make a living as a sparrow, and the existing species are already doing them all. So new sparrow species will only evolve rarely. But if there’s only one or two species, there are lots of opportunities, so they may well proliferate into lots of new species.
This is not an absolute rule – a 2023 study found many animal groups where it seemingly doesn’t apply. But for some groups, like birds, there is decent evidence.
So van Holstein and Foley wondered if the same was true for hominins. If there are lots of hominin species living in a region, does that make it less likely that new species will emerge? “In human evolutionary studies, we haven’t asked this question,” says van Holstein.
The pair focused on three genera of hominins. The first was Australopithecus, which includes a variety of hominins living in Africa roughly between 4 million and 2 million years ago. The second was Paranthropus, a small-brained hominin that survived well into the era of large-brained hominins. And the third was Homo, the genus that includes our species as well as famous ones like the Neanderthals and the “hobbits” (Homo floresiensis). The team excluded the other four hominin genera, basically because of a lack of data: there aren’t enough specimens for us to be confident how long they existed or how many species each contained.
To estimate how speciation rates changed, van Holstein and Foley used two datasets. One was a “family tree” of hominins, showing how the various species are thought to be related. The other was the individual fossils themselves, with their approximate ages. Each dataset has big gaps and uncertainties. But, says van Holstein, “they give the same results”.
Competitive people
In Australopithecus and Paranthropus, more diversity of species meant fewer new species evolving. “That’s the classic pattern,” says van Holstein.
But in Homo, the trend was reversed. “You see positive diversity-dependent speciation,” says van Holstein. “The more species, the higher the speciation rate.” In other words, when there were multiple Homo species living close by, they were under pressure to evolve, and new species were more likely to emerge.
This is extremely unusual. Similar patterns have only been confirmed in a handful of groups, such as beetles living on islands. So the dynamics of evolution in Homo seem to be meaningfully different to other hominin genera and pretty unusual in the context of the wider animal kingdom.
What’s going on? “There’s potentially multiple explanations,” says van Holstein, and they aren’t mutually exclusive.
One possibility is range expansion. Homo was, as far as we know, the first hominin genera to expand beyond Africa. This would have created lots of opportunities, enabling new species to form: think of Homo luzonensis in the Philippines and the hobbits on the island of Flores in Indonesia.
This does raise the question of why Homo was able to expand its range when Paranthropus and other hominins couldn’t. Maybe it was because Homo made and used more or better tools?
On that note, technology could also be part of the explanation for the new species evolving. Homo populations that could invent new tools would be able to create new niches for themselves, for instance by accessing a food that nobody else was using. “I don’t think dispersals explain everything,” says van Holstein. “You do get speciation in places where there’s other members of Homo existing.”
And thirdly, the actions of one Homo species could create new niches for another. Think of beavers, which modify their environment by building dams and thus changing the flow of rivers – creating new niches for insects and other organisms in the process. If a Homo species was hunting large animals or cutting down certain plants, that could create new spaces and opportunities for hominin evolution. “It’s changed the landscape and has created new niches for others to live in,” says van Holstein.
It’ll be tricky to sort out how or if each of these played a role. But the broader point is simply the interactions between Homo species were important, not just for the fortunes of individual populations, but for the origin of new Homo species – potentially including our own.
The grand narrative
How do we fit this inter-species competition in Homo into the broader story of human evolution?
We need to think about the other overarching explanations that have been given for the hominin fossil record. Many of these are environmental: essentially, something in the environment changed and this forced hominins to adapt, leading to the evolution of bipedality or bigger brains or whatever.
The grandaddy of these environmental narratives is the savannah hypothesis. Our ape ancestors lived in forests, often climbing in the trees – but later the climate shifted, the forests shrank and grassy savannahs expanded, so our ancestors came down from the trees. “That was a model for a long time, expansion of grasslands driving human evolution,” says van Holstein.
However, the savannah hypothesis has been challenged by reconstructions of past ecosystems. The expansion of African savannahs began before hominins existed, and in any case many habitats had a mix of grass and woodland. A 2014 paper even asked in its title if the savannah hypothesis was “a dead concept”. However, it concluded that the answer was “no”, provided you accepted that a savannah wasn’t pure grassland but rather a mosaic of habitats – something we seem to be particularly suited to.
In the 1980s, palaeontologist Elisabeth Vrba at Yale University set out the turnover pulse hypothesis. The idea here is that sharp environmental changes can cause a pulse of extinctions. Specifically, she suggests that a cooling climate 2 million to 3 million years ago led to rapid extinctions among African mammals, followed by the evolution of new species – including among hominins. Again, the data reconstructing past environments quickly gets knotty.
More recently, since the 1990s Richard Potts at the National Museum of Natural History in Washington DC has argued for the variability selection hypothesis. Here the idea is that the African climate became more unpredictable, so hominins had to adapt to this changeability. The best way to do this would be to become more adaptable, for instance by becoming more intelligent and inventive.
If all these ideas sound sensible to you, that might be because they are. A 2015 review suggested that multiple evolutionary mechanisms had acted on hominins.
And that’s, I think, where van Holstein and Foley’s study comes in. If hominins were faced with a gamut of evolutionary pressures, from expanding savannahs to an unpredictable climate, then competition among Homo species is one more to add into the mix.
Certainly that’s how van Holstein sees it. She emphasises that she isn’t trying to argue that climatic and environmental factors are unimportant in human evolution. “Your environment obviously determines what niches are available to you,” she says. “If you get massive climate turnover and old niches becoming unavailable and new niches being created, it changes how those biotic interactions will operate.” Her point, which is rather more nuanced, is that the interactions between the hominin species might be equally important.
And so here we are, with a narrative about the evolution of humanity that is extremely multi-stranded. Old ideas like the savannah hypothesis have the apparent simplicity of a classic myth, which is both why they’re appealing and why they’re incomplete at best. The story we’re now unfolding looks more like George R. R. Martin’s A Song of Ice and Fire: very long, lots of characters, a growing profusion of subplots and so far, painfully unfinished.