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A surprising discovery about human evolution began with a research study about, surprisingly, birds.
Five years ago, evolutionary biologist Laura van Holstein at the University of Cambridge read a study on the role of competition in songbird evolution. The work, from an international team of evolutionary biologists, found that competition between interacting species of songbirds—over food, mates, and territory—drove the evolution of new traits, such as beak shape, across long timescales and large distances in Central and South America. For years after reading the study, a question lingered in van Holstein’s mind: Did competition also drive the evolution of hominins, the evolutionary branch that includes humans and our ancestors?
Competition is a major part of natural selection and central to the theory of evolution, yet no one had formally investigated competition between hominin species before, for two likely reasons. First, most human evolution studies focus on climate as the main driver of hominin evolution. Key events in hominin evolution—such as the first use of stone tools and increases in brain size—coincided with major changes in climate, such as the rise of the dry savanna in Africa. So, scientists have spent a lot of time studying how fluctuations in Earth’s climate may have led to adaptations that shaped our species. Second, there simply hadn’t been enough hominin fossils overlapping in time and place to think that competition among those species was a big deal.
However, over the past 15 years, many more hominin fossils and species have been discovered, so van Holstein finally had the chance to investigate her lingering question. Using computer software and mathematical models, she and evolutionary anthropologist Robert Foley analyzed a wealth of fossil data to explore competition in hominin evolution. They focused on three subgroups of hominins: the small-brained, ape-like Australopithecus, the huge-headed Paranthropus, and our own group, Homo.
What they found in our own branch of the evolutionary tree came as a shock—the complete opposite of a pattern found in other vertebrates. For Australopithecus, Paranthropus, and other vertebrate groups, as the number of species in a group went up, competition increased, and the rate of forming new species slowed down. In Homo, as the number of species went up, competition increased, and the rate of forming new species got even faster. The results, published in the journal Nature Ecology & Evolution, challenge previously held beliefs about the history of our species, and raise new questions about how competition formed us into the species we are today.
For the study, van Holstein gathered location and carbon dating information for fossils from 17 hominin species, then used that data to model when a species originated, how long it existed for, and when it disappeared. Next, she ran a mathematical model to calculate and compare how competition—that is, having more than one species in the same location at the same time competing for the same resources—affected speciation and extinction of the three hominin subgroups.
What they found in our own branch of the evolutionary tree came as a shock.
The first results emerged for Australopithecus, a genus made famous by “Lucy,” a petite, well-preserved fossilized skeleton from Ethiopia, and Paranthropus, whose broad molars and gorilla-like skull suggest these animals had huge chewing muscles. Based on the researchers’ estimates, competition drove the evolution of these two subgroups as expected: As the number of species increased, competition over resources such as food, water and shelter increased, and fewer new species evolved while more species went extinct. In other words, as the number of species increased, the rate of forming new species slowed down.
Then van Holstein ran the model for Homo. The results were so surprising, she thought, “This can’t be.” So she ran the model again. Same results. She put the data into a different model, and another. Each time, she saw the same, unusual pattern. Homo was an outlier.
(The timeframes in the illustrations of each species—spp.—are drawn from the scientists’ recent estimates.)
Unlike Australopithecus, Paranthropus, and other previously studied vertebrates, such as birds, the more species of hominins that emerged, the faster new Homo species formed and fewer Homo species went extinct. And that, van Holstein says, is “really weird.”
As with most unexpected discoveries, the scientists then asked, “Why?” They formed two hypotheses.
First, perhaps as there were more hominin species and competition increased, ecological niches where one could find food and shelter became scarce, and there was pressure to make new niches. Instead of going extinct or no longer producing new species, Homo responded with technology, such as the use of primitive tools and fire. In this way, a Homo species could have used technology to create a new niche in the local environment—such as being able to access a new food source—and then adapted to that new niche, forming a new species. Competition begets more species.
Another possibility arises from the fossil evidence that Homo species spread out geographically, the only hominin genus to expand its range outside of Africa. It may be that increased competition led to increased Homo expansions into new habitats, prompting new adaptations and more speciation.
“At a fundamental level, it’s always cool when a new result challenges how we think about our own story,” says van Holstein. Next, she plans to investigate competition between hominins and other species, such as pigs and monkeys, and how competition influenced the evolution of hominin traits such as brain size and body mass.
And there are always more lingering questions, such as what changed when Homo sapiens, our own species, finally evolved. “What the hell did Homo sapiens do? We exist in every single niche. How does our species fit into this narrative of evolution?” asks van Holstein.
The final narrative has yet to be written, but the story so far is rich with possible outcomes. Of at least 20 known hominin species that evolved over 6 million years, all but one disappeared. Starting around 40,000 years ago, Homo sapiens became the sole survivor. Some evolutionary biologists believe Homo species had an edge over other hominins, such as Paranthropus, by living in complex family groups, building shelters and using fire—traits that likely made Homo more resilient and adaptable than other hominins. Once Homo became dominant, Homo sapiens may have outlasted other Homo species through increased cognitive powers, such as problem-solving and building more, varied tools. Or, it’s possible that other Homo species succumbed to natural disasters, disease, or climate shifts. In that case, the mighty Homo sapiens was the last species standing, purely by chance. 
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