“The more species of Homo there were, the higher the rate of speciation. So when those niches got filled, something drove even more species to emerge. This is almost unparalleled in evolutionary science.” These are the words of Dr. Laura van Holstein of the University of Cambridge describing a pattern in our own evolutionary record that overturns decades of suppositions regarding how new species emerge in the animal kingdom and refutes the very models paleontologists have employed to map the branching of the human family tree.
Over the years, the consensus among most scientists was that climate change was the main driver of the evolution and extinction of hominin species. The traditional perspective, taken from research on Darwin’s finches and African cichlid fish, is that when ecological niches are available, species diversify quickly, but as competition becomes more severe and those niches become occupied, speciation decelerates and extinction accelerates. This “Darwinian diversity dependence” is a vertebrate macroevolutionary theory staple and is backed by wide ranging data across birds, lizards, and mammals.
Yet, when van Holstein and colleagues assembled a novel database of 385 fossil occurrences and applied advanced Bayesian modeling, they uncovered a “bizarre” reversal in the Homo lineage. Instead of competition stifling the emergence of new species, it seemed to fuel it. “The effect of climate on hominin species is only part of the story,” van Holstein explained. “We have been ignoring the way competition between species has shaped our own evolutionary tree.”
This result is in stark contrast to what happens in most vertebrates, where negative diversity dependent speciation prevails. That is, as more species pack an ecosystem, the likelihood of further speciation tends to decrease. But in Homo, the trend is positive more diversity, more speciation. The nearest analog in evolutionary literature occurs in island dwelling beetles, where insular environments and severe competition can create similar runaway diversification.
The technical foundation of this study is as cutting edge as its findings. Fossil history is notoriously incomplete, preservation being strongly geology, climate, and research dependent. As van Holstein points out, “The earliest fossil we find will not be the earliest members of a species.” By simulating not only the known fossil discoveries but also the probable population sizes at the beginning and end of each species’ lifespan, and by factoring in effects of the environment on fossilization, the team was able to more precisely estimate origination and extinction dates for 17 species of hominins.
This method unveiled that some species believed to have developed in a linear progression so called “anagenesis” potentially “budded” instead, with new species emerging as their ancestors continued. Australopithecus afarensis and Australopithecus anamensis, for instance, were formerly assumed to be a direct descendant ancestor pair but more than likely coexisted for half a million years. Such discoveries tell us that the human lineage was not a linear ladder of advance but more likely a web of coexisting and possibly competing species.
But what distinguished Homo from its australopith and paranthropine relatives? The solution, it seems, is in taking up technology. Early hominins adapted bodily changing teeth and jaws to process novel foods but people in Homo relied more and more on behavioral innovations. “Adoption of stone tools or fire, or intensive hunting techniques, are extremely flexible behaviours. A species that can harness them can quickly carve out new niches, and doesn’t have to survive vast tracts of time while evolving new body plans,” van Holstein said.
The archaeological record indicates this. Stone tools show up as early as 3.3 million years ago, before even Homo itself (Philosophical Transactions of the Royal Society B, 2015). Control of fire, leaving barely a trace in the record, reshaped diets and enabled hominins to survive in new niches. Archaeological evidence from Wonderwerk Cave and Gesher Benot Ya’aqov indicates that fire management was well established by 1 million years ago, and hearths and heat treated stone tools were widespread throughout Africa and Eurasia by 400,000 years ago.
The flexibility granted by technology enabled Homo species to escape the ecological limitations that restrained other vertebrates. Rather than having to wait for genetic mutation to create new niches, hominins were able to create their way into them. This resulted in an explosion of species, each trying out various strategies some specializing, others generalizing, and many persisting together over long periods. As van Holstein contends, “An ability to use technology to generalise, and rapidly go beyond ecological niches that force other species to compete for habitat and resources, may be behind the exponential increase in the number of Homo species detected by the latest study.”
The consequences of this research ripple out beyond anthropology. For the evolution of vertebrates, positive diversity dependent speciation is extremely uncommon, and its occurrence in Homo implies that extrinsic properties (such as climate) and intrinsic characteristics (such as technological advancement) may be interactive in unforeseen manners. The work also indicates the significance of careful fossil modeling. As recent work on fossilization bias illustrates, even minor coding errors regarding the presence or absence of anatomical features can strongly skew evolutionary trees. Van Holstein’s approaches, which combine preservation variability and occurrence data, provide a stronger framework for reconstructing the mode and tempo of human evolution.
This new view provokes other questions. How did environmental developments feed back into biotic processes? Did the emergence of the “ultimate generalist” Homo sapiens cause the extinction of other species of Homo through continuous competition? And can analogous trends be found in other clades in which culture or technology dominate?
At least for the time being, the history of human evolution seems less a procession of advance and more a turbulent, competitive test one in which innovation, competition, and resilience fashioned a singularly convoluted family tree. As Dr. van Holstein summarizes, “Perhaps most interestingly, in our own genus [competition] played a role unlike that across any other vertebrate lineage known so far.”
