The Many Humans of the Globe

As our species (Homo sapiens) evolved and spread across the globe, they were contemporary with several other hominins. These include the best known of our evolutionary cousins, the Neanderthals, but also Denisovans, Homo floresiensis (mainly in Indonesia), Homo luzonensis (discovered in the Philippines), and Homo naledi (known from South Africa).

Perched from atop our ivory tower, it’s easy to think that we’ve always dominated this planet, taming beasts and molding the environment to do our bidding — and that we were alone on this journey. Not so. It’s astounding that not one, but at least five other species of humans were at some point sharing this pale blue dot of ours.

These humans didn’t appear out of the ether. We all trace our origins to a common hominin ancestor that appeared sometime about six million years ago. Scientists know of about 20 hominin species in total — but there must have been many more. Half of these species were discovered in the last 30 years alone, and more are bound to be unearthed given time (and funding).

How did all these humans evolve? Why did some lineages become successful while others withered and eventually went extinct?

A Shift in Evolutionary Dynamics

The consensus among paleontologists is that climate change is the primary factor that shapes the emergence and extinction of hominin species. However, a new study from the University of Cambridge has unveiled a strikingly unconventional pattern in human evolution. Researchers found that the rate of emergence of new species in our lineage is unparalleled among vertebrates — and it’s all got to do with interspecies competition.

The study found that unlike other vertebrates where competition generally suppresses speciation after ecological niches are filled, the Homo lineage shows an unusual trend where increased competition coincides with an increase in the formation of new species.

“We have been ignoring the way competition between species has shaped our own evolutionary tree,” said lead author Dr. Laura van Holstein, a University of Cambridge biological anthropologist.

“The effect of climate on hominin species is only part of the story.”

Analyzing the evolutionary patterns of early hominins, the researchers found a familiar cycle. First, species emerge rapidly when ecological competition is minimal, then they plateau and decline as competition intensifies and niches fill. Yet, the Homo genus, which includes modern humans, defied this trend. “The more species of Homo there were, the higher the rate of speciation. This is almost unparalleled in evolutionary science,” van Holstein notes, adding that the findings were “bizarre”.

This pattern is somewhat reminiscent of island-dwelling beetles, which also exhibit unusual speciation dynamics due to their isolated environments.

Tracing Hominin Speciation

Over recent decades, researchers have uncovered several new hominin species, from Australopithecus sediba to Homo floresiensis. Van Holstein has developed a novel database cataloging “occurrences” in the hominin fossil record, totaling around 385 instances where species samples have been found and dated.

Van Holstein points out that fossils are not always a reliable indicator of the duration of a species’ existence. “We won’t necessarily discover the earliest members of a species with the first fossil we find,” she explains.

The success of fossilization is influenced by several factors, including geology and climate conditions — whether the environment is hot, dry, or damp. Furthermore, since research is predominantly concentrated within specific global regions, some younger or older fossils likely remain undiscovered.

To counter these issues, van Holstein employed data modeling to incorporate probable population sizes at the start and end of their existence and environmental impacts on fossilization. This approach helped redefine the temporal boundaries for most known hominin species.

Her findings challenge the previous understanding that some species evolved through “anagenesis” — the gradual evolution into another species without branching. Instead, these species may have “budded,” meaning a new species branched off while the original continued to exist. For instance, it was once thought that Australopithecus afarensis evolved directly from Australopithecus anamensis through anagenesis, but new data suggests they coexisted for about half a million years.

This analysis also revealed that more hominin species coexisted and possibly competed with each other than previously thought. While early hominins like Paranthropus may have evolved physically to exploit new food sources, the evolution within our own genus, Homo, likely centered on technological innovations.

Technology As an Evolutionary Tool

The researchers argue that adopting technologies such as stone tools, fire, or advanced hunting techniques allows a species to quickly adapt and occupy new ecological niches without the lengthy process of evolving new physical traits. This capability to harness technology and generalize beyond traditional ecological niches likely spurred the rapid increase in the diversity of Homo species.

Ultimately, this led to the rise of Homo sapiens—the quintessential generalists. Competing as flexible generalists in nearly every ecological niche might have driven the extinction of other Homo species.

“These results show that, although it has been conventionally ignored, competition played an important role in human evolution overall. Perhaps most interestingly, in our own genus it played a role unlike that across any other vertebrate lineage known so far,” added van Holstein.

The findings appeared in the journal Nature Ecology & Evolution.