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Homo sapiens and Neanderthals were becoming reproductively isolated
We know that our own ancestors interbred with Neanderthals, but evidence suggests that we were still distinct species.
By using this as a case study, researchers have probed what it means to be a separate species and how it can impact what we think about our own origins.
When our ancestors migrated out of Africa, they entered a world already filled with a range of other human species.
But questions about when and how these all evolved mean that it can be tricky to trace our origins and pinpoint the exact steps that resulted in our own species’ evolution. Part of the problem comes down to issues in deciding when to name fossils as new species.
Professor Chris Stringer is one of our experts in human evolution, and has been grappling with this thorny problem in a newly published paper. Using our own species and Neanderthals as a case study, he’s helped show how these conversations can be addressed by combining morphological, behavioural and genetic evidence.
Along with his colleague Andra Meneganzin, they’ve argued that we need to identify the two hominins as separate species in order to progress with understanding our own origins.
“Speciation is a process, in which one parent species creates categorically different descendant species,” explains Chris. “In the context of Neanderthals and Homo sapiens, we need to regard speciation as a gradual process that occurred over more than 400,000 years.”
“It is known that the two interbred in areas where they coexisted. But over time, differentiation continued to a point where the two were distinctly different species. This helps to explain how Neanderthals died out around 40,000 years ago, in the final stage of the speciation process, and Homo sapiens continued to persist up to today.”
The full discussion can be read in the Evolutionary Journal of the Linnean Society.
The evolutionary history of humans is complex, with lots of different species living alongside one another ©IR Stone/Shutterstock
When does a lineage become a species?
Deciding when one species becomes another has long foxed scientists.
While on the whole, we can look at most animals and plants and be certain that they are distinct, as soon as you start to zoom in on the details things often get a bit muddier.
A good way to think about this is with a colour spectrum. For most of us it’s fairly easy to tell the difference between yellow and green, for example, but at what point on a spectrum does one colour become the other? It is a similar situation when scientists start to look at the relationships between closely related species.
Historically, scientists thought that only within a species were individuals able to breed and produce fertile offspring. Known as the “biological species concept”, this has subsequently been found to be flawed (or at least, not quite as clear-cut). This makes it surprisingly tricky to produce a single definition for what makes a species. As a result, different scientists will work to different definitions.
“There are over 30 different species concepts,” says Chris. “The biological species concept is clearly a faulty concept for many mammals and birds in practical terms, given all the genetic data we’ve got.”
“Species exist, but they’re humanly created concepts, of course, and nature doesn’t always play along with the simple views that we have.”
Even within the field of human evolution, different people use different definitions for what makes individual hominin species. For some species for which there are only a few fragments of bones, this is understandable. If researchers have only unearthed a few isolated human bones, it can be tricky to say which species it belonged to, or even if it is a new one altogether.
Despite splitting from a common ancestor around 400,000 years ago, Neanderthals are still known to have bred with modern humans about 50,000 years ago.
But these questions have also dogged one of the most well studied of ancient hominins.
Our own species and Neanderthals split from a common ancestor at least 400,000 years ago. Neanderthals were well adapted to the temperate and cooler environments of Europe, evolving a distinct anatomy and genetics to better survive in conditions very different from Africa.
Yet there are some who would argue that they should be classed as a subspecies of our own, as Homo sapiens neanderthalensis. This is largely because we know the two human lineages interbred.
Are Neanderthals a separate species?
Chris suggests that while interbreeding was happening around 50,000 years ago, there is evidence that regardless of this the two human lineages were continuing to diverge as separate species.
“What we argue in the paper is that Neanderthals were showing signs of reproductive differences and there were genetic incompatibilities showing up,” Chris explains. “Parts of the genome were not spreading successfully between the two species.”
“These differences were building up and we were heading to reproductive isolation with Neanderthals, but it hadn’t been achieved before they went extinct.”
These conversations about whether Homo sapiens and Neanderthals were separate species might seem fairly esoteric. But when it comes to figuring out our past it can have a profound influence.
If we can’t define when a species first appears, it makes it hard to recognise patterns in evolution which can create the picture of how we came to be.
“We need to make sense of human evolution,” says Chris. “We have to look at the big picture to see how things have changed through time. How does brain size get bigger or not, how do teeth change size, how does the skeleton evolve towards the form we have today? Unless you have distinct groups [ie species] in the analysis it will make discerning any patterns of change very difficult to determine.”
“We’ve got to be able to look at the fossil record and say, yes, these groups have smaller brains, and these ones, which are later in time, have bigger brains so there has been an evident evolutionary change. To do that, we’ve got to know what a Neanderthal is, and how they fit into the wider patterns.”
Chris and Andra want to open up the debate about what makes a species. By doing so, they hope to advance discussions about our own origins across the African continent.
