Science & Technology

Why Are We the Only Human Species?

Biological classification can get hairy. You probably know of the two main types of elephant (Asian and African), but there are actually many species and sub-species within those classifications. It's the same way with dogs: The Canis genus includes every single dog-like species from the wolf to the fox to the jackal to your beloved Fido. It seems like every organism is part of a great big clubhouse of different species, so why is there only one species of human? Well, in fact, there used to be a lot more than just Homo sapiens — and there's an incredible prehistoric hybrid that's proof.

Modern Stone Age Family

You've certainly heard of Homo neanderthalensis (aka the Neanderthal), and maybe you even have an idea of what it sounded like. But we're guessing you're not as familiar with the mutual cousin of Homo sapiens and the Neanderthal: the Denisovan. In 2008, paleoanthropologists searching in the Denisova Cave of Siberia's Altai Mountains discovered an adult's tooth and a child's pinkie bone, both 40,000 years old. Two years later, they announced that an analysis of the pinkie bone's mitochondrial DNA (which is only passed down by the mother) suggested that the 5- to 7-year-old girl was almost, but not quite, Neanderthal. There's still some debate over whether Denisovans are a new species of human or a sub-species of Neanderthal, but the distinction was enough to make paleoanthropologists around the world sit up and take notice.

But simply discovering a whole new type of human wasn't enough. In 2016, Samantha Brown from the University of Oxford published the incredible results of her genetic analysis of yet another bone fragment found in the Denisova Cave. Based on the mitochondrial DNA, this 13-year-old girl had a Neanderthal mother. Besides mitochondrial DNA, the fragment offered nuclear DNA for analysis. Nuclear DNA is inherited from both parents, allowing scientists to paint a portrait of the father as long as they also have the mitochondrial DNA to compare it with. This year, Viviane Slon from the Max Planck Institute for Evolutionary Anthropology did just that and discovered that this prehistoric teenager had an even split of Neanderthal and Denisovan DNA. Her father was a Denisovan.

Hybrid Humans

The idea that ancient human species, including Homo sapiens, may have interbred isn't a new one, but this discovery casts it in a whole new light. After all, we have only a handful of Denisovan bones to analyze. The fact that the sample size is so small and includes a first-generation hybrid suggests that interbreeding (at least in this particular cave) was quite common in the Pleistocene.

Indeed, Neanderthals aren't likely the only ancient humans that Denisovans mingled with. Denisovan DNA survives today, and in a surprising place: the Pacific Island region of Melanesia. About four to six percent of Melanesian DNA can be linked to Denisovans. However, unlike Europeans, Melanesians show no signs of Neanderthal DNA, suggesting that the Denisovan DNA was carried by interbreeding between Homo sapiens and Denisovans.

Can There Be Only One?

So far, we've mentioned three different species in the Homo genus, each of which was successful enough to have its genes live on to the modern day. But there were actually many more prehistoric humans than that, and the reason why only one species survived isn't entirely clear. According to a new theory put out by Patrick Roberts from the Max Planck Institute for the Science of Human History and Brian Stewart from the University of Michigan, the answer isn't in our bigger brains, language ability, or technological developments. After all, we know that plenty of other humans developed fire, and by the time Homo sapiens made it out of Africa, they found Neanderthals in Europe making use of technology like aspirin and paint. We probably didn't out-think the Neanderthals. So how come we survived?

It all comes down to generalization. The niche that researchers suggest humans fill is that of "generalist specialist" — we play the role of those who can slip into virtually any role available. While some of our cousins may have excelled at dwelling in some extreme climates, we found a way to, if not excel, at least adapt and spread. And as we did, we mingled with the local population and carried their genes into the future.

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Our ancient relatives lived much more complicated lives than we give them credit for. Pick up Dimitra Papagianni's "The Neanderthals Rediscovered" on Audible (free with your trial membership) and find out how. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Written by Reuben Westmaas September 6, 2018

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