Science

A non-expert’s guide to the hype: Why is the Denisovan-Neanderthal “hybrid” such a big deal?

Estimated reading time: 9 mins (1906 words)

So, you may have heard that a paper was recently published on the genome of a girl who lived around 90,000 years ago and was half-Neanderthal and half-Denisovan.

It kinda made the news…

Lots of people were excited, not just geneticists and researchers working with human fossils. And my favorite thing about a science story going viral is that it brings out the comedy gold:

 

To be fair, even Nature’s story on this publication was feeling cheeky:

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But the ultimate tweets were without a doubt the Monty Python references:

 

 

And while the news articles that ran on this story gave a good overview of why researchers were excited about it, without the historical context of how we’ve understood human evolution until now, it can be really hard to understand just how incredible this discovery is.

So let’s back up a little bit…

I’d like to take you back all the way to when I was a wee baby undergrad…

 

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Are you ready for story time, kids?

 

Back in 2011 – an entire 7 years ago – I was just starting my first year at university. Going to all these lectures and learning about archaeology, social anthropology, and biological anthropology was really exciting.

But, the thing I loved most, was all the time we got to have with the professors in small group settings. To this day, I truly believe that this aspect of Cambridge education is why I feel so comfortable in academia, but that’s a story for another time.

One of the professors I spent the most time with, was the Archaeology professor at my college, Sir Paul Mellars.

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This is a picture of him getting knighted by Prince Charles. I don’t know why I find it so funny, but I can’t help but laugh that I knew someone who went through such a fancy ritual.

 

I remember being terrified to meet him when I came to interview at Cambridge, but once I was actually in and after I’d had a few supervisions with him, that went away.

Paul Mellars
I know y’all be like “pics or it didn’t happen”, so hereby, pics of me, Paul Mellars and the other lovely anthropology students at Corpus Christi College.

 

Professor Mellars is probably one of the most entertaining people I’ve ever met. Mostly because, in my eyes, he’s this fancy unicorn who is everything you’d imagine a Cambridge don to be.

Professor Mellars has this low booming voice, and he would often be smoking his pipe or a cigar in his office. Part of me kind of imagines that this is what Santa looks like in his time off,  which is something I obviously never mentioned to his face (and since I don’t expect he’s online much, I don’t think he’ll ever find out).

But to get to the point, in our supervisions, he would always tell stories. It was one of my favorite things about him because, unlike other supervisors, he wouldn’t necessarily grill us with questions and I could rely on him doing most of the talking!

One of the things I distinctly remember talking with him about was Neandertals and why they had gone “extinct”.

Professor Mellars wasn’t just any random archaeologist, after all. What he was known and knighted for was, in part, his work on Neanderthals. He was an expert on stone tools of the Middle-Upper Paleolithic (~300,000-10,000 years ago).

By studying various stone tool industries across Europe, archaeologists are able to say something about who might have been making them and when certain people came into contact with others or were perhaps replaced.

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What we can learn from stone tools is awesome, but god I hated all my stone tool lectures and reading because looking at variously carved rocks for hours is something I simply don’t have the attention span for.

So based on the fossil evidence, the archaeological/stone tool evidence, and the genetic evidence (at the time), it appeared that Neanderthals had simply vanished at some point. Or in any case, at some point, the skeletal morphology and stone tools we associate with Neanderthals was gone, and that time seemed to coincide with what was interpreted as the arrival of “anatomically modern humans”.

There was a lot of debate, but very few people who studied human evolution believed that Neanderthals had contributed to the modern human gene pool.

Back in 2012, during my second year, I remember writing essays about why Neanderthals might have disappeared, and evaluating the evidence about them having gone extinct vs having interbred with us. I remember asking Professor Mellars what he thought, and he said it was unlikely Neanderthals interbred with us.

Most of what I read about the disappearance of Neanderthals was that “modern humans” must have outcompeted Neanderthals with our superior cognitive skills or replaced them after they disappeared due to their inability to adapt to the changing climate.

I couldn’t put my finger on it at the time, but I felt weird about the language that was being used around Neanderthals.

It also felt really uncomfortable that all explanations about this apparent disappearance of Neanderthals seemed to be centered on a post-hoc explanation of why “we” were simply better than them. Despite being wrapped in the language of natural selection, it seemed oddly similar to 19th-century arguments naturalizing colonialism and the “inevitable demise” of certain peoples. And I am certainly not the first person to draw the link between natural selection, eugenics, and colonialism.

But over the last few years, a lot of things have changed.

 

Theories on how humans evolved: one origin or multiple?

Before genetic evidence really came onto the scene, most of what we had to base our understanding of human evolution on was fossil evidence.

Looking at this fossil evidence, during the late 20th century, two camps had formed: multiregionalists and those who thought modern humans were the product of a single-out-of-Africa expansion (clearly, the latter had a much less catchy group name, but I digress…)

Multiregionalists saw continuity between the skeletal anatomy of archaic hominins (i.e. Neanderthals in Europe and Homo erectus in Asia) and the respective modern populations in each continent. For example, some researchers emphasized the similarity between the “shovel-shaped” teeth found in Homo erectus, and the presence of similar dental morphology in modern East Asian populations.

On the other hand, proponents of the single ‘Out of Africa’ hypothesis saw similarities between the skeletal morphology of late African Homo specimens and all modern humans. Some even went as far as to claim that all modern humans were the result of a ‘cognitive behavioral revolution’ around 70,000 years ago.

In the late 1980’s, when researchers were finally able to sequence mtDNA (DNA that is inherited maternally), it appeared to support the idea that all humans today traced their ancestry back, exclusively, to a single group based in Africa around 200,000 years ago.

After this, most people abandoned the multiregional hypothesis of human origins, but also any idea that Neanderthals (or other hominins) might have contributed to the modern gene pool at all.

 

The new kid on the block: Ancient DNA

Reading about the history of science is fascinating because it shows the extent to which our knowledge is shaped by the technology we have, but also the questions we choose to ask and the tools we use to answer those questions.

If you’re at all interested in understanding how weird and winding this path to our current understanding of Neanderthal/Denisovan/ancient human genetics was, I’d recommend you read Svante Pääbo’s autobiography ‘Neanderthal Man‘ where he discusses how he developed the methods that allowed researchers to extract and sequence ancient DNA.

But to get back to our story, when we began analyzing ancient DNA, it was a pain and it was difficult. Getting even a small sequence was a huge victory. So when they got a bit of Neanderthal mtDNA sequenced in the late 1990’s, it was something worth celebrating.

When complete Neanderthal mtDNA sequences followed in 2008 and 2009, while exciting, the evidence still was in line with a single out of Africa expansion.

But then, everything changed in 2010.

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What many researchers in human evolution thought was true fundamentally changed when Svante Pääbo’s group published the draft sequence of the Neanderthal genome and concluded that there was evidence of Neanderthal introgression (genetic contribution) to modern day populations outside of Africa.

In the same year, their group also published the mitochondrial (mtDNA) sequence of an unknown hominin, that appeared to group neither with modern humans nor with Neanderthals – we now refer to them as Denisovans.

Once we were able to sequence the nuclear genomes of both Neanderthals and Denisovans, we were able to get the full picture, rather than just the matrilineal side of things.

While there was an “ancient DNA rush” during this time, not everyone was convinced.

In fact, people were proposing alternative hypotheses based on ancient population structure that could explain the genetic similarity we see between non-Africans and Neanderthals. In other words, what we see could be explained by the fact that non-Africans came from an ancestral population that was genetically more similar to Neanderthals, but they did not interbreed with them.

Nowadays, we have commercially available genetic reports like 23andMe that will tell you just how Neanderthal you are. It’s unbelievable to think that only 5-6 years ago, many researchers would have thought that was plain wrong.

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Unfortunately, I’m pretty low in Neanderthal since I’m 50% African, but I’m excited to get my mom’s results!

 

Who is human and who is not?

One of the funny things I recall from undergrad is being told that I shouldn’t refer to Neanderthals as “humans” – that was reserved for “modern humans”.

When you think about it, that’s what this discussion is all about. From the beginning of anthropology, and even taxonomy, one of the key questions has been “who is human and who is not?” Is that line drawn between white Europeans and everyone else? Is that line drawn between Eurasians and Africans? Or is it drawn at some point in time, between those whose descendants are present today and those who have left no trace of their existence in populations today? And who gets to decide?

This is one of the most difficult questions in human evolution: what does it mean to be human?

This question is the reason why we have such awkward terminology as “modern humans” and “anatomically modern humans” and “behaviorally/cognitively modern humans” – they are all just clumsy attempts to essentialize the differences between groups of closely related people. But you can’t essentialize “humanness” because evolution doesn’t lend itself to clear divisions.

Some people still believe in the biological species concept. Meaning that we can define a species as a group of individuals that can bear fertile offspring. But most biologists know this definition is very limited.

The limitations of species concepts is also why we struggle to talk about this latest discovery – is this truly a hybrid? Is she just an admixed person between two different populations of the same species?

 

 

The truth of the matter is that, at the moment, we don’t know, and that’s very exciting.

We’re learning about just how messy our history is, and how these neat boundaries we have been trying to draw around groups are making us miss the reality of the complex interactions between people.

And thinking back on just how much has changed over the last decade, I’m looking forward to the next decade to come – bring on the discoveries!

 

 

 

 

 

 

1 thought on “A non-expert’s guide to the hype: Why is the Denisovan-Neanderthal “hybrid” such a big deal?”

  1. Lovely stuff. I get really frustrated by the essentialist view of species: even were we to assume that species for the most part form a closed population of mutual breeders we’d surely have to agree that the emergence of any such species must include large numbers of interbreedings before a species is “established”, over the course of a couple of hundred thousand years or so. (I believe another pretty recent species, the polar bear, interbreeds in the wild with the ancestral brown bear, particularly now thanks to climate change pushing each into the other’s territory. Similarly, tigers are most closely related to snow leopards, I recently read, but they are famous for interbreeding with lions – more distant relatives.)

    I would still be surprised, though, if we discover any evidence of sapiens- erectus interbreeding, as per the Chinese orthodoxy.

    Having said which, despite the chromosomal differences, I wouldn’t be entirely astonished if we discovered, some day, a human-bonobo hybrid in the DRC. ..

    Strange and wonderful stuff.

    Like

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