Hunting for hybrid hominins

We carry within our DNA traces of other, ancient human species. But how have hominins – different human species – interbred, and can you identify a hybrid from the shape of its skull? These are some of the questions that Kerryn Warren’s research is seeking to answer.

Humans may look different, sound different and perhaps even think differently, but we have never been more genetically similar than we are today. If, in a thousand years, archaeologists were to compare the skulls of a German, an Ethiopian and an Indonesian, the differences would be slight. But this kind of genetic similarity is, relatively speaking, a very recent event.

If you went back 50 000 or 100 000 years, the human-like inhabitants of our planet would more closely resemble that of JRR Tolkien’s Middle Earth than it would human beings today. Just as the world of The Lord of the Rings was populated with hobbits, elves, dwarves and humans, so was our ancient Earth home to Neanderthals (as in image), humans (Homo sapiens), the Denisovans in Siberia and the Indonesian “hobbits” (Homo floresiensis) – and those are just some of the hominins we know of.

These fellow hominins have left behind traces in the ancient DNA we carry within our genes today. This tells us that, at various points in the history of our species, we mixed and interbred with other ‘proto-humans’.

It is this contact between the various hominin species that Kerryn Warren has focused her research on. The PhD student in Professor Rebecca Ackermann’s Morphometrics Laboratory, Department of Archaeology, is a recent winner of the UCT and National 3-Minute-Thesis competition.

“The question I have been trying to answer”, she says, “is this: If we found the offspring of human–Neanderthal parents in fossil records, would we be able to recognise it?”
 

Building a hybrid model

Our knowledge and understanding of human hybridisation – the interbreeding of different hominin groups – primarily comes from genetics, and this has its limitations.

With very few exceptions, you cannot rely on genetics to study a fossil, explains Warren. This is because fossils are made of stone, and for those that still have organic material left in them (ie haven’t fully fossilised), extracting genetic information is limited to specific cold and dry conditions. Warren is working on figuring out if archaeologists can study the appearance of the bones and fossils to identify human hybrids instead. To this end, research in the Morphometrics Laboratory has focused on trying to build a model of what a hybrid would look like, based on hybrid mammals.

Warren’s research involves the study of three different sub-species of mice, which separated from one another around 600 000 years ago and went on to evolve markedly different characteristics. After their split, the mice experienced population explosions – similar to that experienced by humans and their groups once again made contact and mixed.

Warren stresses that the results of different groups of mice intermixing is widely divergent. Sometimes groups will show no interest in each other and there will be minimal to no interbreeding. Sometimes fertility is a problem and it is rare for them to breed. But sometimes the breeding is incredibly successful, and in some cases the hybrid breeding is so successful they ‘outbreed’ the parent species and form their own hybrid taxon. It is possible that the same varied patterns apply to the interbreeding among different human groups.
 

Big-headed hybrids

Warren set out to discover if there are tell-tale signs in the skulls of the hybrid mice that can be used as identifiers of this interbreeding. The answer appears to be yes. Generally, the hybrids tend to have much larger skulls than either of the parents, and they also have unusual features of their faces (like new bone divisions) that are incredibly uncommon in either parent.

Once again, though, there is variation.

“The hybrids tend to have very large skulls,” says Warren, “but the size depends on how closely related the parents were. Parents who are closely genetically related tend to have intermediate-sized skulls. But when they are a bit more divergent, the hybrids have huge skulls and jaws. Then, surprisingly, as the parents move even further apart genetically, the size of the skull and jaw begins to decline a little.”

This increased variation, and the speed at which the increase in variation happens in the face of hybridisation, is partly what makes this research remarkable. In order to see something completely new and different in the skull of an animal, you don’t need to rely on gradual change through generations over millions of years (as in evolutionary mutation and adaptation); a single incident of contact (gene flow) between two hominin groups can mean an explosion of different traits that did not exist before.

How these character traits carry over also varies.

“In the subsequent generations,” says Warren, “the skull is not always as big as the first-generation hybrid – although sometimes it is – but the variation does carry through into the next generations, be it skull size or shape or other variation.”

There does seem to be evidence that this model for hybrid mammals can be applied to human fossils – but, cautions Warren, these links are tentative.

She points to a few cases in particular where fossils have shown similar characteristics to the hybrid skulls seen in mice. One human specimen, which lived around 40 000 years ago in Romania, had a very strange and large jawbone with very large hind teeth – features consistent with those found in mouse hybrids. The DNA taken from this jaw indicates that, even though it is a modern human, there was a Neanderthal ancestor only four to eight generations back. Evidence from mice and other animals (such as baboons and fish) indicates that these unique features endure for up to eight generations.

Warren says that there do appear to be similar features characteristic of hybridisation in other ancient human specimens, particularly in Europe and the Middle East.
 

Pushing the frontiers of evolutionary knowledge

“What we are trying to do is compliment what the ancient DNA tells us,” she says, “so we can start drawing those parallels with greater accuracy and then begin to push the boundaries of what we know of human evolution, hopefully into contexts where we don’t have the ancient DNA.

“We know that the bulk of our modern genome comes from Africa. We know that hybridisation was happening in the Late Pleistocene, and that it happened between humans and Neanderthals,” says Warren. “But there are still so many gaps to fill. Our DNA indicates that there was integration from other, unknown, hominin groups into what became modern humans in the last 100 000 years.”

Humans, and in fact all primates, evolved from a very bushy evolutionary tree. While we don’t have the evidence to prove it, one could theorise that what makes our tree so bushy may very well have been the high levels of hybridisation between the various hominins – hybridisation that eventually resulted in us.

Image by James St John via Flickr, Creative Commons Licence

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