HUMAN EVOLUTION | Ancient Love:

How the Neanderthals Became Ancestors of the Irish

As February is the month of love, it might be a good opportunity to take a look at some of the stranger couplings of the ancient past which have left a surprising mark in the DNA of the Irish. In recent years, new genetic evidence has revolutionised our view of how the ancestors of modern humans interacted with human species that are now extinct, and it has become abundantly clear that these interactions were of the closest kind. While the Earth was still deep in the Ice Age, modern humans outside Africa were warming themselves with the company of people that didn't belong to their own species, one result of this being that Neanderthals became ancestors of the Irish.

For years, the prevailing theory in our recent evolution was that our species, Homo sapiens, began to leave Africa sometime around 70,000–60,000 years ago, and, as they pushed further and further into the rest of the world, they replaced archaic humans like the Neanderthals (Homo neanderthalensis) without any interbreeding taking place.

However, in recent years, the reclamation of DNA from Neanderthal fossils, and the subsequent reconstruction of the Neanderthal genome, have led various different studies to the conclusion that interbreeding did indeed take place between modern humans leaving Africa and the Neanderthals, and that Neanderthal genes account for between one and four per cent of the DNA of everyone outside sub-Saharan Africa.* This means that the Irish are part-Neanderthal!

This does not mean that the interbreeding between the ancestors of the Irish and the Neanderthals need have been very widespread or frequent, though. Considering that the original population of modern humans to leave Africa may have been as low as 2,000, it would have taken only a small amount of interbreeding with Neanderthals for Neanderthal genes to become widespread in non-African people as their numbers subsequently expanded and they went their separate ways in the colonisation of the globe.

However, it has become increasingly clear in the last few years that the scenario of interbreeding between modern humans and archaic humans like the Neanderthals was not as simple as one episode occurring, likely in the Middle East, followed by the split of non-Africans into the ancestors of the Irish, the Chinese, the Native Americans or the Australian Aborigines, with no further interbreeding taking place.

A study released in 2013 concluded that East Asians have a higher proportion of Neanderthal DNA than Europeans, proposing a scenario where a single, prolonged period of interbreeding with Neanderthals took place in the Middle East after the exit from Africa, while the ancestors of the East Asians experienced even more interbreeding after their population had diverged from the ancestors of the Europeans.

As Europeans, then, the Irish have less Neanderthal DNA than the East Asians. However, it has become increasingly clear in recent years that the complement of archaic genes in the Irish seems even smaller when we enlarge our scope to consider other ancient couplings engaged in by members of our species during the later Ice Age.

This is because, over the last decade, growing evidence has revealed that the ancestors of some non-Africans were getting jiggy with even more archaic humans than the Neanderthals, with recent studies showing that the Australian Aborigines, along with the indigenous people of Papua New Guinea and the islands east of it, can trace between one and six per cent of their DNA to an enigmatic fossil species called the Denisovans.

The Denisovans were only discovered in 2010, when a molar and a finger bone were found in Denisova Cave in the Altai Mountains of southern Siberia. Initially thought to belong to a Neanderthal population, genetic testing showed that these fossils belonged to an entirely new species, and very recent evidence has shown that, although they were similar to the Neanderthals in general appearance, their heads were wider and their jaws more protruding.

For a time, it was thought that the ancestors of the Neanderthals, the Denisovans, and Homo sapiens split from one another around 450,000 years ago when a severe ice age split their parent species, Homo heidelbergensis, into three populations, with each evolving separately in different regions: the Neanderthals in Europe and western Asia, the Denisovans in Asia, and the ancestors of Homo sapiens in Africa. However, the most recent evidence has shown that the split between these three species was even earlier, and that Homo heidelbergensis, long thought to be the common ancestor from which the lines leading to Neanderthals and Homo sapiens sprung, may actually lie only on the ancestral line of Neanderthals.

Although originally known from only their single occurrence in Denisova Cave, it had been suggested from the outset that the Denisovans may also have extended farther south, into South or Southeast Asia, and in 2019 evidence was announced that showed this supposition to be true. It was across this great range, then, that the ancestors of the Aborigines, etc., encountered the Denisovans and bred with them, and when their portion of genes from the Neanderthals and the Denisovans is taken together, these modern people have a full complement of archaic DNA approaching eight per cent or more.

But these don't appear to have been the only modern humans whose ancestors interbred with the Denisovans, with smaller amounts of Denisovan DNA having been identified in East Asians, Native Americans, and Polynesians. In fact, the 2019 discovery suggests that this interbreeding may have conferred certain modern Asians with some remarkable abilities.

The 2019 finding was revelatory in more than ways than one, being not only the first evidence for Denisovans identified beyond Denisova cave, but the first evidence that archaic humans could live at great heights, as these fossils were found at an altitude of over 3,300 metres on the Tibetan Plateau. But, even more fascinatingly as regards the topic we are discussing, it is now thought that modern humans living at great altitudes on the Tibetan Plateau today may have gained the gene to do so from the Denisovans.

Ultimately, then, it seems clear that, as Europeans, the Irish can claim more of a connection with the archaics than sub-Saharan Africans, but far less of one than that held by the indigenous peoples of places like Australia, Papua New Guinea, and the Tibetan Plateau. However, there is one connection with the Neanderthals the Irish can claim that no one else can.

For it was an Irish anatomist, William King, who coined the name Homo neanderthalensis in 1863 after recognising the fossils found in the Neander Valley, Germany in 1856 as distinct enough to count as a separate species, and this name was published the following year. So, although it is undeniable that the Neanderthals fathered many people on Earth besides the Irish, perhaps only the Irish can claim to have fathered the Neanderthals.

* The one sub-Saharan African group, the Maasai, which does have a small amount of Neanderthal DNA is thought to have received this through secondary contact with a non-African population perhaps as recently as 2,000 years ago rather than from direct contact with Neanderthals.