Most people have heard of Neanderthals. In fact, most White, Asian and Native American people carry Neanderthal DNA, up to about four percent. Afrikans tend to have less but it’s recently been found that they have more than was once thought, which makes sense because people do regularly migrate between Afrika and Eurasia. I understand that East Asians have the highest proportion. Neanderthal DNA influences reactions to Covid-19, height, immune system function, hair and skin tone, depression and addiction in people today. The Neanderthals were rediscovered formally by the scientific establishment quite some time ago, in the Neander valley in 1856 CE. Clearly they can only have been rediscovered because we contain their DNA, so there must have been interbreeding and this means Hom. sap. must’ve known about them and they about us, although it will probably never be clear how we perceived each other.
Very recent discoveries, or again rediscoveries, have revealed that there were a few other species, or perhaps subspecies, of human with whom we shared Eurasia in particular, as well as the Neanderthals. The discovery of “hobbits” who died out fifty millennia ago in today’s eastern Indonesia was one celebrated example. These were dwarf humans, about 110 centimetres tall with small heads, who may or may not represent a different species. It’s been suggested that these were the results of the inbreeding of a small population or possibly iodine deficiency, but if that’s so, it doesn’t mean they didn’t adapt successfully to those conditions. Axolotls for example were initially the tadpoles of salamanders who couldn’t mature due to the lack of iodine in their water but are now a species in their own right who reproduce successfully. Homo floresiensis is, however, not what I want to talk about today.
Only eleven years ago, a woman’s little finger bone was discovered in a cave in the Altai Mountains in central and eastern Asia. This cave was known to have been frequented often by Neanderthals but this bone turned out to be different. It contained DNA from a hitherto un-rediscovered species of human, now called a Denisovan after the cave. The DNA showed that they had diverged from the Neanderthals six hundred millennia back and with the common ancestor of H. sapiens and those two species or subspecies eight hundred millennia ago. Thus far, only small parts of skeletons have been found, including a parietal bone (side of the cranium) which indicated that their brains had a capacity towards the upper end of the Neanderthal range, which is itself above that of surviving humans, at 1 800 ml. A third molar was also found which was larger than that of any Homo molar except for Homo habilis and H. rudolfensis, and similar in size proportionately to that of Australopithecines. A hybrid with Neanderthals has also been found.
One of the remarkable things about this is that even though palæoanthropology has been going since the mid-nineteenth Christian century, everyone has been operating in complete ignorance of an entire species of human closely related enough to us to contribute their DNA to ours. This is a major aspect of human prehistory. It turns out also that Melanesians have up to five percent Denisovan DNA and, more surprisingly considering their location, Icelanders had 3.3%. This last is thought to result from Neanderthal ancestry rather than directly from Denisovans. The most Denisovan population of modern humans is found among the Aeta people of the Philippines, who are considered to be a relict population from before the islands were settled by Tagalog and Cebuano speakers or their ancestors. This also seems to imply that there were two waves of settlement into Southeast Asia. Denisovans also crossed the Wallace Line between Australasian and Asian fauna and flora, which suggests that they used rafts to do so although of course other ways in which they ended up there may be involved. I have a pet hypothesis about this. Well, I say that – I’ve only just thought of it.
Denisovan remains have been found far above sea level in Tibet. Their genes include an allele which confers resistance to hypoxia. That is, they could cope well in low-oxygen conditions than most of our species. This would also be useful underwater, so I’m now wondering if this helped them cross the water towards Australasia. Other genes show that their eyes, skin and hair were all dark, by contrast with the apparently light-skinned, blue-eyed and possibly fair-headed Neanderthals. Attempts have been made to reconstruct their facial appearance based on the methylation of their DNA. This is where a methyl group (CH3) is linked to a base such as cytosine which makes it inactive. This happens in life and also in embryos to inhibit the further expression of a gene. It’s incidentally apparently the reason two ova can’t be merged to produce a zygote in humans. In a gamete, all the methylations of the parent are removed from its DNA, so it starts with a clean slate. It’s also involved in the formation of Barr Bodies from X chromosomes. Now this is my guess: I think that if you looked at the genes responsible for the development of the face, you would be able to work out what happened from their later methylation because clearly a face doesn’t continue to develop genetically after adulthood. It does in other ways of course: it succumbs to gravity, acquires wrinkles and the mandible may remodel if teeth are lost. The result is similar to a Neanderthal face.
Five percent of DNA is equivalent to about the proportion you’d expect from a single great-great-great grandparent, a distance from which family resemblance is clearly detectable. The specific genes inherited from Denisovans are to do with muscle and bone rather than the nervous system, which is Neanderthal-influenced in many of us.
One of the startling things about the Denisovan cave is that it has unexpectedly sophisticated artifacts for non-Homo sapiens sapiens humans, and in fact even for us at that time. There’s a forty-five thousand year old bracelet of polished bone which is as sophisticated as our own jewellery from thirty-five millennia later, and also a mammoth ivory button and bone beads. So once again I’m going to go off on one. These are people with bigger brains than us who had technology thirty-five thousand years ahead of our own. Is it going too far to speculate that they might have been more “intelligent” than us too? Brain size doesn’t do that alone of course. Neanderthals are thought to have larger brains because their bodies were bulkier, and Denisovans had an average weight of a hundred kilos. And of course, size isn’t everything.
The Denisovans were in the Tibetan cave, far above sea level, for a hundred and fifteen thousand years or more, dating from 160 000 BP on. Their tools in the Denisova cave show technological progress, becoming more sculpted and sophisticated over the millennia. Also, since they were constantly living at such a high altitude in Tibet, it’s probable that they evolved the low-oxygen gene while they were up there.
The size of their teeth is probably a throwback feature because neither Neanderthals nor we have molars that big. They may have helped them eat tougher food, which raises the question in my mind of whether they had fire. However, their ancestors had it so the chances are they would too. There’s also a question of range. Denisova Cave is 51° north and may represent the limit of their range in that direction. A number of unidentified hominin fossils have been found in China, which may also be theirs, and there’s an arm bone in Kyrgyzstan which is possibly also Denisovan, representing the westernmost find at 41° east of Greenwich. Hence the Denisovans seem to have been mainly central and east Asian and Australasian. One idea as to their origins is that an ice sheet in mid-Eurasia separated the populations which were to become Neanderthals and Denisovans, leading to two distinctive populations of humans, one Asian and one European. Within the Denisovans themselves there may be a genetic subdivision between continental and insular groups, so the Denisovan DNA inherited by the Melanesians may not be from the mainland group. The climatic conditions are of course very different in Indonesia and Tibet. It’s also arguable whether Neanderthals, Homo sapiens sapiens and the Denisovans are different species at all, as they clearly formed a breeding population. I’m personally wondering if the real situation was that we couldn’t interbreed at the extremes of our ranges but blended into a single species at the centre, so to speak, perhaps somewhere in the Middle East.
To finish, I want to step outside this and inject a note of doubt. Whereas I would like there to have been a significant extra species of human unknown until recently, this may well not be a species in its own right and there sometimes seem to be rather a lot of species in our genus. There seem to be about fourteen, of which all but one are extinct. This makes me wonder two things. Firstly, I think it may be a question of people wanting to treat humans as special rather than just another clade of animals. Secondly, I think it’s a good way of getting an impressive publishable unit to do this, and therefore this could be influenced by the “publish or die” mentality. I also wonder about whether being beleaguered by creationists leads palæoanthropologists, and in fact palæontologists in general, to be reactive in their work and insist rather too much on the identification of fossil species. There’s an adapiform called Darwinius masillæ discovered in the Messel deposits for example. Why name it after Darwin? It’s a primate. I dunno, just seems a bit like they’re trying to prove something to people who deny the reality of evolution. All that said, Denisovans are still fascinating and it’ll be interesting to observe what happens in the next few years.
A Box Of Chocolates 