Denisovans

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.

Small – Far Away

Until I was about thirteen, it used to mystify me why distant objects look smaller than close ones. Why wasn’t everything like tunnel vision? If there’s a castle in the middle distance which looks like a mathbox at arms length, why would they seem to be the same size? If they weren’t, vision would be very cluttered. There is, though, probably no way that could happen.

The answer, which it took me a silly amount of time to work out, is of course that the further away your field of vision is, the larger area it occupies, and therefore the smaller the portion of that area is occupied by objects within that field at that distance. I actually had to read Jacob Bronowski’s ‘Ascent Of Man’ before I realised how it worked. I presume practically everyone else who can see has worked it out before they were able to use language, but for some reason not me. It might be due to my poor eyesight, as I’ve been short-sighted for as long as I can remember. My early visual memories are literally blurry.

There’s a famous scene in ‘Father Ted’ about this:

After reaching adulthood, I did have one strange experience on travelling past a field of sheep on a train and not getting any visual cues regarding how distant it was, where they somehow ended up looking about the size of cauliflowers to me. This is odd because there are cues such as mistiness, how you need to focus your eyes and the relative angles of your pupils which ought to tell you what’s going on. Nonetheless, and perhaps mildly embarrassingly, these didn’t come into play on this occasion for some reason. Of course I didn’t actually think they were the size of caulis, but the impression was vivid.

And yes, I hereby acknowledge that I still find Graham Linehan’s writing hilarious.

Another place where “small – far away” comes into play is of course ‘Doctor Who’:

I think there’s a history of dimensional transcendentalism in British children’s literature. Actually that’s quite a pretentious way of saying it, and it’s probably fairer to say that C S Lewis’s 1950 novel ‘The Lion, The Witch And The Wardrobe’ kind of invented it, not ‘Doctor Who’. The wardrobe seems to be bigger on the inside, although it’s because it’s a portal to another universe. Looking at it that way, the TARDIS is dimensionally transcendental because it’s a pocket universe, although ‘Who’ already uses that term to mean something else. As a child, I saw the TARDIS as kind of mushroom-like, in that the physical exterior of the vehicle contained a kind of stalk extending into hyperspace and expanding to a size bigger than the three dimensions of the Police Box. This is similar to the First Doctor’s explanation of the feature. I’ve always presumed that the word “transcendental” turns up in that in the mathematical sense rather than “transcendental meditation” or something else. A transcendental number is one which cannot be expressed using the four standard arithmetical operations or raising to a whole number power. All transcendental numbers are also irrational, since a ratio is one integer divided by another. There is an uncountably infinite set of transcendental numbers, as opposed to integers which are only countably infinite. I’m left with the impression that the dimensional transcendentalism of the TARDIS is achieved by exploiting the points in space between the ones accessible to us in ordinary space, but these two versions can be combined if you imagine the TARDIS to have a kind of four-dimensionally “rough” space along the lines of “If Wales were flattened out it’d be bigger than England” kinda thing. This ignores the Planck Length though.

‘Doctor Who’ is, to say the least, fiction. However, some things really are bigger on the inside. One example is a sphere exactly half the diameter of the Universe. The Universe is said to be 93 thousand million light years across, which reducing it to more manageable units is a diameter of 28.5 gigaparsecs. In Euclidean space, a sphere of half this size would have a volume of a little over 1515 cubic gigaparsecs. However, since space is not Euclidean, this is incorrect and the actual volume of such a sphere is 18 181 cubic gigaparsecs. It actually has a dozen times the volume. This is like the forty-five centimetre diameter Pilates ball in the corner of this room having an internal volume of a ball more than twice its diameter, which would make it a lot slower to inflate and deflate. You’d wonder where all the air was going or coming from.

It isn’t confined to the Universe as a whole either. The distance between the event horizon and the centre of a black hole is greater than the apparent radius of that event horizon. In fact, technically any object with mass is larger on the inside because gravity is a distortion of space, although I don’t think anyone has worked out how much it’s done so. It can’t be significant on the scale of Earth’s mass though because that’s been measured so precisely that the discrepancy is so small that placing the apparent size of the planet in the same volume as if space were Euclidean would not lead to any visible difference even on our comparatively minute scale. Nonetheless it is technically true.

Great Britain, which could possibly do with another name (Albion?) is the eighth largest island, and is accompanied to its west by the twentieth largest, Ireland. There is then, as usual, a kind of power law of islands of diminishing size and variable isolation. I was recently rather shocked to find that I’d been completely unaware of Portsea Island, which is the third largest in terms of population in these isles at two hundred thousand. Or rather, I knew it existed and I’ve even been there, but I failed to recognise the significance of the fact that a large town was largely built on it. Some of the smaller islands are far away, but still count as part of the archipelago: Rockall comes to mind, but also the islands of St Kilda. The question of whether the Faroe Islands count as part of this collection of bits sticking up out of the water also arises. Pear’s Cyclopædia used to say they were part of Scotland, although that was not meant in political terms but geographically. The reason I bring this up here is the evolutionary processes known as island dwarfism and island gigantism, together known as the “island effect” or “island rule”. This is the tendency for small animals to become larger on islands and large animals to become smaller. What, though, do “small” and “large” mean here?

The Canadian biologist J. Foster was one of the first people to study the island effect thoroughly in the 1960s, looking at various groups of mammals, and found that species of subspecies varied in size according to their taxon. Carnivores were smaller, rodents larger, even-toed ungulates smaller and insectivores and lagomorphs smaller. He didn’t look at creatures other than mammals, but the effect does apply to them, as with the Komodo dragon and giant tortoises at the large end and pygmy frogs at the other. One explanation is that smaller creatures get larger due to the lack of predators, whereas larger ones get smaller because of the lack of food. If this is so, the factors involved don’t just apply to islands. I’m personally wondering two things right now: does it apply to lakes, and does it affect plants? And the third thing is, how small does an island have to be before this starts to happen? Does it apply to Great Britain? Are animals and plants on islands far away from the English East Midlands smaller or larger in these islands? It should help to look at more distant examples first.

Possibly the most famous incidence of insular dwarfism is of the Flores Straits Islanders, who were humans of unusually small stature, currently classified as a different species, being around 110 centimetres tall. They died out early in the last Ice Age, although there appear to be folk tales of them still around in the area. Humans currently living in the area are also unusually short, but not to the same extent. They seem to have been a sister species to Homo habilis, the “handy man”, who lived in Afrika about two million years ago. Once again, we’re in the Coral Triangle as Flores Island is in Eastern Indonesia. The island of Komodo itself, where Komodo dragons live, is nearby. On Flores Island there are other examples of dwarfism. As far as the humans were concerned, it was hypothesised that their size was not an adaptation but was associated with pathology, such as Down Syndrome or iodine deficiency. Other primates in this category include an extinct species of macaque who lived on Sardinia and a colobus monkey native to Zanzibar. There was also a dwarf wolf native to Honshu, Kyushu and Shikoku, which strike me as rather large compared to some others, but it’s been claimed that the size of island is not the main factor, which makes no sense to me because in a sense a continent is an island.

Dodos are a good example of island gigantism, and unfortunately also an illustration of island tameness. It’s possible that small herbivores find it easier to hide from large predators, so they become larger in their absence. There were a number of species of giant primate on Madagascar, which is getting on for three times the size of Great Britain. The Pleistocene extinction event, which may or may not be linked to human activity, has led to a lot of larger species dying out. It seems to me also that they may be occupying vacant niches due to lower biodiversity.

There are examples of island gigantism in these isles. For instance, there’s an endangered vole in the Orkeys who is unusually large, a fairly large subspecies of field mouse on St Kilda, but I’m not aware of any insular dwarfism. That said, it does actually seem that the species of animals living in these islands are often smaller than their continental counterparts, and this may be a form of dwarfism. For instance, there are three very small species of native cockroach in the genus Ectobius, each only about a third to a quarter of the length of Blatta orientalis or the larger Periplaneta americana. Likewise with cicadas, native to the New Forest. Cicadetta montana is around a centimetre long, but cicadas elsewhere can be up to five centimetres in length. As is usual with evolution, the size difference probably already existed but became significant in Britain due to the pressure from human activity. That’s just my guess though.

The thing about these islands, though, is that they’ve only been islands at all for the past seven thousand years and are still very close to France and Benelux, so the isolation which might apply to other islands may not pertain to us. Then again, the Orkneys are pretty close to Great Britain too. The Orcadian example, though, might illustrate something about genetics more generally. Orcadians are generally the most distinct population of humans in Ireland and Britain in terms of ancient ancestry here, and the voles of the Orkneys may partly reflect a similar distinction. That does raise another issue for me though: is there another scale factor? Rodents like voles are liable to stay on their islands rather than often surviving to mix with the mainland population, except where they’re helped by humans, but a beaver or capybara is living in a smaller world and less likely to be distinct for that reason. Again, this is just me.

Probably the most shocking example of the island effect is what happened on Haţeg Island in the late Cretaceous. Here, pterosaurs became giants and sauropods (i.e. the likes of Diplodocus and Brachiosaurus) dwarfs. I’ve mentioned the place before. Haţeg Island was about the size of Ireland and was situated in the northern Tethys in today’s Romania. Its pterosaurs were extremely large. Hatzegopteryx, for example, had a twelve metre wingspan. Magyarosaurus and Paludititan, by contrast, were dwarf sauropods, both about six metres long and weighing about a tonne. They might have been preyed upon by the pterosaurs.

Presumably, if at some time in the future Britain was able to stay isolated, after we’ve become extinct, it might be expected to show the island effect, but that would depend on it remaining an island. Although the immediate likelihood of us staying isolated is high due to anthropogenic global warming raising the sea level, in the long term Doggerland, currently below sea level in the German Ocean/North Sea, will rebound against the pressure of the ice sheets and the whole region is likely to return to being a peninsula. Consequently we probably won’t ever be affected much by island gigantism or insular dwarfism, meaning that living in these islands is long going to be like it is now: looking through the wrong end of a telescope and seeing tiny animals here, but giant ones abroad.