Wednesday, May 1, 2019

Denisovan Jawbone Found in Tibet

Behold the largest fossil yet found from the ancient humans we call Denisovans after the Siberian cave where their first fossils were found. Where, in fact, all the Denisovan fossils until this one had been found. We knew Denisovans had to be a widespread species, though, because they interbred with both Neanderthals and modern humans; in fact most of what we knew about them came from studying their genes. Not just the partial genomes extracted from fossils, mind you, since some insights have come from pondering what the Denisovans genes in modern humans seem to do.

It's quite odd, really, that we have discovered (or possibly imagined) so much about these people with no fossil record beyond a toe bone and some fragments. This one jaw increases our knowledge of Denisovan bones tenfold. It was found in Tibet back in 1980 and has been sitting on a museum shelf ever since, paleontologists occasionally wondering what it might be but not coming to any conclusions. It dates to about 160,000 years ago.

As to why Chinese paleontologists think this jawbone is Denisovan, well, that is a tale. Pondering this ancient jaw, which doesn't seem to belong to any known species but isn't all that old as these things go, they at first tried to extract DNA. No luck. So they tried again with proteins, eventually extracting and isolating collagen proteins that could be sequenced. The sequence is clearly not from a modern human, and it matches more closely to those from Denisovan bones than those from Neanderthals.

It's impressive work but this is a new technology and far from certain. Still, that jawbone has to be something, and if it isn't Denisovan then there was yet another human species wondering around out there.

And then there's this:
The altitude of the new Denisovan’s home — 3,280 metres above sea level — surprised researchers, and helps to solve a mystery about Denisovans’ genetic contribution to modern Tibetans. Some Tibetans have a variant of a gene called EPAS1 that reduces the amount of the oxygen-carrying protein haemoglobin in their blood, enabling them to live at high altitudes with low oxygen levels. Researchers had thought that this adaptation came from Denisovans, but this was difficult to reconcile with Denisova Cave’s relatively low altitude of 700 metres. The latest study suggests that Denisovans evolved the adaptation on the Tibetan Plateau and passed it to Homo sapiens when the species arrived around 30,000–40,000 years ago.
Don't ask my why having less haemoglobin helps you survive with less oxygen, but wikipedia's article about EPAS1 says the same thing, so there it is.


G. Verloren said...

Don't ask my why having less haemoglobin helps you survive with less oxygen, but wikipedia's article about EPAS1 says the same thing, so there it is.

A quick bit of Googling confirms what intuition tells us - less hemoglobin does NOT, by itself, help you survive with less oxygen.

When under the effects of hypoxia, the human body produces MORE hemoglobin, not less. This (along with certain other changes) helps oxygen saturation in the blood to be maintained at roughly the same levels.

The effect is most pronounced when first moving to higher elevation, and after a few weeks it begins to wear off, but it does level out at higher than normal concentrations while at higher elevation. And it is firmly established that people living at high altitudes have more hemoglobin than those who live at sea level.

That said, as with most things, too much hemoglobin actually causes problems. With not enough plasma in the blood compared to red blood cells, the blood becomes thicker and more viscous, raising blood pressure and making pumping it around the body more difficult, which can cause chronic mountain sickness.

From what I'm reading, it appears highland Tibetans - despite having higher than normal hemoglobin levels - actually have lower levels than other ethnic groups living in the same region, in particular Han individuals whose genomes originate from lower altitudes.

The supposition seems to be that the Tibetans have lower relative hemoglobin levels because their bodies have adapted to cope with hypoxia in other ways. It seems that rather than just continuing to increase the raw amounts of oxygen available to the body via hemoglobin (and causing major health problems in doing so), instead their organs and muscles developed to make more efficient usage of the oxygen available.

As I understand it, the effect seems to be somewhat comparable to the ability of certain aquatic / oceanic species to hold their breaths for very long periods.

Dolphins, for example, don't have larger lungs (relative to total size) compared to other species - their lungs are just vastly more efficient at gas exchange, and they can extract a far greater percentage of oxygen out of the same volume of held air. Additionally, when diving they actually greatly reduce blood flow to certain non-critical parts of the body, in order to drastically slow their overall consumption of oxygen. They aren't packing more total oxygen into their bodies, they just make much more efficient usage of it.

The same underlying principle seems to be at work with Tibetans and other high altitude peoples. They don't rely on more hemoglobin to increase the amount of oxygen in their systems - they instead have systems that have adapted to use less total oxygen, more efficiently.

The rest of us don't have those adaptations, and so our only way of coping with hypoxia is to create tons more hemoglobin, but that comes with its own problems, which Tibetans and others manage to entirely avoid.

Kpgoog said...

It seems to work in concert with other factors:

Wikipedia (section on Tibetans)-"The genes (EPAS1, EGLN1, and PPARA) function in concert with another gene named hypoxia inducible factors (HIF), which in turn is a principal regulator of red blood cell production (erythropoiesis) in response to oxygen metabolism.[58][59][60] The genes are associated not only with decreased haemoglobin levels, but also in regulating energy metabolism."

May 1, 2019 at 9:14 PM