Discussion in 'Human Science' started by CEngelbrecht, Apr 21, 2018.
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Cool! And good demonstration that such diving adaptations aren't in all humans.
Others are. The ability to "lock down" a lungful of air, for example - that trick of the throat that makes holding the air in almost effortless.
This particular finding is that the spleens in these people are 50% larger than in most human beings. Since the enlarged spleens are found in everyone, including non-diving women and children, the researchers conclude it is an evolutionary adaptation in these people.
What I was left wondering, when I heard about this (on a radio programme last week) is what the selection mechanism is. Can it be that more successful divers have more children? How does that come about?
If success in feeding one's family depends on capable deep diving, sexual selection might operate. https://www.thoughtco.com/blood-shift-spleen-effect-freediving-2962854
Also, maybe the divers with smaller than average spleens tended to die young . Deep diving is dangerous and stressful, and damage to the spleen creates a variety of vulnerabilities (infection, etc).
Yes that's a good point. Trying to follow someone more capable, to depths they could not manage, might make them drown.
Better divers born with larger spleens catch more fish and feeds more offspring.
I find that deep freediving is the least stressful activity on the planet. That's the whole point, if you're stressed out, you burn through your breathhold much faster, shortening your dive.
How big is the spleen in chimps, gorillas and orangutans?
That's not really how it works.
Also, most food is available within the first 30 feet, and all of you can train up to manage that within a fortnight.
Deep freediving is exceptionally dangerous.
Stress wise, you are correct, they have to be able to remain calm at all times. Some describe it as being meditative, as they have to be able to regulate their feelings, mood, stress levels to be able to do it. One Bajau diver, during a series by the BBC was described as entering an almost trance-like state before a dive:
Filmed underwater in real time for the BBC's Human Planet, Bajau fisherman Sulbin demonstrates his techniques off the east coast of Sabah, Borneo. Wearing hand-made wooden goggles and armed with a spear, he first prepares himself mentally.
"I focus my mind on breathing. I only dive once I'm totally relaxed," says Sulbin, who goes into a trance-like state before entering the water.
This degree of mind control is crucial, says freediving instructor Emma Farrell, the author of One Breath, A Reflection on Freediving. "You have to be warm and relaxed - you don't want to hyperventilate before taking your last breath."
It would be interesting to see if the Ama people of Japan and other groups across Asia also have similar genetic mutations that allow them to free dive like the Bajau.
Not to mention, it would be interesting to see if the Bajau tribes across Asia, also have the same genetic mutation, or whether it occurs with just this one group in Indonesia.
However the differences discovered indicate selection pressure was operating, so there must have been something like that going on.
Sure, 'cause they're an aquatic culture and thus subjected to such selection across a few generations. But you can all get to pick food from 30 feet of water in surprising little time.
You miss my point. Mere cultural practices do not create selection pressure.
Selection pressure requires that individual with greater success at the activity in question have more offspring than those who are less successful. I am trying to address this by thinking about how this might have arisen.
The obvious one would be sexual selection for good providers and accomplished performers. Check out the number and maternal variety of children fathered by the average NBA athlete.
Less immediately visible would be the deleterious effects of sustaining even mild chronic injury - from the bends, spleen damage, silent brain injury from subpar spleen performance, etc - from diving over time. These would affect reproduction in all kinds of ways, from sexual selection to disease harboring to relative malnutrition in bad times.
One factor missing from the reports: children in these amphibious communities become self-supporting at a much earlier age than in most cultures - I recall an estimate that the expected age at which a child could feed and provide for themself in such a culture was about 8. A larger spleen would be especially advantageous at an age in which the spleen is smaller, and immune systems first challenged, and growth dependent on health, and all possibility of reproduction hanging in the balance.
Aspect of that: clearly children would not be as deeply diving. So there is a standard ecological intraspecies factor, commonly observed, in play: a deeper diving adult, especially father, is thereby not competing with his children for forage.
That actually has happened elsewhere. Mountain cultures like the Sherpas or the Incas have evolved to better utilize the oxygen in the thin air high in the mountains. The Inuit have adapted to an Arctic almost void of plant foods to feed largely on animal matter, traditionally seals and whales. This also means, that Inuit people find it hard to digest plant matter, which is why they shouldn't drink alcohol, 'cause they have evolved away from the proteine that digests both plant matter and the ethanol molecule, so drinking for them is like them being on Antabuse doing it.
But, it needs to have happened across a certain number of generations for such selection to take effect, for the Inuit several millenia.
You have to really insist to get the bends freediving, especially if you stay within the first 100 feet of depth. Basically, all you need to do to eliminate the risk of the bends is to take adequate pauses at the surface after each dive (~2min.) to flush out excess nitrogen from the blood, which otherwise causes the bends. It's only the sport freedivers going towards 300 feet and beyond, that needs to ascend slowly to eliminate nitrogen from the blood as the pressure decreases (while they're still holding their breath!). And then wait a few days before venturing to those depths again. There has been a few cases of the bends in spearfishers going repeatedly towards 100 feet and only staying at the surface for 10 seconds at a time, 'cause they needed to get that damn fish, and then suddenly their joints starts to ache.
And in terms of brain damage, the point of vasoconstriction is to concentrate the blood flow away from the limbs and keep the oxygen supply high to the brain and internal organs, minimizing the risk of brain damage during breath hold diving.
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Not comparable at all. Both your examples are environmental features to which a whole population is subject and for which an enhanced ability to cope would have obvious advantages, enabling them to remain healthy and active - including sexually - while others would be ill. This is untrue of these sea nomads, obviously.
Yes I think those are good explanations, thanks.
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