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Monday, February 21, 2022

On Human Evolution

This post is brought to you buy the anti-science, anti-intellectualism movement that continues to exist in today's world; specifically, the anti-evolution movement that fails to understand evolution in any capacity.

More specifically, it is brought by the comment that spawned this TikTok video by @science_is_real.

    I saw this a few days ago and the mathematics have been plaguing me since. Why? Because this is an example which can be converted into a path that can be followed.

There has been a lot of research into the correlation between spleen size and ability to do longer, deeper dives in humans: fro reference:
And even to causality of spleen size variance during breath-holding experiments: (this study only had 12 participants so it may not be entirely representative of the overall human population).
Interestingly, the spleen size has a wide variance in the human population and, unlike the other organs, is very malleable; it changes in response to, well, everything. 

The malleable nature of the spleen makes it difficult to clearly and absolutely analyze; difficult, but not impossible. 

This is the point where I remind you that I'm no one special. I am merely inquisitive and like answers. I like data. I like seeing how the world works.

So, for a hypothetical examination of how the divers outlined in the video have been evolving we have to start with a hypothetical starting point. Let us say that there were 2,500 individuals as part of a nomad tribe. When they decided to settle into their lives of huts built on stilts they were no different from the average human population. So their spleens would have had the wide variance of 93 to 253 mL in volume as a resting, healthy, size. Furthermore, during the breath-holding experiments the variance in size was recorded spanning a range of 7% to 20% in spleen volume reduction. 

Despite the small sample size of the study we can still use these numbers as a baseline for a crude model that can show how populations change over time. We have two variables to track: resting size and contraction during breath holding. Because the mechanism of longer divers seems to be that their spleens release red blood cells into the blood during the breath holding I will presume that the larger contractibility of the spleen correlates to better survival rates during long dives.

Now, let's presume that the figures mentioned above are our 1 standard deviation posts, meaning that 2/3 the population will have their normal function within those ranges for both sets. 95% of the population will within twice the range (this is a guess since I don't have an actual standard deviation to work with) and only extraordinarily great or poor spleen performances will be outside those ranges at a rate of 2.5% of the population each.

Free Diving, as a sport, has an absurdly high death rate (as in, it's so high why would anyone do it?) of 1 in 500 dives among the recreational population. At expert levels, which the current peoples of aquatic cultures would certainly rival, the rate is reduced to 1 in 50,000 dives due to the extensive levels of safeguards in place at competitions ( ).

As tempting as it is to say that the original stilt-house divers would have the same death rate as today's recreational freedivers we cannot denigrate them to that level. It would have caused them to go extinct quickly if the entire population were dying at a rate of 1 death per 500 dives whilst diving several times a day each. So quickly, in fact, that they would suffer 4x the number of diving deaths as they have people if they only dove 5x a day - keeping in mind that each free dive is ONE breath.

I will, instead, give a blanket rate of 30% deaths per generation due to diving accidents and presume that a reasonable percentage of the population dies per generation due to unrelated causes (50% is what I am using for the remaining mathematics as a 50% death rate over the course of 20 years seems reasonable through age, accidents, and disease in a culture that is not readily prepared with modern safety equipment and medicine). Couple it with a growth rate of 110% and there are more babies made per generation than there are deaths, so the community grows.

The 50% death rate really has no direct impact on the specific mutation of spleen enlargement that we are looking to follow so we can discount it completely when it comes to the genetics involved, but it has to be considered for overall population size. Which means that, among the 30% who died in dive deaths, we can presume that the majority of them died due to their inability to perform the dive tasks and that that is a direct result of their spleen size and lung capacity. Therefore, their removal from the gene pool will remove those at the bottom of the spleen functionality end of the gene pool before they can generate many offspring.

From these assumptions we have the following crude simulated population growth:

If Natural Selection were not at work on this population through the adversity of the constant free diving then the spleen size among the population wouldn't be altered but, because that excessive stress on the population exists we can see how it affects portions of the population differently.

The model presumes that the worst 95% of the small-spleen crowd die each generation from dive-related injuries with 95% of remainder of those deaths coming from those who have average spleens. As you can see it only takes a single generation to wipe those with smaller spleens off the population chart.

Which leads to a different population breakdown as early as the first generation of people born in to the environment.

What fascinated me the most about this is how quickly such an adverse condition can dramatically change a population. 
I expected, when I set out to make this model, that it would take at least 500 years (25 generations) to show any significant change to the population and I am quite surprised that it only took two generations to eradicate the smaller spleens from the group with a fatality rate that is less than 10% that of the recreational free diving death rate among the world today. 

While "what doesn't kill you makes you stronger" is really not applicable to individuals, it certainly is to populations.

Isolation plus adversity drives Natural Selection and, in turn, evolution. 

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