Human organ evolution (Introduction)

by dhw, Sunday, April 22, 2018, 13:18 (2189 days ago) @ David Turell

DAVID: Deep divers have big spleens:
https://www.the-scientist.com/?articles.view/articleNo/52347/title/Free-Divers-From-Sou...

DAVID’s comment: This is an organ adaptation, not a change in the human species.

dhw: Yes indeed, but it confirms a pattern which runs through so many of these threads. Adaptation to the environment causes changes to the body, but we don’t know the extent to which the body can change itself. The divers didn’t tell their spleens to get bigger. Of course it may be that natural selection caused bigger spleens to outlive smaller spleens, but an alternative would be the process already observed elsewhere, i.e. that concentrated usage results in expansion of the organs used. With musicians and taxi drivers it’s parts of the brain, and with the divers it’s the spleen. One can envisage the same process occurring when land animals entered the water, and limbs became fins, or vice versa, with fish evolving legs out of fins. Not proven, of course, but there is a satisfying consistency in this hypothesis, and it still allows for your God as the inventor of the mechanism that makes it all possible.

DAVID: It is possible that the explanation lies with the first that dived. Those with larger spleens were more productive and produced more divers. The process repeated over and over made the folks we have today. Pure Darwin which makes sense in this case.

Yes, as I said above (now bolded), that is natural selection. Nice to see you defending Darwin for a change! But the alternative origin seems equally possible to me: that usage resulted in expansion, which was passed on to following generations.

DAVID: I see no parallel in those mammals that took to water. That has to be saltation.

If particular usage can change the structure of the brain, and exercise can expand muscles, I don’t see why it can’t change other structures too, but the question is always to what extent, and in this case how quickly. The answer is that we don’t know. Given the choice between your God changing pre-whales’ legs into fins before they entered the water, and pre-whales entering the water with legs, and legs then changing into fins, I would opt for the latter, saltation or not. But of course that is a subjective view.