To quote Robert A. Heinlein, "specialization is for insects". Lower organisms, for the most part, are stuck in one place. There are exceptions like monarch butterflies, which migrate hundreds of miles each year, but most organisms never get farther than a few hundred meters from their place of birth. Rivers and canyons effectively block them from travelling, nevermind oceans. Therefore the selection pressure adapts them to the locale where they live.
Homo sapiens, on the other hand, is intelligent and can build tools. Shoes, stored food, weapons, and clothing allow humans to travel large distances across land and water. Mobility alone is good at spreading genes.
There's also a meta selection factor. Lower organisms are only selected for survival in their locale. Perhaps humans are selected to receive useful genes from far away. If you live in a rain forest, it doesn't help your child much to get a gene for water conservation from a desert visitor (this is oversimplified, but it makes the point). But your distant, wandering descendents will certainly someday meet a drought and be helped by that gene. Likewise, a gene for buoyancy doesn't help your desert-dwelling child, but your great-grandkids cannot use boats if they sink. I think the genes for a generalist body and brain are self-reinforcing: once you have the brain power, what's the point of being 95% efficient at living along a river when you can be 60% efficient at rivers, plains, sea shores, mountains, and deserts.
Compare the laws of survival to a polynomial equation, and biology to a mathetician. Some solutions (bacteria) are trivial to find. Other solutions are harder to find by trial and error, but are nonetheless possible, and in the long view inevitable (bugs, trees). And some solutions require a higher level of sophistication, such as complex numbers (generalism), but once that level is attained a variety of unusual solutions are available (apes, humans, AIs).
I don't want the world, I just want your half.
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