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Mammals live all over the place. After the dinosaurs died out some sixty million years ago,
mammals underwent a spectacular process of expansion into different environments. Many lived on the ground; some burrowed into it. Some, like the whale, returned to the water where life had originated eons earlier. Others took to living in the trees; these are the primates, from which we humans recently sprang. (Our grasping hands, rotating arms, and stereoscopic vision all reveal our tree-dwelling roots.) A few mammals even took to the air, as the bats of today remind us.
When a life form expands into a new environment, any traits that happen to help individual organisms survive and reproduce there will grow more common as generations pass. Assuming the form does not die out, then, it will be modified, by natural selection, for living ever more effectively in the new environment. When a single form of life successfully expands into many environments, the process is termed "adaptive radiation."
Adaptive radiation ordinarily involves the development of new species in the new environments. That is, the cumulative effects of natural selection eventually make the populations in different environments so different from one another that interbreeding has become impossible. Mammals share a common ancestral form, but bats and whales cannot interbreed to produce, say, "whats" or "bales"!
Now, a few human traits, such as skin color, do appear to reflect natural selection in different environments. Near the equator, where the sun's rays strike the earth directly year round, a dark skin aids survival and reproduction by affording protection from skin cancer; far from the equator, though, a light skin seems to offer protection against rickets, a bone disorder which can result from too little exposure to sunlight. Yet this, and other geographically based differences between human populations, are quite superficial--"skin-deep," so to speak. The ultimate biological proof of this superficiality is the fact that a healthy male and female from anywhere in the world are capable of mating to produce fertile offspring. Clearly, humans have managed to go "all over the place" while remaining a single species.
That a single species--especially a large-bodied one--should have done this is remarkable indeed from a zoological and ecological standpoint. Other large-bodied species remain confined to relatively narrow environmental ranges. Chimps and gorillas, our closest living kin, inhabit still the tropical forests of our early ancestors.
Clearly, the secret of our success is culture. Humans have adapted to new environments, for the most part, not biologically but culturally. Culture allows us to create, within hostile environments, a "little environment" friendly to us. Control of fire, for example, meant we could create little enclaves of warmth in the coldest corners of the earth. Now, half a million years later, we live with the fish not by evolving fins and gills, but by surrounding ourselves with submarines; and we are venturing into airless space not by evolving the ability to do without oxygen, but by surrounding ourselves with space shuttles and stations. It even is conceivable that we will be able to modify other planets to suit our needs.
Biological evolution adapts species to environments; cultural evolution adapts environments to species. The inestimable value of culture in promoting the survival and reproduction of culture-bearers appears, scientifically, to be the very reason for its evolution in the first place. Long before the advent of space stations, Herbert Spencer had seen deeply into the profound significance of culture as an adaptation allowing our species to venture into new environments:
These various orders of super-organic products . . . form what we may consider . . . a
secondary environment, which eventually becomes more important than the primary environments--so much more important that there arises the possibility of carrying on a high kind of social life under inorganic and organic conditions which would originally have prevented it. [Spencer 1897, I-1:13-14]
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