Human habitation has been, and is increasingly, playing a direct role not only in the extinction of species, but in their evolution. By our own actions, we may be accompanied into the future by ever more diverse pests and pathogens, and may leave behind what we value most—elephants, tigers, and others of the earth's great megabeasts.
Evolution is often thought of as a slow process relative to our life spans, one that we have played no part in. We imagine it to have occurred in the far distant past. Until recently, the study of big evolutionary changes has rested on an examination of fossil remains and molecular evidence of the deep past. But, from a biological perspective, we can see that evolution is actually happening now and more quickly than we had previously assumed. Moreover, the new centers of evolution are neither tropical forests nor east African lakes but, instead, those habitats and resources most closely allied with us—our human habitats and ourselves.
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First, some context. A consideration of previous periods of speciation suggests that the evolution of new species occurs most rapidly in big habitats with lots of resources. Where are those habitats now? In the last five thousand years the earth has gone from a place dominated by forests and grasslands to one dominated by humans, agriculture, and cities. The Atlantic forest of Brazil, for example, fragmented and dwindling, is unlikely to be an important source of new species in the future. The Amazon and a few other large native habitats may still be important, but less so than they have been historically. Due to our destruction of habitat, we have already extinguished hundreds of birds and mammal species, not to mention the other multitudes. As it stands, up to 95 percent of all the terrestrial world is actively managed for human uses.
The world, as we have rendered it, is now chiefly comprised of our crops, the consumers of those crops (including we humans), our own pathogens at the top of the food chain, and, on the bottom, as it were, the decomposers of our waste. These groups now account for the vast majority of the living matter on earth.
More than half of the species on earth are parasites and, for a subset of those parasites, we represent a tremendous and growing resource. Humans are now six and a half billion strong and those billions represent pounds of resources for needy parasites. We are bodies full of unexploited niches (along with a number of exploited ones). As we expand our numbers, we are expanding evolutionary possibilities for microbes that can live on us and in us. At the same time, we are introducing new selection pressures which are working to speed the evolution of those microbes. We are covered in antibiotics, antimicrobials—anti-everything—which exert strong selection for the evolution of resistant and more virulent forms. We have seen, in the last 60 years, bacteria, protists, helminthes and other parasites all independently, and frequently, evolve resistance to our anti-parasite treatments. In addition, we are witnessing the origin of new human pathogens, such as HIV, either when pathogens switch hosts to take advantage of the resource humans represent, or through the divergence of human pathogens.