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.
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.
If the lesson that parasites offer is insufficiently clear, we can turn to our commensals, the rodents, fruit flies, lice, and doves of the world for an even clearer picture of our recent past and perhaps future. As we spread and our cultures change, we have affected not only our microbes but actually caused the speciation of our commensals. We know, for example, that house mice evolved a commensal relationship with humans early in our history and since then, as they spread with us around the globe and adapted to new habitats, have speciated into no fewer than seven species. Drosophila melanogaster (the common fruit fly) appears to have evolved from a forest species in Africa and also moved with us as we’ve migrated. It is no longer capable of breeding with the populations from which it apparently originated and this in relatively few human generations. The list goes on. Rats and mice that we have introduced to islands, via our ocean-going vessels, have evolved traits over just a few hundred years which ultimately allow them to take better advantage of island resources.
Where we have industrialized agriculture, weeds have evolved to chemically mimic our crops to avoid the herbicide. Insect pests have evolved resistance to DDT and to the pesticides that have followed. We have countered with genetically engineered crops. Already there are insect species resistant to the defenses of those crops. When we add new species of crops, insects in turn rapidly switch to those. Even our most degraded landscapes offer possibilities. Many independent plant lineages have evolved tolerance to heavy metal pollutants. Insects have, in response, evolved resistance to the heavy metals those plants sequester in their leaves.
The more we look at the world around us, the more it seems to be evolving at our hand, albeit without our meaning it to. As we inadvertently introduce thousands of species to new habitats, species evolve. In some of the most detailed studies to date, researchers in Australia have shown that the poisonous cane toad, which was introduced by humans from Central America, has exerted a selective pressure on the local snakes, killing those that eat cane toads. Now, apparently, since the cane toads’ introduction, because snakes with bigger mouths ate cane toads, died, and passed on no genes, at least one species of snake has evolved a smaller mouth. Those are the ones that have survived.
What we must begin to come to terms with is that we may be seeing the beginning of a new adaptive radiation, a new burgeoning of life—but it is not necessarily the one we might hope for. The big creatures we value so highly—indeed treasure—will not be able to regain a stronghold in the face of our encroachments. Indeed, they breed, and so evolve, more slowly than the species mentioned here. Instead, the small will inherit the earth, if it is not already theirs. The evolutionary future is pathogens, pests and guests, at least as we have currently written the story.
Wallace and Darwin met opposition when they revealed their theory of natural selection. Today, such opposition, has been “born again” as it were in the form of creation science or intelligent design. But whether one “believes” or does not believe in evolution, individuals go on mating and dying. Through time, some genes are favored and others are not. The new forms that have evolved in our anthropogenic landscapes don’t care if we believe in them.
If you want a more bucolic version of the ecological future, consult a paleontologist. The paleontologists look further into the future to a time when the great evolutionary opportunities are not agricultural habitats, but are, instead, vast forests—to a time when the seas are again filled with large species—to a time when new large vertebrates roam new kinds of plains. They look forward in time to a world more interesting to us than our present evolutionary future. The paleontologists can do all this because they begin their discussions of future evolution with the statement, “once humans go extinct.”
—Rob R. Dunn is an assistant professor in the department of zoology at North Carolina State University in Raleigh.
Originally published January 7, 2007