Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [108]
Our intuitive biology did not evolve because it was true. It evolved because it was useful. It allowed our ancestors to make good decisions based on the information they could gather, and those decisions raised their odds of surviving and reproducing. But intuitive biology is not a reliable guide to the deep truths of life. What is the essence of E. coli as a species, for example? It’s not being a harmless, sugar-feeding, flagella-producing microbe. Within the species we call E. coli, you can also find aggressive defenders of the gut that shut out disease-causing pathogens. You can find many pathogens equipped with weapons not found in harmless strains. Some strains straddle the divide—they are beneficial, but they also carry many of the genes that make other strains killers. And many of these strains evolved by being infected with viruses that show no respect for our beloved species boundaries. There is no immutable essence that unites E. coli.
Our intuitive biology fails us when we try to understand E. coli, and it also fails us when we try to understand ourselves. Like all other living species, humans are the product of evolution. If we weren’t, the entire controversy over biotechnology would not exist in the first place. If human nature were truly distinct, it would be impossible to plug human genes so easily into E. coli or to grow human brain cells in a mouse’s skull. The essence of being human is as much a construction of our minds as the essence of E. coli.
New research on human evolution makes it impossible to believe that a thing either is or is not a whole human being, as Robert George has claimed. Consider a gene called microcephalin. There are several versions of the gene floating around our species, but one is far more common than the others, found in 70 percent of all people on Earth. Scientists at the University of Chicago decided to trace the history of this version of microcephalin. They found compelling evidence that it entered the human genome long after Homo sapiens had evolved.
About half a million years ago, our ancestors split off from the ancestors of Neanderthals. The split probably occurred in Africa. Afterward, the ancestors of Neanderthals spread across Europe, while the forerunners of our species stayed behind in Africa. Homo sapiens evolved about 200,000 years ago. It was only after our species emerged that humans evolved full-blown language, abstract thought, the capacity for art, and many of the other qualities that are at the core of what we call human nature.
About 40,000 years ago, Homo sapiens expanded their range into Europe. And there humans encountered Neanderthals. Neanderthals became extinct about 28,000 years ago, but it appears that before they disappeared they interbred with humans. Most of their genes disappeared over the generations, but at least one survived: their version of microcephalin. It didn’t just survive, in fact—it spread like wildfire. Something about it was strongly favored by natural selection, with the result that it now can be found in the majority of humans alive today. And microcephalin isn’t some minor gene for growing nose hair or coloring toenails. It plays a central role in the development of the brain. Thanks to natural engineering, most humans carry this nonhuman gene, which is involved in building that most human of organs, the brain.