Where Is Human Evolution Heading?

The human brain, which has evolved into a cognitive machine unique in the world, is likely to change even more in the future. Our niche in nature, says Stephen Pinker, an experimental psychologist at Harvard University who studies the evolution of language and the mind, is the "cognitive niche." In research published last year, Wray identified genes that control glucose metabolism in the brain as among those most recently evolved. Those changes may have been essential to fueling the human brain's growth to a size twice that of our nearest cousin, the chimpanzee. "If you make a big brain, it's an energy hog," Wray says. "It's like putting a V-8 engine in a tiny little car." It could also help explain why chimpanzees don't get diabetes, while humans do.

Tinkering. Take that souped-up brain and put it in the texting, Twittering, 24-7 world we've recently created for ourselves, and it's easy to imagine that we will become superspeedy multitaskers—or more complacent cubicle dwellers. However, this progress comes too slowly for some. "The world is changing so rapidly that biological evolution is not where the action is," says Nick Bostrom, a professor at the University of Oxford and cofounder of the World Transhumanist Association, which seeks to use science to improve humankind. He, for one, doesn't care to wait through a few hundred generations for improvements. Genetic engineering will help short term, he says, and then nanotechnology will step in, altering the biochemistry of the human body at the flip of a switch. "If we're thinking several hundred years out, then much more radical intervention may be feasible."

Unfortunately for those like Bostrom, who see humans as one big fixer-upper project, the human genome has so far proved to be remarkably resistant to tinkering. Since 1990, when gene therapy was first tested in humans, doctors have been trying to repair defective genes by injecting healthy ones. The method has shown only limited success and has failed to deliver as a treatment for common conditions such as heart disease. And gene therapy fixes only somatic genes, which aren't inherited. Germline therapy, which would create heritable mutations, is a far more complex—and contentious—challenge.

Notwithstanding the obstacles, Bostrom's wish list for improved human traits includes a longer "health span," with fewer years of human life spent struggling against cancer, heart disease, and dementia. Enhanced cognitive abilities would be nice, too. "Perhaps physical attractiveness would be a popular trait," he says.

There's as yet no way to select for attractiveness, but parents can choose a few of an offspring's genes if they're willing to try preimplantation genetic diagnosis. In PGD, doctors carefully vacuum a single cell from a 3-day-old embryo and test certain genes before deciding whether to place the embryo into a woman's uterus. The technique, which must be used in combination with in vitro fertilization, was invented almost 20 years ago as a way to reduce the odds of a child inheriting a deadly genetic disorder, such as Tay-Sachs.

It didn't take long for prospective parents to realize that the same method could be use to sort embryos for other reasons. Since 2000, parents have been able to use PGD to choose an embryo's tissue type, so that the ensuing child could serve as a stem cell or bone marrow donor to a sick sibling. More recently, a few have used PGD to reject embryos that have genes that merely increase the risk of disease in adulthood, such as the BRCA breast cancer genes. A few parents with disabilities such as deafness have used PGD to choose a deaf child. And PGD is increasingly used to reject embryos that have no problems at all—unless you consider being the wrong sex a problem. A number of fertility clinics in the United States advertise PGD to parents who want to be guaranteed the child will have the sex they choose. One California clinic boasts of "over 3,800 cases: 100 percent sex selection success." With PGD largely unregulated in the United States, it doesn't take a Nobel Prize in genetics to imagine that babies could soon be ordered up in custom sizes and colors, like a Mini Cooper.