t violent—than the norm.

All of this helps answer that fundamental evolutionary question about how risk alleles have endured. We have survived not despite these alleles but becauseof them. And those alleles haven’t merely managed to slip through the selection process; they have been actively selected for. Recent analyses, in fact, suggest that many orchid-gene alleles, including those mentioned in this story, have emerged in humans only during the past 50,000 or so years. Each of these alleles, it seems, arose via chance mutation in one person or a few people, and began rapidly proliferating. Rhesus monkeys and human beings split from their common lineage about 25 million to 30 million years ago, so these polymorphisms must have mutated and spread on separate tracks in the two species. Yet in both species, these new alleles proved so valuable that they spread far and wide.

As the evolutionary anthropologists Gregory Cochran and Henry Harpending have pointed out, in The 10,000 Year Explosion (2009), the past 50,000 years—the period in which orchid genes seem to have emerged and expanded—is also the period during which Homo sapiens started to get seriously human, and during which sparse populations in Africa expanded to cover the globe in great numbers. Though Cochran and Harpending don’t explicitly incorporate the orchid-gene hypothesis into their argument, they make the case that human beings have come to dominate the planet because certain key mutations allowed human evolution to accelerate—a process that the orchid-dandelion hypothesis certainly helps explain.

How this happened must have varied from context to context. If you have too many aggressive people, for example, conflict runs rampant, and aggression is selected out, because it becomes costly; when aggression decreases enough to be less risky, it becomes more valuable, and its prevalence again rises. Changes in environment or culture would likewise affect an allele’s prevalence. The orchid variant of the DRD4 gene, for instance, increases risk of ADHD (a syndrome best characterized, Cochran and Harpending write, “by actions that annoy elementary-school teachers”). Yet attentional restlessness can serve people well in environments that reward sensitivity to new stimuli. The current growth of multitasking, for instance, may help select for just such attentional agility. Complain all you want that it’s an increasingly ADHD world these days—but to judge by the spread of DRD4’s risk allele, it’s been an increasingly ADHD world for about 50,000 years.

Even if you accept that orchid genes may grant us flexibility crucial to our success, it can be startling to ponder their dynamics up close and personal. After I FedExed away my vial of saliva for genotyping, I told myself more or less to forget it. To my surprise, I managed to. The e-mail that eventually arrived with the results, prom

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