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Hybridization is not the only way foreign DNA got into our cells. Some 3 billion years ago our single-celled ancestors engulfed oxygen-breathing bacteria, which became the mitochondria on which we depend. And, like E. coli, our genomes have taken in virus upon virus. Scientists have identified more than 98,000 viruses in the human genome, along with the mutant vestiges of 150,000 others. Some have donated their DNA to our own biology, such as the placenta. If we were to strip out all our transgenic DNA, we would become extinct. Some of these viruses inserted copies of themselves after our split from chimpanzees. Some are found in Asians and Europeans but not in Africans, suggesting that they infected the human genome only after some humans emerged from Africa 50,000 years ago. When people acquired this foreign DNA, did they lose their human nature?

It is awkward to think this way. It feels unnatural. The unnaturalness is in the workings of our minds, however, not in nature. But we will probably get used to it, in the same way we have gotten used to thinking of matter as being made up of subatomic particles. Our repugnance toward breaches in the species barrier and toward the modification of genes is shifting even now. The lack of angry mobs trying to burn down insulin-producing factories to preserve the natural order of things is proof of that.

This sort of change may well disturb a critic like Leon Kass. In 1997, he testified before Congress in favor of a ban on human cloning, declaring, “In a world whose once-given natural boundaries are blurred by technological change and whose moral boundaries are seemingly up for grabs, it is, I believe, much more difficult than it once was to make persuasive the still compelling case against human cloning. As Raskolnikov put it, ‘Man gets used to everything, the beast!’”

There’s a contradiction here. On the one hand, our wisdom of repugnance is supposed to be a deeply anchored, reliable guide to what is fundamentally right and wrong—not what happens to be right and wrong this afternoon. On the other hand, Kass is angry that this sort of repugnance can disappear as times change. It’s hard to see how he can have it both ways.

We can be overwhelmed by our emotional reactions to scientific advances. In some cases, we eventually recognize that we were probably right—or wrong—to have those feelings. In other cases, our perception of essences triggers feelings of disgust when those essences seem to be corrupted. That disgust may be triggered by E. coli carrying human genes, or in vitro fertilization, or a person receiving a heart valve from a pig. But as we come to recognize the benefits or risks of those developments, as we see the world not coming to a Pandora’s-box end, our sense of disgust fades.

We don’t become Dostoyevskian beasts along the way, though. With the advent of organ transplants, we did not slide down a slippery slope into a world in which paraplegics have their livers yanked out against their will. There are certainly new choices to make—to allow the sale of organs or not, for example—but we continue to make them seriously.

Chimeras and various sorts of genetic engineering will become more common, but they will not, I suspect, produce a moral meltdown. For one thing, a lot of the most startling nightmare scenarios we hear about today have little basis in science. Mice with human neurons will not cry out, “Help me, help me!” There is much more to being human than possessing a peanut-sized clump of neurons. Yet we may decide that engineering such a mouse is cruel to the animal itself. (Repugnance at cruelty toward animals is actually a new sort of disgust many people have acquired, rather than lost, over the past 200 years.) And some chimeras will probably be banned because the challenges they pose to our moral treatment of humans and animals don’t justify the procedure.

I suspect—or at least I hope—that as we make these decisions, we will come to a deeper understanding of what it means to be human: not as an inviolable