The recombination of man and beast.

Science, technology, and life.
June 22 2007 6:31 PM

Animal Farm

The recombination of man and beast.

Mouse. Click image to expand.

If you've been laughing at those Neanderthal presidential candidates who still don't believe in evolution, it's time to sober up. Every serious scientist knows we evolved from animals. The question now is whether to put our DNA and theirs back together.

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right. Follow him on Twitter.

We've been putting baboon hearts, pig valves, and other animal parts in people for decades. We've derived stem cells by inserting human genomes in rabbit eggs. We've made mice with human prostate glands. We've made sheep with nearly half-human livers. This week, Britain's Academy of Medical Sciences reported (PDF) that scientists have created "thousands of examples of transgenic animals" carrying human DNA. According to the report, "the introduction of human gene sequences into mouse cells in vitro is a technique now practiced in virtually every biomedical research institution across the world."


Why have we done this? To save lives. If you can't get a human heart valve, a pig valve will do. If you can't get human eggs to clone embryos for stem-cell research, rabbit eggs will do. If you can't use people as guinea pigs in gruesome but necessary experiments on human tissues, guinea pigs will do. All you have to do is put—or grow—the human tissues in the guinea pigs. You're free to inflict any disease or drug on a human system, as long as that human system lives in an animal.

In stem-cell research, moreover, human cells are the therapy. Under FDA rules, you have to test them in animals before you test them in people. That means implanting them to see how they change the animals. Meanwhile, we're using hamster cells to make a human protein to treat anemia (PDF). We're using mice to make humanized antibodies that produce cancer drugs. We've grown human kidney tissue in rats.

So far, our mixtures are modest. To make humanized animals really creepy, you'd have to do several things. You'd increase the ratio of human to animal DNA. You'd transplant human cells that spread throughout the body. You'd do it early in embryonic development, so the human cells would shape the animals' architecture, not just blend in. You'd grow the embryos to maturity. And you'd start messing with the brain.

We're doing all of those things.

According to the British academy's report, "researchers have constructed ever more ambitious transgenic animals"—some with an entire human chromosome—and it's "likely that the process of engineering ever larger amounts of human DNA into mice will continue." Four months ago in Nature, biologists outlined several ways to pursue this, starting with "genetic modifications to humanize the host strain further." We're transplanting pluripotent stem cells, which proliferate and grow many kinds of human tissue. We're doing it early in mouse embryogenesis, and we're implanting the resulting embryos in "foster mice" so they can develop.

We're not doing these things because they're creepy. We're doing them because they're logical. The more you humanize animals, the better they serve their purpose as lab models of humanity. That's what's scary about species mixing. It's not some crazy Frankenstein project. It's the future of medicine.

Now comes the brain.

Neurological disorders affect 1 billion people and kill nearly 7 million per year. To study these disorders, we're doing to brain tissue what we've done to liver and kidney tissue: We're replicating it in animals. We've made humanized mice with Alzheimer's symptoms. We've put human neural stem cells in monkey brains. We've put human stem cells in the brains of fetal mice and grown them into adult mice with human neurons. According to the British academy, it's now standard practice to test human neural stem cells by assessing whether they "integrate appropriately into mouse or rat brain."