The recombination of man and beast.

Last month, ethicists from Stanford University and the University of Wisconsin detailed a proposal by a Stanford scientist to substitute human brain stem cells for dying neurons in fetal mice. "The result would be a mouse brain, the neurons of which were mainly human in origin," they reported. The payoff, if the fetuses survived, would be "a laboratory animal that could be used for experiments on living, in vivo, human neurons." Imagine that: a humanoid brain network you can treat like a lab animal, because it is a lab animal.

The Stanford experiment wouldn't actually produce a human brain. Most brain cells aren't neurons, and the experiment called for inserting human cells after the mice had constructed their brain architecture. But last year in Developmental Biology, researchers proposed to insert human stem cells in mice before this architectural stage. The resulting "mouse/human chimeras," they argued, "would be of considerable value for the modeling of human development and disease in live animals."

When Stanford's ethicists first heard the proposal for humanized mouse brains, they were grossed out. But after thinking it over, they tentatively endorsed the idea and decided that it might not be bad to endow mice with "some aspects of human consciousness or some human cognitive abilities." The British academy and the U.S. National Academy of Sciences have likewise refused to permanently restrict the humanization of animals.

If you want permanent restrictions, your best bet is the senator who tried to impose them two years ago. He's the same presidential candidate now leading the charge against evolution: Sam Brownback, Republican of Kansas. He thinks we're separate from other animals, "unique in the created order." Too bad that isn't true of the past—or the future.

A version of this article also appears in the Outlook section of the Sunday Washington Post.