In most respects, Abigail and Brittany Hensel are normal American twins. Born and raised in a small town, they enjoy a close relationship, though each has her own tastes and personality. But the Hensels also share a body. Their two heads sit side-by-side on a single torso, with two arms and two legs. They have not only survived, but have developed into athletic, graceful young women. And that, writes Mark S. Blumberg, opens an extraordinary window onto human development and evolution.

In Freaks of Nature, Blumberg turns a scientist's eye on the oddities of nature, showing how a...