The pushy side of mammalian brains.

The bones of the middle ear make a strange journey in growing mammals, one that has puzzled developmental biologists for almost 200 years. The tiny ear ossicles start out as part of the jaw. As the embryo matures, the ossicles tear away from the jaw and migrate backward, eventually attaching to the skull. Paleontologist Timothy Rowe of the University of Texas at Austin thinks he has an explanation for the movement: Our bulging brains are to blame.

Rowe started his study with a few facts. In the reptilian ancestors of mammals, the bones of the middle ear remained connected to the lower jaw. But when the earliest mammals appeared in the fossil record 160 million years ago, they showed the novel ear arrangement. They sported other new features as well, among them a greatly expanded brain. Rowe wondered whether the two had some connection.

Examination of opossum embryos provided a test. The paleontologist followed brain growth and ossicle position from early life through maturation. While the ossicles stopped growing after 3 weeks, the brains continued to enlarge for another 9 weeks, putting pressure on the ear bones.

"The growth of the brain tears the ear ossicles from the jaw and pushes them backward until they reach adult position," says Rowe. He reasons that the evolution of a more specialized brain in early mammals caused the middle ear to split from the jaw.