Most animal appendages (including antennae, wings, legs, fins, and arms) start out as simple extensions of the body wall—essentially as direct outgrowths of the embryonic tissue. Moreover, similar genes are expressed during the early development of appendages in animals as different as humans, fish, birds, and insects. This suggests that a basic “program” for appendage outgrowth evolved hundreds of millions of years ago and has been redeployed many times in the course of evolution.
But what about the star-nosed mole’s novel snout appendages? While we do not yet know the genes involved, we have been able to document the mechanics of the star’s development. As it turns out, the star’s appendages develop unlike those in any other animal, suggesting that it had unique precursors and an entirely independent evolutionary history.
Working in collaboration with Kaas and Glenn Northcutt, of the University of California, San Diego, I examined star-nosed mole embryos at various stages of development. We quickly found that all but the very earliest embryos have a protostar (as well as huge embryonic forelimbs destined to become the digging arms of the adult mole), but that instead of forming as outgrowths of the embryonic nose, the star’s twenty-two appendages first appear as slight, elongated swellings on the embryonic face. In later stages, when the swellings are more pronounced, it almost looks as if the star has been folded back against the side of the face. This impression is not quite accurate but does foreshadow events to come.
During most of the mole’s embryonic development, nothing separates the swellings from the side of its face. But just before birth, a new layer of epidermis grows underneath the swellings. At this point, the appendages become separate cylinders, though they are still attached to the face by this new skin.
Shortly after birth, the back end of each cylinder detaches from the face and swings forward, remaining attached only at its front end (a bit like peeling a banana). What was once the hindmost part of each cylinder thus becomes the forward-facing tip.