Moles get two big thumbs-up for their digging skills. These tunneling mammals sport what looks like a sixth digit on each front paw, a seeming exception to the rule limiting land vertebrates to 10 fingers. Now, a new study of embryonic moles shows that this doppelganger thumb is no thumb at all, but a co-opted wrist bone that starts to elongate relatively late in development.
Although not as famous as the panda's similar extra thumb, a heavy tool for grasping bamboo, the mole's sixth digit is an evolutionary curiosity. This thumb, a solid piece of bone on the outside of the hand that can wiggle but not bend, certainly seems useful. The extra bone widens the mole's paws, presumably making them better for scooping dirt. Primitive mole species less inclined to tunnel-building tend to have stubbier second thumbs. Whether the extra digits are useful, most land vertebrates, with the exception of rare polydactyl humans, cats, and other animals, stick to a maximum of five digits per hand or paw.
To unearth whether the mole is an evolutionary rule-breaker, Marcelo Sánchez, an evolutionary biologist at the University of Zürich in Switzerland, and colleagues went back to the thumb's beginnings. The team compared the stubby and fingerless limbs of embryonic Iberian moles (Talpa occidentalis) with the limbs of closely related shrew embryos, looking for molecular clues to early digit growth. One such clue, a gene called Sox9 that often turns on before bone growth, clearly identified the would-be digit as a finger impostor. Sox9 didn't kick into gear at the site of the future second thumb until the mole embryos were 18 days old, after the gene's activity had peaked in the true fingers. "It forms like a finger forms but later," Sánchez says.
But more importantly, the second thumb forms at the wrong place—not in the limb tissue reserved for growing fingers but where the wrist should be. The digit is actually a wrist bone, Sánchez says. Over evolutionary time, it extended like a claw until it reached the other fingers. The researchers report their findings online today in Biology Letters.
Hands haven't always been limited to five fingers. Nearly 400 million years ago, the earliest land vertebrates could have given each other high-sixes, sevens, and even eights, not just fives, to celebrate leaving the oceans. But over tens of millions of years, creatures with seven and eight fingers died off, leaving only their five-fingered brethren behind. Moles couldn't break the precedent, but they could sneak around it, Sánchez says.
Testosterone may be partly responsible. True to their oddness, many female moles grow not only ovaries but also some testicular tissue, hinting that they have too much of the hormone, Sánchez says. Testosterone is well known for building bones, and some evidence suggests that human polydactyly—people can occasionally develop genuine sixth fingers—coincides with high levels of maternal testosterone.
The study is "a very exciting development," says Robert Asher, an evolutionary biologist at the University of Cambridge in the United Kingdom. But he cautions that scientists have a ways to go before they know all the details of how the second thumb develops.
Wrists are especially complex developmentally, adds Michael Coates, an evolutionary biologist at the University of Chicago in Illinois. Understanding the formation of extreme wrist structures, such as the mole's second thumb, is tricky given how little scientists know about how more banal wrist bones organize, he says. Still, he says, it's satisfying to finally know where those tunnel-worthy thumbs sprout from.
*A previous version of this story listed Robert Asher as an evolutionary biologist at the University of Chicago in Illinois. His affiliation is with the University of Cambridge in the United Kingdom.