杏吧原创

EVER wondered why we have five fingers, not six or seven? By studying mice
with extra digits, a British researcher thinks he has found the answer. If
animals grow more than five digits, he says, their other limb bones get
distorted to such an extent that they can鈥檛 walk or run properly, dramatically
affecting their ability to catch prey and flee from predators.

鈥淚t seems that five is the maximum,鈥� says Chris Hayes, who studied the
abnormal mice at the Medical Research Council鈥檚 Mouse Genome Centre in Harwell,
Oxfordshire. 鈥淭he growth of other limb bones is compromised to accommodate any
extra digits.鈥�

Hayes has found that extra digits stress the limbs of mice so much that they
are forced to grow in an arc instead of a straight line. The animals are then
unable to put their feet flat on the ground, and they become angled to cope with
bowed leg bones. 鈥淚t affects [the] mobility and gait of animals, and that
dramatically affects their ability to survive,鈥� he says.

The more extra digits, the worse the bowing and immobility, Hayes found. For
example, a mouse he studied with a natural mutation called Doublefoot,
giving it 10 toes on each foot, was severely affected. 鈥淭hey end up walking on
their ankles,鈥� he says. Other mice studied by Hayes had a defective gene called
Spag luxate. These animals also have too many digits, plus abnormal
tibia bones which cause the limb to rotate.

But animals can easily cope with fewer than five digits, it seems. Hoofed
animals such as horses essentially walk on the tip of a finger that has been
covered with a hoof in the course of evolution, says Hayes, who now works at
Merck Sharp & Dohme鈥檚 Neuroscience Research Centre in Harlow, Essex. 鈥淪o you
can decrease digit number and still increase evolutionary advantage,鈥� he
says.

He believes that if the reverse were true, animals with more than five digits
would be abundant in nature. For example, the panda has five digits on each paw
plus an extra pad on its wrist for steadying bamboo shoots as it eats. Why,
Hayes asks, didn鈥檛 the panda simply develop a couple of extra digits to hold the
bamboo? 鈥淭here must be some reason why that animal would not have prospered with
more than five digits,鈥� he says.

In people, however, extra digits have no impact on survival and so are quite
common. 鈥淭he biggest problem for humans is probably psychological, rather than
any disruption to motor abilities,鈥� says Hayes.

He says that limb deformities, which affect 1 in 500 live births, are the
second most common malformations behind heart defects, which affect 1 in 200.