Why do we have five digits on each hand?

THAT'S THE WHY:Most of us are born with five digits on each hand: four fingers and an opposable thumb. You probably don't think much about this until something goes wrong with one or more of those digits, and you suddenly realise how much we use them in everyday life.

But why five? If they are so useful, why is it not the norm to have six, seven or even 10 digits per hand? Molecular genetics can offer a clue.

Our digits start to form during embryonic development when cells at the ends of future limbs die away in a certain pattern.

Much of what we have identified about the genetic control of body development comes initially from studies of the fruit fly; scientists have been able to pinpoint particular types of genes that affect body development in the fly, and then have found the corresponding genes in other organisms, such as ourselves.

Studies indicate that “homeobox-containing” or Hox genes have roles to play during limb development in vertebrates – animals with a backbone. And a paper from way back in 1992 in the journal Development outlines how Hox activity could help determine the five digits on paired limbs.

“Hox gene expression patterns divide the embryonic limb bud into five sectors along the anterior/posterior axis,” wrote Clifford Tabin from Harvard in the paper.

“Because there are only five distinct Hox-encoded domains across the limb bud, there is a developmental constraint prohibiting the evolution of more than five different types of digits.”

So that’s a high five to the Hox genes.