Why people have 5 fingers?

Have you ever wondered why you have five fingers?? We have been using our hands for more than twenty years and you may have the same question as i do which is why we have 5 fingers.

To better understand the reason, let’s look at a special case of “polydactyly“. In short, polydactyly refers to the situation where there are more than five fingers in a hand (or foot). There are two types of polydactyly which refer to Type A and Type B. In Type A, the extra finger can function properly like other fingers and have a complete bone structure.  In Type B, the extra finger is non functional and  seems to be “floating”.

Type A of polydactyl http://https://en.wikipedia.org/wiki/Polydactyly

Type B of polydactyly http://https://en.wikipedia.org/wiki/Polydactyly#/media/File:Polydactyly_Left_Hand.jpg

To answer our question in the beginning, we have to know the cause of Type A. The duplication of the entire finger is due to an abnormal function of a gene called “Sonic hedgehog” gene or”SHH” gene which plays an important role in the growth of digits on limbs and organization of the brain. During the fifth week of an embryo development, SHH gene will synthesize a signalling protein called  Sonic hedgehog protein which act on the ulnar side of the “hand” (at the black dot in the picture) and fingers start to form.

Hand of an embryo
https://dissolve.com/video/Human-embryo-close-developing-face-rights-managed-stock-video-footage/002-D30-11-319

When the concentration of the SHH signalling protein is lower than common level, less than five fingers will form. When the concentration of the SHH protein is too highly, it will result in Type A of polydactyly.

This is the reason of how five fingers formed, but you may still wondering that why five-finger structure is common. This is the result of evolution. About 380 million years ago, quadrupedalism can have six, seven even eight fingers depending on different species. Due to evolution, it has been simplified into a five-finger structure which ensure both the flexibility and the grasping ability. This is the reason why we all have five fingers not six or four.

Written by Xuan Wang