Posted on October 31, 2017 by xuan wang | 5 Comments
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 rolein 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 calledSonic 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 thegrasping ability. This is the reason why we all have five fingers not six or four.
Written by Xuan Wang
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Polydactyly, the condition of having extra fingers or toes, often prompts questions about why humans typically have five fingers. Exploring this, let's delve into the genetic and evolutionary aspects. My expertise lies in genetics, developmental biology, and evolutionary biology, areas directly relevant to this discussion.
Polydactyly is indeed a fascinating condition that sheds light on the formation of our fingers. Two main types, A and B, exist. Type A involves a fully functional extra finger, while Type B manifests as a non-functional, seemingly 'floating' extra digit. This phenomenon is caused by a malfunction of the "Sonic hedgehog" gene, or "SHH" gene, during embryonic development.
The SHH gene is crucial in digit growth and brain organization. During the fifth week of embryo development, the SHH gene produces a signaling protein, the Sonic hedgehog protein, which influences finger formation on the ulnar side of the hand. An abnormal concentration of this protein leads to variations in finger numbers. Insufficient protein can result in fewer than five fingers, while an excess causes Type A polydactyly, where a duplication of the entire finger occurs.
But why do most humans have five fingers? Evolution holds the key. Around 380 million years ago, quadrupedal species had varying numbers of fingers—six, seven, or even eight. Over time, evolution favored a simplified structure, leading to the common five-fingered hand. This design balances flexibility and grasping ability, a trait that persisted through natural selection.
The article touches upon essential concepts in biology, genetics, evolution, and embryonic development. The discussion spans genetics (Sonic hedgehog gene's role), embryonic development (finger formation in the fifth week), evolutionary biology (the shift from multiple fingers to five), and mutation (polydactyly as a genetic anomaly).
This case highlights the intricate interplay between genetics and evolution in shaping the human body, showcasing how anomalies like polydactyly provide insight into our developmental history.
