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Why do we have 5 fingers on each hand?

Why do we have 5 fingers on each hand?
Becky from West Yorkshire (age 15-24)

Why do we have 5 digits on each hand and foot?
Miss Partridge from Northamptonshire
Age 5-14

Why do humans have five toes and five fingers?
Liza Welch from Warwickshire (age 5-14)

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One Response

  1. Becky, I see Liza Welch from Warwickshire has asked the same question. I think there’s two questions, and answers: a) why is 5 fingers/toes the number we observe? and b) how do our bodies count up to 5 when our fingers/toes form?
    So, the first one, why is 5 the preferred number in nature? Well, some people are born with more or less than 5 fingers and toes, showing that nature is able to make more or less if it wanted to. So, if we think about all the 100’s of millions of years of evolution that have passed since early animals with backbones (fish) developed and started developing fins, there’s been more than enough time for nature to experiment with more or less than five digits, and indeed fossils have been found where the fins have more or less. The fact is though, that 5 is the number we see predominantly today across mammals and reptiles etc. So, I would say that evolution finds that 5 is the best number for us, not too many and not too few. It’s probably something to do with being a primate and our ancestors found 5 best for life in the trees, and now we walk on the ground it’s still the best number. That might be different if our circumstances or environment were to change. Think of horses, who have 5 digits if you look inside them, but one finger/toe has become the predominant one that they stand on – if it was better for us to run on four limbs, our hands and feet might evolve in the same way. If we had to start swimming everywhere, our digits would best be webbed.

    Second question is how do our bodies usually form 5 fingers or toes on each limb? When our limbs form they are just simple paddle shapes, and our digits (fingers/toes) are formed by some cells in the ends of the ‘paddles’ dying off, from the outside inwards. So our digits don’t form by growing outwards, rather the gaps between them grow inwards. This is programmed into the cells by the genes they have inside them – when the cells know where they are, and at the right time, they do this. How do our limbs know to make 5 digits? There are various SOX, PAX and HOX genes that organise this sort of development. Changes in these genes (“mutations”) can give rise to extra fingers (polydactyly), less fingers, fingers joined together and suchlike. Have a look on the internet and I hope you find some more information on this. I used to be fascinated by how our bodies were left/right oriented – so heart on the left, and appendix on the right and so on …
    I’m a pathologist who did a PhD in DNA repair, and now specialises in laboratory medical genetics. The Royal College of Pathologists is organising the first National Pathology Week (3 – 9 November 2008) this year. See http://www.rcpath.org/index.asp?PageID=1526 for more details.

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