• Question: how many limbs can one person have

    Asked by beano on 24 Jun 2019.
    • Photo: Kaitlin Wade

      Kaitlin Wade answered on 24 Jun 2019:


      This is a really interesting question and is quite complicated at the same time!

      Firstly, we [humans] have four limbs because of evolution – we [terrestrial vertebrates] evolved from a fish-like ancestor that had four ‘limbs’. There are some exceptions of this but more that evolution and natural selection has allowed for some terrestrial vertebrates to have lost their four limbs (like snakes) – but if you look at their skeletons, you can sometimes see where their limbs would have gone, which is really fascinating!

      If a human is born with more than 4 limbs (for example, there have been instances where children are born with 5 limbs, usually an extra leg between their two legs), this is usually a consequence of a congenital defect (i.e., one or several genetic mutations) that lead to the development of another/more limbs. Sometimes, this can also be due to the degeneration of a second twin (so, imagine you were conceived as a conjoined twin but your twin did not develop properly in the womb and therefore only one leg remained conjoined). This is fairly uncommon, mainly because it’s not a particularly advantage – if having more limbs was beneficial to us evolutionarily, then we would be evolving constantly so that more and more of us over generations would have more than 4 limbs.

      So, I suppose, one answer to your question would be, evolutionarily, it’s probably only viable for humans to have 4 limbs but, realistically, I suppose it would be possible for a human to have many more limbs but the question then would be whether or not that person would be able to survive into old age.

    • Photo: Nina Rzechorzek

      Nina Rzechorzek answered on 24 Jun 2019:


      Good question! Most humans have 4 limbs maximum. Polymelia (supernumerary limbs) is a congenital disorder very rarely reported in humans, though not uncommon in other animals. It can result from genetic mutation or incomplete development of a conjoined twin. There is an Open Access report of a human case of polymelia here:

      https://core.ac.uk/download/pdf/81288450.pdf

      However there is also a lot of exciting research going into developing replacement or even additional robotic limbs; for example at MIT – see here:
      https://www.media.mit.edu/articles/brain-controlled-bionic-limbs-developed-at-mit/

      The other thing to think about is that the brain is very plastic and when a limb is lost, often the part of the brain controlling the remaining opposite limb is expanded to partially compensate. Some people also learn to use other parts of their body for things that they might have once done with a hand – humans and other animals are incredible at adapting! There is also the possibility of ‘phantom limb’ which gives us amazing insight into how the brain works, but can be quite debilitating for the sufferers of phantom limb pain. You can read more about this fascinating condition here:
      https://www.mayoclinic.org/diseases-conditions/phantom-pain/symptoms-causes/syc-20376272

    • Photo: David Wilson

      David Wilson answered on 25 Jun 2019:


      Limb development is really interesting and despite a lot of the work done to actually work out which genes were involved being done in the 80’s the science still has a lot of implications today in how we try to understand repair and possible regeneration of limbs.
      .
      The specific signalling pathway that determines how a limb develops is called Sonic Hedgehog (Shh). Yes, this was the 80’s and fruit fly’s were being used to identify loads of genes and signalling pathways. The genes were often given names that described what the fly looked like when the gene was removed. Sonic Hedgehog gave the flies spiky hair just like Segas favourite computer game character at the time.
      .
      The Sonic Hedgehog gene and signalling pathway plays a key role in in how vertebrates embryos develop, such as in the buds that form go on to form limbs, growth of digits on limbs and organisation of the brain.
      .
      Mutations in the Shh gene can cause polydactyl where someone is born with one or more extra fingers or toes.

    • Photo: Matthew Bareford

      Matthew Bareford answered on 26 Jun 2019:


      In general (as has been alluded to already) people will only have 4 limbs. HOWEVER, it is possible for genetic mutations to occur that can result in a person having more limbs. In theory this could be many, but up to a point this would become incompatible with life, and so that person would die (either before being born or shortly after).

      There have been instances of people being born with multiple extra limbs more recently, such as four legs and 3 arms. These will generally not work as well (if at all) and will then be removed if safe to do so.

    • Photo: Deepak Chandrasekharan

      Deepak Chandrasekharan answered on 27 Jun 2019:


      Most people have an above average number of limbs…

      This is because whilst most people have 2 arms and 2 legs, some will have fewer from birth or losing a limb in later life so the mean number of legs is less than 2, and same for arms!

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