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Why don’t we have three legs?

Given that tripods are inherently more stable than bipods or quadrupeds, what is the evolutionary advantage of bipedalism and quadripedalism in legged vertebrates?
Stephen Tonkin from Kent (Age 55+)

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3 Responses

  1. Hello Stephen. I think the answer to your question is not that there is an actual evolutionary advantage of bi- or quadripedalism over tripedalism but that evolution can only work with what’s there and within the constraints of what has come before. To try and explain, evolution works through random mutation and selection of the new features that lead to best survival. A mutation coding for a third leg is very unlikely because of how we (and other animals) are coded to develop. The majority of animals (and all vertebrates) are bilaterally symmetrical, that is they have a central line of symmetry with each side roughly reflecting the other, or a left and right. Having two or four legs (or additional pairs) is more easily coded for given how the animal is already developing, but having three would require something more complicated such as a fusion of two at the front or one to be added somewhere centrally and this would be very unlikely to arise through random mutation.

  2. Thanks for that, Pam, but please permit me to push the point. 🙂

    You wrote:
    “The majority of animals (and all vertebrates) are bilaterally symmetrical,”

    Indeed, but that just pushes the question back to “given that other symmetries exist in the animal kingdom, why have the majority of animals (and all vertebrates) developed bilateral and not trilateral (or some other) symmetry?”

  3. Hm, you’re pushing me to the reaches now! My explanations will no doubt get more dubious! I will propose that it is now fairly well accepted that multicellular organisms as we know them today arose, through evolution, from the flat-worm lineage, which has a bilateral symmetry (indeed many varieties can produce a new left or right half if chopped that way). This alone can explain why things are bilateral. If you ask me why flat worms are bilaterally symmetrical I’ll suggest it is to do with the single cell at the start of life divides to create new cellular layers, organs and limbs. I’m afraid I can go no further than this though!

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