Why are some mutations, like additional limbs or more than 50 chromosomes, are just not possible for humans to have?


Why are some mutations, like additional limbs or more than 50 chromosomes, are just not possible for humans to have?

In: Biology

They are possible. We arrived at the number of limbs we have (additional limbs) through random mutation and natural selection. They are unlikely, but all the ways we are are now are the result of very improbable events paired with lots and lots of reproduction and a very long period of time, all creating some preference for survival in the context of when and where the occurred. If we were now to randomly spawn another limb it would have to be advantageous such that the person with the extra limb was more likely to reproduce than others around them – otherwise it’s not an advantage. Evolution doesn’t have opinions or human values about what is cool or good, it only cares about what maximizes survival toward and through reproduction.

Evolution works by making small changes over time. It would be theoretically possible for humans to have additional limbs or 50 chromosomes, but there would be a transition period over thousands of generations where humans developed a bud, over a few more thousands of generations, the bud grows out more. A thousand more, the bud develops muscles that allow it to move a bit. After millions and millions of years you might have what you could classify as a limb.

[A classic example that demonstrates the limitations of evolution is the eye.](https://www.youtube.com/watch?v=Nwew5gHoh3E) Our eyes, and the eyes of all land animals are filled with fluid. If it were to be filled with air instead, it would be able to see strictly better. That was too big of an evolutionary jump for the sea animals transitioning to land to make though. They had water filled eyes, which saw blurry on land at first but were able to incrementally adapt them to see better on land over generations. Any mutation that ditched the fluid for air would still have lens that are meant for being fluid filled, and so would be a step backwards in vision. They’d have to somehow miraculously ditch the fluid *and* focus for air, which is just too far fetch to ever happen.

Your premise is incorrect my dude. Multiple limbs happen, although not always as a mutation. Its called “polymelia”.

And extra or less chromosome disorders occur as well, the most well known likely being Down Syndrome. Disorders with more than the usual number of chromosomes exist as well, but to a less common extent.

Extra limbs is actually a mutation that can and does happen. I assume that you mean a fully formed and functional extra limb, however. That’s not just some random mutation in a single gene somewhere. I’m no expert, but it would take perhaps dozens or hundreds of genes all mutating in exactly the right way at exactly the same time to get a fully formed and functional limb.

Short of that, in the best case scenario you’re going to have a malformed appendage that’s completely useless. More often than not it’s going to be in the way and cause you a lot more harm than good. This reduces your chances of passing your genes along and so future generations don’t have the opportunity to perhaps mutate another gene that improves the limb’s function.

And that all neglects the fact that the human body is not equipped to host another limb anywhere else on the body. Beyond just the limb, there are dozens more required changes. Support structures for the limb to attach to, etc.

So like others have mentioned, your question is based on false assumptions. These things can and do happen, but they’re detrimental to whoever has them and so the mutation does not propagate.

In addition to what others have said about needing multiple mutations, the body has an aggressive failsafe against big changes occurring to the genes. If the body detects that it’s carrying a child with too many or not enough chromosomes, the child is aborted rather quickly. There are some exceptions where small chromosomes or the sex chromosomes are duplicated, but normally everything proceeds normally.