That’s a great question. Aristotle wondered about
this in Ancient Greece. Modern biologists usually
try to answer questions like this by looking at
the costs and benefits.
Living things don’t get mutations because they
need them. Mutations are random. But once
the genes exist for something, they can give an
individual an advantage, making the genes
likely to become more and more common in future
generation. Or they can be neutral, and not
be any more or less likely to spread. Or they
can be a disadvantage, and be likely to
Having a tail that breaks off easily helps
lizards escape from predators. That would give
lizards who had the ability to re-grow tails a big
advantage, so they would be likely to leave more
offspring with genes that help to re-grow tails.
With other species, like a cow, losing a tail
might be so rare that an individual with the tail
re-growth genes may have no advantage, and the
genes never spread. Or losing a tail may be so
terrible that individuals don’t live long enough
to re-grow the tail, which would also mean the
genes would have no benefit.
Are there any costs to re-growing tails?
Well, a new tail is a big investment of energy
and nutrients. For lizards, the investment is
worth it. The tail may save their lives another
day. For other animals, it might not be.
There might also be some really complicated
reasons that have to do with the control of cell
division and how cells change to become different
types of cells.
Think about the costs and benefits listed above.
Why do you think humans can’t re-grow a lost
arm or leg?
You might be interested in studying developmental
biology or animal physiology.
Thanks for asking,
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