[flamesabers]

Certain animals can change sex at will, why the hell can't we????

i've often wondered about that myself.....

If I'm not mistaken, I think the trend is the more complex the body is, the more difficult it is to do things like regenerate limbs or change physical sex. Having big brains and being warm-blooded comes with a price-tag.

Creatures that regenerate limbs and organs have stem cells that keep this ability throughout their life cycles. If a salamander’s leg is cut off, for example, its stem cells rush into action and form a fast-growing mass of undifferentiated cells called a regeneration blastema, which eventually will differentiate and form the various structures of a new limb.

But like most mammals, by the time we’re born, those pluripotent cells are replaced by somatic -- adult stem cells, which can maintain and to a limited degree repair the part of the body in which they’re found.

But humans can’t make an entire new arm. “There must be something in humans that prevents the regenerative process from going that far,” Gardiner says.

Some scientists think it may be an evolutionary tradeoff.

“If an amphibian chews off one of its arms, it could hide away for weeks without eating and regenerate,” Enrique Amaya, a developmental biologist at the University of Manchester in Great Britain, recently told BBC News. “That’s out of the question for an animal whose high metabolic rate requires it to feed constantly. It has to heal quickly and dirtily.”

http://news.discovery.com/human/evolution/why-cant-humans-regenerate-body-parts-130823.htm