Comments by "An13" (@An-kw3ec) on "The Biology of James Cameron’s Avatar" video.

Losing limbs is easier and faster than gaining new ones based on how evolution works, but they need to become so useless that their absence goes unnoticed by the environment, just like cetaceans evolved, in the initial stages of their evolution they were coastal creatures, so they still needed their limbs to rest and mate on land like modern sealions. Marine reptiles like mosasaurs or plesiosaurs kept their limbs due their different locomotion. Gaining new limbs is very difficult unless it's a genetic mutation which tend to be unpredictable, our limbs They went through a very long modification process, from bony fins to limbs with defined joint, creating a new structure requires a lot of energy.
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