> creatures can lose limbs through evolution, but never gain them
Wait, so where did the limbs come from anyway, if they could not be gained?
Wait, so where did the limbs come from anyway, if they could not be gained?
It seems that vertebrates have certain period when this layout was 'cast in stone' [1]
[1]: https://biology.stackexchange.com/posts/21793/revisions
Take two runners, give one a 15 kg weight in one hand, and give another 15 kg weights in both hands (30 kg total). I think the runner with weights in both hands would run faster than the one with one weight.
Thus, moving species evolve to be bilaterally symmetric.
If the organism has no need to move in certain direction, it doesn't really have bilateral symmetry, like trees or bushes (though big trees generally tend to have their center of mass close to their vertical axis).