Submitted by violetmammal4694 t3_1231x4h in askscience
CyclicDombo t1_jduanij wrote
You mentioned penguins but forgot all the other birds. We all (those of us with spines at least) evolved from 4 legged things. The only reason to get up on two legs is if you have a really great use for those two front legs that makes it worth the mobility and stability sacrifice.
hal2k1 t1_jdubtu1 wrote
Kangaroos and wallabies stand erect if they are watching out for threats or just trying to see what is going on. They can see further when they stand erect.
When they are grazing their spine is horizontal but their front limbs are not used for mobility or stability. Their tail does the stability function.
AnattalDive t1_jduxjzy wrote
he mentions penguins specifically because in contrast to other birds their appearance is a much straighter one, like humans.
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