I know how humans and flightless birds evolved bipedalism, but could life on land and so on evolve with bipedalism form the start? Try to think of something more than just two finned fish equivalents.
closed as too broad by RonJohn, Ash, elemtilas, JBH, JohnWDailey Nov 11 '18 at 21:14
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Perhaps a sea creature with forelimbs & tail starts elaborating its neck, such as did plesiosaurs. Gradually the animal moves into a mangrove environment and the limbs begin to function as grapplers thru tight spots, gradually lengthening, becoming weight-bearing, allowing the limbs to descend below the body cg.
You might then have something that could conceivably trundle about on the surface.
An unlikely scenario in the extreme, but its all I can come up with.
You might also think of some reason to inhibit quadrapediality from arising, since ergonomically that makes vastly more sense.
The current scientific vision on the evolution of terrestrial animals it that they descend from a fish who first ventured on dry land ages ago. And a fish having generally four fins lead to all its descendants having four limbs.
If you want a bipedal ancestor to start from, you can tweak the story and, instead of having the Tiktaalik or the like, have a sort of seahorse move the first step on land.
Seahorses have already a vertical posture and can conveniently attempt to balance using their rear two fins and their tail. This may even lead to the atrophy of the two upper fins, resulting in no upper/front limbs for the descendants.