I recently said that we didn't have a model were
populations became waterside then left only to
return to the waterside. I said that groups had to
push inland periodically, for a variety of reasons,
and those groups adapted to their inland
conditions only to later come back into contact
with the waterside population, exchanging DNA
thus adaptations.

Both models work, actually.

I've thrown out the idea that Dinosaurs were
secondarily flightless. That, they common ancestor
of dinosaurs and pterosaurs was a pterosaur and
this is why the basic dinosaur shape is so birdlike.

"Flightless Pterosaurs" should be a thing, should
they not? But so far only a single one has ever
been identified, that identity is hardly universally
accepted, and it dates to the late Jurassic when
Pterosaurs emerged in the Triassic.

https://pterosaurheresies.wordpress.com/2011/07/21/meet-the-first-flightless-pterosaur-sos-2428/

That's a pretty long time! Birds, on the other hand,
were already diversifying & evolving a number of
secondarily flightless species by the early cretaceous,
while at least one researcher claims to find a
secondarily flightless bird in the Jurassic!

Anyway, the point is that there was oodles of time
for multiple events where pterosaurs evolved into
secondarily flightless animals... and secondarily
flightless would be as good or better excuse as any
for dinosaurs to evolve bipedalism.

So I'm not averse to the thought that our ancestors
could have left the water and then returned. I just
see that route blocked after a certain point.

"That niche is filled."

So if it did happen, I would suspect that it happened
rather early own, certainly before Homo.




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