I understand the evidence and the virtually iron clad case for evolution and so I know it happens and it is how we got here. Nevertheless, it remains extremely non-obvious to me if I try to actually comprehend how it could actually have happened.

Imagine some human male, just old enough to be capable of fathering a child.

Now imagine standing next to him his father, from when his father was just old enough to be capable of fathering children.

Next to him, put his father, at that age, and so on. Go back to their earliest male direct paternal ancestor for which it still makes sense to say it is male (so a little ways past a billion years ago, when sexual reproduction appeared).

Visualize that long line of organisms, each fathered by the one before it, and father to the one after it. The only apparent differences between each organism in that line and its father or child, even under a detailed internal examination, are either very very minor or are cosmetic or size differences.

Yet if you take two organisms far apart in the line, you get massive differences. On one end you have a human, and a good distance down the line you've got something that swims around in the ocean, and differs drastically from the human. Different number of heart chambers, something other than lungs for breathing, and more.

Now go somewhere between those two, where you can find something that lives on land, but uses four legs, not two legs and two arms.

It is not at all obvious how this is possible, because remember, to get from that thing in the water to that thing on four legs, and from that thing on four legs to us, the changes from father to son every step of the way have to essentially be continuous.

Intuitively, that would seem to mean that between the four legged ancestor and the two leg two arm ancestors there had to be many generations where the individuals had two limbs that were somewhere between legs and arms. They would be not as good at four legged things as "normal" four legged animals, and not as good at arm things as "normal" armed animals. Similar argument for every other aspect the differs significantly between them. And yet, despite those disadvantages, everyone in line managed to make it at least long enough to successfully reproduce [1].

It just seems that so many low probability sequential things had to happened for this to work that I don't find it obvious that it is is possible, even taking into account that this possible evolution space was being explored by a huge number of organisms in parallel and taking into account that our end of the line is not special from an understanding evolution viewpoint--it just seems special to use because it is our end.

It's essentially for me like geometry theorems in higher dimensional spaces. I can understand their proofs and know they are true, but cannot find them obvious the way 2D and 3D geometry can be.

[1] Which is kind of sobering, since I do not have kids, meaning that I dropped the ball and ended a direct line of father/son descent going back over a billion years.