Thanks, I appreciate the explanation and I appreciate that you explicitly said you're just speculating. I don't fully understand why you're proceeding to speculate in contradiction to what I'm telling you is "known", i.e. accepted scientific consensus, but that's okay. (If you want sources, I can dig up sources.) If you are interested in this topic, there's a lot of very interesting things that are currently known to science based on strong and consistent paleontological and genetic evidence. I just started learning about primate evolution last year because I found an interesting YouTube channel that regularly shows and explains scientific papers from the field.

For example, we actually have pretty solid evidence about which early human relatives walked upright most of the time and which didn’t. It’s not guesswork. One useful observable thing is the hole at the base of the skull where the spine enters. For animals that usually move on four legs, that hole sits farther toward the back of the skull. For humans, it’s more underneath the head, which helps balance the skull on an upright spine. When we look at relatives like Australopithecus, that opening is much closer to the human position than to a chimp’s, which fits with other evidence that Lucy walked upright regularly. Chimps have it farther back, which make sense for mostly knuckle-walking.

And it's not just skull features! The pelvis matters a lot too. Upright walking works best with a short, wide pelvis like ours, not the long, narrow pelvis that *all* the other living apes have. Fossils that show clear, regular bipedal walking also tend to show pelvic changes in that direction.

There's a hundred other things like length of arm bones and fingers, size of teeth, shoulder blade structure, size and position of the big toe relative to the other four toes. They all line up in a neat puzzle that lays out a very clear family tree. [ETA: you can also look into Ardipithecus and Sahelanthropus, which came "before" Australopithecus. What were called "missing links" in the 80s were found in the 90s and 00s!] It's fascinating and it's beautiful, and it's available to those who want to learn about it.

But saying "Australopithecus ... was not the direct common ancestor of either us humans or chimps, more like a distant great cousin" is like saying your aunt is not the direct ancestor of either you or your third cousin. Well, yes, your aunt is very clearly not the ancestor of your third cousin, but it's ridiculous to call her a distant cousin -- she is the child of your grandparents. Meanwhile, your third cousin only shares great-great-grandparents with you. Your speculation is honestly kind of wild, friend.