Funny about hitting the back of the knee. That’s what they do to you in the Army when they have you standing at parade rest for long periods to catch those people who have their knees locked (and will in due course fall out because of it).
But as you see with the plantaris, the best we seem to have is an “it might be good for...” and that’s about it. Nobody has ever shown that those who are born without it, or who have it removed, are in any way hampered by its absence. Having a larger one doesn’t seem to have a benefit either.
There's something about the evolutionary model that doesn't make sense to me. First it suggests that natural selection drove a relocation of the plantaris muscle from one position to another (assuming other primates represent our ancestral plantaris orientation). But then, conversely, that function was lost entirely. If it is functional in the ape orientation for ape locomotion, and not functional in humans in erect locomotion, then why would natural selection first push a change and then abandon it?
I haven't seen anyone suggest what it could have been useful for in the past in the current orientation. Yet both creationists and evolutionists would agree that there must have been some function for it in the current orientation at some point!
My father would also be interested in evolutionary insights, as noted below:
******************
In comparative anatomy between humans and animals there are all sorts of muscles that have the same name but because God designed the animals to function differently (four footed animals compared to us upright humans, for example)the muscle(s) might have different leverage characteristics and therefore quite different functions.
I remember noting many such comparisons in my cat comparative anatomy class as an undergrad at Eastern Ill. We used human models and texts but of course had no human cadavers to dissect so cats were used so we could gain a better understanding of how muscles operated and how the various joints functioned.
Both humans and cats might have a trapezoid shaped back muscle named "trapezius" that looks very similar but because of normal orientation to gravity would likely play far different roles.
What's in a name? Similar differences in lengths of tendons, how the tendons are routed around boney "pulleys", and differences in types and points of attachment could all influence function. I choose to believe these differences exist by design, not by some process of evolution. ...
I've done very little reading on the opposing viewpoints regarding such anatomical anomalies but would like to consider it further. As you come across such material, perhaps you can share it with me.