I'm pleased to see that Sqad doesn't agree that this is the sort of operation that requires "detailed knowledge of anatomy" to the point that puts it beyond the reach of a plausible skill-set of ancient civilisations.
Everything below is my own musings, which therefore isn't meant to be definitive at all, just hopefully contributing to the discussion.
Still, these are the sorts of questions I'd have *before* wanting to take the "they must have had outside help", ie aliens, as a fruitful line of study:
1. As Sqad mentions, is it really necessary for this to be such a complex operation? Also, maybe the survival rate is low, but you only need one in a thousand to recover, and then you find that one in a thousand, and voila you have an amputee who survived whose body could be found 30,000 years later. What would be interesting -- but, sadly, unlikely -- to see is if it's possible to track the "failed attempts" at this surgery, ie to really get a handle on the progress of medicine at this time.
2. Following on from that, I think we tend to underestimate still how tough the human body is. We see it in animals a lot, they can recover from all sorts of horrific injuries -- sometimes because the sheer "will to live" carries them through, other times because of some stroke of luck that the injury didn't affect anything vital. Perhaps the wound was cauterised or something, so that there wasn't too much blood loss.
3. Also, in terms of sewing it up, I think we can expect that this was in the ability of such societies, because at one level it's just an extension of sewing for clothing, and I think even rudimentary stitching would be doable. Say, you know that leather is skin, leather can be stitched into clothes, skin can be stitched. It's not too much of a stretch, I think, to see this leap in logic being made.
4. Just, generally, I think it's also easy to underestimate the sophistication of ancient societies either because of a sort of "arrogance" about our modern world, or because if you don't have access to any written records then you simply can't measure it. Maybe this is the sort of thing that was at the time only taught by oral tradition -- gradual improvement at how to deal with the injury, a lot of failures before success, but when the last person in that culture dies the secret is lost with them.
5. Oh yes, also, while I think about it, isn't trepanning quite an ancient surgery technique? Break off a piece of the skull, and I can't imagine that being anything other than extremely dangerous without proper equipment and sterilisation. Apparently, it doesn't date as far back as this amputation, ie there's approximately a 20,000-year gap between this and the earliest known cases of trepanning, but we know that people quite often survived this complex, painful, and also often rather pointless operation. Is it really so much of a stretch, again, to imagine that the more logical "chop off a damaged leg" operation couldn't have occurred to someone to try, and much earlier at that?
I guess what I'm saying is that, while I can see that this could indeed be suggestive of "detailed knowledge", it's also just a single case, and could as well be ascribed to finding the one lucky survivor of a dangerous procedure out of the many hundreds of failed attempts.
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It's all fascinating, though, and shows just how much we have to learn about the prehistoric world. Like most areas of study, I'm sure we'll never run out of new things to discover!