Okay, there are a handful of exceptions to the skeleton, but you have to admit there's a vast amount of similarity between -most- critters who have backbones.
Let me back up though, because you clearly know much more about this than I ever will, and my points aren't really addressing the problem.
The problem is one of perception, and plausibility. The reality is that no expert can ever be fooled by fiction concerning his specialty. This is why police officers tend to hate police dramas, lawyers can't get into courtroom dramas and so on. The requirements of fiction intended for laypeople are in many cases incompatible with how experts in the field understand the reality of that area.
My suggestion therefore would be to try to approach this situation not as a challenge against what you know, but a challenge FOR what you know.
Consider the phenomenon as it is placed before you; a galaxy that is not our own but is similar to it, in which many organisms on apparently completely unrelated worlds show remarkable similarities. Why? No one knows. It's a mystery. Theories abound, but no one's come up with one that adequately and fully explains all the details observed. Your character may in fact be one who is researching this very issue. Perhaps that's what he's doing in space...seeking out fossils and primordial chemical soups and discoveries about the origins of life in the cosmos, and why it seems so impossibly convergent...at least as far as sentient species are concerned.
In short, turn what you know into something that helps you get -into- the setting, rather than something that forces you to reject it. Approach the situation as a scientist; make your observations, then try to explain them. In this case, the GM is telling us what we observe. It falls to us now to understand why it's so.
Of course, this may only be delaying the issue. If Jemal ever tries to actually answer that mystery within the game, then you come right back to, "But that's not how things really work!" Hopefully by then though, you'll be willing to set aside your understanding of life sciences for the narrative as easily as the rest of us put aside our notions of causality and physical law.
And who knows? Perhaps you yourself might turn your knowledge of the subject to help the GM, suggesting possiblities that seem plausible to you, that also fit within the framework of the game. I'm sure Jemal wouldn't mind a few ideas thrown his way, as long as you leave him room to introduce surprises.