CRUSHER: We thought it would be like working on Vulcans, but there are subtle differences. Too many of them.
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PICARD: You haven't found a compatible ribosome donor?
CRUSHER: The lab is still processing the tests. Early results indicate humans have far too many bio-rejection factors. I've also ruled out the Vulcans we've tested.
The thing is, the two quotes put together nicely show that the blood compatibility issue transcends species borders - Crusher isn't limiting her search to the Vulcans aboard, or apparently even starting out with them!
Healing the poor sap appears to be running into all sorts of problems before the blood compatibility issue arises. But that the Romulans are biologically distinct doesn't appear to be related to the compatibility question at all.
Given the separation of the two peoples is on the scale of millennia at most, it's pretty remarkable that they are biologically so different. It would be like having an Ancient Briton in a London hospital today and being unable to find a donor.
But our good doctor there
is convinced that modern donors exist - it's just that those Londoners and a handful of Manchesterites inside the hospital walls that day aren't compatible, and there doesn't appear to be any guarantee that somebody from modern Colchester would be a better match to this patient who hails from from ancient Camulodunum.
This doesn't mean it
wouldn't be remarkable that the old-timer is biologically distinct, in some ways that don't affect the compatibility issue. Something weird must have happened. Luckily, though, Star Trek offers more potential solutions than the scenario above, just as you say.
Potential explanations would include some kind of significant mutation or genetic engineering post separation (on either side), or that the two were already separated going back much further than their leaving for Romulus.
Yup. Imagine the poor doctor being amazed at the old man's sickle cell anemia because she has never seen a black man in London, England or indeed the entire United States of Brittany, due to that rift going back to the Roman times...
It could also be that this particular Romulan had some characteristic which made him a very rare match, and Crusher was wrong to assume it was a species based problem.
This has the added advantage of taking into account our TOS heroes already having a working knowledge of the average differences between Vulcans and Romulans (they did say the sensor readings in "The
Enterprise Incident" were distinct in careful analysis). Something "beyond the norm" must be surprising Crusher here. Either the patient is a freak to a degree, or then the Romulans did something to themselves that only took effect after the TOS era.
Timo Saloniemi