I have long held that in saying that Spock is half-human, half-Vulcan, a person is verbalizing imperfectly a rather complicated concept in a manner that is accurate in the general but not in the particular. In general, because Spock’s father is Vulcan and his mother is human, then Spock is half-and-half. In particular, though, it doesn’t work that way at all.
Canon states clearly who Spock’s parents are. Canon is absosmurfly silent on how Spock was conceived. Perhaps because I come at Trek from a hard SF perspective, I can’t accept the idea that Sarek and Amanda did the nasty, and Spock was an accident. To my mind, it’s more likely that Worf could mate with a Targ and produce fertile offspring than the idea that two species that evolved on different planets under different conditions could produce an offspring from a sexual encounter.
“The Chase” provides one explanation–the Progenitors seeded the galaxy four billion years ago. That sounds reasonable. Unfortunately, what the Progenitors left on Earth didn’t do anything for three billion years. It wasn’t until the Cambrian Explosion, some 750 million years ago, that life on Earth progressed past the single-cellular stage, and then for most of that time, the dominant lifeform wasn’t mammalian, it was reptilian. For the Progenitor experiment to have worked, it would have required much intervention on their part over billions of years on hundreds of planets to guide evolution in a particular direction; simply leaving single-celled organisms on proto-life planets would be staggeringly against the odds to produce anything resembling anything else over the timescale of four billion years, let alone be genetically compatible enough to be able to produce offspring through unassisted sexual contact. I could believe in god before I could believe in this, and as I don’t believe the former, there’s no frelling way I can believe the latter.
So, how would I explain Spock?
Suppose Vulcan geneticists took Amanda’s genome, and created from the ground up a Vulcan genome that, if it were the basis of a living being, would produce a Vulcan with all of Amanda’s genetic traits. This created genome is used as the basis for a cell, which is allowed to undergo meiosis and produce ova, and the resulting ova are artificially inseminated with Sarek’s sperm. The end result is a person, Spock, that can be called half-human/half-Vulcan but who is, for all intents and purposes, fully Vulcan on the genetic level.
None of this precludes, to my mind, the scene in Star Trek V of Amanda giving birth to Spock. If the Vulcans could craft a genome from the ground up, they could surely create a placenta that would enable Amanda to carry Spock to term in the natural manner yet not be subject to rejection by the body just because it contained non-human genetic material.
And none of this means that Sarek and Amanda couldn’t have had an active, and pleasurable, sex life, only that their sex was recreational, not procreative.
I’m not saying that hybrids aren’t possible, because we know that they are. But I need a more rational explanation than an ancient Progenitor race.