Captain Shatner wrote:
how does this explain the fantastic preponderance of Terrestrial sailors, captains, and admirals in Starfleet? It would seem that (in Starfleet, at any rate), Earth is vastly overrepresented in comparison to other planets.
There are a few different ways of explaining it.
We've seen many non-Human humanoids, so this could explain the seeming large number of Humans, they are in fact not Humans at all, but a variety of alien species from both inside and outside the federation, who are serving in Starfleet.
Or, a different explanation could be drawn from the United States military, there are people from all the American states and territories serving in the US military, plus people from outside the US serving in the enlisted ranks. But a disproportion number of people in the service are from the American south-east. It could be somewhat similar with Starfleet, it isn't that it composted entirely of Humans, but Humans are a much higher percentage than any of the other federation member species.
Or, Humans could be so culturally different than the majority of federation species, that it necessary to place us in our own ships.
Or, Some federation members might not volunteer for Starfleet at all, they'll do other things within the UFP organization.
Or, as has been proposed in the past, If a member species breathed some atmospheric gas other that ours (and we've seen people with masks before), or are comfortable with extremes in temperature, they could serve aboard their own ships, same with aquatic lifeforms analogist to octopi and dolphins.