William Howells, formerly professor of anthropology at Harvard University, wrote a book about human evolution for a lay audience back in 1959. "Mankind in the Making" is a product of its time, from the sexist title ("MANkind") to its racist terminology (people from Asia are "Mongoloid"). But toward the end, Howells considers the question of what possible aliens might look like, and here (pages 352-353 in the 1967 edition) he offers some interesting speculation.
Now for a big question. Will They be standing up and walking around like us? They would not look very human otherwise. But might they not have two hands and four legs, that is, three pairs of limbs? Insects have. Do we do anything well that a centaur could not do better? We are reasonably content with our own seating arrangements, but perhaps we are making the best of a bad job. You know already why we go around on only two legs: if we wanted hands to use, we had no choice.
The choice existed once, in the early fishes. But the lobe-fins and amphibians chose to keep only four limbs out of an original larger stock of fins. Unimaginative beasts, what was good enough for a bedstead was good enough for a labyrinthodont. And so these feckless ancestors nearly slammed the door on hands entirely, since later hand-users (or wing-users) among the vertebrates had to manage by balancing on the two remaining legs. Supposing that ancient vertebrates had found some simple use for an extra pair of forelimbs, like the insects, while they still had the chance. Then those forelimbs might have continued being sufficiently adaptive for evolution to hang onto, as their possessors came out on land. Had this happened, we might all have avoided the problems which turn up in the blueprints of bipeds. There might, in fact, have appeared on Earth many intelligent, hand-using, four-footed animals. So I will lay a small bet that the first men from Outer Space will be neither bipeds nor quadrupeds, but bimanous quadrupedal hexapods. (I have just invented that last word in the hope that it means "six limbs.")
I just love the philosophical foundation of this notion: "Do we do anything well that a centaur could not do better?" Howell's asks, and clearly he thinks we do not, after all, "we are making the best of a bad job." And I love how he blames ancient lobed fishes! Those "unimaginative beasts" and "feckless ancestors"! They blew our chance at all being taurs ourselves! Everyone who ever wanted to be a taur, just blame those darn lobed fishes! By golly, we almost didn't even get hands -- thank goodness we learned to balance on two feet, freeing up our forepaws.
Odds are, Dr, Howell's says, extraterrestrial beings we meet in the future will have lucked out with the more logical body-form choice, and so we'll be face-to-face with taurs -- I mean, bimanous quadrupedal hexapods -- some day. Anyway, here I am with Dr. Howells' book, with gratitude to the Eastchester Public Library via interlibrary loan.
At the time the book was released, it caused quite a stir Down Under. Which is to say, a newspaper enjoyed sensationalizing Dr. Howell's speculation in a page nine news story, quoting from his book as if he had been actually interviewed. Check out this article in The Sydney Morning Herald for Nov 8, 1959: