DuskMoonPenguin wrote:What exactly defines the human realm?
There are quite a few definitions of the human realm. My favourite is found in a Vinaya sub-commentary: that realm of beings for whom the brahmacariyā is a possibility.
Is it just based on the appearance of four-limbed bipedal intelligent primates,
In the Mahānidāna Sutta's teaching on the seven stations of consciousness humans are classed among those beings who are "variegated in body and variegated in perception." But nothing is stated about precisely how variegated human bodies can be. Nor about how much a being might differ from apes like you or me while still counting as a human.
"Four-limbed bipedal intelligent primates" won't really do as a definition. A primate is a mammal and a mammal is an animal that suckles its young. But in the Mahāsīhanāda Sutta (MN. 12) although conception in a womb is the typical way of generation for humans, there are said to be certain humans who are "spontaneously-generated" (opapātika). That is to say, they just spring up fully-grown and presumably would need no suckling. What spontaneously-generated humans might look like, and in what biological class, order and genus they might fit, is anyone's guess.
My own guess is that they probably look rather like ourselves but without belly-buttons.
Or if they do have belly-buttons they will be of the non-functional sort like the Brahmā gods' non-functional noses and tongues (materiality that looks to all purposes like a nose or a tongue, but is not able to serve as the physical basis of olfactory or gustatory consciousness).
or more so on our sapience and sentience, and the balance of both suffering and happiness in the realm.
The texts sometimes hint at something along these lines. For example, a common commentarial definition of humans is the pun, manaso ussannattā 'manussā' ti ("They are called 'humans' because of their superiority of mind" MA. ii. 37). But strictly speaking this is not a definition of humans but only a nirukti of the word "human".
would a species of intelligent reptilian hominids constitute a portion of the "human realm"?
I don't see why not, especially as we seem already to have an instantiation of this in the person of the current U.S. Secretary of State.