Are hobbits human and just really short? Or are they some entirely other species, like a gold-hoarding dragon? In high-school biology class they teach you to define species in terms of interbreeding. A horse is something that can make nonsterile babies with other horses; it may mate with a donkey, but since their offspring are sterile mules, horses and donkeys count as separate species. By that standard, the most relevant J.R.R. Tolkien passage comes from Appendix A of the Return of the King:
There were three unions of the Eldar and the Edain: Lúthien and Beren; Idril and Tuor; Arwen and Aragorn. By the last the long-sundered branches of the Half-elven were reunited and their line was restored.
It helps to recall here that Eldar is another word for elf and Edain is another word for human. Tolkien is saying here that there were two human-elf pairings in the backstory to the Lord of the Rings. One between Lúthien and Beren and another between Idril and Tuor. Both Arwen and Aragorn are descendants of one of these pairings. So when they get together in the course of the series, they reunite the half-elven lines.
Given that these pairings are clearly possible and fruitful, one might wonder why they seem to be so exceptionally rare. Chalk it up to magic, if you like, or perhaps it’s a question of prejudice.
What about other species? Aragorn says he fought half-orcs under the control of Saruman at Helm’s Deep. Fans discussing the possibility of hobbit or dwarf hybrids on Tolkien forums don’t come up with any textual references to specific instances, but they do offer some strong evidence that they are possible. The prologue to the Fellowship of the Ring states that “it is plain indeed that in spite of later estrangement Hobbits are relatives of ours: far nearer to us than Elves or even than Dwarves.” If human-elf hybrids are possible, and dwarves and hobbits are taxonomically more closely related to humans than elves are, then it seems plausible that human-hobbit, human-dwarf, and hobbit-dwarf hybrids would also be possible. By that standard, perhaps, hobbits are humans—just short ones.
On the other hand, perhaps the existence of occasional half-elvish children is a magical phenomenon, and despite the even closer human-hobbit genetic relationship, no interbreeding is possible under natural conditions.
Modern DNA sequencing technology does allow us to answer related questions about our closest real relatives (unfortunately, all long-lost). Recent paleontological research indicates that there was substantial cross-breeding between our species and Neanderthals as well as with the recently discovered Denisovans. There’s even a group of very small, relatively recent hominids classified as Homo floresiensis and nicknamed hobbits. Existing methods haven’t allowed for the recovery of H. floresiensis DNA to test for interbreeding. But these discoveries are perhaps enough to make us think twice about what we even mean to ask when we ask whether hobbits (or elves or dwarves or Neanderthals) are human. Is the issue at hand really one of chromosomal compatibility? We say that people have human rights, rights that it seems fairly certain we would want to extend to other creatures with intellectual and emotional capabilities similar to our own. It seems unlikely that the decision about whether such a principle would cover H. floresiensis really ought to come down to an issue of sexual compatibility and viable offspring. Reading the adventures of Bilbo, Frodo, and friends, it’s clear that hobbits are moral persons in all the ways that matter.
Correction, Jan. 2, 2014: The caption for this story originally stated that Arwen and Aragorn are half-elf and half-human. Their lineage is apparently much more complicated than that.