hominids
Explanation
The Joke
A group of early hominids are observing a large, imposing figure (likely a Gigantopithecus or similar large primate). One hominid, Charlotte, asks if "that guy" is bigger than everyone else. Another hominid explains that no, he is just closer to them, and that Charlotte is "a total phony" for not understanding perspective. A third hominid suggests that maybe the large creature really is big, to which Charlotte responds with affection: "No Charlotte! He'll break your heart!"
The comic then jumps forward "A Million Years Later" to a modern-day scientist at a podium explaining that it is not clear why Gigantopithecus grew so large, but that it resulted in back pain, sprained ankles, tailbone problems, and arthritis -- essentially the same problems modern tall people face.
The Humor
The comedy works on multiple levels. First, there is the absurdity of prehistoric hominids having a soap-opera-style conversation about a large primate, complete with romantic warnings ("He'll break your heart!"). The joke treats the evolutionary development of gigantism as if it were a personal lifestyle choice motivated by romantic attraction rather than natural selection. Second, the punchline in the final panel deflates the evolutionary mystery by listing mundane modern ailments associated with being large, suggesting that the "reason" Gigantopithecus evolved to be big was as pointless and inconvenient as being tall is today. The comic plays on SMBC's recurring theme of undercutting grand scientific questions with banal human concerns.
References
Gigantopithecus was a real genus of giant ape that lived in Asia from roughly 2 million to 300,000 years ago. It is the largest known primate to have ever existed, estimated to have stood up to 3 meters tall. The reasons for its extreme size and eventual extinction remain subjects of scientific debate.