tl;dr "the body works a certain way so morality works a certain way" / "morals are advanced now because we're more tolerant"
Basically what he's saying is we are biologically wired to have certain morals and they are converging on some kind of ideal position.
This seems so obviously false that I must be missing something. Human civilization and society has to do with biology, for sure: we get hungry, scared, envious, hateful, desireful, etc; we need protection so we gather together and help each other; when we gather together a form of competition arises along with a form of collective. Clearly humans do certain things because of how our bodies are wired. But within those structures (the "limits" Chomsky was talking about, I imagine) we have an almost infinite capacity for imagination that dictates our behavior (morality being an idea that inspires behavior). You can come up with almost any kind of reason to justify your own kind of morals and what actions are the result. "I need to eat your child's still-beating heart so the imaginary man in the sky will bring rain for my crops!" There's no biological narrative behind that, this dude just made some shit up and everyone went along with it because collective behavior.
Human civilization and society isn't relative, but morality is.