“When we’re looking at things that are uniquely human, we can’t look to big brains or bipedalism because our extinct relatives had those. But they didn’t have chins. That makes this immediately relevant to everyone.” Indeed, except in rare cases involving birth defects, everyone has chins. Sure, some people have less pronounced ones than others, perhaps because their lower jaws are small or they have more flesh around the area. But if you peeled back that flesh and exposed their jawbones—and maybe don’t do that—you’d still see a chin.
Also interesting: TIL the word spandrel, an incidental byproduct of evolution.