This is a long thread, so it's possible this is a repeat. That said:

The Croods takes place in the aftermath of an apocalyptic war on a low gravity alien planet, which involved extensive genetic modifications to both humans and a lot of the other invasive species they brought from Earth, plus a good long time for weird adaptations to crop up from a seriously screwed up ecosystem.

It makes so much more sense than the implicit stone age caveman setting, and there's absolutely nothing there that contradicts it.