Two Neanderthals need a bow and some arrows. Grrmt can build a bow in 5 hours and arrows in 4 hours. Aaaargl can build a bow in 2 hours and arrows in 3 hours. Thus, in order to build what they need, Grrmt will take 5+ 4 = 9 hours, and Aaaargl will take 2 + 3 = 5 hours. 14 hours total will be spent by the both of them.
Now what happens if our primitive friends talk to each other, specialize in what they do best, and trade? Something extraordinary. Aaaargl should have no interest doing that, as he's faster than Grrmt in everything, right? Right?