Matt Ridley pens a hilarious simile in his great book, Nature Via Nurture (published as The Agile Gene in the United States) that I think you might actually need to be an evolutionary anthropologist to fully appreciate.  And I quote:

Just as sex enabled mammals to combine two great inventions -- lactation and the placenta -- so trade enabled early people to combine draft animals and wheels to better effect. (Ridley 2003: 228)

Just like it.  Awesome!