page 140 "But even without knowing the mechanics of it, we can be pretty sure that ravens do have an ACS that has somehow achieved displacement"  bickerton 
 page 145 "We know our forebears scavenged large dead animals, we know they faced fierce competetion from other scanvenger/predetors, and it's a reasonable inference that they could have succeeded only by recruiting sufficent numbers.  And as I see it, there's only one way they [humans]could have done this, the same way ants and bees did it, by breaking the ACS barrier and achieving displacement."