Surely, if some local wild mammal species of those continents had been domesticable, some Australian, American, and African peoples would have domesticated them and gained great advantage from them, just as they benefited from the Eurasian domestic animals that they immediately adopted when those became available.
The suggestion here is that the adoption of domesticable animals and plants follows a process similar to evolution by natural selection. By random chance, some groups of humans would have accidentally domesticated an animal, noted its usefulness, and gained an advantage over other groups. This advantage would have been passed on to their successors.
Other groups, noting this advantage, would have then adopted the practice themselves.
