After giving this some thought, these are some possibilities that have occurred to me, which I'm sure anthropologists will be confirming any day now:
"The Dream"- I'm certain that there had to be this one guy, and you probably already know at least one guy like this yourself. You know, the "Big Idea Guy". This guy comes up with a grand scheme out of nowhere, usually after a couple of drinks, and he is instantly convinced that it will work, and he sets out to convince everyone else too. What the idea is doesn't matter, just as long as he can get everyone on the bandwagon. I suspicion it went something like this:
"Oog, things are getting tough around here. You know what we need to do don't you? We need to shake the dust of this crummy little cave off of our heels and move two valleys over. I know there have got to be some chicks over there, and the hunting! Wow, there wouldn't be any need to hunt! At dark, the food just wanders into your cave and you club it over the head. Not to mention there is this kick-ass waterfall! And right next to the waterfall is this fire that never goes out, and right next to that grows this plant that you can make some mind-bending tea with! We could just lay around, surrounded by hot chicks, eat good, and drink wicked tea! That would be awesome!"
At this point, Oog suggests that the guys who live there now might not want the two of them horning in on their good thing. Besides, the next valley over has lots of big, deadly snakes.
Now the conversation can go one of three ways here. The first is very simple, as the "Big Idea Guy" continues to sell his original plan, unchanged, to Oog until Oog finally gives in just to shut him up. Now the other two options in the conversation involve the "Big Idea Guy" taking on one of two persona's that have been wildly successful throughout history.
"The Warrior"- "Oog, just look at yourself man! You are a strapping, hulking specimen of caveman! You could kick a saber-toothed tiger's butt! Those pussies in the next valley have grown soft. They wouldn't stand a chance against a motherf*cker like you man! We grew up in the poor valley and we've had it tough, and we know what it means to be mean! Besides, we come from a long line of smart ass-kickers. Hell, it was my great-uncle who suggested we come down from the trees, and your dad could stomp a hole in a wholly mammoth. What with my brains and your good clubbing arm, those chumps wouldn't stand a chance man! All we gotta do is make sure we make a lot of noise and stomp a lot as we got through the valleys. Yea! That will scare the shit out of the snakes and the snakes will head into their valley and they will beat feet, cause who the hell wants to live somewhere that is crawling with snakes? And if any of those bastards want to hang around, we can kick their asses pronto!" This has been a popular stance of Attila the Hun, Napoleon, Hitler, and street gangs the world over, just to name a few.
But the last option of how the conversation turns seems much more likely, but I'm not sure why.
"The Priest"- "God wants us to take that land because he promised it to us. If you don't help, he will kill you after making you suffer really bad. If you have any questions, face this way, hold your hands just so, and call out to my great-uncle until you find I'm right. If that still doesn't convince you, try some of this tea." The examples for this model a too numerous to site and I'm sure you could think of quite a few that I wouldn't have thought of, but you get the picture.
So what did the first man sell? Tea, and lots of it, and in the immortal words of Lewis Carrol's Mad Hatter, "Clean cup, clean cup! Move down, move down!"