Horses make it less costly to move resources from point A to point B. The same is true of our ancestors walking upright. As the cost of allocation decreased the frequency increased. More frequent allocation meant more frequent allocation problems.
When monkeys, lions, whales and birds migrate they generally don't have to worry about what they'll take with them. It was a different story with our ancestors. They were able to carry many resources at the same time so they were forced to figure out what to take.
Our ancestors needed weapons for defense and for hunting. If they carried too few weapons then they might be killed by wild animals or other groups. But if they carried too many weapons then it would mean carrying inadequate amounts of food and other necessary resources. They constantly struggled to get the balance right. Getting the balance right depends on information. Groups that were better at storing, processing and sharing information were more successful than other groups. Hence the development of language and trade.
Trade provided more reliable information about the importance of things. This information helped our ancestors make better carrying decisions. Clearly it's better to carry more, rather than less, important things. So groups that traded more were more successful than other groups. Trade is facilitated by horses, which the native Americans didn't have, which is why they were far less successful than the groups that did have them.
Nowadays we still worry about what to carry when we go somewhere. But good or bad carrying decisions rarely influence our reproduction. So we've reached peak intelligence.
We still struggle with the balance of defense. Most people incorrectly believe that small groups of leaders are better than trade is at determining the social importance of defense...
Eventually enough people in one country will realize that taxpayers should have the opportunity decide how many of their own tax dollars to trade for defense and all the other goods supplied by the government. This country will be more successful than other countries. Other countries will quickly follow suit.
Summary...
Theory 1: Our exceptional intelligence was caused by our exceptional carrying ability
Theory 2: Low level of development in Americas was caused by absence of horses to carry things
Theory 3: Better balance of defense would be achieved by taxpayers having opportunity to use their tax dollars to help determine the social importance of defense and other public goods.
