The gist of the “ideal free distribution” is that individuals in the best of all possible worlds should distribute themselves in the best of all possible ways. They should sort themselves out across space and time so as to avoid predators, find prey, get mates, and leave as many descendants as they can behind. Where information is imperfect, the best spots will be missed; and where mobility is blocked, distributions will be “despotic.” But where information is unlimited and mobility is unrestrained, distributions will be “ideal” and “free.”
The idea is intuitively obvious, and it has predictive power. It works for aphids. It works for sticklebacks. And it works for us.
Over most of the long stretch of the human past, our distributions were more or less ideal-free. We usually moved around with our prey, following the plants we collected and the animals we tracked. Some foragers, even now, are more footloose than others: On the Kalahari, hunters with access to waterholes, or n!oresi, make more claims to being a big man, or n!a, and have bigger families. But across continents—from Africa to the Americas to Australia—the most reproductively successful forager fathers raise children in the low double digits; and the most reproductively successful forager mothers do too. Genetic evidence backs that up. In contemporary populations including the Khoisan, elevated levels of diversity on the human X chromosome, and very low diversity on the human Y, suggest consistent sex differences in reproductive variance. A larger fraction of the female population has reproduced; a larger fraction of the male population has not. But overall, those differences are small.
After plants and animals were domesticated across the Fertile Crescent, our distributions became more despotic. Agriculture spread up and down the Nile Valley; and within a millennium Menes, who founded the first dynasty, had created an empire that endured for around 3000 years. When the 18th-dynasty pharaoh Amenhotep III married a daughter of the king of Mitanni, she brought along a harem of 317 women, with their hand-bracelets, foot-bracelets, earrings and toggle-pins, says a letter dug up at Amarna; and from ostraca and scarabs in and beyond the 19th-dynasty pharaoh Rameses II’s tomb, archaeologists have uncovered the names of ninety-six sons. From one end of the map to the other—from Egypt, to Mesopotamia, to the Ganges, to the Yellow River, to the Valley of Mexico, to the Andes—overlords rose up wherever the subjected were trapped, and they left behind hundreds of daughters and sons. Again the genetic evidence matches. Geographically diverse samples of Y chromosome sequences suggest a couple of reproductive bottlenecks over the course of human evolution. One from around 40,000 to 60,000 years ago coincides with our moves out of Africa into Eurasia, when the effective breeding population among women became more than twice the effective breeding population among men. A second bottleneck, from around 8,000 to 4,000 years ago, overlaps with the Neolithic, when effective breeding population among women became seventeen times the effective breeding population among men.
Then in 1492, Columbus found a New World. And over the last half-millennium, the flow of bodies, and the flow of information, grew in unprecedented ways. Despotisms collapsed. And distributions from sea to sea became relatively ideal, and free.
Our pursuit of the ideal free distribution is as old as we are. We’ve always pushed back against ignorance, and forward against borders. From our migrations around Africa; to our migrations out of Africa and into Eurasia; to our migrations out of Europe, Africa and Asia into the Americas; to our first intrepid earth orbits aboard Friendship 7, and beyond; we’ve risked our lots for a better life. I hope we never stop.