XO in XY

History Unfolds in Our Genes

Posted Feb 14, 2017

NIH/Wikimedia Commons
Source: NIH/Wikimedia Commons

“All you need is love

All you need is love

All you need is love, love

Love is all you need.”

--The Beatles

As much as anybody, John, Paul, George and Ringo should have known.  They met 3,000 wildly excited groupies when they first invaded in ’64 at JFK; and they played to another 55,000 crying and screaming women (and so on) in ’65 at Shea.  Their family sizes were more modest.  John, Paul, George and Ringo had, between them, 9 wives, and 11 known children.

Which put them pretty squarely in the caveman tradition.  On the basis of gene sequences from 26 worldwide populations assembled by the 1000 Genomes Project, my friend Melissa Wilson-Sayres at Arizona State, along with a small army of human geneticists, is unfolding the history of human mating from our sex chromosomes.  As far back as the African Exodus of around 50,000 years ago, they've found, the effective breeding population of women was around twice the size of the effective breeding population of men.  But by the late Neolithic, around 4,000 years ago, after civilization began, that ratio had swollen to 17/1.  From a modest start as mostly monogamists, some of us had become highly polygynous men.

Across forager populations including the Khoisan—a group of Africans that includes the Kalahari hunter-gatherers, who live on the border of Namibia and Botswana, and have been variously known as the Ju/’hoansi or !Kung—genetic estimates of the breeding sex ratio are consistently biased in favor of women.  A larger number of women has had children by a smaller number of men; but the ratios are usually only around 2/1.  

That fits with what demographers have found.  When the University of Toronto sociologist, Nancy Howell, put together her Demography of the Dobe !Kung, she showed that fatherhood fairly was egalitarian.  In her sample of 35 men over 50, nobody had never been married, and just one man, who'd been married 3 times, had as many as 12 children.  Howell’s sample of 62 women who were 45 and older followed pretty much the same pattern: one woman had 9 children; other women averaged only half as many.  Most foragers were married, and their families were usually small.  But even on the Kalahari, reproductive variance was higher for males: variance for men was 8.60; variance among women was 4.87.

But after we first got out of Africa, variances like that went up.  Over the course of human evolution, there have been a couple of reproductive bottlenecks.  One from around 40,000 to 60,000 years ago coincides with our move out of Africa into Eurasia by a small number of colonizers, after which the effective breeding population among women was, again, around twice the effective breeding population among men.  But those differences took off with the Neolithic.  A after a second bottleneck, from around 8,000 to 4,000 years ago, which overlaps more or less with the origins of civilization, the effective breeding population among women became as high as 17 times the effective breeding population among men.

That fits with the accumulated evidence of 5000 years of history.  A couple of millennia after Menes founded Egypt's first dynasty, the 19th-dynasty pharaoh, Rameses II, remembered the day when his father gave him a crown: “He spoke of me, his eyes filled with tears, so great was the love for me within him; he furnished me with a household from the royal harem.”  In this Rameses' tomb, in the Valley of the Kings at Thebes, there are chambers for dozens of his children; and on bas reliefs and ostraca, statues and scarabs from all over his empire, the names of 49 “king’s sons”—generals, hereditary counts, chiefs of secrets, scribes—survive from his reign, with roughly another 50 inscriptions from unnamed sons. 

More than a millennium later, when the first histories were put together in the Bible, Near Eastern patriarchs, like Abraham, were remembered as the fathers of children who numbered in single digits; judges like Gideon were credited with dozens of progeny; and kings, like Rehoboam, had close to a hundred daughters and sons.  Solomon, who was Rehoboam's father, had 1000 women; and David, who fathered Solomon, was supposed to have written some Psalms.  "In many-colored robes she is led to the king, with her virgin companions, her escort, in her train."

In India, a few centuries after Solomon, the first dynasty founder, Chandragupta Maurya, was succeeded by Bindusara, his son, who was succeeded in turn by the great emperor, “the Sorrowless” Ashoka.  Legend had it that Ashoka won his throne over the bodies of 99 brothers; on rocks and pillars all over the subcontinent, he left edicts that promised: “I have given this order, that at any time, whether I am eating, in the women’s quarters, the bedchamber, the chariot, the palanquin, in the park or wherever, reporters are to be posted with instructions to report to me the affairs of the people.” 

Less than a century after that, Qin Shihuangdi, who unified China in 221, left an inscription on the top of Mt Kuaji, in stone: “I will prohibit and stop licentiousness and dissipation.”  But after the First Qin Emperor connected his 270 palaces together, he filled them with more than 10,000 women, beautiful women with bells and drums that he’d taken from the feudal rulers—said China’s first historian, Sima Qian.  Some men had very large families; but many men lived alone.

Times seem to have changed.  John, Paul, George and Ringo aside, we’re apparently less civilized, and a little more like cavemen, these days.  Some civilized men had large families, but many lived alone.  Cave people, on the other hand, tend to be family women and men.




Betzig, L.  2016.  Mating systems.  In Encyclopedia of Evolutionary Social Science, T. Shackelford and V. Weekes-Shackleford, eds.  Berlin: Springer.

Howell, N.  2000.  Demography of the Dobe !Kung, 2nd edition.  New York: Aldine.

Karmin, M. et al.  2015.  A recent bottleneck of Y chromosome diversity coincides with a global change in culture.  Genome Research, 25: 459-466.

Poznik, G. D. et al.  2016.  Punctuated bursts in human male demography inferred from 1,244 worldwide Y-chromosome sequences.  Nature Genetics, 48: 593-601.