Humans' adoption of agriculture in Mesopotamia around 12,000 years ago led to a large increase in birth rates for those who practiced it.
We can possibly see this increase in fertility as agriculture's most profound and direct evolutionary effect on our species. Our farming ways have shaped other aspects of our bodies, like our teeth and our ability to process lactose in milk after childhood. But the effects on fertility are profound. Agriculture is here today because it allowed more humans to practice it, which in turn led to more agriculture.
Agriculture increased fertility but not necessarily happiness
But agriculture's boost to human fertility wasn't necessarily because of good times. As anthropologist James C. Scott has argued, in the earliest settled agricultural communities in Mesopotamia, people crowded into large cities for the first time, and diseases killed a high proportion of the population. Infant mortality was likely much higher than among hunter-gatherers of the time. But the city dwellers were still able to grow their populations in part because women had many more children than in previous times. These children were fueled by and, in turn, helped cultivate crops.
Often, or perhaps always, agriculture was under duress. As Scott (and others before him) have observed, the agricultural revolution partly consisted of humans enslaving and subjugating themselves. The emergence of "domesticated" humans coincided with the domestication of goats, sheep, and wheat, which all produced larger numbers of offspring than their wild cousins. Indeed, an increase in fertility under domestication has been observed in many animals, but again, this is not necessarily because they are glad to be under our control. Yet, from an evolutionary point of view, an increase in fertility constitutes a success.
There seems to be a cycle of expanding use of storable and nutrition-dense grains and livestock, which in turn requires unfree labor and spawns disease, which people try to outrun by having more babies, often against their will, with those offspring going on to expand agriculture still further, starting the cycle again.
Are we at another turning point for fertility and agriculture?
This cycle may be approaching an end. As statistician and epidemiologist Shanna Swan of Mount Sinai School of Medicine has documented in reviews of a century of studies, men around the world have experienced a dramatic plunge in sperm counts. She argues that a major cause of this decline is chemicals that disrupt normal hormonal processes that influence sperm production. Because sperm counts are lower in men living in rural areas, she believes a prime culprit is herbicides.
The possible effects of herbicides on fertility are over and above other negative health impacts of modern agriculture, from diabetes to obesity, not to mention the risks of catastrophic crop failures due to the fact that humans cultivate only a tiny handful of seed strains in almost all farm fields. So agriculture might, on balance, stop providing us with an evolutionary advantage at some point.
Where we may be headed
It's important to note that Swan's conclusions are based on a string of correlated measurements. In her studies, Swan analyzes data on ano-genital distance in newborns, which is correlated with adult sperm count, which is correlated with fertility, which is correlated with chemicals in the environment via poorly understood biochemical processes.
It is not known whether sperm counts will drop further. Moreover, it is not at all clear that it would be catastrophic even if they did. In fact, many have welcomed the prospect of a continuing slide in fertility, given looming environmental crises.
What we can possibly conclude in considering the beginning of agriculture's effect on our fertility—and our possible future—is that, at the population level, having more offspring is not necessarily a good thing, nor is lower fertility necessarily a bad thing.
Scott, J. C. (2017). Against the grain: A deep history of the earliest states. Yale University Press.
Swan, S. H. (2003). Do environmental agents affect semen quality?. Epidemiology, 14(3), 261-262.