Sex: What Our Genes Have to Say

Guest post by Doug Hornig, from the Casey Daily Dispatch 

Human sexuality. It's complex and the subject of never-ending debate within our society. Could genetics help shed some light on the issue? Turns out, yes.

Specifically, it has something to say about what our "natural" pattern of mating behaviour might be.

Yes, I put "natural" in quotes because there probably is no such thing. Given human cultural variability, any kind of generalizing is about as useful as classifying flowers by stalk length. But there are a few things of which we are certain.

First of all, humans are unusual in that we're not slaves to an oestrous cycle, as most other mammals are. We get to choose when and where to have sex, with whom, how often, and for what reason.

We also bear children who cannot survive on their own for far longer than is the case with any other species, mandating the continued presence of one or both parents—or some other suitable surrogate—in their lives.

And finally, we are surely the only creature in this corner of the universe whose view of sexuality is so complicated that we feel compelled to make rules about it, frequently knotting it up with legal systems and religious beliefs.

It's that last (as well as the second) point that generally comes into play among people who contend we're biologically meant to form monogamous nuclear families, and the first is usually invoked by those who argue that we're innately promiscuous.

Now traditionally, when we wanted to theorize about how something evolved over the hundreds of millennia of our species' development, we looked at three sources of potentially relevant information: our closest mammalian relatives; the "stones and bones" evidence left by our ancestors; and present-day cultures that seem to most closely mirror prehistoric ones.

With regard to sex, we can study the behaviour of the other apes. The Lar gibbon, for example, has long been thought to be one of the only primate examples of monogamy. While it's true that they tend to form a strong pair bond and live in close-knit families, more recent research has shown mates occasionally philandering and even "dumping" the other. Orangutans are solitary, mate very infrequently, and then go their separate ways; it's kind of a wonder they've survived at all. Gorillas live in social groups, but sex is limited to contact between one dominant male and the females in his "harem."

Our closest relatives, the chimpanzees and bonobos, both have sex with multiple partners. But otherwise, they are very different (the two species are believed to have diverged about two million years ago). Chimps live in a hierarchical society with an alpha male at the top, other males below him, and females at the bottom. Sex is strictly about procreation. The alpha male may become aggressive—involving displays of, or actual, violence—about defending his sexual "rights." This does not prevent females from mating with other males in the group, though it is often done outside of his attention.

Bonobos are a real curiosity. Often politely described as "hypersexual," they engage in recreational sex—like humans and unlike chimps. They do it a lot. There is equality between the sexes and strong female bonding. Sex is used for conflict resolution and as a means of social communication. Bonobos practice both hetero- and homosexuality.

That's a pretty wide range. So not much help there in understanding who we are. The stones and bones don't add much either. They indicate people have lived in groups for a long time, and we can infer that there must have been strong bonds among them, due to the lengthy maturation process for humans and the need to protect them from predators until they could in turn reproduce. A strong male at the centre of things would help ensure survival of the species, but we don't yet know whether he was more likely to exert his influence over one or several mates.

Contemporary "primitive" cultures are likewise varied. Most are monogamous, a significant minority practice polygyny (one male, several females), a few practice polyandry (one female, several males), and a very few don't believe reproduction results from sex at all (the gods decide who gets pregnant and when).

Where does genetics play into all this? In her recent book, Paleofantasy, Marlene Zuk—a professor of ecology, evolution, and behaviour at the University of Minnesota—explains.

First, Dr. Zuk dispels a common misconception, the use of "evolution" and "natural selection" as synonyms. Evolution is the change in a species over time, whereas natural selection is one of the four mechanisms by which evolution proceeds. The other three are genetic drift, gene flow, and mutation.

Genetic drift is the alteration of gene frequencies through chance events. Suppose a population has an equal number of big-eared and small-eared people, with no evolutionary advantage conferred by ear size. One year, while the big-eared people are having their annual conclave, a tornado comes through and kills them all. But the small-eared people, having their own party a mile away, are spared. Henceforth, just by chance, small ears will predominate. That's genetic drift.

Gene flow is just the movement of individuals and their genes from place to place, thereby altering the gene frequencies of the group they move in and mate with.

Mutations, in Dr. Zuk's words, are "changes in genes that are the result of environmental or internal hiccups that are then passed on to offspring." These alterations are "usually harmful, simply because random changes to complex machinery are rarely an improvement."

While the other three are important, it's natural selection that primarily drives evolution.

Another misconception Dr. Zuk notes is the widespread belief that humans are fully evolved, and that we are only trivially different from our cave-dwelling ancestors (an attitude that has given rise to the current fad for the so-called "paleo diet," which is supposed to be more "natural," in light of our history).

This is demonstrably untrue, Dr. Zuk argues. Evolution doesn't have a purpose, nor does it strive for perfection. It had no intent to produce such an amazingly adaptable species as humans. Evolution just keeps on keeping on. Natural selection decrees that traits that improve the likelihood of being passed on (or are neutral) tend to survive; any that diminish that possibility tend to fade away.

Moreover, the time frame for dramatic change can be significantly shorter than many people think.

As an example, Dr. Zuk cites lactose tolerance. As infants, we're raised on mother's milk, which requires the ability to digest lactose. That's accomplished by an enzyme called lactase, the production of which is genetically controlled. However, Zuk notes, "lactase production in all non-human mammals, and in most humans as well, grinds to a near halt sometime after weaning."

Thus the "natural" human state is to be lactose intolerant as adults. Which was inconsequential, until we started domesticating animals about 8,000 - 9,000 years ago. At that time, people began consuming the milk of cows, goats, and sheep, and later made fermented milk products like cheese and yogurt. To do that successfully, they needed lactase persistence, which results when "the gene responsible for producing lactase continues to be active because of a mutation in another genetic region that ordinarily curtails the enzyme."

How lactase persistence became widely established—it's exhibited by about 35% of the modern world's population—was a long-time puzzle that wasn't solved until this century, when we became adept at reading the messages in our genetic structure. The details of the research are a bit technical, but the conclusion is simple: it was an adaptation due to natural selection that favoured individuals who could already digest milk, rather than a chance occurrence of genetic drift. Genetic flow was also involved, as newly lactose-tolerant individuals migrated into areas where people didn't already use milk.

And it all happened in the merest blink of the evolutionary eye, which turns out to be not that hard. Anthropologists have calculated that as little as a 3% increase in the reproductive fitness of those with lactase persistence would result in the widespread distribution of the gene after only 300-350 generations—8,000 or so years—roughly the amount of time animal milk has been available as a food source.

But back to sex, and the eternal question.

As science historian Eric Johnson colourfully puts it: "Were our ancestors polygamists, monogamists, or happy sluts?" Disappointingly (for those who've been expecting a payoff), we still don't know for certain. But genetic studies provide at least a partial answer.

Some current researchers in the field have been looking at genetic diversity in human chromosomes. As most people know by now, a person's sex is determined by the interplay between X and Y chromosomes. (Precisely how that works is something that isn't fully understood to this day, but selection for males seems largely due to the action of a single male-directed gene, SRY, on the Y chromosome.)

We all have one pair of sex-related chromosomes, called allosomes, along with 22 pairs of non-sex-related chromosomes, called autosomes. The smallest chromosome contains about 300 genes, while the largest contains about 8,000—for approximately 25,000 genes in total. And within that genetic makeup lies our diversity. There are differences between any given individual and the next, and among groups. Natural selection acts upon this diversity, with characteristics appearing and disappearing according to how likely those who inherit them are to make it to reproductive age.

Women have two X chromosomes (XX) and men have XY, so mothers always pass an X on to their children, while men pass one only to their daughters. Thus, women contribute disproportionately to the genetic diversity on the X chromosome, and what they do contribute will remain relatively stable, no matter their mating behaviour. On the autosomes, however, the genetic diversity will vary according to the number of men contributing. More men mating with a given female population = greater diversity; fewer men mating with that same population = reduced diversity.

So, one way of determining who was doing it with whom, historically, is to examine the genetic variability of autosomes as it relates to that of the X chromosome. This involves complex mathematical analysis, the gist of which is: If they track each other closely, then that means people were mating either monogamously or promiscuously, i.e., in both scenarios any given male had a roughly equal chance of producing children, compared with any other male. If guys made it to sexual maturity, they probably reproduced with someone.

But when the researchers picked our chromosomes apart, what they found was a relatively lower level of diversity among the autosomes, as compared with diversity among the allosomes.

This means that fewer men were contributing to the pool, which means that many were shut out entirely, which means we now know that some variation of polygyny has been a very common form of mating behaviour. It's written in our genes.

It doesn't mean, however, that prehistoric social groups were the gorilla-like beings of popular mythology, with alpha males hauling more than their share of available mates back to the cave.

Keep in mind that research in this area is very new, and at the moment it only extends back 10,000 years or so. That time frame marks the dawn of the agricultural revolution, fixed settlements, and the stockpiling of food and material goods. These are developments that favour polygyny, as the more affluent males accumulate the means to support multiple wives or mistresses and the power to keep them, while other men are left childless.

Delimiting what sort of sexual behaviour prevailed before that time will have to await further research, as we decipher the genetic code of ever more ancient humans. It could go either way. It's possible that the strong genetic evidence for polygyny only emerged since the invention of agriculture—as we saw with lactose tolerance, changes can happen quite fast—and that, prior to that time, we were mostly monogamous. But it's also possible that the genetic remnants of our polygynous past have actually declined over the past 10 millennia, as more and more groups began to adopt monogamy, for whatever reasons.

We're left with an intriguing mix of fact and speculation. But whatever the case, overall, the science of genetics is booming like never before…

Doug Hornig is the senior editor at the Casey Daily Dispatch, where this post first appeared.