Standing Up for Sex

Humans evolved the ability to walk on two legs because it allowed them to more accurately size up prospective mates. Or did they?

By | December 1, 2013

UNIVERSITY OF CHICAGO PRESS, OCTOBER 2013It happened years ago, but the event was so traumatic that I remember it as if it were yesterday: an elderly professor physically pinned me against a wall and berated me for rejecting his paper on why human ancestors got up on their hind legs and walked. “The reason,” frothed the empurpled sage, “was to make it easier for mothers to carry babies close to their chests.” See? So blindingly obvious that anyone, even I, could understand it.

Manuscripts seeking to explain the evolutionary roots of human bipedalism land in my in-box at Nature with monotonous regularity. We became bipeds so that we could carry food, or tools; so we could see farther; so we wouldn’t expose so much of our skin to the Sun; so we could wade better in rivers and lakes. They all make good stories, but they all share the same error—they are explanations after the fact, and, as such, betray a fundamental misunderstanding of how evolution works.

Natural selection, the mechanism of evolution, operates without memory or foresight. It has no intention. It is we who choose to interpret evolutionary purposes as such later on. The features of living things, therefore, do not evolve for any preconceived purpose that we can discern. I explain how such misunderstandings color our understanding of human evolution in my latest book, The Accidental Species: Misunderstandings of Human Evolution.

But none of this stops me having my own go at understanding why humans came to walk on two legs. In my view, it all happened by accident.

Bipedalism is just one of the many peculiarities of human anatomy and behavior that set us apart from our closest relatives, the great apes. We are also much more social than they are, we have unusually large brains, we have much more body fat, and we are much less hairy. The differential distribution of fat and hair happens to be strongly correlated with sexual dimorphism.

On the subject of sex, women’s furless breasts are prominent at all times, not just when women are lactating. Unlike female chimpanzees, our closest living primate relatives, women do not advertise estrus—the time of maximum fertility—by the swelling of the sexual organs. And while we’re talking about advertisement, men have the largest penises, relative to body mass, of any ape. A male gorilla might weigh twice as much as an adult human, but he’s lucky if he ever gets an erection more than inch long.

But if humans’ prominent breasts and big penises are made obvious by hairlessness, they are made more so by bipedalism, which displays everything for all to see. In which case, standing upright could be a by-product of sexual selection, in which mates choose one another on the basis of features that might represent outward signs of inward genetic health.

Some sexually selected features, though, appear have been chosen at random when, by chance, a trait in one sex becomes associated with the preference for that trait in the other, leading to runaway positive feedback, survival value be damned. The massive train of the peacock is a good example. It looks flashy and attracts mates, but costs a great deal of energy to make and maintain, and hobbles a peacock trying to flee from predators. The transition to bipedalism might be seen in the same way: it was selected because it better advertised our sexual wares, but hobbled us in other ways.

The imposition of walking upright on a fundamentally quadrupedal design has prompted a thorough reworking of the entire human body, making back pain one of the single biggest causes of worker absenteeism in the world. Rather than an adaptation, bipedalism could be a dreadful kludge, forced on us by sexual selection in defiance of gravity and common sense.

Now, I advance the above more than half in jest. It’s possibly no better or worse than any other idea, but I’m not going to pin anyone against a wall and shout about it.

Henry Gee is a senior editor at Nature, and the author of Jacob’s Ladder: The History of the Human Genome, In Search of Deep Time, and The Science of Middle-earth. Read an excerpt of The Accidental Species.

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Posts: 1

December 5, 2013

Henry, Good article, but you should think about the following. 

Magic traits: These are traits that can serve as both signals for sexual selection (and species recognition)  and which impact fitness. Magic traits greatly accelerate speciation. I have proposed that upright posture is such a magic trait.

Sexual selection vs species recognition: These are often confused in the literature. One must first identify a conspecific, and only then comes the opprotunity of selecting  among possible mates via sexual selection. Since upright posture is both a male and female trait is more likely a species recognition trait than a sexually selected one.

For more detail look at my book "Why We Have Sex: Solving the Darwinian puzzles of sex and speciation." This is available in ebook and paperback at Amazon.

Richard Weisenberg


Avatar of: Mike Bennett

Mike Bennett

Posts: 19

December 5, 2013

To say that something evolved in order to have a particular function can be viewed as meaning that animals carrying genes enabling that function had more progeny.

I would comment on Richard's second point that species recognition is not essential for sexual response. Many examples come to mind, but an obvious one is interspecific hybridization.

Avatar of: Sergio Meirelles

Sergio Meirelles

Posts: 4

December 5, 2013


Natural selection, the mechanism of evolution, operates without memory or foresight. It has no intention.

but ...

....mates choose one another on the basis of features that might represent outward signs of inward genetic health...

Yes, we use to teach that but I dont believe in nature without noise and most of the apparent logic can be a product of our dream of scaping a meaningless world. I think that mates choice can be a way to keep futile an sometimes undesirable traits in the population by leaving clues in the phenotype. Traits  that confer fitness advantages can be rewarded by high frequency in the population.

Sex is so important that rend us unable to understand the meaning of sex.

Sergio Tadeu


Avatar of: Mounthell


Posts: 48

December 5, 2013

Everyone, as far as I can tell, overlooks life's propensity to connect socially, and from which elaborates the plethora of interesting variation even the most crass reductionist can glimpse, Lord Henry of Naturedom.

Given that, and except for the fact that variation from the norm offers its own delights* (the proof of the pudding, as it were), the rest is mere detail.

*one of the several problems arising from our predisposition to connect is that of demonstrating one's uniqueness which of necessity includes choosing partners among the outgroup or outspecies.

Now go, connect!

Avatar of: PastToTheFuture


Posts: 84

December 5, 2013

I once argued that evolution is never towards something, it is always away from something. Much later, I realized that that is not actually always true. A specie does evolve towards being more adapted to their niche. So if a specie like humans enters a new niche that they can survive in, they will evolve towards being better adapted to it. All it takes is a little start and may not even be the adaptation you see later in the niche. It may have been a sexual selection thing initially (I doubt it) and then that opened up a foraging potential that became the focus of adaptation. Considering all the adaptations of humans to walking, distance is the greatest ability it all confers together. What of these adaptations existed before walking? Eyesight. Human eyesight is the best in the animal (and plant) kingdom. Maybe it was the ability to see far that prompted development of ability to walk far... Just off the top of my head.

Avatar of: Mark Riggle

Mark Riggle

Posts: 3

December 5, 2013

Some problems are here.  Bipedalism likely started 6 MYA, or at least in our line.  It also had a begining 230MYA with the first bipedal dinosaur -- the theropods which are the ancestors to modern bipedal birds.  So any mutation that can lead to bipedalism must be rather rare.  Also, Ardipithecus ramidus shows that there were not ever our ancestors who walked with a bent-knee bent-hip gait; so whatever mutation caused bipedalism moved us to a fully upright gait immediately. It would have made our ancestor a highly skilled biped.  In almost any environment other than a completely arboreal environment, a skillful bipedal ape will out compete its still quadruped cousin.  Those cousins will need to vacate the non-aboreal environments and fully physically adapt to the arboreal environment -- which would give us the chimpanzee line.

The real question is not what the added advantage was that kept that upright gait ability (there are so many), but rather what neural mutation could cause it;  that is, what is the neural mechanism that with a simple change (the mutation) could change that ancestor so radically.  Whatever that mechanism is, an analog should also exist for the theropods -- some quadruped dinosaur learned to walk on two feet which then let it out-competed its cousins. (I'm going to guess that breasts had very little to do with that).

So what simple neural change (assuming it is a neural change) can lead to an upright bipedalism in the human ancestor?  Open for ideas.

Avatar of: James V. Kohl

James V. Kohl

Posts: 412

December 5, 2013

Re: "Natural selection, the mechanism of evolution, operates without memory or foresight. It has no intention."

It also seems to operate outside any biophysical constraints on organismal complexity. Besides, no experimental evidence exemplifies what is selected in mutation-initiated natural selection. For contrast, natural selection of nutrients enables amino acid substitutions that are clearly represented in species diversity. 

Obviously, others have determined that increased organismal complexity associated with amino acid substitutions is a problem. They are attempting to change mutation-initiated natural selection to mutation-driven evolution and are probably hoping that no one will notice that mutation-driven evolution "just happens".

Carl Zimmer wrote:  "Others maintain that as random mutations arise, complexity emerges as a side effect, even without natural selection to help it along. Complexity, they say, is not purely the result of millions of years of fine-tuning through natural selection—the process that Richard Dawkins famously dubbed “the blind watchmaker.” To some extent, it just happens."

Similarly, in the introduction to a theme issue of Philosophical Transactions of the Royal Society B: Biological Sciences, we read that: Each of these papers, in one way or another, consolidates the idea that there will probably be no fixed law, like gravity, to explain at the molecular level how endless forms most beautiful and most wonderful have been, and are being evolved.

Avatar of: mightythor


Posts: 68

December 5, 2013

Or: bipedalism evolved first, and afterwards large breasts and large penises evolved to compensate for the negative anatomical and postural effects of bipedalism on mating and mate selection.  Whenever you have covariance, there is always the perplexing problem of what is the cause and what is the effect.  In the laboratory sciences you can tease things apart experimentally.  In evolutionary biology, not so much.  So in the end we are all left with our opinions, which we should all try to respect -- as opinions.


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