Sunday, February 14, 2021

Descent of Man, 150 years later (with link to video)

Yesterday The Leakey Foundation and Jeremy "Jerry" DeSilva put on a special Darwin Day event focused on Darwin's book Descent of Man, these 150 years after its initial publication. There are several short talks and some rich Q&A discussions in between them, featuring some of the authors of chapters in Jerry's edited volume A Most Interesting Problem: What Darwin's Descent of Man Got Right and Wrong about Human Evolution If you are at all interested in human evolution or Darwin, it is probably worth your time. 

The Leakey folks recorded it and posted it here on youtube:

I've pasted the script to my 10 minute spiel, about halfway through the event, below. 

I self-plagiarized from my chapter in A Most Interesting Problem, from the talk I've recently been giving wherever they'll have me (about sex differences in the skeleton), and from my book manuscript (I AM EVOLUTION)....

Start here...

Until we wrote this book with Jerry, I just saw Descent of Man as the source for Darwin’s formulation of sexual selection, using powerful evidence from across the animal kingdom. What I hadn’t done, until I wrote my chapter, was pay much attention to Darwin’s ideas about humans. I had only peeked at a few of those pages, had seen the word “savages,” and then couldn’t bear to actually pay attention. 

What for? Being based on such outdated and limited evidence, those pages wouldn’t advance my understanding of human evolution, so I didn’t think they were worth my time. But to write this chapter, I had to give those pages my time, and while I read and re-read and raged, I realized what I already knew but hadn’t yet faced: Descent of Man’s greatest legacy is not the scientific power of sexual selection; it’s Darwin’s racist and sexist narratives of human evolution. 

Descent of Man is just the absolute pits for so many people, like me. His book was better for birds, beagles, and baboons than it was for billions of humans! 

So, for scientists, Descent of Man is a foundational theoretical masterpiece, albeit riddled with errors that Darwin can be forgiven for making in his time, without these 150 years of scientific progress that we enjoy today. But for humanity, Darwin’s book has been a curse.

 The scientific value of Descent of Man is impossible to untangle from the oppression that it inspired.


Darwin’s scientific contributions in Chapters 19 and 20 of Descent of Man look like Victorian Age-appropriate explanations for sex differences in visible traits in peoples across the globe—like, for how our naked, colorful skin developed out of our ape ancestry. Because around the world, men tend to be darker-skinned than women, Darwin proposed that variation in human skin color around the world evolved due to sexual selection via local beauty standards.

But that people with deep ancestry in the tropical regions of the planet have some of the most pigmented skin, is best explained by natural selection, not sexual selection. This adaptation likely materialized as our hominin ancestors lost a significant amount of their protective fur covering. In its place, a melanin-rich epidermis shields the body from the sun’s harmful UV rays, particularly at the Equator.

At high latitudes, natural selection is also the dominant explanation for human skin color. Our bodies require UV radiation to synthesize Vitamin D, so permitting some UV radiation into the skin is beneficial. Both skin color extremes have strong adaptive explanations. In between UV extremes, that clinal or spectral continuous variation, from highly pigmented to depigmented skin, is maintained by gene flow connecting the populations and by selection for intermediate UV levels in between. [That last phrase in the previous is nonsense and I correct it on the fly in the video.] The evidence for this explanation for the evolution of skin color variation around the world outweighs the evidence for Darwin’s hypothesis of sexual selection via different beauty standards.


One of the strongest testaments to Darwin’s influence is that the mere existence of sex differences is enough to assume that sexual selection brought them about. Take, for example, sex differences in human height, where in all populations, men are on average taller than women. 

Darwin drew inspiration from the size and strength of male, or silverback, gorillas, and scientists in his wake have helped to build the case, which is now canon, that sexual selection explains sex differences in body size. Big males won greater mating opportunities by physically dominating competitors and mates, and by females preferring to mate with these big winners, so the thinking goes. 

But sex differences in human height are at least as much about the evolutionary importance of estrogen. Estrogen is a major driver of long bone growth in all humans and it’s biphasic, so at even higher levels, estrogen ends long bone growth with the fusion of the growth plates. When estrogen peaks, teenage girls stop gaining stature. This estrogen peak at puberty is fundamental to ovarian development and the initiation of regular menstrual cycling. Boys, who are growing in step with girls until puberty, stop gaining stature just a few years after girls do, because there is nothing to stop them earlier; higher estrogen would be incompatible with male reproductive development and function. Eventually young men have enough estrogen in their more senescent skeletal system to also stop growing.

There are no male or female genes for height and there are no male or female bones; there is only common biology of skeletal growth shared among humans, where similar processes significantly controlled by estrogen play out differently in different bodies during development. As of now, sex differences in the duration of long bone growth are a byproduct of the way that the human reproductive system evolved to function, thanks in large part to estrogen. Still, sexual selection for tall men dominates the evolutionary story. It sure is a compelling one. 

Perhaps more than any other science, evolutionary science is a collection of stories, or fictions, about facts. For example, FACT men are on average taller than women FICTION because of their big winning male forebears. Fictions are supposed to be difficult to establish as fact in science, and they’re far too easy to establish as fact in the zeitgeist. If we are better aware of the precarity of our fictions… if we improve the scientific explanations of visible sex differences, like height, then that science will be less likely to inspire unscientific beliefs about invisible, imagined sex differences. Fewer minds will leap illogically from ‘men are taller’ to ‘men obviously evolved for competition and dominance’ as if women did not.


Underneath Descent of Man’s ambitious scientific contribution to human evolution lies much more than surface beauty. This is Darwin begetting every caveman-inspired nugget of dating advice and every appeal to innate gender roles at home, in the workplace, in science and tech, and on Wallstreet. This is where Darwin first turned his concept of sexual selection loose on humans, launching the evolutionary narrative that dominates pop culture starring QUOTE “the strongest and boldest men… in contests for wives”.

In Descent of Man, Darwin parlayed visible anatomical differences between sexes—like those in skin color and height—into the evolutionary logic behind why Man and Wife perform differently, in matters of love, sex, parenting, cognitive feats, comedy, and seemingly everything else, according to the contemporary world views he continues to shape.

But, pop culture has been slow to adopt new knowledge that has complicated and overturned old facts. Much of the novel insights on human evolution, including especially the evolution of human skin color variation, these have come from women, despite persistent beliefs like Darwin’s that,  QUOTE “Man is more courageous, pugnacious and energetic than woman, and has a more inventive genius.” Darwin even concludes, as if by scientific logic, that, QUOTE “man has ultimately become superior to woman.” For Darwin, women were wives but men were so much more than husbands, this seeded his science of sex differences. From his view of life, Darwin penned nature’s seal of approval. 

Darwin naturalized gender differences, made them biological, made them adaptive, made them morally valuable. All of us suffer from bias, but powerful men seem to share one particular form of bias when it comes to our species’ shared origin story—fuhrers and ex-presidents… their oppressive human evolutionary narratives sound a lot like Darwin’s, which directly inspired the exclusion of women from higher education and from science. Today, there is an increasing understanding that beliefs in gender essentialism, in natural rigid gender stereotypes of masculinity and femininity, like boys are active and dominant and girls are sexual and maternal, these beliefs are correlated to poor educational outcomes in boys and girls and to sexual assaults by boys and men.

Getting humanity’s origin story right matters a great deal to humanity.


The stories we tell about the facts of evolution are in dire need of diversity. The way to get there is not merely to be more correct or less biased than Darwin, it is not merely to be the best version of ourselves, it is to be proactively bigger than any one person can possibly be and… to be proactively bigger than even science can be.  We do not need science to demonstrate that women and people of color are not inferior… to demonstrate that we are not less evolved, less brilliant and inventive, less deserving of opportunity, power, influence, admiration, and freedom* than white men.

While many scientists and scholars have met Darwin’s bias and prejudice with science, science is not the only, or even a necessary way to demonstrate humanity.  That is partially because science does not yet represent humanity. Science will continue to write a better story of human origins and evolution, but if science were all it takes, then we would have that better story by now. Science is not the sole or even primary author of humanity’s shared origin story, humanity is.

Let’s write a great story together. Let’s write one that’s worthy of the entire species, a story that everyone can embrace. 

*I'm not positive but I think that my use of "freedom" inspired a discussion in the live chat about Darwin and his family being abolitionists. Yes, they were. But my use of freedom here refers to something other than the abolition of slavery. I'm referring to the freedom that more wealthy white men enjoy than anyone else. Who wants to live in a world where freedom is merely the absence of slavery!? Freedom is attained by the things I listed before it and so much more...