What Makes Us Human

And one percent Neanderthal

Which Made Us Human, Losing Penis Spines or Gaining Brain?

Genome research yields new results but guess where reporters focus?

Ars Technica titled its story "Humans are losers (when it comes to penis spines)". A raft of newspapers followed a similar line.

The new research reported compared the human genome with that of other mammals and found that areas deleted-- and thus distinctive of humanness-- were mostly "regulatory DNA, which alters the expression of nearby genes":

"One of them lies near the androgen receptor, which mediates signaling by testosterone and its chemical relatives. Humans have a very large deletion (60,000 bases, or 60 kilobases) in the area, which eliminates a highly conserved five kilobase region. When injected into mouse embryonic stem cells, this region could drive expression of a gene in a couple of very specific regions: the developing facial whiskers and the developing male genitalia. Obviously, humans don't develop whiskers, but we do have genitals. What we don't have is penile spines, while chimps and mice both do."

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So, what the media picked up on is the possible loss of penis spines at some point in human evolution.

Now, I would be the last person to diminish an evolutionary change that reportedly improved what Ars Technica coyly calls "male duration during sex". And it is true that the original sober research article in Nature singled out, in somewhat more technical language, this deletion, noting that it

removes a sensory vibrissae and penile spine enhancer from the human androgen receptor (AR) gene, a molecular change correlated with anatomical loss of androgen-dependent sensory vibrissae and penile spines in the human lineage.

But Nature also discussed a second change that led to "expansion of specific brain regions in humans". Given that reorganization of neuroanatomy has been and remains one of the major foci for defining what literally made us human, it seems like that might have been worth rather more commentary than it received in most news coverage.

Again, quoting Ars Technica

For their second gene, the authors look at the rather unfortunately named "growth arrest and DNA-damage-inducible, gamma," or GADD45G, which is involved in controlling how rapidly cells divide. Here, 550 bases of a three kilobase region are sufficient to drive a gene's expression in the brain (specifically the vental telencephalon and diencephalon), specifically in an area that generates inhibitory neurons. If GADD45G is doing its normal thing-slowing down cell division-then the loss of this DNA in humans could cut down its expression and increase brain cell division, contributing to the expansion of our mental resources.

Stanford University neurobiologist Robert Sapolsky was quoted in October as saying that what makes humans special is "the sheer quantity of available brain power – at least 300,000 brain cells for each neuron in a fruit fly brain". Estimates of numbers of neurons for humans (95 to 100 billion) testify to the importance of releasing inhibition on brain cell division.

Comparison of cortical neuron counts in humans (11.5 billion) and chimpanzees (6.2 billion) might be one way to measure the gap that opened up between us and our close cousins, and cortical neuron count is in fact the one measure that Gerhard Roth and Ursula Dicke found ranked humans highest in their 2005 study "Evolution of the Brain and Intelligence".

But what we do with our brains seems far less fascinating to reporters than what males of our species no longer do with their genitalia.

Genital vibrissae have been described as possibly serving in sexual competition to clear out the sperm deposited by other males in previous copulation.

But that is not what the press coverage emphasized. SyFy News even proposed that "sex would be a very different, not to mention painful, proposition" with penile vibrissae, something not suggested by findings on chimpanzee sexuality, which emphasize the enthusiastic participation by females in sexual intercourse. But then, that's because the reporters are left to fantasize about what sex with spines might have been like.

Injecting a little scholarship in this discussion would be helpful, and University of Wisconsin anthropologist John Hawks does just that in his blog entry about the new research. He points out that there is great variation among primates in how developed such spines are, and that they are not uniformly associated with fast copulation. In fact, he notes, the largest spines are found on gibbons-- which pair-bond for life (loosely, they're monogamous) and have "virtual sex marathon sessions lasting two hours or more".

It turns out that when you survey primates, chimpanzees already are at the reduced end of the penis spine spectrum-- only humans (with an entire lack of these structures) are less prickly.

So maybe we haven't advanced all that far, really, since we still seem to be thinking with something other than our brains.

Rosemary Joyce, Ph.D., is a professor of anthropology at UC Berkeley.


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