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Human Nature Decoded–No Whiskers, and No Penile Spines! March 10, 2011

Posted by Nina Rosenstand in Nina Rosenstand's Posts, Philosophy of Human Nature, Science.
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I’ve written a book about the Philosophy of Human Nature, The Human Condition. I’ve given talks, and written papers and blogs, and tweeted about the subject. I’ve been devouring every morsel of information about human evolution I could get my hands on since I was 13 years old. I teach two classes per year focusing on Phil of Human Nature. And what do I read this morning in the CNN online Health section? Brand new research about a significant difference between apes and humans: Ape males have penile spines and human males don’t. First thing I thought was, “And a good thing, too!” Now wipe the smirks off your faces—this finding turns out to have seriously philosophical consequences:

We know that humans have larger brains and, within the brain, a larger angular gyrus, a region associated with abstract concepts. Also, male chimpanzees have smaller penises than humans, and their penises have spines. Not like porcupine needles or anything, but small pointy projections on the surface that basically make the organ bumpy.

Gill Bejerano, a biologist at Stanford University School of Medicine, and colleagues wanted to further investigate why humans and chimpanzees have such differences. They analyzed the genomes of humans and closely related primates and discovered more than 500 regulatory regions — sequences in the genome responsible for controlling genes — that chimpanzees and other mammals have, but humans do not. In other words, they are making a list of DNA that has been lost from the human genome during millions of years of evolution. Results from their study are published in the journal Nature.

…[The scientists] found that in one case, a switch that had been lost in humans normally turns on an androgen receptor at the sites where sensory whiskers develop on the face and spines develop on the penis. Mice and many other animals have both of these characteristics, and humans do not.

“This switch controls the expression of a key gene that’s required for the formation of these structures,” said David Kingsley, a study co-author at Stanford University. “If you kill that gene — smash the lightbulb — which has been done previously in mouse genetics, the whiskers don’t grow as much and the penile spines fail to form at all.”

To sum up: Humans lack a switch in the genome that would “turn on” penile spines and sensory whiskers. But our primate relatives, such as chimpanzees, have the switch, and that’s why they differ from us in these two ways.

So what does it matter, other than, presumably,  a different female sexual experience, and a lack of ability to sense things a few inches from our faces?

The other “switch” examined in this study probably has to do with the expansion of brain regions in humans. Kingsley and colleagues believe they have found a place in their genome comparisons where the loss of DNA in humans may have contributed to the gain of neurons in the brain. That is to say, when humans evolved without a particular switch, the absence of that switch allowed the brain to grow further.

The earliest human ancestors probably had sensory whiskers, penile spines and small brains, Kingsley said. Evolutionary events to remove the whiskers and spines and enlarge the brain probably took place after humans and chimpanzees split apart as separate species (Some 5 million to 7 million years ago), but before Neanderthals and humans diverged (about 600,000 years ago), Kingsley said.

So there you have it: We were on the fast track to becoming Homo Sapiens when the switch for sensory whiskers and penile spines was turned off! Make of that what you want, in this Women’s History Month! For me, that story made my day!

(I thought of calling this blog post “Of Mice and Men”, but that would be unfair to Steinbeck.)