First published 3/10/11 on The Doctor Weighs In
This blog is posted by popular demand. Well, what I really mean, my popular spouse’s demand. She couldn’t wait for me to open my eyes this morning and greet me with, not her usual cheery “Good Morning” or “another day in Paradise”, but with the news that humans of the male persuasion are bereft of spiny penises! I thought she quit LSD decades ago. Anyway, she demanded an explanation, and I had to bow to “the popular’s” demand. So here goes.
What happened on the way to Homo sapiens?
Here is a way we all think: Q: “What makes us human?” A: Study the human genome and find genes that are unique to us, and are missing in chimpanzees –the species closest to us, and from which we diverged about 5-7 million years ago. But this is linear thinking.
Here is a “lateral thinking” way of looking at the problem: compare the human and chimp DNA and find the areas that chimps have and we don’t. In other words, maybe what we don’t have makes us human. Today’s Nature (March 10, 2011) reports on this kind of ingenious thinking. Two Stanford molecular geneticists, Bejerano and Kingsley, looked for areas of DNA that were present in Chimpanzees and are missing in humans. These are called deletion mutations. They found 510 such areas, but to their surprise most of them were not genes coding for specific traits. They were what is called non-coding DNA, areas between genes, that make up 98% of the total DNA; genes make up only 2% of our DNA. These endless stretches of non-coding DNA were dubbed ‘junk DNA’. Well, like all disparaging labels for things we don’t understand, it turned out to be wrong. We now know that non-coding DNA is actually very important –it controls the expression of the genes next to it. It is a very important part of the command and control part of an animal’s genome.
Of the 510 areas of deletions Bejerano and Kingsley chose 2 that looked interesting to study in depth. One is next to the AR gene, which codes for the androgen (or testosterone) receptor. The other from near GADD45G which is a gene involved in tumor suppression. They then inserted the sequences into mouse embryos to see what kind of functions they control. The first sequence (adjacent to the testosterone receptor) caused the mice to develop the hard spines that chimps have on their penises. The second sequence acted as a brake on the development of certain brain areas.
Comments on this research
As you might expect, this research attracted more than casual interest. Commentary is already flooding the scientific literature, as well as the popular press (an item of which greeted me as I barely woke up).
What is the function of those penile whiskers? And why did we lose them? Evolutionary biologists have theorized that they function as a broom, sweeping out sperm of other males who had paid a visit to the missus, thereby increasing the odds of their own DNA getting the prize. Maybe. Would that we could ask the female what she thinks.
I think the second deletion is potentially the more interesting finding. By removing the sequence that is inhibitory to the development of certain brain regions the human brain was free to develop the neocortex, the outer layer of neurons that carry much of our human traits, including cognition and meta- cognition. Chimps do have a neocortex, but it is greatly attenuated compared to ours. Apparently, deletion of DNA next to the GADD45G released the neocortex from its developmental shackles, and violá–a evolutionary star is born. Well, not quite; the emergence of Homo sapiens is going to be a lot more complex, but it’s a good start.
So, even in evolution you win some, and you lose some. But I would still opt for gaining a piece of brain tissue and lose a mop of whiskers on the penis. To begin with, I doubt the “eliminate the competition” theory if for no other reason than it sounds suspiciously anti-free market. But even if true, we still came out ahead. Lost your penile whiskers? No problem; our over-developed brain invented the French ticklers, thank you very much. There are no evolutionary theories about their utility, and no double blind studies testing their efficacy. But the testimonials are ecstatic, both figuratively and literally.