Let me be honest: when I started doing paleoanthropology, I really did not expect I'd be talking about Neandertal penises.
And yet, here I am. Cory McLean and colleagues  combine a straightforward genomic analysis of human-specific deletions with a couple of transgenic mice, and take us straight to penis spines.
You see, most primates, and indeed many mammals, have at least some spines on their penises. "Spine" means more or less what you would expect: little projections that are covered in hard material, generally keratin, curving toward the base of the penis. These spines are sometimes called "horny papillae."
No, I cannot make this stuff up.
The morphology of these spines varies among primates. They overlie sensory receptors, and they intensify or enhance sensations accompanying intromission of the penis. Like a KY commercial, except they don't enhance sensations for the female. The net effect in some species is to reduce how long it takes the male to ejaculate. For example, a 1991 paper  by A. F. Dixson...
No, I cannot make this stuff up.
...removed the penile spines of several male marmosets, finding that they took twice as long to achieve penile intromission after starting pelvic thrusts. Of course, "twice as long" in marmosets only means 15 seconds. The spineless males took 2 seconds to ejaculate, compared to only 1.73 seconds for those who had a "sham surgery" -- that is, they got the same depilatory spine-removal procedure without the active ingredient. That's some evidence in favor of the idea that losing penile spines might be related to longer coital duration.
But penile spines don't always mean fast sex. Galagos have penises covered in long hook-like spines, which they use in virtual sex marathon sessions lasting two hours or more. Prosimians tend to have much more elaborated spines, in contrast chimpanzees' spicules are comparatively minor -- in a broad comparison across primates, Harcourt and Gardiner  rated chimpanzees along with humans as having insignificant penile spinosity.
Let me just say that the comparative data don't convince me of an adaptive model for loss of penile spines in humans. Evidence from mutilated monkeys is not all that persuasive. I mean, really, how fast do you think you would manage after the "operation"? More important, the differences among hominoids run against the hypothesis -- gibbons have the spiniest penises among the apes, despite their monogamous, pair-bonded social habits.
And I'll pause to savor the surreality: I'm here making value judgments about genital cacti.
One thing that is definitely well-known about these penile spines is that their development depends on testosterone. Castrated monkeys do not develop the characteristic spines, and they lose them if already present. The androgen receptor (AR) locus is surrounded by promoter/enhancer sequences that are tissue-specific, capable of being flipped on or off as development proceeds within different parts of the body.
Within this system, the genetics in humans and chimpanzees are simple: A long (60 kilobase) deletion of DNA in the human lineage has knocked out a 5 kb conserved region that enhances AR. That enhancer is specific to the follicles around the developing facial whiskers (vibrissae) and in the skin layers of the penis. This specificity was discovered in transgenic mice, in which a reporter gene is inserted with the enhancer, and embryos display expression of the reporter wherever the enhancer is active. Very straightforward, very cool science.
One more thing: The chimpanzee version can drive expression when implanted into transgenic human foreskin fibroblasts. That indicates that the overall genetic system to make penile spines is still there lurking in our genomes. If we could turn on the gene at the right time, replacing the function of the enhancer, we can still grow penile spines.
Just saying -- there may be a market there. Maybe the "male enhancement" companies will hit that next. I can only imagine what the wrapper on the NASCAR circuit will look like. OK, I know, don't encourage them. It's bad enough that we have labs full of foreskin tissue with chimpanzee genes floating around.
I couldn't make this stuff up if I tried.
Finding the deletion was straightforward genomics: They scraped the human genome for parts missing from chimpanzees and macaques, and then extracted from that set all deletions that included sequence conserved in other mammals. Others have done similar comparisons for conservation and human-specific changes; this is a clever twist on the same problem. It does fit an ongoing theme -- many essential aspects of humans may involve the loss of genes or functionality from our ape ancestors.
Ok, so where do Neandertals fit in? They have the sequence deletion just like the rest of us do. If that deletion rules out chimpanzee-like spiky penises, then Neandertals could glide like the rest of us.
All in all, it's a nice short paper, and very straightforward. The only questionable part to me is the social model. The genetics and expression data are solid.
Speaking of Neandertals and the androgen receptor (AR) locus, my genome appears to have a Neandertal-derived haplotype across that gene. I'll expose this fact at greater length later, but I thought it worth sharing that the current paper is not the end of the story. Neandertals may not have had penis spines, but some functional polymorphisms in testosterone response might still have come into our population from them or other ancient people.
UPDATE (2011-03-11): Eric Michael Johnson gives us the real dirt on this story ("Penis spines, pearly papules and Pope Benedict's balls"). He points out the relatively small extent of these features of the chimpanzee penis compared to other primates, and adds detail about the lack of association between their presence and sexual system in hominoids.
He also reveals a shocking fact: a fairly large fraction of men still have the chimpanzee-like pearly papules.
Scicurious also takes on the topic "Friday Weird Science: Penis Spines, what are they REALLY?", reviewing the original Osman Hill study of chimpanzee penis morphology. I think the Nature paper is very misleading in its use of galago illustrations for these spines, the chimpanzee version is comparatively minor.
- Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature [Internet]. 2011 ;471:216–219. Available from: http://dx.doi.org/10.1038/nature09774
- . Penile spines affect copulatory behaviour in a primate (Callithrix jacchus). Physiology & behavior [Internet]. 1991 ;49:557–562. Available from: http://view.ncbi.nlm.nih.gov/pubmed/2062934
- . Sexual Selection and Genital Anatomy of Male Primates. Proceedings: Biological Sciences [Internet]. 1994 ;255. Available from: http://dx.doi.org/10.2307/49837