Skip to main content

How the human penis lost its spines

By Elizabeth Landau, CNN
Scientists are seeking to understand the underlying reasons why humans and chimpanzees have key differences.
Scientists are seeking to understand the underlying reasons why humans and chimpanzees have key differences.
  • Humans lack a switch in the genome that would "turn on" penile spines, sensory whiskers
  • Some scientists theorize that the absence of spines encourages pair bonding
  • Chimpanzees and mice both have penile spines and sensory whiskers

(CNN) -- You've read the headline, and it probably made you giggle. Go ahead. Get it out of your system. Then take a deep breath and consider how evolution affected a few specific body parts, and why.

Humans and chimpanzees share more than 97% of DNA, but there are some fairly obvious differences in appearance, behavior and intellect. Now, scientists are learning more than ever about what makes us uniquely human.

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.

Think of it like light bulbs and their switches, where the light bulbs are genes and the switches are these controlling DNA sequences. If there's no bulb, the switch can't turn the light on. Now imagine there's one bulb and five switches to turn it on at different times in different places. If you take one of the switches away, the bulb still works in the four other contexts, but not in the fifth.

This study looks at two particular switches. Bejerano and colleagues took the switch information from a chimpanzee's genome and essentially "hooked it up" to a reporter gene, a gene whose effects scientists can track as an organism develops. They injected the reporter gene in a mouse egg to see what the switch would do.

Planet of the walking apes?

They 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."

Humans have kept the "light bulb," however -- we have androgen receptors, but ours don't produce whiskers or penile spines, he said. Chimpanzees do have small sensory whiskers, not as externally obvious as in cats or mice, but we don't have them 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.

And humans are somewhat exceptional in this regard -- a lot of male primates have bumpy penises; mice, which are rodents, have them, too.

The basic idea of natural selection is that over many generations, an animal species loses some traits that are disadvantageous to survival or reproduction (or just don't do much, in some cases), and develops features that carry benefits. Traits that allow members of a species to have more children will eventually become more widespread, as they are passed on genetically to more and more offspring. In humans, this process takes place over hundreds of thousands to millions of years. So, there must be a good reason that the guys you know look different.

In fact, speculation abounds about what purpose the spines serve. One theory is that they are used in sperm competition; if the male's goal is to get his mate pregnant, he will want to take out her previous partner's sperm if she's recently had sex. The bumpy penis may be better for removing that sperm from the female, scientists theorize.

There's probably less debate about why humans reap benefits from having larger brains than chimpanzees, Kingsley said.

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

We know that Neanderthals had big brains like ours. They probably didn't have penile spines, either. There are traces of the Neanderthal genome in humans today, meaning Neanderthals and humans probably mated.

"The fact that Neanderthals were also missing penile spines is at least consistent with the idea that the mating structures of Neanderthals and modern humans were compatible enough that some interbreeding occurred," Kingsley said.