Megan Scudellari summarizes new research:

Hundreds of deletions in non-coding DNA have helped sculpt human evolution, including an increase in brain size and the loss of sensory whiskers and penis spines, proposes a study published this week in Nature.

Jen Phillips goes deeper into the science with the study's lead author, Cory McLean:

Humans (like mice and chimps) have an androgen receptor gene that's necessary to develop penis bumps (penile spines, in scientific terms). Humans, at some point along the evolutionary road, lost the DNA needed to activate that receptor, and thus, do not have "spines" on our penises anymore. ... Human penile spines, if they existed, would probably be similar to chimps', which have a polka dot-esque distribution and are made of keratin, the same tough-yet-yielding substance that makes up our hair and nails.

The first time I heard "penile spines" I thought "ouch". But then, when McLean told me they were made out of keratin, I thought, hmmm, maybe the bumps increased female pleasure rather than diminished it. Think of ribbed condoms or bump-laden vibrators

Stephanie Pappas connects the dots:

Penile spines are exactly what they sound like: small spines on the head of the penis of many animals. Plenty of animals sport the spikes, including a type of beetle called the bean weevil whose hard, sharp spikes scar the female beetle's reproductive tract during sperm delivery. Many rodents, primates, such as marmosets, and even pythons whose Y-shaped hemipenis is often spined in order to grip the walls of the female's opening, known as a cloaca.

In species with penile spines, Kingsley said, females tend to mate with multiple males. Penile spines may have evolved to clear out a competitor's sperm – or to abrade the female's vagina, making her less likely to mate with others. Either way, Bejerano said, "the loss of the spines is most often seen in species that have gone more the monogamous way."