Humans share 99.9 percent of their DNA with chimps. What makes up the 0.01 percent difference?

According to a series of studies on the evolution of the male penis, a large portion of what separates us from monkeys is our giant, pulsating dicks. No joke.

The average adult gorilla's penis is only about 1.5 inches fully erect. Chimps have it a little better at around 3 inches. Of all the primates, humans are packing the most heat, with an average erect penis length or between 5-7 inches.

Sure, 7 million years of evolution separate us from apes … but how did we get so big while they stayed firmly rooted in micropenis-land?

A few theories:

1. Your great, great, great grandma liked 'em big

The prevailing theory as to why humans are the most well-endowed primate is that girls simply find a well-hung guy dreamier than not.

In 2013, an Australian study found that women who were shown computer-generated images of men with various body types and penis lengths generally rated the more-endowed men as better-looking. 

“Before we were wearing clothes, this is one way women could have judged a man’s attractiveness,” said study author Brian Mautz, Ph.D.

However, even though our female ancestors mated with increasingly larger dicks over the course of evolution for the sheer fact that they look cool, there's paradoxically still a large portion of women who don't give any shits whatsoever about penis size.

“Although women saw a larger penis as more attractive on average, there are women who have no preference at all,” Mautz said.

So, inchworm cocks of the Internet, rest easy. Women find many other physical traits as, or more, attractive than a monster schlong. In fact, broad shoulders and tall height were both better predictors of attractiveness than penis size in the Australian study, meaning the size you should be worrying about is your vertical one. And that's what these are for:

2. Larger penises increase the chances of vaginal orgasm

Whether we're talking about a thousand years ago or right this second, women will always select men who can make them come as mates. Two reasons for that …

First and most obviously, orgasm rules. Ever had one? Try it.

Second, and more importantly, the contractions from a vaginal orgasm help propel semen towards the cervix, increasing the chance that sperm will reach an egg and fertilize it. Thus, men who can make women come increase a woman's chance of fertilization. This has important implications for evolution because men with large penises have a better chance of passing on their well-hung genes to their spawn.

Several studies have attempted to explore this. In one, Scottish researchers found that out of a sample of 300 women, approximately 50 percent experience vaginal orgasms during the course of a month …  and one third of them preferred large dicks that could reach deeper inside them during intercourse.

A University of Texas study also found that 90 percent of women said a thicker penis was the most important thing when it came to their sexual satisfaction.

Of course, women have perfectly glorious orgasms with smaller or average penises too, and less-endowed men aren't technically less fertile than well-hung ones. It's just that women's orgasmic preference for big cock has lead to men evolving bigger members.

3. Mushroom tips help ensure you are the father

The reason why human boners are so super-massive could also be purely mechanical. Some scientists think that the mushroom tip of the penis, which is unique among primates, may function to scrape out the semen of a lady's previous sexual partners during intercourse so that their own semen has a better chance for fertilization.

Researchers at the University of Albany tested this idea by using differently shaped dildos on willing volunteers. The anatomically correct ones with a larger head managed to sweep out 91 percent of previous semen, compared to just 35 percent in the headless models. Dying to know how we can volunteer for this … anyway …

“By filling the vagina, a longer penis would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity,” said study author Gordon Gallup, Ph.D., an evolutionary psychologist at the University of Albany.

So, massive dong = Dad status. Dads pass on massive dong genes, and the cycle continues.

All this to say that size does matter, but only in the sense that large penises are, you know, responsible for humanity as it is today. Without well-endowed men being naturally selected for throughout evolution, the world might look like a very different place … and you might need a microscope to see it. But, don't worry if you're not packing a baseball bat-sized object in your pants; a California State University study found that 85 percent of women are perfectly happy with their man's peen. You should be too; at least it's bigger than an ape's. Good work, kid.