Pornography is often portrayed as one of the ills of today's society, evidence of modern moral decay brought to you by video cameras and broadband access.

As it turns out, modern times have got nothing on the past. Pornography existed long before video or even photography, and many researchers think evolution predisposed humans for visual arousal (It's a lot easier to pass on your genes if the sight of other naked humans turns you on, after all). Whichever way you slice it, the diversity of pornographic materials throughout history suggests that human beings have always been interested in images of sex. Lots and lots of sex.

"Sex has always played a super-important role for human beings and their relationships," said Seth Prosterman, a clinical sexologist and licensed therapist in San Francisco. "What people do sexually has always been a curiosity, and of interest." [RELATED: New Technologies Let Pornography Producers Stay On Top]

Ancient erotica

The definition of "pornography" is famously subjective. After all, one man's Venus de Milo is another man's masturbation aid. But researchers generally define the genre as material designed solely for sexual arousal, without further artistic merit.

By that standard, the first known erotic representations of humans might not be porn, in the traditional sense, at all. As early as 30,000 years ago, Paleolithic people were carving large-breasted, thick-thighed figurines of pregnant women out of stone and wood. Archaeologists doubt these "Venus figurines" were intended for sexual arousal. More likely, the figurines were religious icons or fertility symbols.

Fast-forwarding through history, the ancient Greeks and Romans created public sculptures and frescos depicting homosexuality, threesomes, fellatio and cunnilingus. In India during the second century, the Kama Sutra was half sex-manual, half relationship-handbook. The Moche people of ancient Peru painted sexual scenes on ceramic pottery, while the aristocracy in 16th century Japan was fond of erotic woodblock prints.

In the West, many early explicit materials were political, rather than exclusively pornographic, said Joseph Slade, a professor of media arts at Ohio University. French revolutionaries, in particular, satirized the aristocracy with sexually charged pamphlets. Even the Marquis de Sade's famously brutal and erotic works were part philosophical.

"They were political invectives disguised as pornography," Slade said.

Porn is born

In the 1800s, the idea of porn for porn's sake began to spread. Erotic novels had been in print since at least the mid-1600s in France (though being identified as the author of one meant a sure trip to jail), but the first full-length English-language pornographic novel, "Memoirs of a Woman of Pleasure," also known as "Fanny Hill" (Oxford University Press) wasn't published until 1748.

Despite the reserved public attitudes toward sex at the time, pornographic novels held little back. The author of "Fanny Hill" managed to cover bisexuality, voyeurism, group sex and masochism, among other topics. By 1888, the anonymous author of "My Secret Life" was writing about sex with words that would make a modern television censor squirm.

Technology drove innovation in the porn genre. In 1839, Louis Daguerre invented the daguerreotype, a primitive form of photography. Almost immediately, pornographers commandeered the new technology. The earliest surviving dirty daguerreotype — described by Slade in a 2006 paper as "depicting a rather solemn man gingerly inserting his penis into the vagina of an equally solemn and middle-aged woman" — is dated at 1846.

Video followed a similar path. By 1896, filmmakers in France were delving into the erotic with short, silent clips like "Le Coucher de la Marie," in which an actress performed a strip tease. Hard-core sex started showing up after 1900. These "stag films" were usually shown at all-male gatherings, and they were tame by today's standards, Slade said.

"They look like your grandparents having sex," he said. "They were quaint, but it was real intercourse."

Pornography gets popular

For a long time, stag films remained stagnant, both in content and in quality. Then, in the 1970s, changing social mores opened the door for public showing of explicit films. The Internet and the invention of the digital camera lowered the barriers to porn-making so low that entire websites are now devoted solely to non-professional videos.

The shift from publically viewed stag films to privately viewed rentals and internet downloads drove changes in the types of acts shown on-screen. Privacy, Slade said, made men more willing to watch fetish films depicting specific, sometimes odd, sexual behavior. A 1994 Carnegie Mellon study of early porn on computer Bulletin Board Systems (a precursor to the World Wide Web), found that 48 percent of downloads were far outside the sexual mainstream, depicting bestiality, incest and pedophilia. Less than 5 percent of downloads depicted vaginal sex. This could have been because magazines and pornographic films had traditional sex covered, and people went to their computers for images they couldn't find elsewhere, Slade suggested.

Today, porn is all over the internet, but the actual size of the industry is a mystery. No one keeps official records, and few studies have made a stab at the economics of porn. Adult Video News, a trade industry journal, made annual estimates of porn sales and rentals, along with sales of magazines and sex toys. In 2007, according to an AVN senior editor Mark Kernes, retail sales reached $6 billion a year. However, AVN's figures have been widely disputed. And even if they were reliable, the numbers wouldn't take into account all of the free amateur videos uploaded to sites like XTube or the photography site Flickr.

Regardless of how much money is being made, porn is attracting eyes. A 2008 study of 813 American university students found that 87 percent of men and 31 percent of women reported using pornography. The study was published in the Journal of Adolescent Research. And in 2009, University of Montreal researcher Simon Louis Lajeunesse made headlines when he announced that he had attempted a study on the impact of pornography on young men's sexuality, but he couldn't find a control group. In other words, good luck finding a man in his twenties who hasn't seen porn.

Controversial content

So what is all that porn doing to us? The question is a hornet's nest of controversy. While most mainstream Internet porn today doesn't rise to the level of those early Bulletin Board images, critics argue that competition between pornographers has led to an upswing in dominance and verbal abuse of women depicted in films made for straight men.

"They need to always put out something new, something enticing, to attract people," Chyng Sun, a professor of media studies at New York University and director of the film "The Price of Pleasure: Pornography, Sexuality and Relationships," told LiveScience. "The degradation, the aggression levels, that is something you can create, something a little bit new to offer to the audience."

By analyzing best-selling pornography films, Sun has found that physical and verbal aggression are present in 90 percent of mainstream porn scenes. Films directed by women are no less likely to contain aggression than films directed by men, she reported in a 2008 paper in the journal Psychology of Women Quarterly.

Sun argues that these aggressive images are harmful to people's sex lives and that they help cement negative stereotypes about women. Others disagree. Prosterman, the San Francisco sexologist, points out that research has failed to draw a clear link between porn and criminal sexual behavior. And, he said, porn is one way for people to explore their own sexual desires.

Debates about pornography have been ongoing since at least the Victorian era (no word on whether stone-age people hid the fertility statues under the mattress), and they're not likely to cease anytime soon. Nor are people likely to stop looking at pictures of other naked people.

"Most people like to have sex," said the AVN's Kernes. "A not-too-much-smaller segment of them like to watch other people have sex, and that is what the adult industry delivers."