Love in the Lab: Top 5 Historical Discoveries About Sex

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Are Plato and Aristotle talking about sex, perhaps?

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Introduction to Top 5 Historical Discoveries About Sex

The most esteemed thinkers of ancient Greece and Rome had sex on the brain -- a lot. And it isn't any wonder, really, considering that the human instinct to mate is as potent a drive as hunger pangs in the belly or a parched tongue in need of liquid relief. Moreover, back then in the BCs, the birds and the bees know-how that's taken for granted today remained shrouded in myth and mystery. But that doesn't mean men didn't take a stab at sussing out what all of the sexy fuss was about.

Fourth century B.C. thinker Aristotle, for instance, believed that the degree of heat a man produces during sex determines a baby's sex. If he's warmer than the woman, a boy it shall be; vice versa gets a girl. Greek contemporary and physician Hippocrates attributed female patients' mental health instability and anxiety to hysteria, or a wandering womb [source: Catonné]. And in "Natural History," written in the 1st century A.D., Roman writer Pliny warned that intercourse would taint lactating women's breast milk.

As science advanced from philosophy to empirical knowledge over time, groundbreaking discoveries throughout the centuries -- like the five on this list -- revealed the reproductive equipment inside the human body and even where monogamous love between two sexually active people comes from.

Human sperm, magnified 600 times.

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5: Leeuwenhoek Spies a Sea of Sperm

Anton von Leeuwenhoek didn't exactly know what he'd see when he took a gander at his own semen sample in a homemade microscope. It was 1677, and science had yet to figure out the ingredients of ejaculate, not to mention that von Leeuwenhoek got into microscopes on a lark as a hobby from his day job as a draper, or cloth salesman [source: RadioLab]. Peering into his newfangled microscope that magnified the fluid 300 times, von Leeuwenhoek spied "a multitude of live animalcules more than a million, having the size of a grain of sand," as he wrote to London's Royal Society in November 1677 [source: Karamanou].

Although with this observation, human spermatozoa had been revealed for the first time in history, it would still take a while to get the record straight about how sperm contribute to the baby-making process. Von Leeuwenhoek's contemporaries, in fact, imagined each sperm contained a tiny person, wishing and hoping to be delivered safely to a female womb [source: RadioLab].

The motherlode, literally.

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4: Wild Goose Chase for Female Egg

Since female eggs aren't nearly as observable at male semen, it took a staggering 150 years after Anton Von Leeuwenhoek saw sperm for Prussian-Estonian embryologist Dr. Karl Ernst von Baer to follow up with the human ovum [source: CBC News]. In the meantime, erroneous theories of embryonic development and differentiation abounded; some believed a fetuses' future hinged on which testicle produced the semen and where the fluid ended up in the uterus [source: Price].

Contemporary scientists had tirelessly investigated birds' eggs, questioning whether a similar mammalian seed existed, and William Cumberland Cruikshank had claimed the honor of identifying rabbit eggs in 1779 [source: Jansen and Mortimer]. However, von Baer's observations of dog embryos led him to correctly outline the development of the human egg and explain the ovum's cross-mammalian reproductive role in his landmark 1827 paper entitled "On the Genesis of the Ovum of Mammals and of Man" [source: von Baer].

Ironically, this egg pioneer still dismissed sperm as useless in fertilization, exemplifying the simmering uncertainty over how exactly the ovum and those eel-tailed swimmers interact [source: Bernard].

Necessary requirements: one sperm and one egg.

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3: Two Become One

When reproductive fertilization piqued the interest of German scientist Wilhelm August Oscar Hertwig in the early 1870s, the field was dominated by two conflicting -- and ultimately incorrect -- theories [source: Brind'Amour and Garcia]. One camp posited that the mechanical vibrations of so many wiggling sperm around an egg triggered embryonic development, like a clap light switching on in response to the auditory transmission. The other predominating view maintained that sperm deposit a chemical compound into the egg, providing the crucial ingredient to kick-start the process.

In 1872, Hertwig closely observed fertilization in a transparent species of sea urchin and witnessed the fusion of sperm and egg, effectively disproving his academic cohorts [source: Brind'Amour and Garcia]. Not only that, Hertwig also discovered that fertilization isn't a group effort on the part of the 150 million spermatozoa released in the average ejaculate. He realized that, instead, it only requires a single sperm to find its way inside the egg.

The menstrual cycle remained a mystery until the late 1920s.

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2: The Other Big O: Ovulation

The journey from the exit out of the male body to the outer gates of the female egg is like a dance marathon for sperm. When the starting gun sounds, 150 million swimmers head for the finish line, but only 15 percent of them on a good day are fit enough to make it past the uterus to the Fallopian tubes where the prized egg awaits [source: Angier]. By that time, only 1,000 or so have survived, and from there, just the strongest dozen reach the final destination [source: Freudenrich and Edmonds]. Not to mention the biggest wild card of all in the reproductive landscape: whether the ovaries have even released an egg into a Fallopian tube.

Before the late 1920s, doctors had modeled women's fertility on the mammalian estrous cycle, in which females go into heat, since the role of hormones was unknown at the time. In some species, if the cyclical window of fertility doesn't result in pregnancy, endometrial lining will shed, similar to having a period. By that logic, scientists presumed that menstruation and ovulation were likewise co-dependent, or that ovulation merely happened spontaneously [source: PBS]. The research team of Edgar Allen and Edward A. Doisy isolated and identified the roles of the hormones progesterone and estrogen in 1927 and 1928, respectively, by examining the ovarian tissue in female rats [source: Phillips]. By unearthing those key hormones, scientists were able to map out the menstrual cycle as well as ovulation.

How do sex and love relate?

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1: Piecing Together Sex and Love

With the biology of the birds and the bees firmly in place by the latter half of the 20th century, the compulsion to have sex with a monogamous partner was widely considered a matter of the metaphorical heart, rather than the physical body. Although sex and love intermingled in the bedroom, how they intermingled in the brain remained unclear until neuroendocrinologist C. Sue Carter began playing with a bunch of prairie voles in the early 1990s [source: Johnson].

The rodents were known for their monogamous, lifelong pair bonding -- a rarity in the animal kingdom, where promiscuity is the rule, rather than the exception. Carter correctly suspected the voles' fidelity had something to with how their brains processed oxytocin, a neurochemical in the brain associated mother-child bonding that's also released during sex [source: Carter]. Picking up Carter's cue in 1998, neurobiologist Thomas R. Insel analyzed prairie vole brains and noticed a striking distinction. Unlike their nonmonogamous montane vole cousins, the prairie voles oxytocin and dopamine receptors were clustered together, linking together the neurochemicals' effects of bonding and pleasure [source: Young, Wang and Insel]. That brain chemistry translates to a physiological incentive to not just find a sexual partner for a night, but rather one to grow old with. Further neurological research confirmed a similar oxytocin receptor arrangement in the human brain, thus explaining why the deep, passionate love between two people that Greek philosophers had long-pondered has persisted through the ages.

Lots More Information

Related ArticlesSources
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  • Bernard, Raymond W. "The Mysteries of Human Reproduction." Health Research Books. March 01, 1994. (Feb. 17, 2012) http://books.google.com/books?id=thbvifQ5DhwC&dq=1875+human+fertilization&source=gbs_navlinks_s
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