This isn't so much a fascinating belief as it is a new scientific truth: in the history of nakedness and why we started wearing clothes, some of the most interesting work has been done via the study of lice, and specifically how they evolved to live on our bodies. In 2011, scientists managed to sequence the precise moment in time when head lice evolved into body lice: it was 170,000 years ago, and they reckon that must have been the first time in history that humans regularly wore clothing of any kind, as the lice needed a safe environment to encourage them to evolve.
Researcher Becky Wragg Sykes explained in The Guardian that these new lice likely spread between the three kinds of humans around at the time, and that the history of clothing rapidly developed from there: we have evidence of animal skins being tanned for clothing from 100,000 years ago, and jewelry started turning up 75,000 years ago. Why did we suddenly decide to get clothed? Likely temperature: the development of body lice coincided with an Ice Age. Brrr.
We've got the Greek writer Plutarch to thank for the following: his reflections on the state of Sparta, and its very unique legal system, give us an insight into one of the most peculiar city-states in world history (if he can be believed). For the Spartans, Plutarch said, nakedness had a couple of societal functions, and they ricocheted madly between awesome and terrible.
Young men and women were encouraged to parade naked and go to feasts without clothes on, and sing lots of songs about war glory to inspire the male warriors. "Nor was there anything shameful in this nakedness of the young women," Plutarch added; "modesty attended them, and all wantonness was excluded. It taught them simplicity and a care for good health, and gave them some taste of higher feelings, admitted as they thus were to the field of noble action and glory." But it wasn't all good-natured military nudity hijinks. Unmarried men were ordered to "mark naked themselves around the marketplace, singing as they went a certain song to their own disgrace". In winter. Yeesh.
Hesiod was an ancient Greek poet with a few interesting opinions about how to conduct yourself while farming, including the apparent belief that you should inspire fruitfulness in your crop by doing all the hard work without any clothes on. "Sow naked, and plough naked, and harvest naked," he recommends, "if you wish to bring in all Demeter's fruits in due season." Demeter was the goddess of seasons and harvests, and why exactly she'd be fond of a bunch of naked farmers running around doing their work without proper health and safety regulation clothing remains unclear; but his advice is pretty unequivocal. It wasn't just a poet being fanciful, either; we have artefacts from ancient Greece devoted to Demeter in which men are shown ploughing without a stitch.
One of the most famous incidences of nudity in cultural history is, of course, the Greek athletic tradition of competing, showing off and doing sports naked as men; but it wasn't necessarily all about the glory of the male form. The origins of the nakedness in ancient Greek sport, particularly at the Olympics, stems from a legend about an athlete called Orsippus of Megara, but the myth goes one of two ways.
In one, Orsippus is competing in the Games, loses his loincloth, and wins his race, prompting copycats to discard theirs in pursuit of victory and starting a centuries-long trend. In another, though, Orsippus's loincloth falls as he's sprinting, gets caught under his feet, and causes him to fall, crack his head open, and die. The loss of the loincloth may have been for practical reasons, but possibly not the ones you think.
The modern Western ideas about "naughty" areas of the nude body are pretty definitive: genitalia, buttocks and, in women, nipples and breasts. It may surprise you to know that these particular ideas aren't embedded in European cultural history. In fact, for certain periods in European fashion, the naked female breast was all the rage as a completely non-shocking addition to an outfit.
Queen Elizabeth I may have had an outfit that showed the entirety of her breasts, including a slice of flesh underneath, depending on how you interpret a letter from a slightly shocked ambassador at her court; 18th century French fashion was all about the sudden coquettish exposure of a nipple; and we have records of fashionable court dresses for royal women of the 17th century that exposed the entire breast, but covered all the arms and legs to prevent even a sliver of flesh being seen.