Yes, the title is a bit jokey (but attention grabbing, no?), but seriously... For most of Western history men wore pants and women wore much looser clothing in the form of robes, gowns, dresses, and skirts. The idea of pants as male gendered clothing was heavily baked into the culture all throughout Europe and other parts of the world. It's only begining in the 19th Century that we first start seeing women wearing pants, to the point it's notable as distinctly desgined for women and not used as a disguise.
From what I can tell, pants first became acceptable for women to wear under their skirts so they didn't show any of the naughty bits like their kneecaps when riding horses, and it seems to have grown from there, and even have been a major part of 19th Century Feminist activism. In fact, 'bloomers' are named after Amelia Bloomer, a suffragette who designed them herself. So, how did we get from dresses to denim?
The story of women’s trousers in the west could be said to start in the very early 18th century, when Lady Mary Wortley Montague traveled through and visited the Ottoman Empire as wife of the British ambassador. When she reached Adrianople in her English riding habit (very common traveling dress), she shocked the Turkish ladies with her close-fitting stays, while she was intrigued by their dress and eventually donned it herself, mostly to travel around the city without attracting attention:
The first part of my dress is a pair of drawers, very full, that reach to my shoes, and conceal the legs more modestly than your petticoats. They are of a thin rose-coloured damask, brocaded with silver flowers. My shoes are of a white kid leather, embroidered with gold. Over this hangs my smock, of a fine white silk gauze, edged with embroidery. This smock has wide sleeves hanging half-way down the arm, and is closed at the neck with a diamond button, but the shape and the colour of the bosom is very well to be distinguished through it. The antery is a waistcoat, made close to the shape, of white and gold damask, with very long sleeves falling back, and fringed with deep gold fringe and should have diamond or pearl buttons. My caftan, of the same stuff with my drawers, is a robe exactly fitted to my shape and reaching to my feet, with very long strait falling sleeves. Over this is the girdle, of about four fingers broad, which all that can afford it, have entirely of diamonds or other precious stones; those, who will not be at that expence, have it of exquisite embroidery on satin; but it must be fastened before with a clasp of diamonds. The curdee is a loose robe they throw off, or put on, according to the weather, being of a rich brocade (mine is green and gold); either lined with ermine or sables; the sleeves reach very little below the shoulders. The head-dress is composed of a cap called talpock, which is, in winter, of fine velvet embroidered with pearls or diamonds, and, in summer, of a light shining silver stuff. This is fixed on one side of the head, hanging a little way down with a gold tassel, and bound on, either with a circle of diamonds (as I have seen several) or a rich embroidered handkerchief. On the other side of the head, the hair is laid flat; and here the ladies are at liberty to shew their fancies; some putting flowers, others a plume of heron's feathers, and, in short, what they please […] The hair hangs at full length behind, divided into tresses braided with pearl or ribbon, which is always in great quantity.
Lady Mary wrote home of this, and when she returned to England she had herself painted in it on multiple occasions. She exchanged the “drawers” for petticoats, as did the many women who were painted in “Turkish costume” after her, but the fact that women outside Europe were wearing some form of trousers was still known.
Roughly 150 years later, Turkish drawers were topical again. (I should probably address the official portraits of Marie Antoinette and Catherine the Great in masculine riding habits with breeches, but these really were one-offs, done for their own purposes, and did not have influence on anyone else.) The Bloomer costume, worn by Amelia Jenks Bloomer (but technically invented by Elizabeth Smith Miller - Bloomer helped to spread the word through her newspaper, The Lily^(1)) and a few other dress reformers in the early 1850s, consisted of an ordinary bodice worn with a shortish skirt and full trousers based on those known to have been worn across the globe. It was tremendously unpopular with the mainstream, of course, because it was “masculinizing” women, but it didn’t knock it out of the park even with all of the feminists at the time, because it was not very attractive. The bodice was usually somewhat loose and wrinkled, and the skirt had no supports like the hoop, worn since the late 1850s, or even the many layers of petticoats worn before that. The women’s rights and dress reform movements dropped the outfit by the end of the decade, and dress reform later in the century would generally involve alterations of fashionable dress (gathered instead of darted, unboned corset waists instead of corsets, etc.) rather than entirely new, unfashionable garments.
However, there were other women (in France) around this time period who wore something very like the Bloomer costume: vivandières/cantinières, women who sold food and goods to soldiers to keep up morale. Due to the stresses of their jobs, they had been wearing cobbled-together civilian/military outfits for some time, often including pants, but some of these women who worked in/with the Army of Africa somewhat spontaneously created a uniform for themselves following the masculine one in the 1830s – blue tunic and red trousers – and on their return to France, the idea spread to women in other parts of the army, who created variations based on their regiments. The cantinières were essentially protected from public scorn by the fact that they were so beloved by the soldiers, and it likely helped that they don’t appear to have been particularly feminist outside of their desire to wear a jacket/skirt/trousers outfit: they viewed themselves as part of the military, and felt that that gave them the right to dress how they liked and to show their affiliation with their regiment. While cantinières seem to have been more of a widespread institution in France, they’re also known to have served in other countries’ armies, including in the United States during the Civil War. As far as I can tell, the popularity of these women and their presence in popular culture had little effect on the acceptability of women’s trousers outside of their specific context.
Bloomers did come to mainstream fashion … eventually. With the rise of bicycling, swimming, tennis, gymnastics, and outdoor sports in general, a whole new category of dress was devised: athletic costume. In some cases (such as for tennis), women just wore dresses made of sturdier, cleanable fabric, but the popularity of bicycling led to a certain amount of debate on acceptable clothing. While today it seems like a no-brainer – just wear pants – a lot of women in the 1890s were not enthusiastic about bifurcation. It seemed immodest, it could lead to unwanted attention, it might lead to you being prevented from entering a shop or restaurant. Some women did wear plain, undisguised bloomers (as I discussed in a previous answer on bicycling fashion), but the garment really just had another “moment” in the middle of the decade and then faded out again. Divided skirts that looked like skirts but functioned like trousers, or devices that could turn a full skirt into something more like trousers, were much more commonly used, but only in the context of sports.
So, how did pants get to be something women could just wear if they felt like it? The answer is basically “college students”. Early twentieth century collegiate women and men both tended toward a “sloppy” style to distinguish themselves from the grown-ups. For women, this meant no hats, no gloves, oversized sweaters without girdles underneath, ugly shoes … and pants and shorts. (To go on a tangent, I love the fact that both college boys and college girls wore shapeless/unflattering/dirty clothing to display their own identities, but at the same time, complained about each other – the men thought the women who dressed this way were hideous and unfeminine, and the women found men in unwashed corduroys and messy hair appalling.) Prior to World War II, colleges did not tend to regulate when and where pants could be worn, but in general student handbooks recommended that they be saved for very informal extracurricular activities, like bicycling, boating, or hiking, but not in classes or dining halls. Pants then became very commonly worn for “war work” in the 1940s, by both collegiate and non-collegiate women, and hung around afterward on campuses enough for school administrations to finally start making rules about them. These rules could be laxer or stricter depending on geography (with rural/Californian colleges tending to be more lax and colleges with city campuses being stricter), religious affiliation, etc. Around this time, shops and magazines tapered off advertising specifically to college students, and started to offer clothing for women of the college age, whether they were in college or in a career, which focused on making money by selling popular styles (including pants) rather than attempting to tell young college women what was appropriate for campus wear, which had been becoming more and more of a joke to the collegiates. Going forward into the late 1950s and 1960s, the rise of cool, unfashionable (off-campus) counterculture certainly drew from the “sloppy” look – including the use of casual pants – and then gradually became even more normalized.
If I can ask a follow-up question /aside, when and why did trousers become male coded to begin with?
I remember that in Europe the Romans and Greeks considered trousers a sign of barbarianness. Not sure about other parts of the world. Did the tabboo exist outside Europe before substantial contact with the West?