Everyone who has looked through portraits or pictures of people from earlier times notices how mediocre the women look compared to now. That's also true for hunter-gatherers -- it's rare to find a group whose women knock your socks off. Moreover, the progress seems to be very recent, like within the past 100 or 200 years at the furthest. Before then, it doesn't seem to matter if you look at women in 1800 or 1300; all are not very exciting to behold.
The first guess is that it has to do with better health, meaning better nutrition or access to medicine or prevention of infectious diseases. But none of those work because hunter-gatherers have great nutrition -- better than our nearly de facto vegan diet of the past 20 to 30 years. Plus they live in low-density groups so that most crowd diseases are unknown, though they still may suffer from those spread by winged insects or snails in their water source. And it's really not even a close call -- hunter-gatherer women are nothing close to your typical middle-class American woman in looks. If it were a matter of nutrition, effective medicine, and prevention of infection, then American women of the 1950s should be the ones who look the best, yet that doesn't seem true when flipping through old photo albums.
There could have also been natural selection for better looks, although that usually implicates a greater pathogen burden, as good looks signal good genes, and these are more important where just making it through childhood without being overwhelmed by bugs is crucial. Our pathogen burden has only lightened over the past 100 to 200 years, so that can't be why today's women in America look better than those of earlier times.
A simpler explanation is that women now put off marriage and having children much later than historically, and they have far fewer of them even when they begin. The idea is that women's good looks are designed to help her attract a mostly lifelong mate -- once that's done and she's started raising a family, who needs to preserve good looks if that's at all costly? Better to pull those resources out of maintaining good looks and invest them where they'll be of greater use for mothering. Her children's fitness will not depend on how cute she looks, and assuming her appearance doesn't go to hell too much too soon, the husband won't desert her either. So once motherhood begins, looks slowly (and then quickly) fade.
The number of children she has also matters: the more she has, the stronger the demand for mothering-related expertise rather than looking good. Also, the more she has, the more secure she feels that her husband isn't going to ditch her if her looks begin to fade.
Therefore, if women start to delay marriage and child-bearing, and if they have fewer kids once they start, their bodies are getting the signal that it's rough going in the mating market for some reason. After all, this late and so few kids, possibly none at all? Shoot, guess we can't turn off her looks just yet. We'll have to keep up whatever looks she has for awhile, rather than re-invest those resources to making her more nurturing and maternal. (Another characteristic of modern women is that they don't seem very motherly or wife-like even by age 30.)
So if we could compare 17 year-olds from England today vs. 500 years ago, we might not notice too much of a difference, since looks only start to fade once the body expects motherhood to begin. Yet we don't have many pictures of teenagers from that long ago, and more recent ones usually show them in some body-covering work uniform in some dim factory. Still, relying on paintings of adolescent and young-adult girls (Botticelli, Fragonard, and Bouguereau spring to mind), they seemed to look about as lovely as they do today.
However, most of our intuition about what women look like in various time periods is based more on women who are around 25 or older. For most of human history, they'd nearly all be mothers -- and even if they weren't, their bodies would reflect such a past, and would start withdrawing resources from good looks if they sensed it was too late, that she'd had her chance and blew it. After the demographic transition, a good deal of these women may not even be married, let alone be mothers, and even then they wouldn't have had many children. These signals all tell her body to keep up whatever looks she had before, at least for awhile longer in the hopes of finally snagging a husband and pumping out babies.
This idea would not only explain why women from roughly the same population have gotten much better looking after the demographic transition -- not simply "over time," since there seems little improvement before the transition -- but also why modern women look orders of magnitude better than hunter-gatherer women, despite the latter having equal or better nutrition and freedom from crowd diseases. And it would also explain why, fixing the place and time, higher-class women tend to look better than lower-class women. I remember little difference in middle or high school -- there were tons of cute girls who weren't from upper-middle backgrounds. Putting that aside, though, somehow when you see them in their 30s, their looks have decayed much more than have the looks of the upper-middle girls, compared to where each group started out as teenagers. Higher-status women are purer exemplars of the demographic transition, so their bodies are more likely to think that they've got to keep up the looks and not get all maternal, nurturing, and wife-like just yet.
The same idea applies to men, although they don't age as awfully as women do, so the effect is harder to notice. Why don't men age as quickly? Recall that a woman's Darwinian fitness doesn't depend much on her looks once she starts raising a family -- her kids don't respond to her based on how pretty she is, and provided she doesn't look like a scarecrow right away, the husband isn't going to ditch her. Those resources are re-invested into mothering traits. But the father's Darwinian success can still be boosted by impregnating lots of other females, either in a polygamously sanctioned way or just on the sly, and no young babe is going to sleep with a much older guy if he looks decrepit. Fathers don't father nearly as much as mothers mother, so the need to re-invest resources from looks into fathering traits isn't that strong to begin with. This also explains why men tend to have higher energy levels, higher sex drives, and are just overall more fun and exciting to be around as they age, relative to aging women. After all, if they weren't, they wouldn't be able to stay in the mating market and keep picking up younger girls.