You certainly won't regret reading Breasts. The historical vignettes are droll and judiciously few. "The reasons why the breasts of women are on the chest," Henri de Mondeville wrote to the King of France in the 14th century, "whereas other animals more often have them elsewhere, are of three kinds. First, the chest is a noble, notable and chaste place and thus they can be decently shown. Secondly, warmed by the heart, they return their warmth to it so that this organ strengthens itself. The third reason applies only to big breasts which, by covering the chest, warm, cover and strengthen the stomach". It's like asking a three-year-old what he thinks of the sun.

There is much more uncharted territory in the world of breasts than there is mature debate, but where the latter has emerged, Florence Williams's book makes a solid, readable précis of it. The story begins with evolutionary biology: why do we have breasts in the first place? The sex-selection theory is that breasts signify health and youth, so a man who is prepared to survey them quite closely will be less likely to waste his seed on barren ground. This might explain why men supposedly prefer large breasts, since they show the sag; small breasts, less susceptible to gravity, might lead one egregiously to suppose a 45-year-old is 35, and that's 300 million swimmers you'll never get back, caveman. It's hard to falsify, especially since  unlike opposable thumbs  breasts leave no fossil record.

But there are clearly holes in the argument, since studies have shown no consistent male preference for one size over another. Furthermore, "if big, firm breasts tell a man that a woman is fertile and ready for sex, then why would her breasts be at their biggest and firmest when she's already pregnant or lactating?" Once Williams has asked that question, almost any competing theory becomes likelier than the breast as a seed-and-time-saving signal for men. The anthropologist Gillian Bentley has developed the "flat face" theory  "in order for newborns to get through our unusually narrow bipedal hips, their faces need to be flat. Flat faces and flat chests don't work well together." There's the camel theory, that breasts are fat deposits, which render a woman's fertility and lactation more resilient to a bout of famine. There's a theory about the specific suckling technique the breast engenders, which develops the muscles needed for speech. Williams becomes convinced  and has convinced me  that breasts evolved as a driver of female and infant survival, and men's obsession with them arrived not "in lockstep", but some time later. "Perhaps, all along, the breasts were calling the shots," she concludes.

Williams has set herself a template that works well, sometimes arrestingly well  there are many cultural assumptions made about the breast, and she deftly shows almost all of them to be wrong. Breast milk, and the narrative of breastfeeding, is one example: it is presented as pure, a means by which the mother-infant dyad can lock out all the toxins of modernity while they get on with their bonding. However, the composition of breastmilk is a little more complicated: sending hers to a German lab, Williams, an American, discovers that it has levels of flame-retardant (a class of chemicals known to accumulate in fat) 10 to 100 times higher than those of European women. The composition of the breast makes it a magnet for environmental toxins, so they "carry the burden of the mistakes we have made in our stewardship of the planet".

Williams's interviews are rangy, thorough and unhurried. She tracks down Jean Lindsey, the first woman to have a modern silicone implant, and unearths this lovely story: "They asked me, would I like to be in a study to have implants?" She'd "never even dwelled on" her breasts: "I was okay with what I had. After six children I guess they were kind of saggy. I said, 'you know, what I really want is to have my ears pinned back'  They said, 'Yeah, we'll fix your ears too.'" Not only is the voice right in the room with you, but there's a neat portrait of the manufacture of self-hate: how do you build up your silicone industry? How do you persuade people to hate a body part? You have to horse trade with their existing hatred of a different body part. Capitalism really is ingenious.

If anything lets this diverting book down, it's the relentlessly conversational tone, which strikes a note somewhere between a local news report ("To understand why our breasts so easily consort with molecules of bad repute, I needed to learn how cells work") and Woman's Own solidarity-schmaltz (Girls! Has he left his socks on the floor again? Has culture sought to denigrate your status by the attribution of false traits to your secondary sexual characteristics? Or maybe it just really likes breasts!) All the interviewees are described by their outfits and haircuts, at needless length, especially given that they were pretty well all wearing a button-down shirt and a tan jacket. Intended to make the book more accessible, the chattiness just interrupts the flow and makes it harder to read. But this is what your skim function is for: the meat of it is more than worthwhile.