The way we eat today— in large portions, away from home, with lots of energy- dense snacks in between meals, maybe with a superfood thrown in here or there— is only part of the reason why so many of us are overweight. Perhaps nothing has contributed more to our weight gain than the clash between our drinking habits and our biology.

Our genus, Homo, separated from other hominids between two million and three million years ago. Our species, Homo sapiens, appeared between two hundred thousand and one hundred thousand years ago. But it’s likely that until wine and beer were invented— about eleven thousand years ago— we didn’t drink anything other than water or breast milk. Our hunter- gatherer predecessors drank breast milk for the first few years of their lives, and after that only water.

But during the last century we’ve seen huge changes in the kinds of beverages consumed throughout the world— changes our biology isn’t prepared for. Imagine what would have happened to a hunter gatherer if his or her hunger was satiated by drinking water. He or she wouldn’t feel the need to forage for food, and wouldn’t have stored essential body fat for times of famine. It’s obvious that if water consumption alleviated our hunger pangs we, as a species, might not have survived. Another way of looking at this is that those who survived did not cut their food intake after drinking. And for us today the implications are clear— we drink a lot of our calories, but we don’t cut our food intake as a result. Recent studies confirm this. We can have a sixteen- ounce bottle of Coke or three beers before dinner— and not eat any less because of it.

The relatively recent addition of caloric beverages to our diet— shown in the illustration below— provides a sense of the role of beverages in the obesity pandemic. Compared with the millions of years during which we evolved into Homo sapiens, the span of time in which we’ve been consuming caloric beverages has been very short. Our genes take a long time to respond to such changes— we’re living in a rapidly changing world but with genes adapted to an earlier period.

Before we examine some of the new high- calorie beverages of the past few decades, let’s take a look at the brief history of the beverages that have been around for a while— water, milk, coffee, tea, beer, and wine— and the roles that they play in our lives today.

Water is the basis of life for all mammals. Before we developed agriculture, water was rarely contaminated. Then, with agriculture and subsequent urbanization, feces (animal and human) and other contaminants created health problems related to water consumption. Pathogens periodically led to outbreaks of cholera and dysentery; more recently, toxic chemicals have caused problems.

Today, bottled water has been the savior in countries where public sources of water are contaminated. Elsewhere, the rise of designer bottled waters has been a steady and healthful trend. Nonetheless, the disposal of these plastic water bottles has become a legitimate environmental concern, revealing how the positive goals of different groups can collide. Some municipalities are attempting to ban water bottles, and a New York Times editorial in August 2007 spoke against bottled water. But I think it’s inappropriate to single out one source of plastic bottles without condemning other— much less healthy— sources of plastic bottles.

People have been conditioned to carry bottled beverages around— and we need that to be water. It would be better for schools and other public and private facilities to sell water rather than caloric beverages, such as soft drinks, fruit juice, whole milk, energy drinks, and heavily sugared teas. Today, the need to have safe, clean, accessible water is, of course, not limited to the United States. Mexicans, for example, drink as much, if not more, soft drinks than we do. The typical portion sizes in Mexico are one and two liters, as opposed to our sixteen- ounce portions (about a half-liter). Consequently, obesity levels in Mexico are as high as those in the United States. Mexicans don’t have safe tap water, so they need to have access to bottled water as an alternative to sodas.

In a fast- paced society, we need people to think first of water— wherever they are. Too many Americans will drink Red Bull, Pepsi, or sugared vitamin waters— all with excessive calories. Drinking water, whether it comes from a faucet or a bottle, is an easy step we can all take toward better health. Bottled water should not be pitted against tap water, however. This is a false choice. We should talk about the essential need we all have to consume more water. And of course, we should push for the complete recycling of bottles and other containers. But we shouldn’t single out the healthiest beverage we have— we shouldn’t ignore other beverage containers and take- out food containers that are equally harmful to the environment but which contain less healthy beverages and foods. They are equal scourges to the Earth’s future— and their health benefits are clearly not as important as those of water.

In the West, we tend to think milk, from cows, sheep, or goats, is critical to life. However, animal milk is a recent addition to our diet. Historians haven’t been able to definitively determine when milk was first consumed. The earliest reliable record of dairy products is a tablet left by a Sumerian farmer in approximately 4000 BCE, which details the size of a herd and tells us of cheese and butter production. Of course, this doesn’t guarantee that the farmer drank milk.

The consumption of milk is very interesting from an evolutionary standpoint. The enzyme lactase is required to be able to digest the lactose in milk, and most of the people in the world lose this enzyme after they’re weaned from their mother’s breast. Genetic and archaeological research has found that the first known population of adults to be fully able to ingest milk was the Funnel Beaker culture, which flourished five thousand to six thousand years ago in central Europe. Today, the descendants of these people— the Swedes, Danes, and other north- central Europeans— have the enzyme lactase and thus can digest milk without the side effects experienced by people who are lactose intolerant. Geneticists have recently shown that as we move away from northern and central Europe toward southern Europe and the Middle East, people don’t possess the correct genetic makeup to break down lactose. However, some exciting studies have found several populations in eastern Africa that also possess the lactase gene. Complex genetic analyses suggest that the gene mutations in eastern Africa arose between 2,700 and 6,800 years ago. This is one of several recent discoveries that have revealed how rapidly our genes can change at the local level as a result of environmental pressures. It’s clear that northern and central Europeans and eastern Africans needed essential nutrients from milk and thus developed the ability to digest it. Americans who descended from northern and central European peoples are today some of the small subsets of the world population who have adequate amounts of the enzyme lactase. Most American blacks came from areas in Africa where milk was not consumed, and thus are lactose intolerant, as are many Asians who have immigrated to the United States.

Until the beginning of the nineteenth century, milk consumption was limited even for lactose- tolerant peoples. Before the French scientist Louis Pasteur invented in the 1860s the process we now call pasteurization, which uses heat to destroy microorganisms, milk could transmit diseases such as cholera, polio, anthrax, scarlet fever, bovine tuberculosis, and botulism. By the 1900s, milk had become much more popular in the United States and Europe, and producers made huge advances in handling, processing, and distributing pasteurized milk to places far from its source.

More recent developments in heat treatment, and the invention of a now widely used alternative to pasteurization called ultra- high temperature, or UHT— in which milk is raised to a temperature of 138°C (280°F) for about two seconds— have allowed the distribution and storage of milk without refrigeration. As UHT- treated milk does not require refrigeration, it has become increasingly used in Europe, Asia, and Africa. It’s very safe— but it doesn’t taste as good to the American palate as does fresh milk.

Milk consumption patterns are at present changing rapidly in the United States. We are tailoring milk to compete with sweetened beverages; there are an increasing number of new flavored and sweet milk products, including chocolate and strawberry milk, in stores and vending machines. These sugary and higher- fat milk products are no better than other sugared beverages; the adverse effect of sugar offsets the nutritional benefits of milk.