Why are humans the only species on this planet that drink milk from another lactating species and well past two yrs of age? Is this natural? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Why are humans the only species that drink milk from another lactating species well past two years of age?

I’ll leave the question “is it natural” to the philosophers among us. Are we the only animals that do it? Probably.

The ability to digest milk sugars beyond infancy (any milk, never mind that of other species) is an evolutionary adaptation to our practice of animal husbandry, or the keeping livestock for food. “Lactase persistence,” as it’s called, offers sufficient survival advantages that it has evolved at least four times in different populations, all within the last 9,000 years. That roughly corresponds to the rise of agriculture and the shift from hunting and gathering to settlement. Even so, the majority of today’s human population is lactose-intolerant.

So what gives? It’s really very simple. The shift from nomadic hunting and gathering to the purposeful keeping of livestock allowed humans to take control of their food supply in a way that no other animal ever had. The ability to consume dairy products and eggs extended that control, by essentially allowing our ancestors to use living animals to warehouse food energy the same way we use refrigeration and other food storage technologies today. This was highly advantageous because it gave people the flexibility to feed off livestock while they were still growing to maturity (essentially converting grass and weeds, which we cannot digest, into dairy products we can) and then to feed on their meat at maturity. This was vastly more time and energy efficient than simply following prey animals through the wild and killing them when we could, or even keeping them exclusively for meat.

It also had the advantage of letting us stretch the supply of food through harsh conditions: during a cold winter, a drought, or a long expedition into new and hostile lands, we could supplement our food stores with dairy and eggs until our animals grew weak, then butcher them and live to find new animals when conditions improved.

The reason so much of the world’s population remains lactose intolerant is simply that their ancestors found more clement homelands, where such extremes of survival were more rare.

This question originally appeared on Quora. More questions on Quora:

* Evolutionary Theory: Why are scientists so defensive when someone says evolution is a theory?

* Human Biology: How did the human race manage to survive the past without vaccinations?

* Science: What are some clever uses of the placebo effect?

Photo Credit: ronstik/Getty Images