Milk Drinking Still a Mystery
The mutation for milk-drinking evolved in different parts of the world over the last 10,000 years as a result of strong natural selection, but why was it so advantageous?
The mutation for milk-drinking evolved independently in different parts of the world over the last 10,000 years as a result of strong natural selection, but why was it so advantageous?
Among the more momentous developments in human evolution was the ability to digest milk beyond early childhood.
Mutations that enabled lifelong milk drinking appeared independently in several parts of the world over the last 7,500 years, according to growing evidence. And those genes spread rapidly. Today, about a third of adults around the world can drink milk without stomach problems, a trait known as lactase persistence.
But why was milk drinking so advantageous to humankind?
A new study debunks one leading theory: that milk provided a valuable source of vitamin D, which would've helped people absorb its calcium.
Newly analyzed human skeletons from an ancient site in Spain show that the milk-drinking gene spread just as rapidly in that sun-drenched climate as it did in other places, suggesting that milk must have been beneficial there for some reason other than its vitamin D content.
"Throughout the years, I have heard so many evolutionary hypotheses about lactase persistence because they are so fun to coin," said Oddný Sverrisdóttir, an evolutionary biologist at the University of Uppsala in Sweden. "For decades now, people have hypothesized that it was because of lack of sunlight in the north of Europe that people would have had to supplement the lack of calcium and vitamin D by drinking milk."
"Now, looking at this picture from Spain," she said, "the calcium-assimilation hypothesis either didn't affect the evolution of lactase persistence at all, or other forces were there as well."
Sverrisdóttir has long been interested in how and why Europe's early farmers began drinking milk, so she was excited when she got her hands on well-preserved samples of skeletal remains from eight people who lived in northeastern Spain about 5,000 years ago. That was well after the milk-drinking mutation had appeared in northern Europe, and she was eager to find out if those ancient Spaniards were drinking milk, too. So the first thing she did was test their DNA for lactase persistence.
"I thought at least one would have the mutation," since so many of today's Spanish adults can drink milk without health consequences, Sverrisdóttir said. "None did."
To figure out whether the recent and rapid spread of lactase persistence in Spain was a fluke or if natural selection was at play, Sverrisdóttir and colleagues compared the mitochondrial DNA of modern Spaniards with the ancient samples. Mitochondrial DNA changes very slowly, making it ideal for tracing family trees over time.
And, the researchers report today in the journal Molecular Biology and Evolution, analyses showed that the ancient cave dwellers were indeed ancestors of people who live and frequently drink milk in Spain today.
Then, the team used computer simulations to see what kind of genetic shifts were necessary to get from a population where no one could digest milk past childhood to one where about a third of adults could drink it over the course of just 5,000 years.
Those simulations ruled out the possibility that the mutation reached its current level just by chance and instead showed that there was strong selection for it: Something gave people who had the milk-drinking gene a big advantage over people who didn't.
But if it wasn't the vitamin D that made milk so beneficial in sunny Spain, then what was it?
The new findings can't answer that question, but Sverrisdóttir has a favorite theory. Early farmers were eating cheese and yogurt long before they could drink milk because fermented dairy products are easier to digest. But in times of famine, when crops failed and all of the processed dairy foods had been consumed, people would have turned to milk out of desperation.
Those who happened to have a lurking mutation that helped them digest it would've thrived while those who were lactose intolerant would've ended up with life-threatening diarrhea.
"During normal times, if you were well-fed and you had diarrhea for days, it wouldn't matter much," Sverrisdóttir said. "But if you were already starving, this would mean the difference between life and death. People would have not lived long enough to get their genes into the next generation. This was the new super-food for people who could tolerate it."
It's still possible that milk got its value from vitamin D's calcium-absorbing powers in some places, said Pascale Gerbault, an evolutionary geneticist at University College London.
But the new study suggests that there may have been multiple reasons why milk was so pivotal in changing the course of human evolution and that those reasons varied through location and time.
"It makes sense that milk was a good food resource at different points in our evolution," Gerbault said. "But what were the situations that triggered these pressures? They're not quite known yet."
Extreme weather events, financial collapse, political unrest: With today's overabundance of apocalyptic worry, now is a good time to start thinking about what you’ll do if and when the bottom falls out. In a survival situation, shelter, fire and clean drinking water should be your top priorities, said Tom Brown, founder of Tom Brown Jr.'s Tracking, Nature and Wilderness Survival School. And, even though people can survive for up to three weeks without food, Brown said, extreme hunger can make you crazy. So it's worth stocking up on canned foods and other non-perishables. Read on to find out what else you can -- and really shouldn't -- eat when the cans run out.
DO: Pet food People end up eating pet food often enough -- and sales tend to go up during recessions -- that FDA standards require food made for animals to be suitable for humans to eat too, said Cody Lundin, founder and director of the Aboriginal Living Skills School in Prescott, Ariz. In an episode of the Discovery Channel show "Dual Survival", Lundin eats dog food cooked over a campfire -- and while he expresses hope that they'll catch raccoon for breakfast, he lived to tell the tale.
DO: Rodents It's easy to catch rats and other rodents, said Brown, author of "Tom Brown's Guide to City and Suburban Survival." Simply bury a five-gallon bucket in the ground up to its edges. Cover the mouth of the container with sticks and wood scraps, and wait for a startled mouse or chipmunk to scramble under the jumbled objects. The animal will fall right into your trap. Next, burn the hair off your prey, skin them, gut them and throw them into a stew pot with water and any grains, vegetables or flour you might have on hand. "Don't even bother filleting them or getting rid of the bones," Brown said. "Bone marrow is high in nutrition and protein."
DON'T: Leather During their infamous struggle against starvation, the Donner Party ate a wide variety of unappetizing objects, including leather, which is made from animal hides. Long ago, people used the tannins in oak tree bark to turn animal skins into leather, making it a safe food item. But modern leather products are tanned with chemicals that are surely poisonous, said Lundin, author of "When All Hell Breaks Loose: Stuff You Need to Survive When Disaster Strikes." Your belts may look as good as fruit roll-ups when you're really hungry. But it's best to leave them in the closet.
DO: Bugs Grasshoppers, cockroaches, ants, tarantulas: Virtually all insects are edible. Just make sure to cook them well enough to kill the wide variety of diseases they can carry, Brown said. You can even eat bees and scorpions as long as you remove their stingers first. One easy way to catch insects is to fill a sink with a little water and some food crumbs. Hungry bugs will go for the bait and either drown or get stuck in the tub. Ounce for ounce, Brown added, insects have up to four times more usable protein than other animals. Instead of a pound of beef, a quarter-pound grasshopper burger will do the same job.
DO: Weeds "Food plants grow everywhere," said John Kallas, director of Wild Food Adventures, an educational company, and author of "Edible Wild Plants: Wild Foods from Dirt to Plate." "All you need to do is go out in your backyard." You also need to build up some detailed knowledge about botany before all hell breaks loose because eating the wrong plants or the wrong parts of plants can kill you. Common vitamin-rich weeds include wild spinach, cattails, field mustard, garlic mustard, nipplewort and dandelions. No matter how hungry you are, Kallas warned, only eat a little amount of any one kind of vegetation at a time. "Dandelions have some vital chemicals that are great for you in small amounts, but too much will give diarrhea," he said. "That's what you don't want in a survival situation."
DON'T: Cardboard and Paper Cardboard boxes may seem appealing because they contain cellulose from wood pulp, which is used as a thickener, stabilizer and source of fiber in a variety of food products. And along with paper, cardboard can counter hunger pains by taking up space. But people cannot adequately digest the cellulose in cardboard and paper, Brown said. Also, many of these products are treated with chemicals that can be toxic.
DO: Acorns Like any nut, acorns can be delicious and filling, but you can't just pop them in your mouth like cashews. To make acorns edible, Brown advised, first take them out of their husks. Next, drop them in a pot of just-boiled water and let them steep for a couple hours. Drain and repeat this process two to four times until all of the bitter tannic acid is gone. At last, you can eat the acorns plain. You can roast them. Or you can grind them into flour that will accentuate your rodent stew. Play the "Dual Survival" challenge, featuring survival experts Cody Lundin and Dave Canterbury.