Smithsonian Magazine
You aren’t what you eat, exactly. But over many generations,
what we eat does shape our evolutionary path. “Diet,” says
anthropologist John Hawks, of the University of Wisconsin-Madison,
“has been a fundamental story throughout our evolutionary history. Over
the last million years there have been changes in human anatomy, teeth
and the skull, that we think are probably related to changes in diet.”
As our evolution continues, the crucial role of diet hasn’t gone away. Genetic studies show that humans are still evolving, with evidence of natural selection pressures on genes impacting everything from Alzheimer's disease to skin color to menstruation age. And what we eat today will influence the direction we will take tomorrow.
Got Milk?
When mammals are young, they produce an enzyme called
lactase to help digest the sugary lactose found in their mothers’ milk.
But once most mammals come of age, milk disappears from the menu. That
means enzymes to digest it are no longer needed, so adult mammals
typically stop producing them.
Thanks to recent evolution, however, some humans defy this trend.
Around two-thirds of adult humans are lactose intolerant or
have reduced lactose tolerance after infancy. But tolerance varies
dramatically depending on geography. Among some East Asian communities,
intolerance can reach 90 percent; people of West African, Arab, Greek,
Jewish and Italian descent are also especially prone to lactose
intolerance.
Northern Europeans, on the other hand, seem to love their
lactose—95 percent of them are tolerant, meaning they continue to
produce lactase as adults. And those numbers are increasing. “In at
least different five cases, populations have tweaked the gene
responsible for digesting that sugar so that it remains active in
adults,” Hawks says, noting it is most common among peoples in Europe,
the Middle East and East Africa.
Ancient DNA shows how recent this adult lactose tolerance is,
in evolutionary terms. Twenty-thousand years ago, it was non-existent.
As of 2018, about one-third of all adults have tolerance.
That lightning-fast evolutionary change
suggests that direct milk consumption must have provided a serious
survival advantage over peoples who had to ferment dairy into yogurt or
cheese. During fermentation, bacteria break down milk
sugars including lactase, turning them into acids and easing digestion
for those with lactose intolerance. Gone with those sugars, however, is a
good chunk of the food's caloric content.
Hawks explains why being able to digest milk would have been
such a boon in the past: “You're in a nutrition limited environment,
except you have cattle, or sheep, or goats, or camels, and that gives
you access to a high energy food that infants can digest but adults
can't,” he says. “What it does is allow people to get 30 percent more
calories out of milk, and you don't have the digestive issues that come
from milk consumption.”
A
recent genetic study found
that adult lactose tolerance was less common in Roman Britain than
today, meaning its evolution has continued throughout Europe's recorded
history.
These days, many humans have access to plentiful alternative
foods as well as lactose-free milk or lactase pills that help them
digest regular dairy. In other words, we can circumvent some impacts of
natural selection. That means traits like lactose tolerance might not
have the same direct impacts on survival or reproduction that they once
did—at least in some parts of the world.
“As far as we know, it makes no difference to your survival
and reproduction in Sweden if you can digest milk or not. If you're
eating out of a supermarket (your dairy tolerance doesn’t affect your
survival). But it still makes a difference in East Africa,” Hawks says.
Wheat, Starch and Alcohol
These days, it isn’t uncommon to find an entire grocery
store aisle devoted to gluten-free cookies, bread and crackers. Yet
trouble digesting gluten—
the main protein found in wheat—
is another relatively recent snag in human evolution. Humans didn't start storing and eating grains regularly until around 20,000 years ago, and wheat domestication didn't begin in earnest until about 10,000 years ago.
Since wheat and rye became a staple of human diets, however,
we've have had a relatively high frequency of celiac disease. “You look
at this and say how did it happen?” asks Hawks. “That's something that
natural selection shouldn't have done.”
The answer lies in our immune response. A system of genes
known as the human leukocyte antigens take part in the fight against
disease, and frequently produce new variations to battle ever-changing
infections. Unfortunately, for individuals with celiac disease, this
system mistakes the human digestive system for a disease and attacks the
lining of the gut.
Yet despite the obvious drawbacks of celiac disease, ongoing
evolution doesn't seem to be making it less frequent. The genetic variants behind celiac disease seem to be just as common now as they've
been since humans began eating wheat.
“This is a case where a selection that is probably about
disease and parasites has a side effect that produces celiac disease in a
small fraction of people. That's a trade-off that recent evolution has
left us and it wasn't an adaptation to diet—it was an adaptation in
spite of diet,” Hawks says. Unintended trade-offs are common in
evolution. For example, the genetic mutation to red blood cells that
helps humans survive malaria can also produce the deadly sickle cell
disease.
Other examples of our continuing evolution through diet are
intriguing but uncertain. For instance, Amylase is an enzyme that helps
saliva digest starch. Historically, agricultural peoples from West
Eurasia and Mesoamerica have more copies of the associated gene. Were
they selected to digest starches better? “That makes a compelling story
and it may be true. But biology is complicated and it's not totally
clear what's at work or how important it is,” Hawks says.
More than one-third of East Asians—Japanese, Chinese and Koreans—
have a flushing reaction when they metabolize alcohol,
because the process creates an excess of toxic acetaldehyde enzymes.
There's strong genetic evidence that this was selected recently, during
the last 20,000 years, Hawks notes.
Because its appearance in the genome may roughly coincide
with rice domestication 10,000 years ago, some researchers
suggest that it stopped people from over indulging in rice wine.
The timelines aren't precisely determined, however, for either the
mutation or rice domestication. It has also been suggested that
acetaldehyde
offered protection from parasites that were unable to stomach the toxin.
“It mattered in some way, to past populations, because it
wasn't common and now it is,” says Hawks. “It's a big change, but we
really don't know why.”
More Important Than We Think?
Even the color of human skin may be
shifting, at least in part, as a response to diet (other factors,
studies suggest, include sexual selection). The current diversity of human skin colors is a relatively recent development. The
standard hypothesis
focuses on the prevalence of UV rays at equatorial latitudes. Our
bodies need vitamin D, so our skin produces it when soaked by UV rays.
But too much UV can have detrimental effects, and darker skin pigments
are more effective at blocking them.
As humans moved into darker, colder latitudes, the idea
goes, their skin no longer needed protection from too much UV and
lightened so that it could produce more beneficial vitamin D with less
sunlight.
But DNA studies comparing modern Ukrainians with their prehistoric ancestors
show that European skin color has been changing over the past 5,000 years.
To explain this, another theory suggests that skin pigmentation could
have been under the influence of diet, when early farmers suffered from a
lack of vitamin D their hunter-gatherer ancestors once got from fish
and animal foods.
Nina Jablonski, a skin color researcher at Penn State University, told Science
that new research “provides evidence that loss of regular dietary
vitamin D as a result of the transition to a more strongly agricultural
lifestyle may have triggered” the evolution of lighter skin.
It's difficult to see evolution in action. But new
technologies like genome sequencing—and the computing power to crunch
massive piles of data—are making it possible to spot tiny genetic tweaks
that can add up over many generations to real evolutionary shifts.
Increasingly, databases of genetic information are also paired with
information like medical histories and environmental factors like diet,
which may allow scientists to observe the ways they interact.
Hakhamanesh Mostafavi,
an evolutionary biologist at Columbia University, authored one such
genome study that analyzed DNA from 215,000 people to try to see how we
continue to evolve over the span of just a generation or two.
“Obviously
our diet is radically changing today, so who knows what evolutionary
effect that may have,” Mostafavi says. “It may not necessarily have a
direct selection effect but it may interact with genes that control a
trait.”
Mostafavi's genetic research also revealed that some
variants that actually shorten human life, like one that prompts smokers
to increase their consumption above smoking norms, are still being
actively selected against.
“We see a direct effect of that gene on the survival of
humans today,” he explains. “And potentially you can imagine that diet
might have the same kind of effect. We have so many recent dietary
changes, like fast food for one example, and we just don't know yet what
effects they may or may not have.”
Fortunately, thanks to the work of scientists like Mostafavi and Hawks, it might not take 20,000 years to find out.
Brian Handwerk is a freelance writer based in Amherst, New Hampshire.
