evolution: Why milk? Most people in the
world aren’t genetically equipped to digest
dairy as adults. A minority of them—
including most northern Europeans— have
one of several mutations that allows their
bodies to break down the key sugar in milk,
lactose, beyond early childhood. That abil-
ity is called lactase persistence, after the
protein that processes lactose.
Until recently, geneticists thought that
dairying and the ability to drink milk must
have evolved together, but that didn’t prove
out when investigators went looking for
evidence. Ancient DNA samples from all
across Europe suggest that even in places
where lactase persistence is common to-
day, it didn’t appear until 3000 BCE—long
after people domesticated cattle and sheep
and started consuming dairy products. For
4,000 years prior to the mutation, Europe-
ans were making cheese and eating dairy
despite their lactose intolerance. Warinner
guessed that microbes may have been doing
the job of dairy digestion for them.
To prove it, she began looking for places
where the situation was similar. Mongolia
made sense: There’s evidence that herding
and domestication there dates back 5,000
years or more. But, Warinner says, direct
evidence of long-ago dairy consumption was
absent—until ancient calculus let her har-
vest it straight from the mouths of the dead.
Starting in 2016,
in her Jena lab,
Warinner and her
team scraped the
teeth of skele-
tons buried on
the steppes thou-
sands of years
ago and excavated by archaeologists in the
1990s. Samples about the size of a lentil
were enough to reveal proteins from cow,
goat, and sheep milk. By tapping the same
remains for ancient DNA, Warinner could
go one step further and show that they belonged to
people who lacked the gene to digest lactose— just
like modern Mongolians do.
Samples of the microbiome from in and around
today’s herders, Warinner realized, might offer a
way to understand how this was possible. Though
it’s estimated that just 1 in 20 Mongolians has the
mutation allowing them to digest milk, few places
in the world put as much emphasis on dairy. They
include it in festivities and offer it to spirits before
any big trip to ensure safety and success. Even
their metaphors are dairy-based: “The smell from
a wooden vessel filled with milk never goes away” is
the rough equivalent of “old habits die hard.”
Down the hall from the ancient DNA lab, thou-
sands of microbiome samples the team has collected
over the past two summers pack tall industrial freez-
ers. Chilled to minus 40 degrees F—colder, even,
than the Mongolian winter—the collection includes
everything from eezgi and byaslag to goat turds and
yak-udder swabs. Hundreds of the playing- card-size
plastic baggies new mothers use to freeze breast
milk contain raw, freshly squeezed camel, cow, goat,
reindeer, sheep, and yak milk.
Warinner’s initial hypothesis was that the
Mongolian herders— past and present— were using
lactose- eating microbes to break down their many
varieties of dairy, making it digestible. Commonly
known as fermentation, it’s the same bacteria-
assisted process that turns malt into beer, grapes
into wine, and flour into bubbly sourdough.
Fermentation is integral to just about every dairy
product in the Mongolian repertoire. While West-
ern cheeses also utilize the process, makers of
Parmesan, brie, and Camembert all rely on fungi andSPRING
2020PAGE 51“I never expected
to have a
career based
on something
people spend lots
of money trying
to get rid of.”
—CHRISTINA WARINNERS