With
7 billion people now inhabiting the planet — more than at any other time in
history — you’d think we’re having more babies than ever before. But a
millennia ago, birth rates were actually higher in the Southwest than they are
anywhere in the world today, researchers have found. Back then, the regional
population soared — and then crashed eight centuries later. Can modern-day humans
learn anything from the ancient Puebloans’ downfall?
Indeed
we can, says a team of anthropologists at Washington State University, who
report surprising population trends in the first millennial Southwest in as
study published today in Proceedings of the National Academy
of Sciences.
Tracking
the Boom
Sifting
through a century’s worth of data on thousands of human remains found at
hundreds of sites across the Four Corners region, the area where modern-day
Arizona, New Mexico, Colorado and Utah meet, the researchers crafted a detailed
chronology of what’s called the Neolithic Demographic Transition, when people
began eating more grain and less meat.
WSU
anthropology professor Tim Kohler and graduate student Kelsey Reese put the
start of the ancient population boom in the region at around 500 A.D. By that
time, people had adopted two of the hallmarks of civilization: settled
agriculture and food storage. They were growing mostly maize, which had become
a dietary staple, accounting for about 80 percent of calories. With plenty of
food to go around, crude birth rates — the number of newborns per 1,000 people
per year — increased steadily.
But
those halcyon days came to a sudden end around 1300. Within 30 years, the
northern Southwest was virtually uninhabited.
Mysterious
Decline
It’s
likely that Mother Nature played a significant role in the population’s
decline: A major drought that gripped the area in the mid-1100s was the
beginning of the end, Kohler says. Until about 1280, the farmers left and
conflicts raged across the northern Southwest. But the babies kept coming.
“They
didn’t slow down — birth rates were expanding right up to the depopulation,” he
says. “Why not limit growth? Maybe groups needed to be big to protect their
villages and fields.”
As
many as 40,000 people lived in the region in the mid-1200s, but suddenly, it
emptied out. No one is really sure why, but Kohler suspects the population
became too big to feed itself as the climate changed and growing conditions
worsened. And as people began to leave, the community deteriorated, making it
more difficult for anyone staying behind to fight off intruders and build and
maintain infrastructure, he says.
Interestingly,
people in the southern Southwest, who had developed irrigated agriculture, did
not experience the same population boom their dryland-farming neighbors to the
north did.
Consequences
of Growth
Kohler
says the Sonoran and Tonto people, who inhabited what we know today as southern
Arizona, probably didn’t have more children because it would have been
difficult to develop more farmland for them to use given limited surface water
supplies. And water from irrigation canals may have carried harmful
disease-causing bacteria and viruses, he adds. Groups to the north, who relied
on precipitation from the skies to water their crops, would have been able to
expand maize production into new areas as their populations grew — until the
drought came.
Whatever
caused the northern ancient Puebloans’ decline, Kohler says, their fate shows
that “population growth has its consequences.”
He
points to the warning of Thomas Malthus, who warned back in the 18th century
that humans would eventually become too numerous for Earth’s limited resources
to support.
Despite
huge advances in farming and food distribution systems, modern-day humans are
still vulnerable to catastrophic changes, and shouldn’t take for granted the
resources and climate we depend upon for survival, he says. Kohler: “We can
learn lessons from these people.”
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