268 21 OCTOBER 2016 • VOL 354 ISSUE 6310 sciencemag.org SCIENCE

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TFor centuries, tropical rainforests were

seen as the very definition of wil-

derness, largely untouched by hu-

mans. Spanish conquistadors called

the Maya forest landscape a “green

hell.” As late as the 1970s, anthropo-

logists described Amazonian rainforests as

a “counterfeit paradise,” arguing that their

soil lacked the nutrients to sustain agri-

culture or complex human societies. “When

I was starting school, there was this idea

that the tropics were unlivable,” able to sup-

port only small groups of hunter-gatherers

at most, says archaeologist Anabel Ford

of the University of California in Santa

Barbara (UCSB).

It’s now becoming clear, however, that

human ancestors not only lived in the rain-

forest, but transformed it over tens of thou-

sands of years. At Pantropica, a conference

organized here this month by the Max Planck

Institute for the Science of Human History,

presentation after presentation highlighted

how prehistoric people burned the forest,

cleared it, farmed it, nurtured certain of its

tree species, and even built cities in it, leav-

ing lasting, if subtle, marks. “Tropical forests

are long-term documents of human action,”

says environmental archaeologist Chris Hunt

of Liverpool John Moores University in the

United Kingdom.

Although evidence of complex rainforest

societies, and even cities, began emerging

decades ago, the idea that human activity

transformed the forest itself was slow to

crystallize, because data were scarce. Acidic

rainforest soils eat away bone, and tools

and buildings made from organic materials

like wood and bamboo rot quickly in warm,

humid forests. Thick foliage obscures earth-

works and structures.

But archaeologists have found clever

ways to uncover ancient humans’ impact

on today’s jungles. For example, powerfully

fragrant ammonia from bird and bat guano

preserved organic remains for millennia at

Niah Cave on the island of Borneo, allowing

Hunt to analyze a pollen record from inside

the cave. He identified and counted the spe-

cies represented in the pollen, noting some

that sprout after forest fires. He found that

the fires began only after modern humans

arrived about 50,000 years ago, suggesting

that people torched the forest, probably to

make hunting easier and to promote useful

“edge” plants and animals like bearded pigs.

Then, about 11,000 years ago, pollen from

a nearby site showed abundant wild rice

and nonnative palms, showing that humans

were selecting and importing useful species

from other islands.

Thus this region, usually described as

pristine, was in fact managed millennia ago.

“It was shocking to me when I found out it

wasn’t primary rainforest,” Hunt says. Pol-

len records from Australia and from Papua

New Guinea’s Ivane Valley suggest a similar

pattern of burning after humans arrived.

Researchers at the conference argued that

slash-and-burn techniques, once thought to

be environmentally destructive, were actu-

ally sustainable, part of a centuries-long

forest management cycle practiced in many

tropical forests. Modern Maya “forest gar-

deners” in Belize and Guatemala farm burn

plots for a few years, then manage them

for second-growth species like fruit trees,

UCSB’s Ford says. She argues that a combina-

tion of arboriculture and cyclical agriculture

ARCHAEOLOGY

I N D E P T H

By Andrew Curry, in Jena, Germany

‘Green hell’ has long been home for humansBy burning trees and tending crops, prehistoric people left a lasting mark on rainforests

LiDAR images show a ceremonial pyramid

(circled) with rectilinear and circular elements, in

the pre-Columbian city of Angamuco in Mexico.

0 16080

Meters

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A century-old theoretical model of

magnetism is giving rise to a hybrid

computer, part classical and part

quantum, that may be capable of solv-

ing problems that overwhelm conven-

tional computers. The so-called Ising

machine, described in 100-bit and 2000-bit

versions in two reports this week in Science,

could tackle optimization problems that re-

quire finding the best solution among myriad

possibilities, such as predicting how a protein

will fold.

“This is an exciting development and

something that we want to pursue,” says Alán

Aspuru-Guzik, a theoretical chemist at Har-

vard University who has studied using Ising

machines to decipher protein folding. If the

technology can be scaled up just a bit further,

“it’s going to start competing with classical

computers,” he predicts. Others are more ambi-

valent about the technology’s prospects.

The machines take their name from the

Ising model, which was developed in 1920 by

German physicist Wilhelm Lenz and his stu-

dent Ernst Ising to explain how magnetism

arises. In a magnetic material, each atom acts

like a little magnet. At high temperatures,

the atoms point in random directions so that

their magnetism cancels out. Below a certain

temperature, the atoms point in the same di-

rection, magnetizing the material. The Ising

model aims to capture the transition from

randomness to order by envisioning a regu-

lar array of “spins” that point up or down and

randomly flip depending on the temperature.

In the original model, neighboring spins

interact so that they tend to point in the same

direction. Lenz and Ising hoped to show

how the spins would suddenly snap into the

same orientation as the temperature fell—

although the model turned out to be fiendishly

difficult to analyze.

Curiously, many optimization problems

can be mapped onto more general versions of

the Ising model, in which any two spins can

interact, not just neighbors, and the paired

enabled the ancient Maya to support large

populations in a sustainable way, and that

the practice of burning the forest had begun

with the first Native Americans in the area.

“They are regularly burning fields, creating

a cycle for reforestation,” Ford says. “It’s not

that they’re one with nature—they’d been re-

creating the forest across 8 millennia.”

The legacy of ancient burning or clearing

lingers in what may look like primeval for-

est today, Hunt says. Heavy concentrations

of certain tree species, particularly useful

or edible ones usually found near rivers but

appearing far from streams, may signal an-

cient human forest management. One recent

study estimated that 1.4% of Amazonian tree

species make up more than 50% of the rain-

forest, with palm trees and other useful spe-

cies dominating (Science, 18 October 2013,

p. 325); the authors suspect that human

hands crafted the unusual distribution.

Recently, researchers have found other

signs of human disturbance

in the Amazon: large patches

of fertile “black earth,” or

terra preta—carbon-rich soil

prized for farming today.

Some argued that ancient

people intentionally enriched

nutrient-poor rainforest

soils by burning vegetation

and working charcoal and

organic garbage into the

ground. But most terra preta

looks like places where people lived rather

than farmed, with ample shards of pot-

tery and bones, says archaeologist Manuel

Arroyo-Kalin of University College London.

He thinks ancient people could have boosted

soil nutrients accidentally, simply by tossing

their refuse in the same place for millennia.

New evidence, from the bones of early

rainforest people, suggests they did grow

surprising amounts of corn. At the confer-

ence, archaeologist Tiago Hermenegildo of

the University of Cambridge in the United

Kingdom presented data on carbon isotopes

in bone collagen from more than 120 pre-

historic individuals previously excavated

from Amazonian sites. Corn uses a specific

kind of photosynthesis, called C4 rare in rain-

forests, so carbon isotopes from people eat-

ing mostly corn look different from those

who eat a more mixed diet.

At two Amazonian sites, Hermenegildo

uncovered evidence of an unexpectedly corn-

heavy diet. In the Brazilian site of Hatahara

in the central Amazon, dated to between

750 and 1050 C.E., he detected the clear

presence of corn in the remains of people

thought to subsist mostly on fish and river

21 OCTOBER 2016 • VOL 354 ISSUE 6310 269SCIENCE sciencemag.org

NEWS

Pollen from Borneo’s Niah Cave shows that people

began to burn nearby rainforest 50,000 years ago.

Odd computer zips through knotty tasksHybrid Ising machines survey universe of possible solutions for best answer

COMPUTING

By Adrian Cho

turtles, suggesting that they planted corn

to eat or ferment. For farmers from the

Bolivian site of Llanos de Moxos, dated

to between about 600 and 1400 C.E., corn

was thought to be a secondary food source.

But there, too, Hermenegildo found a

strong signature from corn in human

bones, and also in Muscovy ducks from the

site. “They’re using a lot of corn,” he says,

“so much they’re feeding their animals

with it.”

Besides traces of agriculture, rainforests

also bear the imprint of ancient cities far

larger than anyone had guessed. Using laser

scanners mounted on aircraft, or light de-

tection and ranging (LiDAR), archaeologists

have shown that Angkor Wat in Cambodia

sprawled more than 3000 square kilometers

and could have housed more than 750,000

people at its height in the 11th century C.E.

In Honduras, archaeologist Chris Fisher

of Colorado State University in Fort Col-

lins helped use LiDAR to discover traces

of walls, earthen pyramids, and artificially

leveled plazas with stone pavement: a siz-

able “lost city” dating to be-

tween 1000 and 1400 C.E.,

deep in what was thought to

be untouched, impenetrable

jungle. He also directed a

LiDAR project that revealed

the extent of Angamuco, a

major urban center in Mex-

ico built by the Purépucha

culture that rivaled the great

cities of the Aztec empire

from 1000 C.E. until the ar-

rival of the Spanish around 1500 C.E. “We

can use these records to repopulate the

Americas,” Fisher says. “LiDAR enables

us to see” cities in places once thought to

be untouched.

The implications for modern rain-

forest management could be profound, as

the evidence suggests that human activ-

ity helped shape some of today’s supposed

wildernesses. “Humans are not by nature

incompatible with biological diversity,” says

anthropologist William Balée of Tulane Uni-

versity in New Orleans, Louisiana, who was

not at the Jena meeting.

But others say the evidence for wide-

spread human settlement in tropical for-

ests has been overstated. “There was such

a strong belief beforehand that you couldn’t

develop complex societies in the rain-

forest that when examples started to pop up

the field immediately went to the other ex-

treme,” cautions Crystal McMichael, a paleo-

botanist at the University of Amsterdam

who was not at the meeting. “In truth, it’s

probably somewhere in the middle.” j

Andrew Curry is a writer in Berlin.

“Tropical forests are long-term documents of human action.”Chris Hunt, Liverpool

John Moores University

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(6310), 268-269. [doi: 10.1126/science.354.6310.268]354Science Andrew Curry (October 20, 2016) 'Green hell' has long been home for humans

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