THE FIRST Peruvians lived by fishing. But as sea levels rose, at the end of
the last ice age, a splinter group moved inland and began to farm the highlands
and river valleys. Then, 5800 years ago, the two separate cultures started to
merge and become more sophisticated. 鈥淲e begin to see鈥攏ear river
mouths鈥攍arger sites and architecture, with a mixture of wild plants,
domesticated plants and marine resources,鈥� says Dan Sandweiss. These coastal
dwellers also began building temple mounds, their first monumental sacred
places.
鈥淥ne would naturally expect that there were changes, as people moved,
populations grew and technology changed,鈥� says Sandweiss, an anthropologist at
the University of Maine in Orono. But for Stone Age cultures, such changes
normally came at a glacial pace by modern standards. So why was there such a
cultural growth-spurt in Peru? And why does this correspond to a peak in
cultural development worldwide around 5800 years ago? Sandweiss and others
believe that climate change was the spur and they point to new evidence that the
date coincides with the emergence of the El Ni帽o/Southern Oscillation
cycle in the form that still has such an impact today.
Such thinking marks a sea change for both archaeologists and climatologists.
Until just a few years ago, they assumed that climate has been stable since the
ice age ended more than 10 000 years ago. But recent studies revealed strong
climate fluctuations starting about 6000 to 3000 years ago as early
civilisations emerged from the Stone Age. Now Sandweiss and his colleague Kirk
Maasch are coordinating efforts to correlate global climate change with cultural
events in that crucial era, when farming matured and civilisation first emerged.
The cultural leap around 5800 years ago, when El Ni帽o began, provides the
strongest evidence so far that these phenomena are linked. In addition, Harvey
Weiss of Yale University has traced the global impact of a 300-year drought that
toppled civilisations some 4200 years ago (see 鈥淭he fall of Tell Leilan鈥�).
No one claims climate alone shaped ancient history, and many key dates remain
uncertain, but the links are surely there and researchers say that, given
current global warming, we ignore this at our peril.
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Survival strategy
Global climate was generally stable when agriculture emerged as the Earth
recovered from the last ice age. Temperatures were a little warmer than today
during the 鈥渁ltithermal鈥� or 鈥渃limatic optimum鈥� between 9000 and 6000 years ago.
Seas rose to near their present level as meltwater flowed in from the remnants
of the great ice sheets that had covered northern Europe and North America. Some
now-arid zones were substantially wetter than today. Summer rains brought life
to parts of the Sahara, and lake levels were higher in southeastern Australia.
Other places were drier. Lakes in the Chilean Andes, for example, dried up about
8000 years ago and did not reappear until about 3700 years ago.
Cultures changed slowly during that warm era. Farmers refined their
practices, gradually settling in villages and making tools from bone, stone,
wood and ceramics. For centuries there was little change in the stone tools and
pottery that archaeologists use as markers to identify ancient cultures. Then
change began to accelerate. Some anthropologists have suggested that Stone Age
technology had reached a crucial threshold for progress. But the timing and
nature of the change has convinced Sandweiss, Maasch and others that the onset
of El Ni帽o 5800 years ago pushed our ancestors to innovate so they could
survive.
Today, El Ni帽o episodes occur every two to seven years and have their
strongest effects along coastal South America. In normal years, cold deep water
rich in nutrients rises to the surface off Peru as winds from the coast push
warmer surface waters offshore and ocean circulation piles up water on the
Australian side of the Pacific. This leaves the South American coast dry while
monsoon rain deluges southern Asia, northern Australia and a corner of Africa.
El Ni帽o shifts the circulation pattern, depleting fish stocks in the
eastern Pacific, bringing heavy rains to the coast of northern Peru and causing
the monsoons to fail in Australia and Asia.
Historical records trace this phenomenon back hundreds of years and now
environmental markers are beginning to reveal El Ni帽o鈥檚 emergence. Corals
which grew 6600 years ago in the western Pacific lack the annual growth
variations that the oscillation has left in their modern counterparts. But by
5800 years ago sediments, such as flood deposits off the Peruvian coast, are
starting to show the telltale signs of El Ni帽o episodes, says Sandweiss.
At first, these disruptions seem to have come every 40 or 50 years. It was a
couple of thousand years before they became as frequent as they are today.
Melanie Riedinger of Northeastern Illinois University in Chicago has also found
this pattern on the Gal谩pagos Islands. There heavy rains during El
Ni帽o episodes have left distinct layers in lake sediments, which begin
5800 years ago and become more frequent around 3000 years ago. Don Rodbell of
Union College in Schenectady, New York, has found a similar pattern of
striations in sediments from a lake in Ecuador.
On the other side of the Pacific, evidence in corals, lake and river-bed
deposits and glacial moraines paints the same picture. Western Pacific climate
became much more variable between 5500 and 5000 years ago, says Jamie
Shulmeister of Victoria University of Wellington in New Zealand. The effects
differed across the region, with southeastern Australia drying, while cool
westerly winds brought more frosts to New Zealand. These effects are consistent
with El Ni帽o鈥檚 emergence, says Shulmeister. He thinks he can explain what
triggered it.
Pole position
Imagine the Earth鈥檚 annual orbit around the Sun as an egg-shaped path. If
winter in a given hemisphere occurs when the Earth is farthest from the Sun,
then summer will occur when the two are closest, and this hemisphere will
experience strong seasonal variations in temperature. The orientation of the
Earth鈥檚 pole changes in a 21 000-year cycle, tracing out a circle on the sky, so
the degree of seasonality, or the difference between seasons, of both
hemispheres varies over this orbital cycle. About 9000 years ago, the southern
hemisphere had its lowest seasonality, because its summer came when the Earth
was furthest from the Sun. Since then, its summers have warmed while its winters
have cooled. Shulmeister鈥檚 theory rests on the fact that strong seasonality is
linked to strong ocean and atmospheric currents. He says that around 5800 years
ago, ocean circulation became strong enough to drive the El Ni帽o
oscillation.
In Peru, the onset of El Ni帽o would have been good news for farmers.
Sandweiss suggests that they began building temple mounds to give thanks for
their improved conditions. The oldest temples are found in central Peru and date
to this period. Soon afterwards, temple mounds start to appear further north on
the coast and in the highlands. On the central coast, early El Ni帽os
probably increased shellfish populations and brought fish closer to shore.
But further north, and as El Ni帽o became more frequent, there would
have been floods and failure of the fisheries. Sandweiss believes that temple
mounds were built in an attempt to control the environment. Over the next 2300
years, temple building spread north along the coast, only to stop about 3500
years ago, when El Ni帽os became much more frequent. 鈥淚f one of the
functions of the priests was to prevent El Ni帽o, if it鈥檚 every 40 to 50
years you look good. If it鈥檚 every four to five years, you look bad,鈥� he
says.
In North America there were also changes. Archaeologists draw a line between
Middle Archaic and Late Archaic cultures at 5800 years ago. Local climate,
previously warmer and drier than today, shifted to the modern mode. People
spread beyond the river valleys where they had lived as hunter-gatherers, their
numbers grew dramatically, and they began making pottery, says David Anderson of
the Southeast Archeological Center in Tallahassee, Florida. Over the following
2000 years, they made greater use of plants, and agriculture appeared.
On the other side of the Pacific, the onset of El Ni帽o would have made
southeast Australia drier. It is hard to link that event to cultural changes in
the area, admits Atholl Anderson, an archaeologist at the Australian National
University in Canberra. Increasing variation in climate may have contributed to
the wider use of irrigation in the New Guinea highlands between 6000 and 4500
years ago, but Anderson points out that this could also be explained by new
ideas reaching the area from Southeast Asia.
Cultures from the area of the Japan Sea basin did change. There were new
types of food and pottery. Rice cultivation originated in China around 8000
years ago, but did not spread to the Korean peninsula and Japan until around
5000 years ago. But few archaeological sites in eastern Asia are dated
accurately, making it difficult to make connections between the origin of El
Ni帽o and specific events.
Except for dates from tree rings, the most accurate dating method for
prehistoric sites is measuring radioactive isotope carbon-14, which accumulates
in living things and decays after their death. But the results are hard to
interpret. Artefacts made from old wood will appear older than they really are
and contamination can introduce errors in the dating. Even without these
problems, production of carbon-14 in the atmosphere has varied over time, so
careful calibration is needed to convert radiocarbon ages to calendar years. For
example, 5000 radiocarbon years actually was about 5800 calendar years ago.
Large uncertainties in dating are the biggest barrier to creating a global
picture of links between changes in climate and culture.
Stratified society
Nonetheless, a pattern is emerging even as far afield as Europe and the
Middle East. In Scandinavia, evidence from the changing altitudes at which
certain trees are found on mountain sides indicates that temperatures rose by
about 2掳 C around 5800 years ago. Lars Larsson of Lund University in Sweden
has been charting cultural changes during that period. Earlier people had used
some grain, he says, but 5800 years ago marks 鈥渢he onset of agriculture in the
sense that it becomes an important part of the 别肠辞苍辞尘测鈥�. People developed new
styles of pottery and tools, and shifted their taste in seafood from oysters to
cockles, perhaps because of a rise in sea level.
Perhaps the most dramatic impact of climate on culture came in southern Egypt
west of the Nile. There the period following the last ice age brought summer
moisture to the now desiccated area and, for thousands of years, herders brought
their cattle to graze on seasonal vegetation. At the end of the period, it grew
steadily drier, so fewer and fewer people returned until the area was abandoned
around 5800 years ago, says Fred Wendorf of Southern Methodist University in
Dallas.
Archaeologists had thought the herding culture was simple until they recently
started taking a closer look at the two- and three-metre-high stone blocks they
had been sitting on. Excavations revealed that desert sands had covered a circle
of megaliths aligned with the sun to function as a calendar. Wendorf says that
stone sculptures and burial mounds containing the remains of cattle suggest
their religion centred on the animals. The scale of the monuments, as well as
the presence of deep wells in the area, suggest a highly organised society.
Construction 鈥渞equired a lot of social stratification. There had to be somebody,
`the boss鈥�, there to get those people to do all that work,鈥� says Wendorf. 鈥淢aybe
they were praying for rain.鈥�
The fate of the herders is unclear, but Wendorf speculates they may have
migrated to the nearby Nile Valley, where ancient Egyptian civilisation rose a
few hundred years later. 鈥淒id that happen because these people came in with
their already stratified society?鈥� he asks. He notes that ancient Egyptians
depicted many major gods as bulls, although 鈥渢here were no signs before the
beginnings of this complex civilisation that cows were important to their
别肠辞苍辞尘测鈥�.
Wendorf readily concedes the scanty evidence is far from proof that climate
change gave civilisation an early push. Yet the idea has a clear appeal.
Agriculture, irrigation and other cultural innovations may have been adaptive
responses to changing conditions, he says. Like other species exposed to
environmental change, human culture may have been forced to evolve to
survive.
ABOUT 4200 years ago, a severe drought hit the northern Mesopotamian city of
Tell Leilan. Over the preceding 300 years, the city had grown about sixfold.
After the drought hit, residents abandoned the city and most of the towns and
villages in the area, and dust covered the buildings. What had been a thriving
civilisation virtually collapsed. Collapse was the adaptive response to a severe
drought when the local agriculture depended on rain, says Harvey Weiss of Yale
University. Refugees fled to southern Mesopotamia, where irrigation with river
water allowed farmers to grow crops in drier conditions.
The drought lasted some 300 years. Researchers first spotted evidence of the
dry spell in Palestine decades ago, but only recently did anyone recognise its
extent and effects. Historians had ignored the event because written records did
not blame it for the collapse of Tell Leilan, pointing instead to wars and other
historical forces. Weiss, however, has evidence of a long dry spell in a variety
of palaeoclimate records and makes the connection between this and cultural
collapses throughout the region from the Aegean Sea to the River Indus.
The area that nurtured many early civilisations is particularly vulnerable to
climatic upheaval. 鈥淢esopotamia is the meeting place of three climatic regions,鈥�
says Heidi Cullen of the Lamont-Doherty Earth Observatory of Columbia University
in Palisades, New York. Moist air from the North Atlantic meets Siberian
high-pressure zones and the monsoons to the south. Weiss believes a change in
global atmospheric circulation caused the ancient drought by shifting the jet
stream so it redirected the Mediterranean westerly winds that normally brought
rain to Mesopotamia.
Now studies show evidence of the drought around the globe. The record extends
to the Americas, where spikes of wind-blown dust are visible in ice cores from
the high Andes and cores drilled from the bottom of Elk Lake in Minnesota. Weiss
has recently spotted evidence of the event in the tree-ring record from Nevada
and ice cores from Greenland. He draws a parallel between this and current
global climate changes, warning that adding anthropogenic change to a natural
climate variability we do not yet understand poses the risk of 鈥渁 double
飞丑补尘尘测鈥�.