By Alister Doyle

OSLO (Reuters) - Swathes of the Amazon may have been grassland until a natural shift to a wetter climate about 2,000 years ago let the rainforests form, according to a study that challenges common belief that the world’s biggest tropical forest is far older.

The arrival of European diseases after Columbus crossed the Atlantic in 1492 may also have hastened the growth of forests by killing indigenous people farming the region, the scientists wrote in the U.S. journal Proceedings of the National Academy of Sciences (PNAS).

"The dominant ecosystem was more like a savannah than the rainforest we see today," John Carson, lead author at the University of Reading in England, said of the findings about the southern Amazon.

The scientists said that a shift toward wetter conditions, perhaps caused by natural shifts in the Earth’s orbit around the sun, led to growth of more trees starting about 2,000 years ago.

The scientists studied man-made earthworks, uncovered by recent logging in Bolivia, that included ditches up to about a kilometer (1,100 yards) long and up to 3 meters deep and 4 meters wide.

They found large amounts of grass pollen in ancient sediments of nearby lakes, suggesting the region had been covered by savannah. They also found evidence of plantings of maize, pointing to farming.

PRISTINE

The Amazon has traditionally been seen as a pristine, dense rainforest, populated by hunter-gatherers. In recent years, however, archaeologists have found hints that indigenous peoples lived in the thick forest, but managed to clear tracts of land for farming.

The PNAS study suggests a new idea – that the forest simply did not exist in some regions.

The "findings suggest that rather than being rainforest hunter-gatherers, or large-scale forest clearers, the people of the Amazon from 2,500 to 500 years ago were farmers," the University of Reading said in a statement.

Carson said that perhaps a fifth of the Amazon basin, in the south, may have been savannah until the shift, with forests covering the rest.

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In one lake, Laguna Granja, rainforest plants only took over from grass as the main sources of pollen in sediments about 500 years old, suggesting a link to the arrival of Europeans.

The purpose of the earthworks is unknown - they could have been defensive or for drainage or religious purposes.

And understanding the forest could help solve puzzles about climate change.

The Amazon rainforest affects climate change because trees soak up heat-trapping carbon dioxide as they grow and release it when they rot or are burnt. Brazil has sharply slowed deforestation rates in recent years.

Carson said that the growth of Amazonian forests could, for instance, have contributed to the Little Ice Age, from about 1350 to 1850 by absorbing heat-trapping gases from the air.

Michael Heckenberger, an expert on the Amazon at the University of Florida, said the study added to evidence that people living in the Amazon managed nature.

"These indigenous systems were highly sophisticated...There are over 80 domesticated or semi-domesticated crops in the Amazon," he said. "In Europe at the time they were working with about six."

(Reporting by Alister Doyle; Editing by Lisa Shumaker)