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 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.

 

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.

 

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.

 

PHOTO: An aerial view shows a tract of Amazon rainforest which has been cleared by loggers and farmers for agriculture near the city of Santarem, Para State, April 20, 2013.

Photo: Nacho Doce