The green and lush Amazon rainforest wasn't always as we see it today; in fact, parts of it were once much drier and more resembled an African savannah, researchers say.

A significant portion in the Amazon basin was likely grassland until the change in climate conditions 2,000 years ago brought a wetter environment in which is rainforests could form, they say.

That change may have been caused by a natural shift in the orbit of the Earth around the sun, researchers at the University of Reading in England reported in the Proceedings of the National Academy of Sciences.

The grasslands character of the landscape is suggested by the discovery of grass pollen trapped in the ancient sediments in lakes in Bolivia, and the uncovering of man-made earthworks that showed sings of planting of maize, point to farming that would be much more likely to take place in savannah than in rainforest, the researchers say.

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 said in a statement.

One fifth of the region may have been grasslands and savannah before the climate change, with forests in the rest, Reading researchers say.

"The dominant ecosystem was more like a savannah than the rainforest we see today," study lead author John Carson says.

The people living and farming in the region continued to live there as the rainforest began emerging, likely maintaining their farming lifestyle by keeping certain areas cleared as the forest began to encroach, he says.

"It kind of makes sense," Carson says. "It's easier to stomp on a sapling than it is to cut down a big Amazonian tree with a stone ax."

The rainforest of today is likely a product of both natural and human forces, he says; while natural climate cycles caused the rainforest to move into the region, humans stayed on for 1,500 years afterward.

"It's very likely, in fact, that people had some kind of effect on the composition of the forest," Carson says. "People might favor edible species, growing in orchards and things like that, [or] altered the soils, changing the soil chemistry and composition, which can have a longer-lasting legacy effect."

At the time Europeans arrived in the region there were more than 80 domesticated or semi-domesticated crops being farmed around the Amazon, while in Europe during that period farmers were utilizing about six, experts say, strong evidence of the ability of Amazon peoples to manage nature.