The US scientists believe that small populations released carbon emissions as they cleared large tracts of land to produce relatively meagre amounts of food.
They were much less efficient than farmers using today's agricultural practices because there were no constraints on land.
A study published online in the journal Quaternary Science Reviews by researchers at the University of Virginia and the University of Maryland-Baltimore County (UMBC) said that early farmers could have cleared five or more times as much land as they used at any one time.
According to the researchers, today’s population of six billion people uses about 90 per cent less land per person for growing food than the early farming societies.
William Ruddiman, the paper’s lead author and Emeritus professor of environmental sciences at the University of Virginia, said the early farmers were likely to have cleared land by burning forests, planted crop seeds among the dead stumps and moved on to a new area once the yields declined.
“They used more land for farming because they had little incentive to maximise yield from less land, and because there was plenty of forest to burn. They may have inadvertently altered the climate,” he said.
Ruddiman first published a hypothesis five years ago suggesting people began altering the global climate thousands of years ago, with human activity accounting for rises in carbon dioxide that began about 7,000 years ago.
His theory was criticised by scientists who believe the human impact on the climate began with the industrial revolution because earlier populations were too small to influence the level of carbon emissions in the atmosphere. But Ruddiman said that early farming methods, with around ten times the amount of land per person than is used today, could have created an impact on the climate despite the small number of people in early civilisations.
He suggests it was only as populations grew larger that farming technologies improved to increase yields using less land.
His co-author, Erle Ellis, of UMBC, said: “Many climate models assume that land use in the past was similar to land use today and that the great population explosion of the past 150 years has increased land use proportionally.
“We are proposing that much smaller earlier populations used much more land per person and may have more greatly affected climate than current models reflect.”