By flooding fields to grow rice and raising livestock, they may have been fundamentally altering the climate of the Earth.
Washington: Ancient farming practices may have led to a rise in the emission of heat-trapping gasses like carbon dioxide and methane -- a trend that has continued since, a study has found.
Without this human influence, by the start of the Industrial Revolution, the planet would have likely been headed for another ice age, researchers said.
Millenia ago, ancient farmers cleared land to plant wheat and maize, potatoes and squash. They flooded fields to grow rice and began to raise livestock. Unknowingly, they may have been fundamentally altering the climate of the Earth.
"Had it not been for early agriculture, Earth's climate would be significantly cooler today," said Stephen Vavrus, a senior scientist at University of Wisconsin-Madison in the US.
"The ancient roots of farming produced enough carbon dioxide and methane to influence the environment," said Vavrus, lead author of the study published in the journal Scientific Reports.
The findings are based on a sophisticated climate model that compared our current geologic time period, called the Holocene, to a similar period 800,000 years ago.
They show the earlier period, called MIS19, was already 1.3 degree Celsius cooler globally than the equivalent time in the Holocene, around the year 1850.
This effect would have been more pronounced in the Arctic.
Using climate reconstructions based on ice core data, the model also showed that while MIS19 and the Holocene began with similar carbon dioxide and methane concentrations, MIS19 saw an overall steady drop in both greenhouse gases while the Holocene reversed direction 5,000 years ago, hitting peak concentrations of both gases by 1850.
The researchers deliberately cut the model off at the start of the Industrial Revolution, when sources of greenhouse gas emissions became much more numerous.
For most of Earth's 4.5-billion-year history, its climate has largely been determined by a natural phenomenon known as Milankovitch cycles, periodic changes in the shape of Earth's orbit around the Sun -- which fluctuates from more circular to more elliptical -- and the way Earth wobbles and tilts on its axis.
Astronomers can calculate these cycles with precision and they can also be observed in the geological and paleoecological records.
The cycles influence where sunlight is distributed on the planet, leading to cold glacial periods or ice ages as well as warmer interglacial periods.
The last glacial period ended roughly 12,000 years ago and Earth has since been in the Holocene, an interglacial period. The Holocene and MIS19 share similar Milankovitch cycle characteristics.
All other interglacial periods scientists have studied, including MIS19, begin with higher levels of carbon dioxide and methane, which gradually decline over thousands of years, leading to cooler conditions on Earth.
Ultimately, conditions cool to a point where glaciation begins.