When we think of how humans have altered the planet, greenhouse gas warming, industrial pollution, and nuclear fallout usually spring to mind. But now, a new study invites us to think much further back in time. Humans have been altering landscapes planetwide for thousands of years: since at least 1000 B.C.E., by which time people in regions across the globe had abandoned foraging in favor of continually producing crops.
“This is the first project of its kind within archaeology,” says Lucas Stephens, an archaeologist and environmental researcher at the Environmental Law & Policy Center in Chicago, Illinois, and the lead author of a study that presents the expert opinions of hundreds of archaeologists around the world. “There’s never been a global synthesis like this.”
Scientists have long sought to model ancient land use to accurately inform modern climate reconstructions and to set a start date for when humans first made their mark on the planet. The most commonly used model, however, uses estimates of estimates: It takes other scientists’ guesses of the human populations of various regions, then estimates how much land would be required to sustain them. That model places the start date for continuous global farming at only 500 years ago.
To pinpoint a more accurate date, researchers divided Earth’s land into 146 regions covering every continent except Antarctica. They then invited more than 1300 archaeologists with expertise in ancient land use to complete a roughly 80-question survey, asking how humans in each region used land for the past 10,000 years. Questions included when ancient people transitioned from foraging to farming and whether they developed pasturelands for grazing animals. The survey also asked how confident the archaeologists were in their answers.
The 711 responses from 255 archaeologists (researchers could take the survey multiple times if they were experts in multiple regions), were weighted according to confidence and then aggregated. The results of this ArchaeoGLOBE project—which counted 112 of the respondents as co-authors—reveals that hunter-gatherer lifestyles declined globally between 10,000 and 3000 years ago as they were replaced by continuous farming, the researchers report this week in Science. By about 1000 B.C.E., all of the world’s regions that now practice farming were annually cultivating crops.
Those trends also imply that ancient humans may have been influencing climate by raising livestock and razing forests to create croplands. Researchers debate the size of the climatic impact, but by 3000 years ago the effects were significant, “albeit smaller than those of today,” says co-author Erle Ellis, ArchaeoGLOBE’s project leader and an environmental scientist at the University of Maryland, Baltimore County.
The timing suggests the Anthropocene, a newly proposed epoch that marks when humans first started to leave their geological mark on Earth, needs an earlier start date, writes Neil Roberts, who studies environmental change and archaeology at the universities of Plymouth and Oxford in the United Kingdom, in an accompanying editorial.
The Anthropocene Working Group (AWG), a group of mostly earth scientists tasked with defining the epoch’s start, have generally agreed on the mid-1950s, when radioactive particles from hydrogen bombs embedded themselves in sediments across the globe, where they will linger for tens of thousands of years.
AWG chair Jan Zalasiewicz, a geologist at the University of Leicester in the United Kingdom, calls the new study “useful” but says it “doesn’t show anything dramatically new.” There’s no doubt humans have altered landscapes throughout prehistory, he explains, but for the Anthropocene to be recognized as a formal geological epoch, it needs to show a distinct signature in the rock record, which the study doesn’t provide.
The authors do, however, show a unique approach to crowdsourcing data collection, says Ruth Duerr, a data management researcher at the Ronin Institute for Independent Study in Westminster, Colorado, and former head of the Earth and Space Science Informatics section of the American Geophysical Union. She says the robust survey methods, the mass authorship, and the crowdsourced open data make the study a “model” model. “There are lots of crowdsourced citizen science projects out there,” she says, “but here they’re crowdsourcing from experts. The methodology is just so cool.”