A new analysis of the fossil record shows that a deep pattern in nature remained the same for 300 million years. Then, 6,000 years ago, the pattern was disrupted - at about the same time that agriculture spread across North America. "When early humans started farming and became dominant in the terrestrial landscape, we see this dramatic restructuring of plant and animal communities," said University of Vermont biologist Nicholas Gotelli, an expert on statistics and the senior author on the new study.
     In the hunt for the beginning of the much-debated 'Anthropocene' - a supposed new geologic era defined by human influence of the planet - the new research suggests a need to look back farther in time than the arrival of human-caused climate change, atomic weapons, urbanization or the industrial revolution. "This tells us that humans have been having a massive effect on the environment for a very long time," said S. Kathleen Lyons, a paleobiologist at the Smithsonian's National Museum of Natural History who led the new research.
     Gotelli and Lyons were part of a team of 29 scientists who studied plant and animal datasets from both modern ecosystems and the fossil record stretching back to the Carboniferous Period, well before the emergence of the dinosaurs. The scientists looked to see how often a particular pair of plant or animal species was found within the same community. Some pairs of species appear to be 'aggregated,' meaning they tend to appear together in nature more often than one would expect by chance - like cheetahs and giraffes who both depend on savannah habitats. Other species are 'segregated,' meaning that when one is found, it's unlikely to find the other there too.
     For modern communities of plants and animals, recent studies show that segregated species pairs are more common than aggregated ones. But when the team investigated the composition of ancient communities using data from fossils, they were surprised to find the opposite pattern: from 307 million years ago to about 6,000 years ago, there was a higher frequency of aggregated species pairs. Then, from 6,000 years ago to the present, the pattern shifted to a predominance of segregated species pairs. An ancient rule had changed.
     "We don't have direct evidence to show that this pattern change was caused by humans," Gotelli cautions, but the indirect evidence is compelling. The new pattern started emerging about 6,000 years ago, during the great Neolithic revolution when humans developed agriculture and their populations grew and spread globally. From this time until the present, plant and animal communities exhibit less co-occurrence and a greater frequency of segregated species pairs. The scientists explored - and eliminated - many possible reasons for why this new pattern appeared. "So we're left with human impacts," Lyons said. "We think it's something that humans do that causes barriers to dispersal for both plant and animal species."
     "We humans have influenced the landscape, but perhaps for a lot longer than we had previously recognized," says Gotelli. "When we look at landscapes and say, 'this is pristine or unaltered,' that's not necessarily true. We may have changed the rules over a much larger scale than we appreciate."

Edited from University of Vermont PR (17 December 2015)