It’s hard to imagine a global force strong enough to change natural patterns that have persisted on Earth for more than 300 million years, but a new study shows that human beings have been doing exactly that for about 6,000 years.
The increase in human activity, perhaps tied to population growth and the spread of agriculture, seems to have upended the way plants and animals distribute themselves across the land, so that species today are far more segregated than they've been at any other time.
That’s the conclusion of a study appearing this week in the journal Nature, and the ramifications could be huge, heralding a new stage in global evolution as dramatic as the shift from single-celled microbes to complex organisms.
A team of researchers led by S. Kathleen Lyons, a paleobiologist at the Evolution of Terrestrial Ecosystems (ETE) program in the Smithsonian's National Museum of Natural History, examined the distribution of plants and animals across landscapes in the present and back through the fossil record in search of patterns.
Mostly they found randomness, but throughout time, there was always a small subset of plants and animals that showed up in relationship to one another more often than can be attributed to chance. That relationship either meant that pairs of species occur together, so when you find one, you usually find the other. Or it meant the opposite: when you find one, the other is usually not present, in which case they’re considered segregated.
An example would be that where there are cheetahs, you often find giraffes, because they prefer the same habitat. Predator-prey relationships can also cause animals to co-exist on the landscape, as in the case of dire wolves and giant ground sloths in the late Pleistocene. It’s believed that dire wolves may have preyed on baby giant ground sloths.
On the flip side, segregated animals are those that appear together less often than they would by chance alone. Today, Grevy’s zebra and colobus monkeys are rarely found together because they have evolved to exploit different landscapes.
The surprise discovery was that for 300 million years, it was more common for species pairs to occur together—to aggregate on a landscape—than it was for them to segregate. Then the pattern flipped around 6,000 years ago in North America. Around the same time the human population was expanding and becoming dependent on agriculture, plant and animal communities shifted to a pattern dominated by segregation.
Lyons and her colleagues looked at nearly 360,000 pairs of organisms from 80 communities on different continents, but the best data available to them around the time period in question came predominantly from North America. Lyons expects the pattern shift will be evident around the globe if other researchers look for it.
“It’s striking that there’s a community structure that is changing in ways it hasn’t changed before and that appears to be associated with humans,” says Erle Ellis, a professor of geography and environmental systems at the University of Maryland and a member of the International Union of Geological Sciences Anthropocene Working Group. “I would say it’s one of the most interesting indicators I’ve ever seen of a shift in the biosphere associated with humans.”
The scientists can’t say exactly why the shift occurs at this distinct moment in human history, but they’ve gone to great lengths to rule out other possible connections, including examining ice cores to get at past climate conditions. There have been many periods of natural climate variability over those 300 million years, and still the pattern held steady, with an average of 64 percent of species pairs with significant relationships being aggregated.
After the shift 6,000 years ago, the average dropped to 37 percent. Today, a significant relationship between a pair of species is more likely to mean where you find one, you don’t find the other. In other words, species are more segregated than they’ve ever been.
Though there’s no smoking gun, Lyons has thoughts on the role humans played in this change. “We’re living in a lot of areas where species used to overlap their distributions,” she says. “They don’t overlap anymore because they can’t get through the areas where we’re living now.”
Gregory Dietl, a paleoecologist and Curator of Cenozoic Invertebrates at the Paleontological Research Institution in Ithaca, New York, says that this break in a 300-million-year-old pattern signals that we’re living in a new world, and that makes it more challenging to use the past to predict what may happen in the future.
“For me that was the big piece,” he says. “What does this more segregated pattern mean then, ultimately, for how species may adapt or just respond to climate change in the future?”
Dietl wrote a review of the study that also appears in the same issue of Nature. Like many of his colleagues who have seen the paper, he believes it’s reasonable that increased segregation may make species more vulnerable to changes in their environment.
“It probably means species are more vulnerable to extinction because there are fewer connections between them,” Lyons says. Humans have broken up plant and animal populations by destroying and fragmenting habitats. Their ranges are smaller, and no longer overlap in the way they once did.
“And because their geographic ranges are smaller, their abundances are almost certainly smaller.” But understanding how environmental changes will impact species is far more difficult in a world without clear examples from the past to rely on.
Whether more plants and animals adapt or go extinct in the future, this dramatic shift in the past highlights the extent of human influences that have prompted the official naming of a new age: the Anthropocene.
“There’s a tendency to think humans did not become a transformative force until fairly recently,” says Ellis. “But this effect can be placed at the very beginnings of agriculture. So it’s a very early indicator. The process of humans becoming distinct from other species and the way they transformed the Earth is really the cause of the Anthropocene. So this [study] is interesting in terms of asking where and when did this train leave the station?”
However, this study is not likely to help set the date scientists will use to mark the start of the Anthropocene. The Anthropocene Working Group is due to make that decision in 2016, and they’re more likely to rely on the accepted practice of identifying a well-defined line in the sand—or in most cases, the rock—that represents the sum of environmental changes denoting the shift from one time period to the next.
Chair of the working group and professor of paleobiology at the University of Leicester, Jan Zalasiewicz, says that line is likely to have been drawn in 1952, when fallout from thermonuclear weapons tests deposited a distinct radioactive signature in sediment around the world.
“Radionuclides do not represent as big a change to the Earth system as do the changes in population dynamics described in the paper, but they do provide a sharper time marker,” he wrote in an e-mail. And that’s what the working group is looking for. What the current paper contributes to the discussion, however, may be something even bigger on Zalasiewicz’s radar.
“This adds weight to the increasing impression that the Anthropocene is not simply different from the Holocene, but differs in some important respects also from all previous historical episodes on this planet,” he wrote. Zalasiewicz was one of the coauthors on a recent paper in The Anthropocene Review proposing that the significant impacts humans are making to life on the planet could be the start of a long transition to something completely new—a third stage in evolution.
The previous transition from single-celled organisms to complex life took roughly 100 million years, so it’s not unreasonable to suggest that we’re initiating a (very long-term) change in course for the biosphere.
Proponents of such a transition point to the global homogenization of plants and animals, the introduction of vast amounts of new energy into Earth’s system from the burning of fossil fuels, the increasing integration of technology into a global network of human interactions and the dominance of a single species, Homo sapiens, directing the evolution of other species.
If Lyons’s results can be replicated in the fossil record in other parts of the world, it would prove that our global influence on the evolution of life on Earth began thousands of years ago.
“I have to say that this result is so striking that I think it’s going to keep a lot of scientists busy trying to decipher this,” Ellis says. “They’re opening up a door to a whole new way of looking at changes in the Earth system, changes in the biosphere, changes induced by humans. This isn’t the final word, but it’s the opening salvo to a discussion on it.”
UPDATE 12/17/2015: A previous version of this article stated that elephants and giraffes form a "significant pair," when it should be giraffes and cheetahs, and that significant pairs of animals that are aggregated "always" are found together, and segregated animals are "never" seen together.