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Scientists say that 6,000 years ago, humans dramatically changed how nature works

A new analysis of the fossil record by scientists at the Smithsonian’s National Museum of Natural History has revealed that the structure of plant and animal communities changed significantly about 6,000 years ago, around the time agriculture began to spread across North America. The study drew on the museum’s paleobiology collections and focused on 360,000 pairs of organisms in 80 plant or mammal communities on different continents, including groups that existed up to 307 million years ago, as well as modern communities and communities from the recent past. (Image credit: Chip Clark, Smithsonian)

There’s no doubt that humans have changed the face of the planet in countless ways since we first arose as the dominant species on Earth. Many of our most obvious impacts have become apparent in the past century or so, with the rise of concerns about climate change, habitat destruction, pollution and the loss of biodiversity.

But new research reminds us that humans have actually been reshaping the planet for thousands of years, in ways we’re only just beginning to understand.

A paper published Wednesday in Nature suggests that human activities caused a major shift about 6,000 years ago in the way plant and animal communities were structured on Earth — this was after the start of the geological epoch known as the “Holocene,” an era which includes the growth of human populations and their rising influence around the globe. The study compared data from the fossil record with observations from the modern era to reach its conclusions.

“I would say that what we can take from the fossil record, in particular, is an understanding of how communities changed naturally before humans arrived in the picture,” said the paper’s lead author, S. Kathleen Lyons, a paleobiologist at the Smithsonian Institute’s Evolution of Terrestrial Ecosystem’s program. Comparing these findings with observations from after the rise of humans allowed the researchers to better understand how humans have changed the Earth.

What “species pairs” can tell us about nature

The work investigates the structure of plant and mammal communities throughout the past 300 million years by examining species pairs. Scientists have observed that sometimes two species tend to be “aggregated,” or paired, meaning they frequently show up in the same place together.

This probably has something to do with the way the species interact with one another or with their surroundings, Lyons said — either they both depend on the same type of habitat, and thus tend to be found in the same places, or they depend on each other for survival in some way. She listed the cheetah and the giraffe as a modern-day example — they tend to be found together because they’re both savannah organisms and share the same type of habitat.

The opposite can also be true. Some species tend to be segregated, rather than aggregated, meaning they aren’t found together. Either they simply require very different types of habitats, or living too close to each other is bad for them in some way — perhaps they compete with each other too much for food and other resources to live too close together.

The key to remember here is that species can also occur in nature totally at random, sometimes being found together and sometimes not — in fact, this is what’s considered usual for most organisms. In order to be considered aggregated, species need to be found together more often than one would statistically expect to happen by random chance — and in order to be considered segregated, they must occur separately more often than one would expect to happen by chance. When this happens, the statistics suggest that some force or environmental pressure is either keeping the species close together or consistently driving them away from each other.

By looking at the fossil record, the scientists found that aggregated species pairs were dominant on the Earth — in other words, there were more aggregated pairs than segregated pairs — starting from about 300 million years ago and continuing up through the beginning of the Holocene.

Then, about 6,000 years ago, there was a shift, and segregated pairs became dominant. Obviously, something had changed the way plants and animals interacted with each other and with their environments — the question that remained was what might have been the cause. The researchers had two main theories: the first was that changes in the climate were responsible, and the second was that some kind of other biological pressure, likely human activities — which were on the rise during this period in history — was to blame.

The power of human influence

The researchers were quickly able to rule out climatic influences by once again looking at the past. They examined data from ice cores — ice samples taken from the polar ice caps — to see how the Earth’s climate had changed over millions of years. The polar regions contain snow and ice that’s been around for millennia. By examining their composition, scientists can learn what Earth’s temperature and other aspects of its climate were like millions of years ago.

They found that while the planet’s climate varied throughout the past few million years, there were no major shifts in plant or animal community structure — that is, how aggregated or segregated species pairs were — corresponding in time with these climate variations.

Thus, they concluded that biological pressure from the rise of human activity during this time period was the likely cause. The exact mechanisms remain unknown, but Lyons points out that human populations were growing and agriculture was beginning to catch on in North America (where a majority of the data used in the study came from) around this same time.

“I do want to emphasize [that] we don’t have a smoking gun of mechanisms for what activities humans were doing,” Lyons said. But the combination of all their growing activities on Earth during the early- to mid-Holocene was likely causing barriers to the spread of different organisms throughout their habitats, she said. Perhaps settlement by humans, and their cultivation of land for agriculture, was making it hard for some species to settle down in areas they otherwise used to dominate.

Additionally, Lyons noted, some species were probably driven away from certain areas because it was too difficult to compete with humans for resources, such as food or space. The combination of these factors means that certain organisms, which for the past 300 million years were often found together, suddenly were splitting apart after being forced to adapt to the changing landscape that humans were reshaping.

Using the past to understand the future

While one could argue whether the specific shift from aggregation to segregation is good or bad (or totally neutral), the real takeaway here is the power that humans have to bring about large-scale changes in the environment — a power that’s most obvious today, but in fact has existed since human civilizations first started to become dominant on the Earth. A debatable point, however, is how useful this revelation is for making predictions about the way organisms might react to environmental pressures in the future.

In their paper, the authors have written, “Future work comparing the co-occurrence structure of fossil and modern communities should allow us to better understand how this alteration will play out in the future.” In other words, the authors argue that looking at past ecological shifts can help us understand what kinds of changes humans might bring about in the future.

Gregory Dietl, curator of cenozoic invertebrates at the Paleontological Research Institution, takes this stance as well — but he cautions that not all scientists feel this way. Dietl, who served as a reviewer on the new study before it was published, authored a “News & Views” piece (also published Wednesday in Nature) reacting to the new paper. In the piece, he pointed out that the results in this study show a clear difference between modern-day plant and animal communities — which have been reshaped by human influences — and ancient communities. And he noted that some scientists have argued that the past is so different, we can’t necessarily use it to make inferences about the future. As he wrote in his piece, “At stake is whether we can reliably use the past as a guide to an uncertain, anthropogenically modified future.”

Dietl told The Post that he feels the past can still be used as a tool for understanding the future, even if it looks so much different from the present day. Doing so successfully just requires careful and creative thinking.

Scientists often try to gain insight into the future by looking for periods in the past during which conditions were similar to those in the present — these similar periods are called “analogues.” Then, by examining how the environment reacted to those conditions in the past, they can get a glimpse at how similar conditions in the present might affect the future.

By choosing analogues carefully, scientists can gain insight into the future — not by making exact predictions, necessarily, but by coming up with “alternate interpretations of what might happen in the future,” and then making informed inferences about which potential future scenarios are most likely to occur, Dietl said.

And Lyons agreed that this paper, and others like it, are useful for understanding the future. The fossil record clearly shows what plant and animal communities looked like before human influence and what they looked like after, she said.

“What this allows us to do is understand areas where humans are having an effect,” she said. Then, in present-day studies, scientists can examine the ways natural communities are changing and make better-informed inferences about which of these changes might have been caused by human influences.

And, if this study has any say on the matter, human influences are a powerful force on the face of the planet. Knowing the power humans have been able to exert — just from our mere presence on Earth — over the fundamental structure of natural ecosystems is, at the very least, a wake-up call about the changes we may cause in the future.