Wherever you go on this blue, green and white globe of ours, odds are some person has been there before you—and left a mark. That's because the hunting, farming or burning practices of our most distant ancestors have shaped most land areas on the planet, argues an interdisciplinary team of archaeologists and ecologists in Proceedings of the National Academy of Sciences. If we are indeed living in the Anthropocene—a new geologic epoch brought on by the outsized environmental effects of the human species—then this new interval isn't just a few hundred years old, it is older than the industrial revolution.
The researchers set out to investigate just how long human being have been profoundly changing the environment on land. "This is a super important question for the identity of humanity," argues ecologist Erle Ellis of the University of Maryland, Baltimore County, a co-author. "Are we the people who transformed the planet for hundreds of generations, or the people who just recently started destroying things?"
To answer that outstanding question the researchers started with a vast spread of archaeological and ecological data from around the world, particularly micro charcoal records from sediment cores. The charcoal delivers a long-term record of human burning, whether intentional or accidental, that coincides with the arrival of modern humans in a particular area. That arrival also often coincides with the extinction of large predators and large animals, generally.
But how exactly do humans impact a new environment? Scientists have used computer models that aim to estimate how quickly and how profoundly Homo sapiens change the landscape. One option estimates land use simply based on the number of humans around, assuming a minimum acreage required to support a person. The other model has humans relatively quickly sprawl through an entire area, but then contract to intensify land use in support of a larger but denser population. This might be dubbed the laziness principle—humans invest the least amount of work, technology or any other resource as possible to survive and even thrive, these researchers argue. "People are doing the easiest thing, knocking out top predators early on," Ellis explains. "There's a pretty big impact per person to make a living, [because people are] burning big swathes of forest just to make it easier to get some game."
According to this model, and the charcoal record where it is available, a relatively small number of humans began to transform most of the planet's land surface at least 3,000 years ago. "If people can get away with less work, they're going to do less work," says archaeologist Dorian Fuller of University College London, who also contributed to the research.
Take for example rice cultivation in Asia, developed some 6,000 years ago in the Yangtze River Valley but not adopted for another thousand years or so in areas of southern China and Southeast Asia. "You have relatively happy hunter-gatherer-fishers who don't want to put in the effort" to farm rice until population density requires it, Fuller explains.
As the human population swells—as seen in the record of fertilizer use in Europe and Asia—the resources then become more intensively used. This is not confined to agriculture; archaeologists find a similar intensification in the hunting patterns of Paleolithic Europeans after the decline of big game. These proto-Europeans began to hunt a wider range of smaller animals more intensively as well as developing the food preparation technology to extract more food from a larger array of sources. This idea further suggests that humanity has escaped time and time again from the Malthusian trap of population colliding with limited resources by transforming the relationship between human population and the environment through technology, whether through the invention of cooking or modern mechanized agriculture. Humanity simply applies technology to derive more from a given resource, whether it be copper or farmland.
That trend continues into the present day, the researchers argue. The most modern industrial agriculture focuses primarily on the best land it can get. The human population has shifted away from subsistence and low productivity agriculture, collecting in cities as fossil-fueled machines help fewer farmers work the land. "The next revolution is when the majority of people get into cities and are fed by a minority," Fuller explains. This process is already complete in industrialized countries where less than 1 percent of the population feeds the rest, but "we're not finished with that yet," in developing countries such as China and India, Fuller says. Peak farmland may be imminent.
If the human impact is longstanding and widespread, then the landscape is as much in recovery from past impacts as it is enduring new changes. Think of the cutting back of the Amazon rainforest—itself potentially a recovery from earlier, more intensive human use before the arrival of Europeans—versus the regrowth of the forests of the eastern U.S. In fact, the woodland ecosystems of Europe and South America commonly thought of as natural may be the legacy of prior human use. "Most of the forest have had people in them, interacting with them and transplanting species around for thousands of years," Fuller notes. "We have very little in the way of natural forests, which doesn't mean that we shouldn't be trying to reforest environments and have forests." After all, the modern phase of the Anthropocene may be the first time humans can choose intentionally what an appropriate level of impact might be.
Fully answering this question of how long the human impact on land has been widespread requires a broader global synthesis of the archaeological and paleoecological data on human population and land use. Most of that data is available—and has been examined—in a local rather than global context, such as the impacts of humans on the Yucatan Peninsula or Australia. Nevertheless, what data exists suggests that this is a "used planet," in the words of the authors. "We've been husbanding these biomes and creating our own types of ecologies—the cultivated lands, the rangelands—we've been doing this for a very long time," Ellis argues. "We've been living in that Anthropocene biosphere since prehistory."