The Remnants of Prehistoric Plant Pollen Reveal that Humans Shaped Forests 11,000 Years Ago

A tropical forest writes much of its history at large scale, producing trees as tall as skyscrapers and flowers the size of carry-on luggage. But by zooming in, scientists are uncovering chapters in forest history that were influenced by human activity far earlier than anyone thought.

A new study of pollen samples extracted from tropical forests in southeast Asia suggests humans have shaped these landscapes for thousands of years. Although scientists previously believed the forests were virtually untouched by people, researchers are now pointing to signs of imported seeds, plants cultivated for food, and land clearing as early as 11,000 years ago—around the end of the last Ice Age.  

The study, to be published in the peer-reviewed Journal of Archaeological Science comes from researchers led by paleoecologist Chris Hunt, of Queen’s University, Belfast, who analyzed existing data and examined samples from Borneo, Sumatra, Java, Thailand and Vietnam.

Pollen offers an important key for unlocking the history of human activity in a region where dense tropical forests make traditional excavations slow, arduous work, and thick canopies hinder aerial surveys. Reliance on building materials that perish with the centuries (rather than stone or ceramic) can make it difficult to recognize signs of long-gone inhabitants. Pollen, however, can survive for thousands of years in the right conditions and paint a picture of vegetation over time.

In the Kelabit Highlands of Borneo, for example, pollen samples dated to about 6,500 years ago contain abundant charcoal evidence of fire. That alone doesn’t reveal a human hand. But scientists know that specific weeds and trees that flourish in charred ground would typically emerge in the wake of naturally occurring or accidental blazes. What Hunt’s team found instead was evidence of fruit trees. "This indicates that the people who inhabited the land intentionally cleared it of forest vegetation and planted sources of food in its place,” Hunt explained in a statement about the study.

Hunt’s team also looked at the types of pollen reported in cores extracted from very isolated areas where, in all likelihood, humans did not intervene with the succession of plants that would have come about simply because of changes in temperature, rainfall, and competition among species. The patterns in these cores could then be used as a proxy for what to expect without human intervention. When layers sampled from other, comparable sites in the region failed match up, it raised a flag for the researchers that humans may have disrupted the natural succession through burning, cultivation, or other activities.

"Ever since people had the ability to make stone tools and control fire, they were able to manipulate the environment," explained biologist David Lentz, who directs the Center for Field Studies at the University of Cincinnati. "In pre-agricultural times, they would burn forest to improve hunting and increase the growth of plants that were edible—often weedy plants with lots of seeds. This is a pattern that we see all over the world." It’s not surprising, he added, to see it documented in Southeast Asia.

And yet, Hunt said, "It has long been believed that the rainforests of the Far East were virgin wildernesses, where human impact has been minimal.” To the contrary, his team traced signs of vegetation changes resulting from human actions. “While it could be tempting to blame these disturbances on climate change,” he said, “that is not the case as they do not coincide with any known periods of climate change.

This kind of research is about more than glimpsing ancient ways of life. It could also present powerful information for people who live in these forests today. According to Hunt, “Laws in several countries in Southeast Asia do not recognize the rights of indigenous forest dwellers on the grounds that they are nomads who leave no permanent mark on the landscape.” The long history of forest management traced by this study, he says, offers these groups “a new argument in their case against eviction.”

Such tensions have played out beyond Southeast Asia. In Australia, for example, “the impact of humans on the environment is clear stretching back over 40,000 years or so,” says environmental geoscientist Dan Penny, of The University of Sydney. And yet, he says, “the material evidence of human occupation is scarce.” Starting in the 18th century, the British used that fact "to justify their territorial claim" to land inhabited by Aboriginal Australians—declaring it terra nullius (belonging to no-one), establishing a colony, and eventually claiming sovereignty over the entire continent.

This latest study comes as part of a larger discussion about when and how our species began shaping the world around us. “Humans and pre-humans have been present in Asia for a very long time, and there have been a number of studies that point to a very long history of human alteration of the natural environment,” says Penny. Hunt’s work in Southeast Asia, he says, makes a “valuable contribution” to that discussion, and to a broader debate surrounding the timing of what scientists call the Anthropocene—a proposed period in human history when activity began to alter natural processes in a significant way.”

Josie Garthwaite | | READ MORE

Josie Garthwaite is a journalist and editor based in San Francisco. She writes about science, environment and innovation.