It is no secret that modern humans, with our fuel-burning automobiles, the massive practices of agriculture and livestock and the penchant for disposable goods, have had a great Impact on almost all environments across the globe. But new research shows that even our ancestors in the Bronze Age changed the chemistry of the soils they cultivated more than 2,000 years ago. It is one of the first proofs that humans have had a lasting environmental impact on planet Earth.
"This is a new lens in one of the most profound changes in human history: when humans go from being part of nature to being process drivers," said Eric Guiry, lead author of the new study and a candidate for doctorate of anthrozoology at the University of British Columbia.
The finding has implications for how we establish the limits of the anthropocene, the current geological age defined by human actions that have a dominant influence on the climate and the planet's environment. Many relate the beginning of the anthropocene with the Industrial Revolution, the burning of fossil fuels or the testing of nuclear bombs. But now it is evident that even ancient humans could alter the environment in a lasting way.
Specifically, these Bronze Age farmers changed the nitrogen composition of their fields and the surrounding ecosystems. Plants need nitrogen to grow, but in many cases, those plants also deplete soil nitrogen. Although some bacteria and fungi in the soil can fix nitrogen in the air to make it usable for plants, farmers began to fertilize their fields with manure rich in nitrogen to ensure an abundant harvest. Fertilization is still done today, of course, either on large farms with commercial fertilizers or simply by adding some Miracle-Gro to an indoor plant.
"Nitrogen is part of all ecosystems and is a vital component," Guiry told Gizmodo. "Being able to manage it is key to expanding society." And while taking over the nitrogen cycle is not intrinsically a problem for the environment, the excess nitrogen in today's fertilizers can pollute waterways and contribute to greenhouse gases.
In the Bronze Age, Irish farmers probably began fertilizing and growing their soil more frequently and on a larger scale as the population grew, Guiry said. More people ate more crops and had more livestock, such as cattle, pigs, goats and sheep, which meant that more of that livestock could be used to farm plus .
That's why the researchers saw an increase in a certain isotope of nitrogen associated with bone culture in animals of the Middle and Late Bronze Age, about 2,000 years ago. Farmers had interrupted the nitrogen cycle for the first time, changing the chemistry of their soils and the ecosystems that surrounded them, at least in Ireland.
"This is a turning point for an entire ecological system," said Sarah McClure, a zooarchaeologist at Pennsylvania State University who was not involved in the study. "In the Bronze Age, you get these deep and prolonged changes in the nitrogen composition of soils due to human activity that never really disappears."
The research team extracted this key isotope, nitrogen-15, from 712 animal bones that came from 90 different archaeological sites in Ireland and spanned the entire Holocene (the last 10,000 years). Nitrogen signatures in animal bones are a good indicator of what they ate in their last 10 or 15 years of life, so the researchers were hoping to find out if the animals domesticated at that time were eating forage that had been grown in soils with Altered chemistry
"Those buried bones are providing a small capsule of time until those animals were alive," said Gizmodo Fiona Beglane, zooarchaeologist at the Sligo Institute of Technology in Ireland and one of Guiry's co-authors.
Nitrogen-15 signatures should have remained stable, but by the middle or late Bronze Age they jumped to levels similar to those we see in the modern era, as described in the study published today in Science Advances . That was the turning point in which ancient humans in Ireland began to alter the environment of the Earth instead of just living in it.
"We did not realize to what extent the nitrogen cycle had essentially changed forever," said McClure.
In addition, nitrogen 15 also increased in the bones of wild animals such as red deer. The authors argue that this means that farmers at that time were not only affecting the chemical composition of their own fields, but the ecosystems throughout the island of Ireland.
And although this study was limited to Irish bones, McClure and Guiry said the method could easily be applied to other regions of the world. Doing so could help identify when the main actors in agropastoralism – the lifestyle of growing and raising livestock – began to change the Earth chemically.
"Until we understand the effects that people have on the soil and the environment, it is difficult to look ahead and see the effects we still have now," Beglane said. "People argue that what we do today has no effect on seven billion of us, now we show people in the Bronze Age that irreversible changes happen, and that there were only a few million people. of people can have such a fundamental effect on the environment, shows that the figures we are working with today have an impact. " [
[ Science Advances ]