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New fossil research shows how human impacts, particularly through the rise of agriculture and livestock, have disrupted natural mammal communities as profoundly as the Ice Age extinctions.

Fossil bones from six continents have revealed how people have fundamentally transformed mammal communities across the globe, according to new research that traces 50,000 years of animal history.

The international study, last month in Biology Letters, shows that during the last Ice Age, mammal communities formed distinct patterns across continents based on natural climate zones and geographic barriers. But after farming began about 10,000 years ago, just a handful of livestock species spread along with humans and scrambled those natural boundaries forever.

"The study shows how agriculture and hunting combined as powerful global forces to reorganize ecosystems, which still creates conservation challenges today," says Associate Professor John Alroy from Macquarie University, a co-author on the study.

The researchers compared species lists from the last Ice Age—specifically, the Late Pleistocene geological epoch, ending around 11,700 years ago—with lists from the Holocene, our current epoch, which began when this Ice Age ended.

"We examined species lists from hundreds of archaeological and paleontological sites across multiple continents, spanning the past 50,000 years," says lead author Professor Barry Brook, a conservation biologist from the University of Tasmania.

During the Pleistocene, natural factors like climate gradients, and physical barriers such as mountain ranges and oceans, shaped the make-up of large mammal communities. Animals in similar climates tended to live together, creating predictable continental patterns.

But the Holocene brought dramatic changes to species distributions, directly linked to human development of agriculture and domestication of selected species of animals.

Domestic disruption

Examining archaeological records, the researchers found just 12 domesticated species—including cattle, sheep, pigs and horses—appeared in roughly half of the global sites studied, fundamentally altering the composition of animal communities.

"After farming began, just a handful of livestock species spread with humans and scrambled those natural boundaries, reshaping mammal communities worldwide," says Professor Brook.

Domesticated animals which had an outsized impact included familiar farm animals.

"All domesticated species had an impact, including donkeys, sheep, goats, pigs and dogs," Associate Professor Alroy says. "Large ungulates like horses and cows are important because they monopolize food resources wherever they are in high numbers."

While the study excluded birds from the main analysis due to their erratic fossil record, Associate Professor Alroy says that domesticated chickens were also found at 29 of the 350-plus sites, mostly in Europe and the Middle East.

The researchers developed a new computer-clustering method to show that domesticated animals link Holocene archaeological sites thousands of kilometers apart. At the same time, many wild mammals went extinct, in each case following human arrival—not during a particular worldwide climate change episode.

When domesticated animals were spread among geographically distant regions, these ecosystems ended up with similar compositions. For instance, mammal communities in Europe and Africa became more alike after both adopted domesticated species from the Middle East.

Beyond the megafauna

While human impacts were found almost everywhere, the Pleistocene extinctions were more severe in regions with less evolutionary history between humans and local species, such as North and South America, Australia, New Zealand and Madagascar.

After the Pleistocene, the impact of agriculture also varied dramatically by region. Some areas, such as New Guinea and Sri Lanka experienced minimal change, while Europe, the Americas, Australia and parts of Africa saw the highest levels of species turnover—meaning the greatest loss and gain of different animal species.

Previous extinction studies from the late Ice Age have highlighted the disappearance of large-bodied "megafauna"—large animals like giant sloths, wooly mammoths and giant marsupials—but this study shows human impacts continued long after their disappearance.

"When megafauna like mammoths disappeared, we expected the absence of competition for food would see surviving wild species increase their population sizes, but this didn't happen," says Associate Professor Alroy.

Clustering shows species distributions

Using a new method called "chase clustering," the team grouped fossil sites based on lists containing very similar species, regardless of where the sites are located geographically.

"Groupings of domesticated animals link sites thousands of kilometers apart, while many wild mammals in those areas disappeared," says Associate Professor Alroy.

Normally, sites close together geographically have similar animals due to shared climate and environment. But the new method revealed something traditional methods missed: human activity broke that pattern by spreading the same farm animals.

Associate Professor Alroy says this study shows the chase clustering method has potential to be applied broadly in research dealing with fossils.

Modern implications

Some researchers link past species extinctions deep in Earth's history to human hunting or major shifts in climate, despite major climate changes in the Pleistocene being either mostly beneficial to mammals or not coinciding with extinctions.

However, this study shows hunting-related Pleistocene species extinctions were complemented by very different agricultural impacts in the Holocene.

"Over the last 10,000 years or so, humans have overseen the wholesale replacement of native mammal communities with a very limited set of domesticated species," Associate Professor Alroy says.

"Post-extinction ecosystems have not been truly natural for the last 10,000 years or more, so national parks in the hardest-hit regions, such as Australia and the Americas, lack over half of the native large mammal species that would have been present if not for humans."

He provides a dramatic example from Australia: "One excavation layer in Tight Entrance Cave in Western Australia includes 17 species of large mammals, including a thylacine, the Tasmanian devil, Thylacoleo, Zygomaturus, a large extinct wombat, and five different species of extinct short-faced kangaroos, showing an ecosystem unrecognizable compared with modern ones."