For years, scientists have debated whether humans or climate caused the dramatic decline in large mammal populations over the past few thousand years. A new study from Aarhus University confirms that climate cannot be the cause. About 100,000 years ago, the first modern humans migrated from Africa in large numbers. They are so adept at adapting to new habitats that they have colonized almost every landscape, from deserts and jungles to the icy taiga of the far north.
Prehistoric man attacks an elephant. New research suggests that humans, not climate, caused the dramatic decline of nearly all of Earth's megafauna 50,000 years ago. Image source: First published in Bryant and Gay in 1883. Wood carvings by E.Bayard.
Part of the success was due to humans' ability to hunt large animals. With their ingenious hunting skills and specially crafted weapons, they have perfected the art of killing the most dangerous mammals. But unfortunately, our ancestors' great success came at the expense of other large mammals.
It is known that many large species became extinct during the global colonization of modern humans. Now, new research from Aarhus University shows that those large mammals that survived also experienced dramatic declines.
By studying the DNA of 139 living large mammals, scientists found that nearly all species experienced a sharp decline about 50,000 years ago.
Jens-Christian Svenning, director of the Center for Ecodynamics of Novel Biospheres (ECONOVO) at the Danish National Research Foundation and professor at Aarhus University, is the initiator of this research.
"We studied the evolution of large mammal populations over the past 750,000 years. For the first 700,000 years, the populations were fairly stable, but 50,000 years ago, the curve broke and populations declined sharply and never recovered. Over the past 800,000 years, the world has fluctuated between glacial and interglacial periods approximately every 100,000 years. If climate was the cause, we should have seen larger fluctuations when the climate changed 50,000 years ago. But we did not see that. So humans are the most likely explanation."
For decades, scientists have debated the reasons for the extinction or rapid decline of large mammals over the past 50,000 years. One group of scientists believes that rapid and violent fluctuations in climate are the main reason. For example, they believe the woolly mammoth went extinct because much of the cold mammoth steppe disappeared. On the contrary, some people believe that the prevalence of modern humans (Homo sapiens) is the main reason. They believe that our ancestors hunted these animals to the point where they were either completely extinct or largely wiped out.
By far the most important piece of evidence in the debate is fossils dating back 50,000 years. These fossils show that the strong, selective extinction of large animals in time and space roughly coincides with the global spread of modern humans. Therefore, animal extinctions are difficult to link to climate. However, the debate continues.
New research provides fresh data that sheds new light on this debate. By studying the DNA of 139 large mammal species, researchers were able to find that the numbers of these animals had also declined over the past 50,000 years. This development appears to be related to the spread of humans rather than climate change.
Over the past 20 years, a revolution has occurred in the field of DNA sequencing. Mapping entire genomes has become easy and cheap, so the DNA of many species has now been mapped.
Juraj Bergman, the lead researcher of the new study and assistant professor at Aarhus University, explained that genome maps of species around the world are freely available on the Internet, and the Aarhus University research team made use of these data.
"We collected data on 139 species of large mammals and analyzed a huge amount of data. There are about 3 billion data points per species, so it takes a long time and a lot of computing power: DNA contains a lot of information about the past. Most people know about the tree of life, which shows where different species developed and what common ancestors they had. We also have a lot of knowledge about DNA variation. does the same thing. By grouping mutations and building a family tree, we can estimate the population size of a particular species over time. The larger the population of an animal, the more mutations occur. In the case of elephants, each pregnancy has the potential to produce some mutations and pass those mutations on to the offspring."
The 139 large mammal species examined in this study are all living species today. These include elephants, bears, kangaroos and antelopes, among others. There are an estimated 6,399 species of mammals on Earth, but this study selected 139 living megafauna species to examine how their populations had changed over the past 40,000 to 50,000 years, when similar large animals became extinct.
Large mammals are also known as megafauna - defined as animals that weigh more than 44 kilograms as an adult. Therefore, humans are also considered megafauna. In this study, however, the researchers looked at a species that weighed just 22 kilograms, so every continent was represented except Antarctica.
However, the size of the elephant population is not the only factor that affects the amount of variation.
If an area where elephants live suddenly dries up, the animals would be stressed, which could affect the composition of the variation. The same effect would occur if two isolated elephant populations suddenly met and mixed genes.
If it wasn't just the size of the population that affected the amount of variation, you would think the results would be inconclusive. But that's not the case, explains Juraj-Bergman.
"Only 10% of mammalian genomes are active genes. Strong selection pressure from the environment or migration will mainly lead to genetic mutations. The remaining 90% of genes are relatively neutral, so we studied mutations in the parts of the genome that are least susceptible to environmental influences. These parts mainly account for the size of the population over time."
Much of the debate over what led to the extinction or decline of large animals has focused on the woolly mammoth. But that's not a good example, because as Jens-Christian Svenning explains, most extinct megafauna species were associated with temperate or tropical climates.
"The classic argument for climate as an explanatory model is based on the fact that when the ice melted and habitat types disappeared, woolly mammoths and some other species associated with the so-called 'mammoth steppe' disappeared with them," he said. "This is basically an unsatisfactory explanatory model because the vast majority of the species disappeared at that time." "Extinct megafauna species did not live in the Mammoth Steppe at all. They lived in warmer areas such as temperate and tropical forests or savannahs. In our study, we also found that many of the surviving megafauna species experienced sharp declines during this period, and they came from a variety of different regions and habitats."
The end of the debate may not yet be decided, but Jens-Christian Svenning believes that the debate over climate as an explanation is difficult to continue.
"It seems unimaginable that it is possible to propose a climate model that explains why all continents and large fauna groups have experienced extinctions and continued declines since about 50,000 years ago. This selective extinction of megafauna over the past 66 million years, despite huge climate changes, is unique."
Given the wealth of data we now have, it's hard to deny that it was instead due to the global spread of humans from Africa and subsequent population growth.
Compiled from: ScitechDaily