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Decline of large mammals was caused by humans, not climate 

What led to the dramatic decline of large mammals over the past several millennia? Was it human intervention or climate change? A recent study from Aarhus University suggests that humans were the culprit.

Approximately 100,000 years ago, modern humans began their exodus from Africa, spreading across the globe and inhabiting diverse landscapes ranging from arid deserts to dense jungles and the frigid taiga of the far north. 

A key factor in the success of this group was their proficiency in hunting large animals with sophisticated techniques and specialized weapons. However, this success came at a dire cost to the large mammals that co-inhabited these regions.

Focus of the study

The research was led by Professor Jens-Christian Svenning of the Danish National Research Foundation’s Center for Ecological Dynamics in a Novel Biosphere (ECONOVO).

The researchers analyzed the DNA of 139 living species of large mammals, revealing a universal and dramatic decline in their populations approximately 50,000 years ago. This timeline intriguingly coincides with the widespread colonization of the world by modern humans. 

Key insights 

“We’ve studied the evolution of large mammalian 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 fell dramatically and never recovered,” explained Professor Svenning.

“For the past 800,000 years, the globe has fluctuated between ice ages and interglacial periods about every 100,000 years. If climate was the cause, we should see greater fluctuations when the climate changed prior to 50.000 years ago. But we don’t. Humans are therefore the most likely explanation.”

This conclusion contradicts the long-held belief by some scientists that climate fluctuations, such as those affecting the woolly mammoth, were the primary cause of these extinctions. 

Enormous data analysis 

The mapped genomes of species all over the globe are freely accessible on the internet – and this is the data that was used for the study, explained Professor Juraj Bergman, the lead researcher.

“We’ve collected data from 139 large living mammals and analyzed the enormous amount of data. There are approximately 3 billion data points from each species, so it took a long time and a lot of computing power,” said Professor Bergman.

Mutations in the DNA

“DNA contains a lot of information about the past. Most people know the tree of life, which shows where the different species developed and what common ancestors they have. We’ve done the same with mutations in the DNA. By grouping the mutations and building a family tree, we can estimate the size of the population of a specific species over time,” explained Professor Bergman.

“The larger the population of an animal, the more mutations will occur. It’s really a question of simple mathematics. Take elephants, for example. Every time an elephant is conceived, there’s a chance that a number of mutations will occur, and it will pass these on to subsequent generations. More births means more mutations.”

Study significance 

The study encompasses a broad range of large mammals (megafauna), defined as animals weighing over 44 kg, and even includes species as light as 22 kg to ensure a global representation.

One critical aspect of the study was focusing on the neutral parts of the DNA, which are less susceptible to environmental influences, providing a clearer picture of population changes over time. 

Woolly mammoth 

This approach challenges the notion that the woolly mammoth’s extinction, often cited as a climate change consequence, is representative of the broader trend in megafauna decline.

Much of the debate about what caused the large animals to either become extinct or decline has centered around the woolly mammoth. But this is a bad example because the majority of the megafauna species that went were associated with temperate or tropical climates, explained Svenning.

Unsatisfactory model

“The classic arguments for the climate as an explanatory model are based on the fact that the woolly mammoth and a number of other species associated with the so-called ‘mammoth steppe’ disappeared when the ice melted and the habitat type disappeared,” said Svenning.

“This is basically an unsatisfactory explanatory model, as the vast majority of the extinct megafauna species of the period did not live at all on the mammoth steppe. They lived in warm regions, such as temperate and tropical forests or savannahs. In our study, we also show a sharp decline during this period in populations of the many megafauna species that survived and come from all sorts of different regions and habitats.”

Broader implications 

Despite the robust evidence presented, the debate may not be entirely settled, but the new findings make a compelling case against climate change as the primary driver of the decline in large mammals. 

“It seems inconceivable that it is possible to come up with a climate model that explains how, across all continents and groups of large animals, there have been extinctions and continuous decline since about 50,000 years ago. And how this selective loss of megafauna is unique for the past 66 million years, despite huge climate change,” said Svenning.

“Given the rich data we now have, it’s also hard to deny that instead it is because humans spread across the globe from Africa and subsequently grew in population.”

The study is published in the journal Nature Communications

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