Probably the most famous site for extinct mammals is the La Brea Tarpits on Wilshire Blvd. in the heart of Los Angeles. But two big changes happened almost simultaneously with the extinction of the mammoths, dire wolves, camels, etc. of Los Angeles: the end of the Ice Age and the arrival of Siberian mammoth hunters. So it’s hard to be sure which of the two was the cause. The Tar Pits museum traditionally went for blaming climate change, while UCLA professor Jared Diamond figures it was the Indians.

Here’s a press release from Aarhus U. in Denmark promoting a new paper on the perennial topic of what killed off Ice Age megafauna: climate change or Indians and other human hunters having them for dinner. They step back from La Brea Tar Pits mammoth controversy to look at the bigger picture using DNA data from surviving megafauna.

(As I’ve often mentioned, I like university press releases in which a PR staffer creates a readable summary of a professor’s science paper and the academic checks it over before it goes out.)

People, not the climate, caused the decline of the giant mammals

For years, scientists have debated whether humans or the climate have caused the population of large mammals to decline dramatically over the past several thousand years. A new study from Aarhus University confirms that climate cannot be the explanation.

7 December 2023 by Jeppe Kyhne Knudsen

About 100,000 years ago, the first modern humans migrated out of Africa in large numbers. They were eminent at adapting to new habitats, and they settled in virtually every kind of landscape—from deserts to jungles to the icy taiga in the far north.

Part of the success was human’s [sic] ability to hunt large animals. With clever hunting techniques and specially built weapons, they perfected the art of killing even the most dangerous mammals.

But unfortunately, the great success of our ancestors came at the expense of the other large mammals.

It is well-known that numerous large species went extinct during the time of the world-wide colonization by modern humans. Now, new research from Aarhus University reveals that those large mammals that survived, also experienced a dramatic decline.

In other words, it wasn’t just the extinct species that took a beating when modern humans showed up over the last 50,000 years, but also the surviving species.

By studying the DNA of 139 living species of large mammals, the scientists have been able to show that abundances of almost all species fell dramatically about 50,000 years ago.

This is according to Jens-Christian Svenning, a professor and head of the Danish National Research Foundation’s Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) at Aarhus University, and the initiator of the study.

“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,” he says, and continues:

So that’s a suggested date for when anatomically modern humans became modern in behavior—when they started wiping out megafauna around 50,000 years ago. Damaging biodiversity is What We Do.

“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.”

Who killed the large mammals?

For decades, scientists have debated what is behind the extinction or rapid decline of large mammals over the past 50,000 years.

On one side are scientists who believe that rapid and severe fluctuations in the climate are the main explanation. For example, they believe that the woolly mammoth went extinct because the cold mammoth steppe largely disappeared.

There’s a lot of cold country way north. Mammoths survived on Wrangell Island north of Siberia until about 4,000 years ago.

On the opposite side are a group who believe that the prevalence of modern humans (Homo sapiens) is the explanation….

The new study presents brand new data that sheds new light on the debate. By looking at the DNA of 139 large living mammals—species that have survived for the past 50,000 years without becoming extinct—the researchers can show that the populations of these animals have also declined over the period. This development seems to be linked to the spread of humans and not climate change.

DNA contains the long-term history of the species

By grouping the mutations and building a family tree, we can estimate the size of the population of a specific species over time.

The larger the population of an animal, the more mutations will occur. …

“Only 10 per cent of mammalian genomes consist of active genes.

They used to call them junk genes, but then it was discovered that they did some stuff, just a lot less, so the term “junk genes” was discarded, even though it’s pretty helpful for explaining this technique.

“Great selection pressure from the environment or migration will primarily lead to mutations in the genes. The remaining 90 percent, on the other hand, are more neutral,” he says, and continues:

“We have therefore examined mutations in those parts of the genome that are least susceptible to the environment. These parts primarily indicate something about the size of the population over time.”

The woolly mammoth is an atypical case

Much of the debate about what caused the large animals to either become extinct or decline has centered around the woolly mammoth.

You can see why people think more about woolly mammoths then most other extinct Ice Age megafauna. You’re riding down Wilshire Blvd. and there’s Daddy Mammoth sinking into the La Brea Tarpits while Mommy Mammoth and Junior Mammoth bellow tragically on the bank. That tends to seize the imagination of nine-year-olds.

But this is a bad example because the majority of the megafauna species that went were associated with temperate or tropical climates, as Jens-Christian Svenning explains.

… 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.”

The final full stop in the debate has probably yet to be set, but Jens-Christian Svenning finds it difficult to see how the arguments for the climate as an explanation can continue.

“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.“