Unveiling the True Culprits Behind the Decline of Earth’s Largest Creatures: DNA Reveals the Story

For an extended period, scientists have been entangled in a debate, pondering whether the population decrease of colossal mammals throughout the past millennia was attributed to the influence of humans or the changing climate. A recent study conducted by Aarhus University has decisively shifted the perspective, indicating that climate cannot be held accountable for this decline.

Approximately 100,000 years ago, the first wave of modern humans began their exodus from Africa in substantial numbers. Their remarkable capacity to adapt to diverse environments, ranging from arid deserts to lush jungles and frigid northern taiga, played a pivotal role in their success. One aspect of their prowess was their adeptness at hunting large animals, as they honed ingenious hunting techniques and crafted specialized weapons for this purpose. Unfortunately, the zenith of human accomplishment came at the expense of Earth’s other colossal inhabitants.

It is widely acknowledged that numerous colossal species vanished during the period of global human expansion. However, recent research from Aarhus University divulges that those few large mammals that did endure also witnessed a marked decline in their numbers.

One of the most severely affected species is the eastern gorilla, now confined to small pockets in the DR Congo. The greater one-horned rhinoceros, native to India and one of five remaining rhino species, has likewise suffered substantial decimation.

By meticulously scrutinizing the DNA of 139 extant large mammal species, scientists have unveiled a striking revelation: the populations of almost all these species experienced a substantial downturn around 50,000 years ago. Jens-Christian Svenning, a professor and the head of the Danish National Research Foundation’s Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) at Aarhus University, who initiated this study, elucidates, “We’ve delved into the evolution of large mammalian populations spanning the past 750,000 years. For the initial 700,000 years, these populations remained relatively stable. However, 50,000 years ago, a dramatic decline ensued, never to rebound.”

Over the last 800,000 years, our planet has oscillated between ice ages and interglacial periods approximately every 100,000 years. If climate were the primary instigator, one would expect more pronounced fluctuations preceding the 50,000-year mark when climate changes occurred. However, such fluctuations are conspicuously absent, rendering humans the most plausible explanation.

For decades, scientists have engaged in a fierce debate, contemplating the reasons for the extinction or significant decline of colossal mammals in the past 50,000 years. One camp posits that abrupt and severe climate fluctuations hold the key, contending that species like the woolly mammoth succumbed to the vanishing mammoth steppe ecosystem as temperatures rose. Conversely, another faction argues that the ascendancy of modern humans, Homo sapiens, drove these creatures to the brink of extinction through extensive hunting.

Until now, much of the pivotal evidence in this dispute has relied on fossils dating back 50,000 years. These fossils indicate a striking correlation between the selective extinction of large animals and the global proliferation of modern humans, casting doubt on climate as the predominant factor. Nevertheless, the debate persists.

This new study introduces fresh data that casts a revealing light on the debate. By examining the DNA of 139 extant large mammals—species that have endured over the past 50,000 years—the researchers offer compelling evidence that population declines in these animals are intrinsically tied to human expansion, rather than climate fluctuations.

In the past two decades, a revolution in DNA sequencing has unfolded, making the mapping of entire genomes both accessible and cost-effective. The DNA of numerous species worldwide is now readily accessible on the internet, and the research team at Aarhus University leveraged this invaluable resource. Assistant professor Juraj Bergman, the lead researcher behind the study, explains, “We’ve amassed data from 139 large living mammals and meticulously analyzed this vast dataset, encompassing approximately 3 billion data points for each species. This undertaking was time-consuming and demanded substantial computational power.”

DNA harbors a wealth of information about the past. Just as the tree of life delineates the evolutionary relationships among species and their common ancestors, the study extends this concept to mutations within DNA. By clustering mutations and constructing a genealogical tree, researchers can estimate the historical population sizes of specific species.

Population size significantly influences mutation rates. Larger populations witness a greater occurrence of mutations. Even seemingly neutral factors, such as environmental changes or the intermingling of isolated groups of animals, can induce mutations.

However, the study focuses on the DNA regions least susceptible to environmental influences, providing insights into population trends over time. This methodology minimizes the uncertainty arising from external factors.

The study scrutinizes 139 extant large mammals, including elephants, bears, kangaroos, and antelopes, among others. While Earth hosts an estimated 6,399 mammal species, this selection concentrated on megafauna, defined as animals exceeding 44 kg in adulthood. Notably, humans are also classified as megafauna. The researchers extended their investigation to species weighing as little as 22 kg, ensuring representation from all continents except Antarctica.

The study’s comprehensive approach isolates the influence of population size on mutation rates from other extraneous factors, yielding robust results.

While the woolly mammoth has long served as a focal point in discussions about megafauna extinction, its relevance is challenged by the fact that most extinct megafauna species inhabited temperate or tropical climates. Jens-Christian Svenning emphasizes, “The conventional arguments for climate as the primary explanatory model hinge on the idea that the woolly mammoth and other species associated with the ‘mammoth steppe’ vanished as the ice receded and their habitat disappeared. However, this model falls short, as the majority of megafauna species that went extinct did not inhabit the mammoth steppe but rather resided in warm regions, including temperate and tropical forests or savannahs. Our study reveals a pronounced population decline during this period among surviving megafauna species from diverse regions and habitats.”

While the final chapter in this debate remains unwritten, the arguments favoring climate as the sole explanation appear increasingly untenable. The weight of evidence suggests that the expansion of Homo sapiens across the globe, originating from Africa and subsequently experiencing exponential population growth, played a pivotal role in the selective loss and continuous decline of megafauna over the past 50,000 years.

Reference: Bergman, J., Pedersen, R. Ø., Lundgren, E. J., Lemoine, R. T., Monsarrat, S., Pearce, E. A., Schierup, M. H., & Svenning, J.-C. (2023). Worldwide Late Pleistocene and Early Holocene population declines in extant megafauna are associated with Homo sapiens expansion rather than climate change. Nature Communications, 14(1), 1-11. doi:10.1038/s41467-023-43426-5

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More about megafauna_decline

• Aarhus University
• Nature Communications – Research Article
• Definition of Megafauna
• Woolly Mammoth and Climate Change
• DNA Sequencing Advancements
• Tree of Life and Evolution
• Homo sapiens Expansion