When Mammals Were Bigger

In the Natural History Museum’s Hall of Primitive Mammals, a gargantuan skeleton clambers over plaster sand, the viewer safely behind a glass case. On display is the giant Diprotodon, an ancient mammal that inhabited Australia up until about 20,000 years ago. Most megafauna of Australia and North America are now extinct, and their disappearance correlates with first human contact. Homo sapiens developed watercraft to navigate the South Pacific as early as 45,000 years ago. Their arrival on mainland Australia is timestamped in fossil record along with the last appearance of two-thousand-pound lizards and rhino-like marsupials. Across the Pacific, in North America, 80% of megafauna became extinct before the last Ice Age. The Shasta ground-sloth and Harrington’s mountain goat were found near the Grand Canyon as late as 11,100BC.

History of Megafauna

It is widely accepted in the scientific community that human hunting practices led to the extermination of megafauna in both North America and Australia. Climate change may have had an impact in both environments, as a shift in global temperature fragmented habitats and rendered large mammals more vulnerable to attack. When humans stumbled across these giant animals, did they understand the impact hunting would have on populations? Economic theorists posit that the option to kill smaller prey warranted an extermination of larger mammals. A second study in the Journal of Human Evolution suggests that Paleolithic hunters in Europe and Asia targeted only the most abundant prey. The study cites evidence that humans coexisted with Eurasian megafauna for longer than in North America or Australia.

Natural History of Megafauna

Evolutionary biologists have codified a delicate balance between the occurrence of megafauna, and other mammals that share an ecosystem. The Forgotten Megafauna argues that larger mammals have a crucial role in seed dispersal, because of their staggering ability to consume and proliferate fruit. The loss of megafauna in continental landscapes such as South America depleted plant life and made it harder for other fruit-eating mammals to live in the same habitat. The body mass of present-day frugivores, like the Tapir, is a full order of magnitude smaller than the largest extinct megafauna. In vulnerable island environments, the surviving mammals are one hundred to one thousand times smaller than their extinct peers. Humans share plant-eating tendencies with large mammals and compete for the same resources. Mathematical models suggest that co-existence of humans and megafauna can cause ecological meltdown, even when humans aren’t hunting.

Herbivory aside, the force of technological innovation gave humans a leg up over giant mammals. The invention of boats exposed unacquainted animals to spear-equipped troops. The advent of agricultural fires cleared up land for crops but left megafauna out in the open. In examining a present-day mass-extinction, we must ask ourselves if innovation and ecological vulnerability are intertwined.

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