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efforts will prevent a new round of extinctions in most body-size and taxonomic spectra; and (iii) a near-future biomass crash that will unfavorably impact humans and their domesticates and other species is unavoidable unless alternative energy sources are developed to replace dwindling supplies of fossil fuels.

The Quaternary Megafauna Extinction (QME) killed >178 species of the world’s largest mammals, those weighing at least 44 kg (roughly the size of sheep to elephants). More than 101 genera perished. Beginning ≈50,000 years (kyr) B.P. and largely completed by 7 kyr B.P., it was Earth’s latest great extinction event. The QME was the only major extinction that took place when humans were on the planet, and it also occurred at a time when human populations were rapidly expanding during a global warming episode. Thus, the QME takes on special significance in understanding the potential outcomes of a similar but scaled-up natural experiment that is underway today: the exponential growth of human populations at exactly the same time the Earth is warming at unprecedented rates.

Causes of the QME have been explored primarily through analyzing the chronology of extinction, geographic differences in extinction intensity, timing of human arrival vs. timing of climate change, and simulations that explore effects of humans hunting megafauna (Martin, 1967; Martin and Wright, 1967; Martin and Klein, 1984; MacPhee, 1999; Alroy, 2001; Roberts et al., 2001; Grayson and Meltzer, 2003; Barnosky et al., 2004; Trueman et al., 2005; Koch and Barnosky, 2006; Wroe and Field, 2006). Results of past studies indicate that human impacts such as hunting and habitat alteration contributed to the QME in many places, and that climate change exacerbated it. Potentially added to those megafauna stressors was the explosion of a comet over central North America, which may have helped to initiate the Younger Dryas (YD) climatic event, and which may have caused widespread wildfires, although those ideas are still being tested (Firestone et al., 2007).

Whatever the cause of the QME, one thing is clear: there was a dramatic change in the way energy flowed through the global ecosystem. The energy that powers ecosystems is derived from solar radiation, which is converted to biomass. Before the extinction, the energy needed to build megafauna biomass was divided among many species. After the extinction, increasing amounts and proportions of energy began to flow toward a single megafauna species, humans.

Humans are, by definition, a megafauna species, with an average body weight of ≈67 kg for modern Homo sapiens and 50 kg for Stone-Age people, placing us at the lower end of the body-size distribution for megafauna

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