Tuesday, April 19, 2011

Extinction of Ice Age Mammals

It has recently been proposed that a comet or an asteroid may have exploded over northeastern North America around 12,900 years ago. The resulting fireball touched off immense wildfires over much of North America and sent debris into the atmosphere that settled as far away as Europe. A thousand year period of global cooling known as the Younger Dryas began about this same time and may have been the result of the bolide (comet or asteroid) explosion. Previously the Younger Dryas was thought to have resulted from the sudden influx into the Atlantic Ocean of fresh cold water from large glacial lakes that had been forming at the front of the retreating continental glaciation following the retreat of the ice sheet near the end of the last ice age. Perhaps the breakup of the ice was a result of the exploding bolide. 

The theory of an exploding bolide could explain other key events that happened around this same time period. In addition to the sudden mini-ice age recognized as the Younger Dryas, the distinctive Early American Clovis culture seems to have vanished at this time. Also a number of mammal species – mammoths, mastodons, giant ground sloths, giant beavers, saber-tooth cats, horses, and camels - went extinct about this same time. Much of the evidence for a bolide impact is found in a distinctive carbon-rich layer of sediment containing material indicating extraterrestrial origin. This sedimentary layer has been found at a number of Clovis habitation sites in North America. 

 Image Credit: from E.C, Pielou (1991)

However, more recently, the comet theory has met with increased skepticism. The main evidence cited by Firestone et al. (2007) in favor of the bolide theory is magnetic microspherules, carbon microspherules, fullerenes, a helium-3 anomaly, an iridium anomaly, a radiation anomaly, abundant charcoal and nanodiamonds. Haynes et al. (2010), however, argue that these anomalies are not indicative of extraterrestrial origin and use evidence of samples they collected from roof tops to demonstrate the source of anomalous values. Although Haynes et al. cannot support the bolide explosion and resulting extinctions, they admit that their evidence also does not preclude it.

Other extinction theories that have been proposed in the past include human overkill, climate change, and pandemic disease.

Paul Martin of the University of Arizona has been a champion of the overkill hypothesis. Martin’s hypothesis is that the extinction of many of the megafauna near the close of the ice age was the result of overkill by the Early Americans arriving in North America. There is evidence of massive kill sites in Canada and in the lower US where herds of animals such as buffalo were run over cliffs resulting in the death of many more animals than could be used to feed the people.

Climate change has also been proposed as a cause of the extinctions and an abrupt cooling period about 12,900 years ago, known as the Younger Dryas was mentioned above. When the North American ice cap had melted sufficiently to open up drainage of the massive lakes dammed behind the glaciation, it flowed out through the present Hudson Bay area into the North Atlantic. This inflow of water altered the ocean currents in the Atlantic and resulted in a cooling of the Northern Hemisphere.

Pandemic disease among the mega-mammal populations has also been proposed by Kathleen (2004), among others. The idea behind the pandemic disease is that the Early Americans brought a plague-like disease with them when they arrived in the new world that was transmitted to the mega-mammals, and it was this disease that caused the extinction.

For my novel, Ancestors of Gods, I liked the idea of a bolide striking the Earth, because that theory makes for a better story. In the end, however, it may have been a combination of factors that caused the extinctions. This is supported by the fact that the extinctions do not appear to be sudden or restricted to a certain time period. Instead the extinctions seem to have occurred over a period of time. Some of the last woolly mammoths survived on Wrangel Island off the northeast coast of Siberia until about 4000 years ago.


Pielou, E.C. (1991) After the Ice Age; The Return of Life to Glaciated North America; The University of Chicago Press; 366 pages.

Firestone RB, et al. (2007) Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling; Proc Natl Acad Sci USA, 104:16016–16021. http://www.pnas.org/content/104/41/16016.full.pdf+html

Haynes Jr. et al. (2010) The Murray Springs Clovis site, Pleistocene extinction, and the question of extraterrestrial impact; March 2010, 107(9), 4010-4015.  http://www.pnas.org/content/107/9/4010.full.pdf+html

Lyons, S. Kathleen, (2004) Was a ‘hyperdisease’ responsible for the late Pleisotcene megafaunal extinction?; Ecology Letters, Vol. y, Issue 9. 

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