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. 

Sunday, April 3, 2011

Migration Routes for the Early Americans

An article in the recent issue of Science magazine details findings at the Buttermilk Creek archaeological sites in Texas and discusses migration routes of Early Americans into North America. Findings at these Texas sites argue for pre-Clovis Early Americans in North America as early as 15,500 years ago.

During the last ice-age sea level was some 400 feet lower than it is today. The lower sea level exposed a land mass, Beringia, that stretched between Siberia and Alaska and was as much as 1000 miles wide. Most scientists think that Early Americans arrived in North and South America via this route.

The “Clovis-first” proponents believe that Early Americas arrived in the present lower 48 states of North America between 12,800 and 13,100 years ago. Another group of archaeologists believe that Early Americans crossed through Beringia earlier, as early as 40,000 years ago.

There were two general paths of migrations from Siberia into the lower parts of North America. If people were in the southern part of North America prior to about 15,000 years ago, they probably arrived by boat following the Pacific coast. Prior to that the continental ice sheet covered most of present day Canada and joined with the Cordilleran ice sheet that covered the Rocky Mountains out to the Pacific coast. The only migration route available at the time was the coastal route. The ancestors of the people who lived along Buttermilk Creek probably followed this route.

Image credit: Dixon, E. James (2000)

15,000 years ago the ice sheets began to retreat and, in the process, opened up an ice-free corridor along the eastern side of the Rocky Mountains. The “Clovis-first” proponents favor Early Americans arriving in the southern part of the continent via the ice-free corridor route.
Image credit: Dixon, E. James (2000)

Another theory, favored by Dennis Stanford of the Smithsonian Institution and Bruce Bradley of the University of Exeter, suggests that the Early Americans came to North America from Spain, following the edge of the North Atlantic ice cap. Stanford and Bradley’s main argument is that the Clovis points in the Americas appear very similar to the Solutrean points found in southern Europe (see figure below). Also, currently, the preponderance of Clovis sites appear to be located in the eastern US. Stanford and Bradley suggest that ice-age fisherman and hunters sailed to America in small boats made of animal skins about 18,000 years ago; similar to the boats used by Eskimos today to hunt whales and seals.

Evidence suggesting Clovis-age people used the ice-free corridor has been found in Alberta and British Columbia. Some evidence has also been found along the Pacific coast supporting the coastal route theory. However, there may be even more evidence that is now under water. Any possible evidence for the migration route from Europe would now be at the bottom of the Atlantic Ocean. 

There is good evidence of an Early American settlement at Monte Verde, Chile that date back to at least 12,500 years ago and possibly as much as 14,800 years ago. As additional study is being made, evidence supporting earlier arrivals of people into North and South America is increasing. Migrations from Siberia likely happened more than once and each may have followed different migration routes. However, studies of current native North Americans, focusing on language, dental structure, and genetics, pretty well demonstrate that their ancestors came to the Americas via Siberia. So far the evidence for European origins is lacking.

Waters, Michael R., et. al. (2011) “The Buttermilk Creek Complex and the Origins of Clovis at the Debra L. Friedkin Site, Texas”; Science 25 March 2011: Vol. 331 no. 6024 pp. 1599-1603

Dixon, E. James (2000) "Bones Boats & Bison; Archeology and the First Colonization of Western North America"; The University of New Mexico Press; 322p.

Mithen, Steven, (2003) “After the Ice”; Harvard University Press; 622 p.