The Pleistocene Extinction

This extinction is not as catastrophic as the Big Five that I discussed in a separate post, but here we see a massive 73% of large mammal genera disappear from Earth's ecosystems. There are two theories that have been put forward to explain this extinction; climate change and the Prehistoric Overkill Theory. It is possible that both of these worked in tandem to push the mammals to extinction. Due to the disappearance of the megafauna there is a shift in the planet's flora, grasslands becomes woodland, this increases the number of forest fires.

Theory 1 - Climate Change: This is the more popular theory. The warming of the the climate would have melted the vast ice sheets that covered a lot of the Northern Hemisphere, the sea levels would have risen preventing the migration of mammals to southern latitudes.

A warming climate would also alter the plant life; we see a transition from grasslands and confier forests that withstood the cold of the Ice Age to deciduous woodland. The herbivorous megafauna depended on the grasslands as their primary source of food, thus when the grasses disappear the mammals have to move further North following the receeding grasslands. This explains why the last populations of mammoths are found in the Arctic Circle in Siberia.

In part this change was natural, part of the fluctuation between glacial interglacial periods that characterise Ice Ages. The change to interglacial was exacerbated by the increase in forest fires, they contribute to an increase in greenhouse gases and therefore allow for the warming of the climate.

Humans managed to survive the extinction as we had the ability to quickly adapt our lifestyles to the changing climate.

Early humans hunting a woolly mammoth. The bones and skin would be used to
make shelters and the meat would be eaten. But did early man hunt to often?
Image credit sciencemag.org

Theory 2 - Prehistoric Overkill Theory: This theory was put forward by Paul Martin of the University of Arizona. Martin noticed a chronological and casual link between the appearance of humans and the disappearance of the megafauna mammals. The theory suggests that when humans first entered areas such as North America, the megafauna did not recognise humans as a threat as they had not come into contact with humans before. This meant that hunting the large mammals was much easier initially and therefore the humans exploited this through overhunting. The loss of the megafauna is also believed to have been the reason behind the extinction of smaller species as there would have been a major ecological disruption.

This extinction event lasted for 1,000 years. In comparison to the K/T Extinction it was a very rapid event, it is believed that the extinction of the dinosaurs took close to 55,000 years from the impact of the meteorite in the Gulf of Mexico.

There is evidence to support this theory. For instance, some Mastodon bones are found with the scarring of tuberculosis. This is not seen in fossils before the appearance of humans in the area, indicating that these animals were susceptible to new diseases that humans were carrying. Also in Africa where large mammals and humans had coevolved and coexisted for millions of years, there were very few extinctions, only two out of twenty three large mammal genera went extinct.

Which theory do you think is correct? Climate change, Prehistoric Overkill or both? Let me know in the comments.