The Pleistocene, an era which lasted from about 2.5 million years ago until 11,700 years ago, was an era whose beginning was marked by the growth of ice sheets in the Northern Hemisphere. It was a period of time marked by climatic instability. Paleontologists generally divide the Pleistocene into three periods—Early, Middle, and Late. The Late Pleistocene was characterized by many large mammals, known collectively as megafauna. The climate at this time was cooler and wetter than it is today.
Shown above is artist Mark Hallett’s perception of the Pleistocene. This is on display in the San Bernardino County Museum in Redlands, California.
According to the San Bernardino County Museum display:
“Most of the large mammals that lived here [California], like saber-tooth cats, camels, mammoths, mastodons, giant cave bears, ground sloths, and dire wolves, died out near the end of the Ice Ages. This is called the Pleistocene extinction.”
The display shown above is in the San Bernardino County Museum in Redlands, California.
According to the Museum display:
“Mastodons, gompotheres, and mammoths all looked something like today’s elephants, but they were only distantly related. They had a common ancestor 30 million years ago.”
Shown above is a mastodon (Mammut americanum) on display in the San Bernardino County Museum in Redlands, California.
Shown above is a mammoth (Mammuthus columbi) on display in the La Brea Tar Pits Museum in Los Angeles.
Shown above is a dire wolf (Canis dirus) on display in the La Brea Tar Pits Museum in Los Angeles.
Shown above is a saber-tooth cat (Smilodon fatalis) on display in the La Brea Tar Pits Museum in Los Angeles.
Shown above is a camel (Titanotylopus nebraskensis) on display in the Los Angeles County Museum of Natural History.
Shown above is a camel (Camelops hesternus) on display in the La Brea Tar Pits Museum.
Shown above is a large cat (Panthera atrox), often called the American lion, on display in the Los Angeles County Museum of Natural History.
Shown above is an ancient bison (Bison antiquus) on display in the La Brea Tar Pits Museum.
Shown above is Harlan’s ground sloth (Glossotherium harlani) on display in the La Brea Tar Pits Museum.
At the end of the Pleistocene nearly 75% of the megafauna (land-based animals over 44 kg) in North America died out. Ian Lange, in his book Ice Age Mammals of North America, writes:
“Extinction—defined as the end of the evolution of a genetically related group of organisms without replacement—has taken place since organisms first inhabited the earth. And extinction is the destiny of all species.”
With regard to the extinctions following the Ice Ages in North America, Ian Lange writes:
“The extinction of Ice Age mega-mammals was massive. North America lost at least thirty-one genera of animals weighing more than 100 pounds (44 kilograms), including several families and the entire elephant order.”
In his book Humans: From the Beginning, Christopher Seddon writes:
“In North America, 33 genera of megafauna went extinct, a loss of 72 percent; and in South America, the losses were even more severe with 50 genera lost, or 83 percent.”
Several hypotheses have been suggested regarding the cause of the extinction.
Climate change
There is a great deal of evidence showing global climate change at the end of the Pleistocene. During the Late Pleistocene, there were three major periods of ice sheet glaciation separated by lengthy nonglacial intervals. In his entry on the Pleistocene in The Oxford Companion to Archaeology, Alistair G. Dawson writes:
“These climatic changes must have influenced profoundly the movement of people from eastern Asia into the Americas, although it is not clear what effect such climatic changes may have had on the great “megafaunal” extinctions that took place at the close of the Pleistocene.”
There appear to have been two peak periods of decline for the megafauna. In an article on the megafauna overkill theory in American Archaeology, Paula Neely writes:
“The first period occurred about 14,100 years ago, when the climate was extremely arid. That was followed by a period of recovery about 500 years later. The population plummeted again about 12,700 years ago, coinciding with documented temperature and vegetation changes that could have led to deaths caused by dietary stress.”
According to the San Bernardino County Museum display:
“No previous Pleistocene climatic changes had the devastating effect of those 11,000 years ago. This is likely because plants and animals can respond to shifting climate by moving to different latitudes and elevations. Since no major extinctions are known from earlier Ice Age climate swings, such changes were probably not the only factor in the end-Pleistocene extinction of the megafauna, although they certainly played an important role.”
However, Los Angeles County Natural History Museum display points out:
“But similar climatic changes took place at least nine other times during the last 2 million years without resulting in widespread extinctions.”
According to Christopher Seddon:
“…the cause of the extinctions was a trend toward shorter annual growing seasons for plants, and increasing seasonality of climate, particularly in respect of rainfall. These trends were brought about by the long-term decline in global temperatures that began in the late Eocine.”
Christopher Seddon writes:
“Large non-ruminating herbivores such as horses, rhinos, mammoths and mastodons were particularly hard hit. These animals rely on a great deal of diversity of plant forms in their diet to provide a balanced mixture of carbohydrate, protein, fat, vitamins, minerals and fibre.”
Over-Hunting
Since the extinction occurs at about the time there is increasing evidence for modern humans in North America, there are a few people who feel—some quite strongly—that over-hunting led to the extinctions. In 1932, archaeologists working in a site near Clovis, New Mexico, found an ancient stone spearpoint embedded in the rib of a mammoth, thus starting the hypothesis of the Clovis mammoth hunters as the first Americans. In a report in American Archaeology, Tamara Stewart writes:
“Megafauna such as the Columbian mammoth went extinct at about the same time Clovis hunters were becoming established in North America, fueling a debate regarding whether the animals’ extinction was due to human hunting, climate change, or a combination of both.”
Data from the many Clovis sites which have been studied in North America doesn’t really substantiate the idea of Clovis hunters focusing on mammoths: there are only a dozen sites in which Clovis is associated with mammoth remains.
Archaeological data also shows a decline in the megafauna in several areas many centuries before the first human habitation. Christopher Seddon writes:
“Another problem is that some of the extinctions occurred in places where there were no humans. In Alaska, horses declined in size, then in numbers, and finally became extinct—but the extinction occurred some 500 years before the first humans arrived.”
According to the San Bernardino County Museum display:
“There is very little direct physical evidence to support this scenario. In southern California, none of the tens of thousands of fossils of late Pleistocene megafauna show any convincing signs of human interaction. Since the megafauna in our region died out in the apparent absence of human hunters, some other factor must be involved.”
While the over-hunting hypothesis is popular, the archaeological data doesn’t support it.
Human fires
In many places around the world, early hunting and gathering people used fire as a way of managing the ecology which they exploited. There is both historic and prehistoric evidence that American Indians burned prairie lands on the Great Plains, and forested areas in other parts of the continent as a part of their hunting and gathering strategies. Could this burning have altered the ecology enough to have been a factor in the extinction of the megafauna? Christopher Seddon writes:
“However, there is no clear-cut evidence that fire use by humans led to any major habitat alterations in the New World at the time of the extinctions.”
Human disease
There has also been the suggestion that humans brought with them diseases which impacted the megafauna. Again, there is no physical evidence for this.
Extraterrestrial
Another hypothesis that has been suggested is a catastrophic extraterrestrial event, such as that caused by a meteorite, could have caused the mass extinctions.
According to the San Bernardino County Museum display:
“It has been hypothesized that the near-impact of a comet 12,900 years ago over North America ignited continent-wide wildfires, melted glacial ice sheets, changed oceanic currents, and plunged the Earth into a millennium of global cooling. These dramatic changes are inferred to have caused the extinction of the megafauna.”
According to the Los Angeles County Natural History Museum display:
“It has been suggested that the breakup of a comet 11,500 years ago over the Great Lakes region of North America resulted in continent-wide wildfires. These may have adversely affected both the North American large mammals and the humans that were present in North America at that time.”
While this is an interesting hypothesis, it is difficult to provide actual evidence for an extraterrestrial cause for the extinction of the megafauna.
Bison
Another hypothesis focuses on the bison. According to the San Bernardino County Museum display:
“Bison were late immigrants into Pleistocene North America, not becoming abundant until near the end of the epoch. Could the presence of herds of bison have aggravated competition for dwindling food and water at the end of the Pleistocene? Might this, coupled with changing climate, have driven the extinction?”
According to the Los Angeles County Natural History Museum display:
“Bison were not present in North America before 200,000 years ago. Bison are grazing mammals. The introduction of large numbers of grazing mammals can have a major effect on the vegetation of an area. Perhaps the arrival of large numbers of bison caused habitat changes that other larger herbivores could not cope with.”
More Ancient America
Ancient America: Mastodons
Ancient America: Mammoths
Ancient America: North American Camels
Ancient America: Bears
Ancient America: The prehistoric Southwest, 1375-1425 CE
Ancient America: Some Plateau Indian petroglyphs (museum tour)
Ancient America: Northeast Arizona, 560 BCE to 825 CE
Paleontology 201: The North American Dire Wolf
