The San Berdardino County Museum in Redlands, California, has a gallery called the Hall of Earth Sciences. Some of the exhibits in this gallery focus on the California Pleistocene.
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. According to the Museum display:
“Most of the large mammals that lived here, 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.”
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.”
The Pleistocene Extinctions
At the end of the Pleistocene nearly 75% of the megafauna (land-based animals over 44 kg) in North America died out. 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. According to the 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.”
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. According to the 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.”
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: According to the 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.”
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 focus on the bison. According to the 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?”