Mammoths, mastodons, saber-toothed cats, glyptodonts, and other great beasts roamed the land of North America, Europe, and Asia, until something happened about 12,000 years ago. Just what that something was, is subject to great debate. We will focus on the case in North America.
Hypotheses attempting to explain the extinctions include: The mammoth hunters “is wot done it”, disease, normal climate change at the end of the glacial epoch, and abrupt climate change caused by a big flood or a comet strike.
Here is what we do know. Humans (of the Clovis Culture) entered North America at least 13,500 years ago when abundant megafauna inhabited the land. The planet was warming up from the last glacial epoch and had reached temperatures similar to today. Between about 12,900 and 11,500 years ago there was an abrupt cooling episode called the Younger Dryas (after a small Arctic flower), during which global temperatures plunged, in a matter of decades, from temperate climes to near glacial conditions. It stayed cold for about 1,400 years then rapidly warmed again. During that time, most of the megafauna became extinct. Evidence of the human Clovis culture also disappeared.
That point in time about 12,000 years ago, the end of the Pleistocene epoch, is marked in many places with a “black mat” of rich organic material. Below the mat are abundant fossils of megafauna and artifacts of the Clovis culture. No such fossils and few, if any, Clovis artifacts are reported above the black mat.
A study of the black mat at 50 Clovis sites in North America found a “discrete layer with … magnetic grains with iridium, magnetic microspherules, charcoal, soot, carbon spherules, glass-like carbon containing nanodiamonds, and fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at 12.9 ka.”
Let’s examine the hypotheses:
Could a relatively small group of people with spears kill off a continent full of large animals simply by hunting for food? No evidence has been found of mass slaughter. The hunting hypothesis requires that all megafauna be hunted to death everywhere at the same time, prior to the “black mat.”
The hunting hypothesis does not explain the demise of megafauna in Europe and Asia at about the same time.
As a corollary to hunting hypothesis is the contention that Clovis killing of all the mammoths caused the climate change. According to one proposal, the demise of leaf-eating mammoths allowed the spread of dwarf birch trees in the Arctic which changed the planet’s albedo and caused global warming. On the other hand a competing proposal speculates that killing of megafauna greatly decreased the amount of methane produced from mammoth farts and that led directly to global cooling. I’m not making this up.
There are two scenarios related to climate change. The first holds that normal warming from the peak period of glaciation changed the environment and reduced forage for large herbivores which led to their gradual demise. With reduced populations of herbivores, their predators also fell on hard times.
The comet hypothesis holds that “a comet, two and a half to three miles in diameter, that detonated 30 to 60 miles above the earth, triggering a massive shockwave, firestorms, and a subsequent drastic cooling effect across most of North America and northern Europe.” The composition of material associated with the black mat supports this idea.
However, University of Arizona researchers dispute the comet contention. They say the apparent extraterrestrial composition of the black mat can be explained by simple, alternative, earthly processes. For instance, they say the magnetic spherules result from automobile exhaust and power plant emissions. The same spherules found at Clovis sites in Arizona are also found in the dirt on rooftops. Nanodiamonds are a “common ingredient of cosmic dust; nanodiamonds are constantly raining down onto the earth’s surface, rendering them unsuitable as unequivocal evidence of an extraterrestrial impact.” Iridium is common in meteorites and can be produced from volcanoes.
The comet hypothesis was a good candidate to explain the abrupt cooling of the Younger Dryas. But there is another explanation. As the ice sheet in North America melted, vast lakes were formed. At some point, an ice dam broke releasing large quantities of fresh water into the ocean. This is said to have caused a change in the salinity and circulation of the heat-dispersing ocean currents which led to rapid cooling.
There is no compelling direct evidence that climate change was the culprit, only speculation.
This hypothesis holds that pathogens and parasites carried to North America by humans and their domestic animals infected and killed the megafauna. Such a disease would have to kill quickly, would have to affect many species, and would have to be benign to its human or animal carriers. So far, no evidence has been found to support this hypothesis.
There are many hypotheses about the extinction of mammoths and other megafauna, but we still don’t know for sure which, if any, or which combination of things, was the ultimate cause. Climate hypotheses alone, do not explain how the megafauna survived previous interglacial cycles over the last two million years, but not the current interglacial. Was the human influence the difference in this interglacial, or was it the comet?
The extinction remains a mammoth mystery.
P.S. Part of the problem in correlating timing of events is the use (or misuse) of radiocarbon dating. The “years” reported from radiocarbon dating depend on knowledge of how carbon14 has varied in the atmosphere over the time interval in question, and which value for the half-life of C14 the researchers used. Therefore, radiocarbon “years” are not true calendar years. For an explanation of radiocarbon dating and its problems see Radiocarbon Web-info from Oxford University.
For additional information on glacial-interglacial cycles, see my article on Ice Ages and Glacial Epochs.