John Frederick Walker

Were spear-carrying ice age hunters responsible for the disappearance of woolly mammoths, ground sloths, and other large, lumbering animals of the Pleistocene?  No theories that seek to explain their demise—even ones that invoke crashing comets and climate change—seize the public’s attention the way this “blitzkrieg” hypothesis does.  The idea that early humans went on a millennia-long killing spree gives us the shivers and confirms our worst fears about ourselves.  See, the thinking goes, humans have been mucking up the planet since we stood upright….

That our species contributes to global warming is inarguable, but there’s been less certainty over whether we had a role in ice age extinctions.  That question has returned with renewed urgency with the recent publication of “Pleistocene megafaunal collapse” in Science by Jacquelyn Gill, John W. Williams and researchers at the University of Wisconsin-Madison and other institutions. It sparked a flurry of media reports.

The team studied the dung fungus in lake deposits at sites in Indiana and New York left behind by mammoths and their ilk. These humble markers suggest that the giant beasts that once roamed North America began dying out between 14,800 and 13,700 years ago.  That was before the shift in vegetation from grasslands to forest that some theorists argued was the reason for these creatures’ decline.  Now it looks like their disappearance caused the changeover in habitat greenery and the increased wildfires that followed.  If this timeline is accurate, another great critter die-off theory also collapses:  a comet impact on (or over) what became Canada around 12,900 years ago.  Ditto for the supposed killer cold-snap of the Younger Dryas, the return to semi-ice age conditions that some say may have been triggered by the impact.

So let’s take a close look at those spear-shaking humans.

The blitzkrieg hypothesis is associated with Paul Martin of the University of Arizona, who, starting in the 1960s, underscored the association between megafauna die-offs and the spread of the human species.  Butchered mammoth bones from the period 14,700 to 14,100 years ago found at a site in Wisconsin suggests that people were hunting these huge creatures at the very start of their decline.  And a mammoth skeleton with a spear point in its ribs was among the first evidence found of the Clovis People, so named after the artifacts (notably elegant and deadly spear points) of a late Paleolithic hunting and foraging culture uncovered near Clovis, New Mexico.  Those people are thought to have crossed from Siberia to Alaska during lowered sea-levels of the last ice age before fanning out across the continent about 13,000 years ago, when megafaunal extinctions were largely finishing up, and may well have helped hasten the process.

Although humans are omnivorous and can make use of a wide variety of nutritional sources, the concentrated food value of meat in this period made hunting a key survival strategy.  Killing small game, which requires a large expenditure of energy for a relatively small gain, could not match the payoff of bringing down larger prey.  Pleistocene peoples became efficient killers, learning to isolate, trap, and throw spears at their prey before finishing the job at close quarters.

Did they hunt to extinction many of the creatures they encountered? We have the grim examples of man-driven massacres of island species, notably on Madagascar and New Zealand.  In historic times, we have the evidence of the annihilation of the quagga, the dodo, the thylacine, the passenger pigeon, et al.—surely we could have snuffed out whole species back in the days when no one was keeping a list.

But some researchers find the overkill hypothesis distinctly overrated.  Paleontologist Ross MacPhee has pointed out that there are a number of examples of species, including fur seals, various whales, and bison that weren’t wiped out despite ruthless pursuit by later humans with highly advanced weaponry.

It’s been suggested that now-extinct megafauna were doomed by their lack of wariness and their inability to cope with the sudden appearance of two-legged predators streaming into their habitat.  The cow-sized ground sloth of the late Pleistocene was probably slow-witted and relatively easy to bag.  But were woolly mammoths really so naïve that they allowed human hunters to sidle up to them and plunge a stone-tipped spear between their ribs? Wouldn’t they have been leery of approaching humans? If not at first, how long would it have taken the ancestors of modern elephants to develop strategies to deal with these strange new enemies?

Even if it was relatively easy pickings for these hunters—a big if—did they knock off more than they could eat, just for the hell of it, the way 19th century train passengers used to amuse themselves by shooting bison from railway car windows on the Kansas Pacific?  In the harsh Pleistocene, excess slaughter would have been a waste of precious energy, and pointlessly courting injury or death when larger quarry was tackled.  Although various parts—skin, bones, teeth—of the animals slain were utilized, and  mammoth ivory in particular prized as a carving material, we can be confident that meat was the main point of a hunt in pre-history.

It would have taken more than intermittent wanton slaughter for the tiny populations of early man to kill off slow-breeding species faster than they could replace themselves, sending them into an irreversible slide.  It’s possible a few animals disappeared this way, but what about all other extinct species in North America, like bear-sized beavers and western camels? Humans would have had to methodically exterminate millions of such creatures, chasing them down in every corner of the continent.  To explain the global disappearance of megafauna, this unlikely scenario would have to have been repeated on every other continent as well.

MacPhee concedes that the evidence shows that wherever humans arrived, animal extinctions followed, but doubts that hunting alone explains that. In collaboration with Preston Marx, MacPhee hypothesized that humans may have brought diseases with them that killed off indigenous species, although so far no evidence for the hyperdisease model has been uncovered.

Gill said that while her group’s new findings weren’t consistent with the blitzkrieg hypothesis, she and her colleagues couldn’t flatly rule out human involvement.  For other researchers, the idea that Pleistocene hunters singlehandedly knocked off the mammoth and similar megafauna may just be too seductive to let go.  Christopher Johnson, an Australian biologist who wrote an accompanying commentary in the same issue of Science, remains convinced that human hunters were the “sole cause” of “megafaunal extinction.”

Next month’s meeting in San Francisco of the American Geophysical Union will give comet proponents a chance to present new evidence for their extinction-with-a-bang theory. Frankly, it may take the impact of a comet to make those who blame our ancestors for the disappearance of the mammoth to drop the idea.