So sayeth a new paper to be published in the Journal of Economic Behavior & Organization which proposes that the deaths of woolly mammoths and other Ice Age animals 12,000 years ago were the result of humans having chance encounters with megafauna while out hunting smaller game.

Such "converging" toward overkill may come as news to paleontologists.

The authors of the paper, titled "Megafauna Extinction: A Paleoeconomic Theory of Human Overkill in the Pleistocene," are the same trio of economists who wrote an earlier paper suggesting trade among Homo sapiens led to the extinction of Neandertals.

The new paper is an extension of the "overkill hypothesis" introduced by Paul Martin in 1967. This theory suggested that human arrival in North America via the Bering Land Bridge at the end of the last Ice Age led to the demise of 57 species of large mammals.

Critics of overkill say the incoming population of humans was too small to hunt so many animals to extinction, but a 2001 computer simulation by University of California-Santa Barbara biologist John Alroy predicted that a group of 100 hunters with modest population growth could indeed wipe out North American megafauna.

Megafaunal extinctions have also been blamed on climatic warming at the end of the Pleistocene. Yet there were fewer extinctions in previous interglacial periods than at the beginning of the interglacial in which we currently live, the Holocene. The only difference between this one and the others was the arrival of humans in the New World.

The authors of "Megafauna Extinction" posit that if early North American hunters focused merely on hunting relatively small animals such as rabbits and deer, accidental encounters with big megafauna like mammoths were common enough to result in megafaunal kills. The eventual result was extinction of proboscideans, horses, camels and other large New World plant-eaters, as well as the carnivores that preyed upon them.

An alternative food source is assumed for humans, otherwise they would have followed megafauna into extinction once big game were gone. In the models, primitive agriculture as the alternative results in stable co-existence between man and beast because folks are too busy farming to hunt. Since megafauna aren't here today, ergo, agriculture must not have led to overkill. Small prey, or "minifauna" (the authors' invented category), resolves the dilemma by moving humans out of the cave and onto the tundra where they encounter megafauna. Once the megafauna are gone, hunters switch fully to chasing minifauna, which reproductively sustain their populations.

But overkill fails to explain why certain large animals -- megafauna -- still exist in the Americas. Why do American elk and moose, both of which died in Alroy's computer model, abound today? Why are bighorn sheep still with us? Why is the northern elephant seal the only American mammal over two tons to survive the Pleistocene? Why do bison, which were known targets of Clovis hunters, still roam the plains? Why are deer crapping on my lawn?

Proponents of overkill explain this paradox by suggesting many species which entered the Americas long before mankind were "nave" to human hunters when they appeared. Still, the overkill hypothesis, wholly predicated on this "navete," conjectures specifically about extinctions in North America and Australia; it doesn't work anywhere else -- and even Down Under, new evidence suggests humans coexisted with giant kangaroos and other Australian chazzwazzers for at least 15,000 years.

Elsewhere, such as in Africa, it is supposed that development alongside humans selectively produced animals that were leery of hominine predation, so living megafauna such as bison, which entered the Americas in relatively recent times, were apparently instinctually aware of our taste for barbecue. Meanwhile, the mammoths that arrived 1.7 million years ago were ostensibly nave to humans and could not effectively defend themselves when Homo sapiens finally crossed Beringia.

The navete argument is a big pill to swallow, considering North American herbivores knew enough to steer clear of saber cats, lions, wolves and other predators. Those animals who have historically demonstrated complete ignorance of humans, like the dodo, developed in areas where there was a complete absence of any predators, not just the bipedal kind. Even the massive ground sloths of the Pleistocene, which may or may not have been slow-moving, somehow thrived in an environment that bears comparison to the modern African savannah. The 3,500-lb. Harlan's ground sloth had dermal ossicles -- bony bumps -- which effectively wrapped it in armor and gave it a set of brass knuckles on each paw.  While overkill proponents say the hunting skills of Ice Age humans should not be underestimated, neither should the defenses of big animals that could demonstrably take care of themselves.

Assuming they were healthy.

"Recent work suggests that plants were undergoing carbon starvation during the last Ice Age and that plant productivity was very low," said John M. Harris, chief curator of the George C. Page Museum in Los Angeles. "This would have lessened the food supplies for herbivores and that in turn would have imposed stress on the carnivores.

Harris et al. measured carbon levels in samples of juniper recovered from the Rancho La Brea Tar Pits and found them to be severely lacking compared to modern juniper species. Based on examples of other plants that have experienced carbon starvation, this greatly reduced juniper availability.

"Fauna was already under stress due to climatic fluctuation and the arrival of yet another carnivore -- humans -- was the last straw," said Harris.

Giant fish-eating pinnipeds like walruses and elephant seals would have been unaffected by the extinction of terrestrial herbivores. But what about the rest? The ancestors of both elk and bighorn sheep entered North America during the Irvingtonian Land Mammal Age, between 1.9 million years ago and 300,000-150,000 years ago, the same time the first mammoths arrived.

"Does that mean elk were extra fast learners or extra fast breeders?" asked "Megafauna Extinction" co-author Jason Shogren, professor of natural resource conservation and management at the University of Wyoming. "Elk should have been easy prey. This may have little to do with learning by elk and more to do with the types of investments humans deemed to be optimal -- that is, investments that made it easier to target other species, perhaps because those other species were easier to catch and so we specialized more in those."

"The large carnivores became extinct because their large prey disappeared," said Harris. "The larger herbivores digest their food in one of two ways: ruminants have a multi-chambered stomach and chew their food several times, known as chewing the cud, whereas non-ruminants have less complex stomachs and the bulk of their digestion takes place through fermentation in the hind gut.

"When you review the herbivores that survived you'll note they comprise ruminants (bison, deer) and omnivores (peccaries). Horses, ground sloths and proboscideans are hind-gut fermenters that failed to survive."

Harris said ruminants tend to process food more thoroughly, which may have given them an advantage in an environmentally stressed habitat.

Said Harris: "This may be another clue that the late Pleistocene extinction in North America had an environmental component and wasn't solely due to human predation."

I badgered Peter D. Ward, a paleontologist at the University of Washington in Seattle whose 1997 book The Call of Distant Mammoths argues a case for overkill. Why did one species survive and another did not, I asked. Why this one, why not that one?

Ward's response:

"Great questions all. I wish I could answer them. Lots of new info since I wrote that potboiler."