Humans Exonerated of Killing Off the American Giant Animals

We may however have delivered some megafauna species the coup de grâce, especially after a magnetic pole reversal caused global mayhem 42,000 years ago

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Artist's impression of an irritable meeting between a short-faced bear and saber-toothed cat
Extinct megafauna: Artist's impression of an irritable meeting between a short-faced bear and saber-toothed cat Credit: Daniel Eskridge / Shutterstock

It seems that in North America, at least, humans were not key to the extinction of the magnificent megafauna. The chef culprit in North American megafaunal extinction was climate change, claims a paper published in Nature last week. Meanwhile in other extinction news, a separate article in Science suggests that a 1,000-year reversal of Earth’s magnetic poles 42,000 years ago may have created global climatic mayhem that also decimated what megafauna still remained – and may have helped kill off the Neanderthals too, while about it.

The definition of megafauna is loose so let’s just call them big animals, usually defined as weighing over 100 pounds – about 44 kilos. Compared with the past, very few megafauna still exist today: Extant ones include elephants and rhinos, the big hoofed animals and cetaceans . By all means we can count giant reptiles too, such as the Komodo dragon described with affectionate revulsion by The Guardian last week. But the great proportion of megafauna are extinct.

To set the record straight, the Late Pleistocene extinction event began about 130,000 years ago. The ancestors of modern humans hadn’t even left Africa yet (though other hominins had been roaming Eurasia for two million years, and anatomically modern humans actually had started leaving Africa at least 200,000 years ago, but those lineages of early exiters went extinct).

It is true that the Neanderthals were definitely serious hunters, capable of using much the same tools as their Homo sapiens cousins. Some scholars believe, based on morphology, that the Neanderthals were apex predators with enlarged livers who subsisted mainly on meat. That has not been proven.

But we do know that Neanderthals and other hominins doted on eating megafauna, one reason being that large animals have abundant fat, which skinny little rodents do not, and fat is essential to the diet. Humans never did cavil at snacking on rats but it seems our diet shifted to small, lean prey only when there were no other choices.

Looking at a baby woolly mammoth, extracted from permafrost, and models of adult mammothsCredit: Matt Dunham / AP

Also, humans (including hominins) were clearly involved in some megafauna extinctions, and some of them were more recent than is generally realized.

The last mammoth died just 4,000 years ago, the time of the fabled patriarch Abraham. Ditto the last giant sloth. Once widespread through the Americas, the giant sloth featured in cave art – note a stunning example of what look exactly like a mother and baby giant sloth on the cliff face of Nuevo Tolima in Colombia. The awe-inspiring Smilodon saber-toothed tigers and American lions died out just 10,000 years ago, roughly speaking, well after humans had reached the New World. To think we just missed knowing them personally and must settle for skulls in the La Brea tar pits and the like is frustrating.

Evidence for the prosecution

There is evidence against humankind in the fossil record. For instance, the pace of the Late Paleolithic extinction “event” picked up in the early Holocene – the geological epoch in which modern human civilization began (roughly 11,500 years ago). Moreover, one oddity about the Late Pleistocene die-off is that the megafauna were not ecologically replaced with other big animals, as had been the case in previous extinctions, which tends to suggest our responsibility.

Another oddity is that a majority of Late Pleistocene extinctions in Eurasia and the Americas, and especially the final demise of large animals on isolated islands, coincided with the spread of modern humans.

Megatherium: Giant sloth skeletonCredit: Mariana Ruiz Villarreal / Ladyof

But before we leap to blame this whole thing on the insatiable human appetite for meat or theoretical puberty rites involving the ritual murder of giant cats, there is evidently no single explanation for all the megafaunal extinctions in the last 130,000 years. And in North America, climate change, not human population growth, correlates with Late Quaternary megafauna declines, write Mathew Stewart, W. Christopher Carleton and Huw Groucutt of the Max Planck Institute in Germany.

Using the latest modeling technique, Radiocarbon-dated Event-Count, the trio asked whether declines in North American megafauna could be best explained by changes in the climate, increases in human population densities, or both. The model produced no evidence of a persistent relationship between human and megafauna population levels in North America, the authors explain – but they did find evidence that decreases in global temperatures correlated with megafauna population declines.

As the Holocene began, some 80 percent of North America’s megafauna species were gone, the researchers say. But although humans apparently began reaching North America at least 15,000 years ago, apparently we didn’t do it, though overkill has been the popular theory for decades. Most of the animals were gone before we arrived.

The three also point to other signs of climatic “trauma” in the New World, such as a genetic bottleneck created in bison, the extinction of a number of bird groups, and a species of spruce tree. Animals and plant ranges moved to new grounds; several species including elks and bison became smaller.

We add that it is extremely hard to pinpoint when a species goes extinct, and that doesn’t only apply to the coelacanth in the ocean’s depths – thought to have been extinct for 65 million years, and isn’t, bless its fins. We would likely notice if towering megafauna were still roaming the planet surface, but their last appearance is hard to pin down. The three researchers also point out that megafauna could have been in decline and then humans arrived and delivered the coup de grâce.

Komodos, extant megafauna among the reptilesCredit: Achmad Ibrahim / AP
Finding the Coelacanth Credit: Neo Geo Wild

Magnetic poles deliver a body blow

Separately, another group writing in Science suggests that animals – and Neanderthals – were brought low by a global crisis caused by the reversal of Earth’s magnetic poles, as happens periodically, about 42,000 years ago.

The magnetic field protects us to a degree from solar radiation, and a biologically disruptive radiation bath is just one consequence of the resultant instability. The reversal and accompanying transient breakdown of the global magnetic field would also likely have decimated the ozone concentration in the atmosphere and caused climatic dramatics.

The pole flip in question – called a Laschamps excursion – lasted about 1,000 years and coincided with a “solar minimum.” During that time North America was covered in ice, doing the animals there no favors. Eurasia naturally was affected too.

At the time of this “excursion,” Eurasia was occupied by both modern humans, actual ancestors of all non-African modern humans, and other human species, including the Neanderthals and maybe others as well. And although the Neanderthals and especially their cousins the Denisovans are widely considered to have been cold-adapted, they couldn’t live on an ice sheet and there were other constraints.

Extinct American lionCredit: מתוך חשבון הפליקר

So the researchers posit that the magnetic flip combined with the low solar radiation contributed to the demise of the Neanderthals. Like the megafauna, possibly the magnetic pole reversal was a body blow to an already suffering species.

Other postulated contributions to the Neanderthals’ extinction include interbreeding, being out-competed by Homo sapiens on their own turf, and kuru (the human equivalent of mad cow disease) caused by cannibalism. The last known Neanderthals died about 40,000 to 35,000 years ago.

However the megafauna and Neanderthals disappeared, no, they are not likely to be resurrected in any meaningful way, despite sincere efforts to recreate a mammoth, and despite whispers of Neanderthal genes in our own genomes.

Even if scientists can obtain extraordinarily well-preserved ancient DNA from some hapless long-dead animal preserved in the permafrost, for example, the DNA will be damaged; even if they “reconstruct” much of the ancient genome, part will be guesswork and also, the inevitably hybridized animal will have to be nurtured in the womb of a closely related animal. In the case of a mammoth, absent a mommy mammoth with the proper prenatal environment, it will be grown inside an elephant. There are also the cultural elements the baby animal will be missing. It just won’t be the same.