Finally we can stop being guilty about SOMETHING~ctm
New research finds grassland expansion drove the decline of giant mammals over the last 4.6 million years
University of Utah
New research disputes a long-held view that our earliest tool-bearing ancestors contributed to the demise of large mammals in Africa over the last several million years. Instead, the researchers argue that long-term environmental change drove the extinctions, mainly in the form of grassland expansion likely caused by falling atmospheric carbon dioxide (CO2) levels.
Tyler Faith, curator of archaeology at the Natural History Museum of Utah and assistant professor in the Department of Anthropology at the University of Utah, led the study. The research team also includes John Rowan from the University of Massachusetts Amherst, Andrew Du from the University of Chicago, and Paul Koch from the University of California, Santa Cruz.
The study is published today in the journal Science.
“Despite decades of literature asserting that early hominins impacted ancient African faunas, there have been few attempts to actually test this scenario or to explore alternatives,” Faith says. “We think our study is a major step towards understanding the depth of anthropogenic impacts on large mammal communities, and provides a convincing counter-argument to these long-held views about our early ancestors.”
To test for ancient hominin impacts, the researchers compiled a seven-million-year record of herbivore extinctions in eastern Africa, focusing on the very largest species, the so-called ‘megaherbivores’ (species over 2,000 lbs.) Though only five megaherbivores exist in Africa today, there was a much greater diversity in the past. For example, three-million-year-old ‘Lucy’ (Australopithecus afarensis) shared her woodland landscape with three giraffes, two rhinos, a hippo, and four elephant-like species at Hadar, Ethiopia.
When and why these species disappeared has long been a mystery for archaeologists and paleontologists, despite the evolution of tool-using and meat-eating hominins getting most of the blame.
“Our analyses show that there is a steady, long-term decline of megaherbivore diversity beginning around 4.6 million years ago. This extinction process kicks in over a million years before the very earliest evidence for human ancestors making tools or butchering animal carcasses and well before the appearance of any hominin species realistically capable of hunting them, like Homo erectus,” says Faith.
Taking a Closer Look
Faith and his team quantified long-term changes in eastern African megaherbivores using a dataset of more than 100 fossil assemblages spanning the last seven million years. The team also examined independent records of climatic and environmental trends and their effects, specifically global atmospheric CO2, stable carbon isotope records of vegetation structure, and stable carbon isotopes of eastern African fossil herbivore teeth, among others.
Their analysis reveals that over the last seven million years substantial megaherbivore extinctions occurred: 28 lineages became extinct, leading to the present-day communities lacking in large animals. These results highlight the great diversity of ancient megaherbivore communities, with many having far more megaherbivore species than exist today across Africa as a whole.
Further analysis showed that the onset of the megaherbivore decline began roughly 4.6 million years ago, and that the rate of diversity decline did not change following the appearance of Homo erectus, a human ancestor often blamed for the extinctions. Rather, Faith’s team argues that climate is more likely culprit.
“The key factor in the Plio-Pleistocene megaherbivore decline seems to be the expansion of grasslands, which is likely related to a global drop in atmospheric CO2 over the last five million years,” says John Rowan, a postdoctoral scientist from University of Massachusetts Amherst. “Low CO2 levels favor tropical grasses over trees, and as a consequence savannas became less woody and more open through time. We know that many of the extinct megaherbivores fed on woody vegetation, so they seem to disappear alongside their food source.”
The loss of massive herbivores may also account for other extinctions that have also been attributed to ancient hominins. Some scientist suggest that competition with increasingly carnivorous species of Homo led to the demise of numerous carnivores over the last few million years. Faith and his team suggest an alternative.
“We know there are also major extinctions among African carnivores at this time and that some of them, like saber-tooth cats, may have specialized on very large prey, perhaps juvenile elephants” says Paul Koch. “It could be that some of these carnivores disappeared with their megaherbivore prey.”
“Looking at all of the potential drivers of the megaherbivore decline, our analyses suggest that changing climate and environment played the key role in Africa’s past extinctions,” said Faith. “It follows that in the search for ancient hominin impacts on ancient African ecosystems, we must focus our attention on the one species known to be capable of causing them – us, Homo sapiens, over the last 300,000 years.”
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HT/CBB
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I am relieved to know that my great*100,000 (+/-) grandparents are not responsible for the significant die-off in large mammals in Africa some 2.6 to 4.6 million years ago. A heavy weight has been lifted from my shoulders.
An Alternative Theory: Man with Torch did it.
The earliest hearth discovered so far is in Swartkrans Cave in South Africa and is dated 1.5 million years old. However, it is likely that hominids domesticated fire even earlier, as much as 2.2 million years ago. Our jaws and intestinal tracts are adapted to eating cooked food, (not big food). Our upright posture is adapted to carrying stuff in our hands — such as sticks for firewood into the caves where the hearths were.
And if we could make fires in caves, we could absolutely make fire outside the caves, too. Like in the brush under the trees, in the grass, or anywhere we could get the landscape to burn.
It was pre-dudes and pre-dudettes with torches that altered the vegetation — not CO2 levels. The anthro-burned savannas favored some quadrupeds and not others.
Every other theory mentioned neglects the KNOWN propensity for fire starting by the only creature in creation that is ignition-enabled (and indeed totally fire dependent). Apparently all you so-called homo sapiens have forgotten about this.
Those who study extinctions have surely not forgotten. Please see my links to NZ extinctions below. Thanks.
It think I’ve seen it written, that it wasn’t so much that “meat eating” lead to better nutrition and bigger brains…. It was the use of fire to cook raw foods, both meat and vegetable alike to make them more digestible and release more calories and nutrients. Raw meat is high in protein and hard for a body to digest unless it is cooked. It takes quite a bit of energy to digest meat and derive the protein from it.
As for CO2 having an effect on plants rather than climate….. Yep, absolutely. We even see that today with satellites showing “greening” of the Earth, while statistics for crop growth also show CO2’s beneficial effects on Plantlife.
Okay. So a megafauna prey/predator ratio is stable. H. Somebody shows up and starts after mammoth calves. Each time H. Somebody takes a mammoth calf, Smilodon is done out of lunch. H. S. isn’t a sportsman but a businessman, going after the easy ones. Like Smilodon, looking for the sick, lame, lazy, and clueless.
Could a case be made that, first, Smilodon was erased by H. Somebody? Or if not Smilodon, some predator in roughly the same niche?
So, at least at first, the prey/predator ratio might not have changed much, including the take quantified either as individuals or pounds of meat.
Not until H.S. out took the local version of Smilodon would the pressure have led to reduced population.
The reasoning about taking just one calf per H. S. every so often is interesting, but I wonder if it takes into account that a nursing female may not be fertile. Once her calf is dead, she no longer nurses and will be fertile sooner. So a replacement may come along relatively soon. To the extent she is not eating for two, marginally more resources are available for those who are. So that survival would, marginally, be more likely.
A weaned calf is eating, except if it’s been killed, in which case somebody else gets the graze/browse. In a hard season, this may make the difference. Various hunter-gatherers we know of practiced infanticide, abandoning the old, and various kinds of birth control. Including the undergrads’ fave, Ritual Subincision Among the Arunta. Admittely, these are relict cultures driven to the edges of survivability. Not like our sturdy H. S., lord of protein plenty in fat city. There is no reason to dismiss the likelihood of a population of megafauna similarly living on the edge. And, paradoxically, being culled might have been a good thing.
Like American bison hunters, H. somebody would have taken not just calves, but their moms as well.
Humans in historical times have wiped out many species. Ancient hunter-gatherers were also capable of killing too many large, naive beasts. Their weaponry and cooperative hunting skills were up to the task.
PS: The Pleistocene megafauna however weren’t wiped ou by H. somebody, but by H. sapiens sapiens, ie AMH, ie us.
If so, true as to Sap. But the article was talking about pre-Sap eras. So if it happened, it wouldn’t have been Sap.
RA,
As above, I don’t think that Pliocene hominins wiped out any species. H. heidelbergensis, Neanderthal and Denisovan grade humans, maybe. But our species and subspecies, not only yeah, but Hell, yeah!
As to extinction potential, not much difference between anatomically modern humans of 60 Ka and today. Main distinction now is that we’re aware of our devastation potential.
MarkW,
The case of NZ moa and related extinctions: very rapid from hunting and burning. Moa numbers were expanding before the arrival of humans.
https://phys.org/news/2014-12-early-humans-single-handedly-nudged-zealand.html
http://www.pnas.org/content/111/13/4922.short
Not fanciful imaging, not post hoc fallacy, but facts.
JT is right.
Also please note that one individual with a torch can burn a million acres quite easily if the setting is ripe. Every animal doesn’t have to be speared. H. something wasn’t a careful conservationist constrained with hunting and burning regulations. Those were the days…
Also saves the trouble of cooking.
I don’t understand why the role of pathogens is almost always overlooked in the study of mass extinctions.
When a new species, including hominids, enter an area, they likely bring new fungi, bacteria and viruses with them for which native fauna have no resistance.
Humans ourselves probably brought few pathogens with us dangerous to the indigenous fauna of most of North America, there being then few primates north of Central America. Our dogs however might have infected wolves, coyotes or even bears. Humans did however pick up pathogens from American animals. The syphilis spirochete is for instance closely related to that which causes yaws in bears (bacterial Genus Treponema).
Recent (2016) further confirmation of Martin’s 1973 “Overkill” hypothesis:
Test of Martin’s overkill hypothesis using radiocarbon dates on extinct megafauna
http://www.pnas.org/content/113/4/886
Following Martin [Martin PS (1973) Science 179:969–974], we propose the hypothesis that the timing of human arrival to the New World can be assessed by examining the ecological impacts of a small population of people on extinct Pleistocene megafauna. To that end, we compiled lists of direct radiocarbon dates on paleontological specimens of extinct genera from North and South America with the expectation that the initial decline of extinct megafauna should correspond in time with the initial evidence for human colonization and that those declines should occur first in eastern Beringia, next in the contiguous United States, and last in South America. Analyses of spacings and frequency distributions of radiocarbon dates for each region support the idea that the extinction event first commenced in Beringia, roughly 13,300–15,000 BP. For the United States and South America, extinctions commenced considerably later but were closely spaced in time. For the contiguous United States, extinction began at ca. 12,900–13,200 BP, and at ca. 12,600–13,900 BP in South America. For areas south of Beringia, these estimates correspond well with the first significant evidence for human presence and are consistent with the predictions of the overkill hypothesis.
Martin actually first proposed the hypothesis in 1966. With each passing decade, it receives more support and confirmation not just for the Americas, but the Antipodes, parts of Eurasia, Africa and isolated islands and habitats.
This despite the many who, for devotion to CACA, prefer to blame climate change rather than the “A” in CACA, ie anthropogenic factors.
At about the same time (1967), Belarussian climatologist M. I. Budyko, who later became notorious on both sides of the CACA hypothesis, proposed human hunting for the abrupt decline of Pleistocene megafauna in Eurasia:
On the Causes of the Extinction of Some Animals at the end of the Pleistocene
https://www.tandfonline.com/doi/abs/10.1080/00385417.1967.10770945
That’s right! The Soviets invented the overkill hypothesis first! As with all else.