A life-sized display of Archie, a Columbian mammoth, is on display at the University of Nebraska State Museum in Morrill Hall. A new study suggests that massive mammals were much more likely than their smaller counterparts to go extinct in regions occupied by ancient humans. Photo: Troy Fedderson, University of Nebraska-Lincoln
The extinction of large mammals such as wooly mammoths, giant sloths, and saber-toothed tigers has been linked to prehistoric humans, including Homo sapiens and Neanderthals, according to a new study.
The study, which has been published in the Science journal, has found that Homo sapiens, Neanderthals and other recent human relatives may have begun hunting large mammal species at least 90,000 years earlier than previously thought, the University of Nebraska-Lincoln has announced.
The study has been authored by Felisa Smith from the University of New Mexico and Kate Lyons from the University of Nebraska-Lincoln together with Jon Payne of Stanford University and Rosemary Elliott Smith from the University of California, San Diego.
The team has received support from the US National Science Foundation. It has used emerging data from older fossil and geologic records.
Mammal megafauna such as wooly mammoths, elephant-sized ground sloths and various saber-toothed cats roamed the Earth between 2.6 million and 12,000 years ago.
The phenomenon of the megafauna’s disappearance is known as size-biased extinction.
It is pointed out that prior research had suggested that large mammals began disappearing faster than their smaller counterparts in Australia around 35,000 years ago.
Now, however, the new study has estimated that this size-biased extinction started at least 125,000 years ago in Africa.
It reports that by that point the average African mammal was already 50% smaller than those on other continents despite the fact that larger landmasses can typically support larger mammals.
What is more, other size-based extinctions started to be observed in other continents as well as humans migrated out of Africa.
These additional size-based extinctions correspond to the regions and timelines of the known human migration patterns, the researchers have found.
Thus, over time, the average body size of mammals on those other continents approached and then fell well below that of mammals in Africa, and mammals that survived during the span were generally far smaller than those that went extinct.
According to the study, the magnitude and scale of the recent size-biased extinction surpassed any other recorded during in the past 66 million years, i.e. since the extinction of the dinosaurs.
“It wasn’t until human impacts started becoming a factor that large body sizes made mammals more vulnerable to extinction," says study co-author Kate Lyons, Assist. Prof. of biological sciences from the University of Nebraska-Lincoln.
“The anthropological record indicates that Homo sapiens are identified as a species around 200,000 years ago, so this occurred not very long after the birth of us as a species. It just seems to be something that we do," she adds.
“From a life-history standpoint, it makes some sense. If you kill a rabbit, you’re going to feed your family for a night. If you can kill a large mammal, you’re going to feed your village," the researcher elaborates.
Just as it has found evidence that prehistoric humans such as Homo sapiens and Neanderthals have contributed to the extinction of large mammals, the research team found little support for the idea that climate change has driven size-biased extinctions over the past 66 million years.
Large and small mammals seemed equally vulnerable to temperature shifts throughout that span, the authors reported.
“If climate were causing this, we would expect to see these extinction events either sometimes (diverging from) human migration across the globe or always lining up with clear climate events in the record. And they don’t do either of those things," Lyons has emphasized.
The University of Nebraska State Museum’s Elephant Hall highlights the differences in current elephants (left) and mammoths (middle and right). Pictured (from left) is an African elephant; an Asian elephant with a juvenile; dwarf mammoth; Archie, a Columbian mammoth; and a Jefferson mammoth. Photo: Troy Fedderson, University of Nebraska-Lincoln
The team also looked ahead to examine how potential mammal extinctions could affect the world’s biodiversity.
To do so, it posed a question: What would happen if the mammals currently listed as vulnerable or endangered were to go extinct within the next 200 years?
In that scenario, Lyons says, the largest remaining mammal would be the domestic cow. The average body mass would plummet to less than six pounds — roughly the size of a Yorkshire terrier.
“If this trend continues, and all the currently threatened (mammals) are lost, then energy flow and taxonomic composition will be entirely restructured," says Felisa Smith, Professor of biology at New Mexico.
“In fact, mammalian body size around the globe will revert to what the world looked like 40 million years ago," she adds.
Lyons says that the restructuring in question could have “profound implications" for the world’s ecosystems.
Large mammals tend to be herbivores, devouring large quantities of vegetation and effectively transporting the associated nutrients around an ecosystem.
In her words, if they continue to disappear, the remaining mammals would prove poor stand-ins for important ecological roles.
“The kinds of ecosystem services that are provided by large mammals are very different than what you get from small mammals," Lyons said.
“Ecosystems are going to be very, very different in the future. The last time mammal communities looked like that and had a mean body size that small was after the extinction of the dinosaurs.
“What we’re doing is potentially erasing 40 to 45 million years of mammal body-size evolution in a very short period of time," she has stressed.
Felisa A. Smith, Rosemary E. Elliott Smith, S. Kathleen Lyons, Jonathan L. Payne. Body size downgrading of mammals over the late Quaternary. Science, 2018; DOI: 10.1126/science.aao5987