In the past, researchers have blamed humans for hunting these animals to extinction thousands of years ago. But new research suggests that climate change is the likely culprit in the demise of prehistoric mammoths, mastodons and early elephants rather than overhunting by early humans at the end of the last Ice Age.
Waves of extreme global climate change chipped away at the proboscideans over time, eventually causing most of them to go extinct in different parts of the world between 2 million and about 75,000 years ago, the researchers said.
The study was published on Thursday in the journal Nature Ecology & Evolution.
The international group of palaeontologists created a detailed data set analyzing the rise, evolution and fall of the 185 different proboscidean species, which took place over the course of 60 million years and began in North Africa. In order to track these elephants and their ancestors, the researchers studied global fossil collections and focused on traits like body size, skull shape, tusks and teeth.
"Remarkably for 30 million years, the entire first half of proboscidean evolution, only two of the eight groups evolved," said Zhang Hanwen, study coauthor and Honorary Research Associate at the University of Bristol’s School of Earth Sciences, in a statement.
"Most proboscideans over this time were nondescript herbivores ranging from the size of a pug to that of a boar. A few species got as big as a hippo, yet these lineages were evolutionary dead-ends. They all bore little resemblance to elephants."
That all changed 20 million years ago, when a migration corridor that opened up at the Afro-Arabian tectonic plate pushed into the Eurasian continent. This allowed elephants and their predecessors to live in new environments, first in Eurasia and then North America by crossing the Bering Land Bridge — a strip of land that once connected Asia and North America.
Moving out of Africa exposed the elephants to habitats, changing climates and the need for adaptation.
"The aim of the game in this boom period of proboscidean evolution was ’adapt or die,’" Zhang said. "Habitat perturbations were relentless, pertained to the ever-changing global climate, continuously promoting new adaptive solutions while proboscideans that didn’t keep up were literally, left for dead. The once greatly diverse and widespread mastodonts were eventually reduced to less than a handful of species in the Americas, including the familiar Ice Age American mastodon."
As Earth experienced ice ages, the elephants had to adapt: The woolly mammoth, for example, had giant tusks that could plow beneath snow in search of food and a thick, shaggy coat.
"We discovered that the ecological diversity of proboscideans increased drastically once they dispersed from Africa to Eurasia 20 million years ago and to North America 16 million years ago, when land connections between these continents formed," said Juha Saarinen, study coauthor and postdoctoral researcher in the department of geosciences and geography at the University of Helsinki.
While diversity initially increased after these events, that began to fall between 3 and 6 million years ago as the global climate cooled.
"The ecologically most versatile proboscideans, mostly true elephants which were adapted to consuming various plant resources, survived," Saarinen said.
The extinction peaks for proboscideans started around 2.4 million, 160,000 and 75,000 years ago for Africa, Eurasia and the Americas, respectively, according to the researchers.
"It is important to note that these ages do not demarcate the precise timing of extinctions, but rather indicate the points in time at which proboscideans on the respective continents became subject to higher extinction risk," said Juan Cantalapiedra, lead study author and Senior Research Fellow at the University of Alcalá in Spain, in a statement.
These times don’t line up with when early humans began to branch out and hunt giant herbivores, because it largely predates such advances.
"We didn’t foresee this result," Zhang said. "Conservatively, our data refutes some recent claims regarding the role of archaic humans in wiping out prehistoric elephants, ever since big game hunting became a crucial part of our ancestors’ subsistence strategy around 1.5 million years ago."
Chris Widga, paleontologist and head curator at the Gray Fossil Site and Museum at East Tennessee State University, believes this research points to how important it is to track the evolutionary history of animal groups over longer time scales. Widga was not affiliated with this study.
"Those of us who study extinctions are usually most concerned with when and where the last of a certain species blips out," Widga said. "In order to understand whether the final extinction of some of these proboscidean groups was significant, we need to know something about ’background’ extinction rates that occur in response to global ecological and climate changes. This paper shows how proboscideans responded to broad-scale climate changes and how they diversified ecologically to fill new niches. And because proboscideans have such an outsized impact on their surroundings — this is a big deal."
This research also suggests that while 185 different proboscidean species have been identified with more than 2,000 fossil locations around the world, there may have been more.
"As if we needed more encouragement to find and describe more fossil proboscideans," Widga said.
The study authors note that overhunting by humans "may have served as a final double jeopardy" after proboscideans already suffered harsh, changing climates long before.
"This isn’t to say we conclusively disproved any human involvement," Zhang said.
"In our scenario, modern humans settled on each landmass after proboscidean extinction risk had already escalated. An ingenious, highly adaptable social predator like our species could be the perfect black swan occurrence to deliver the coup de grâce."