Climate change – not humans – started decline of ancient elephants, says study
Prehistoric elephants were pushed to extinction by extreme global environmental change rather than being over-hunted by early humans, according to a study.
The research indicates that the extinction of the last mammoths and mastodonts at the end of the last Ice Age was the final part of progressive climate-driven decline among elephants over millions of years.
Previously, it had been claimed that early human hunters slaughtered prehistoric elephants, mammoths and mastodonts over millennia.
Elephants were once a diverse and widespread group of giant herbivores, known as the proboscideans, which included the now extinct mastodonts, stegodonts and deinotheres.
England was home to three types of elephants: two giant species of mammoths and the straight-tusked elephant, just 700,000 years ago.
Now, elephants are restricted to three endangered species in the African and Asian tropics.
The latest research was carried out by group of palaeontologists from the universities of Alcala, Bristol, and Helsinki.
They examined how 185 different species of elephants and their predecessors adapted over 60 million years of evolution that began in North Africa.
Museum fossil collections from across the world, including London’s Natural History Museum and the Paleontological Institute in Moscow, were used as part of the research.
Researchers investigated traits such as body size, skull shape and the chewing surface of teeth. They discovered that all proboscideans fell within one of eight sets of adaptive strategies.
Dr Zhang Hanwen, of the University of Bristol’s School of Earth Sciences, said: “Remarkably for 30 million years, the entire first half of proboscidean evolution, only two of the eight groups evolved.
“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.”
The course of evolution for proboscideans changed dramatically about 20 million years ago, when the Afro-Arabian plate collided into the Eurasian continent.
Arabia allowed a migration corridor for the herbivores to explore new habitats in Eurasia, then into North America through the Bering Land Bridge.
Dr Juan Cantalapiedra, of the University of Alcala in Spain, said: “The immediate impact of proboscidean dispersals beyond Africa was quantified for the very first time in our study.
“Those archaic North African species were slow-evolving with little diversification, yet we calculated that once out of Africa proboscideans evolved 25 times faster, giving rise to a myriad of disparate forms, whose specialisations permitted niche partition between several proboscidean species in the same habitats.
“One case in point being the massive, flattened lower tusks of the ‘shovel-tuskers’. Such coexistence of giant herbivores was unlike anything in today’s ecosystems.”
The researchers found that the changing global climate reduced the once greatly diverse and widespread mastodonts to less than a handful of species in the Americas.
By three million years ago, the elephants and stegodonts of Africa and eastern Asia seemed to be surviving.
However, environmental disruption connected to the Ice Ages left the species forced to adapt to new, more austere habitats.
The study showed the final extinction peaks for proboscideans started about 2.4 million years ago for Africa, 160,000 years ago for Eurasia and 75,000 years ago for the Americas.
Researchers say those ages are not the precise timings of extinction but the points in time where proboscideans on the continents became subject to higher extinction risk.
The ages also do not correlate with the expansion of early humans and their enhanced capabilities to hunt down the large herbivores.
Dr Zhang said this suggests the pattern of proboscidean extinctions could have happened without the impact of early humans, which contradicts claims that they wiped out prehistoric elephants.
“Although this isn’t to say we conclusively disproved any human involvement,” Dr Zhang added.
“In our scenario, modern humans settled on each landmass after proboscidean extinction risk had already escalated.”
The study, The rise and fall of proboscidean ecological diversity, is published in Nature Ecology & Evolution.