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Over the last 60 million years, proboscideans - the order of animals which includes elephants - changed dramatically as they expanded geographically and adjusted to various climate changes.
New research explores how this rich group of giant mammals diversified and became extinct, leaving behind only three species of elephants today.
Elephants - the largest land animals alive today - are the only remaining species of the family Elephantidae, which belongs to an ancient order of Proboscidea.
Proboscideans were a diverse and widespread group of herbivores that first appeared in Africa about 60 million years ago.
Unlike the three endangered species of elephants we know today, not all proboscideans were giants, nor did they look anything like their cousins.
In fact, some of the earlier species, such as the Eritherium which lived in Morocco, were as small as foxes and lacked a trunk.
Steven Zhang, Honorary Research Associate at the University of Bristol, says, 'Remarkably, only two groups of proboscideans evolved during the first half of the proboscidean evolution, which lasted around 30 million years.
'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.'
Steven, along with an international team of paleobiologists explored why and how proboscideans changed so much over millions of years.
Using fossil collections from museums around the world, they studied the evolutionary adaptations of 185 species over 60 million years of existence.
Steven says, 'Some of the most significant clues for gauging the epic proboscidean story were found right here in the collections of the Natural History Museum, one of the very few institutions in the world housing fossil specimens representative of all eight evolutionary directions.'
The research culminated into the most detailed analysis to date of the rise and fall of elephants and their cousins.
Early proboscideans that lived in Africa were slow-evolving with little diversification.
Once the Afro-Arabian Plate collided into the vast Eurasian landmass, an important migratory corridor was formed, which allowed the species to explore new habitats in Eurasia and then into North America. This was via the land bridge which sporadically connected Siberia to Alaska and is now submerged under the Bering Sea.
Over the past 20 million years, the global climate changed frequently and dramatically. Provided with new challenges and habitats, proboscideans that expanded from their range in Africa evolved 25 times faster than the cousins they left behind. This resulted in a variety of forms, with three to four different proboscidean species coexisting in one space. This richness of giant herbivores was unlike anything in today's ecosystems.
But from about six million years ago, the diversity in proboscideans started to decrease following the harsh cooling of Earth. Only the most ecologically versatile proboscideans survived.
The most extreme example would be the wooly mammoth, which had thick, shaggy hair and large tusks for retrieving vegetation hidden under thick snow.
Whether humans or climate change caused the extinction of large animals, including proboscideans, is still up for debate today.
Early humans became effective hunters of large animals around 1.5 million years ago. The team's analysis shows the final proboscidean extinction peaked much earlier, however, at around 2.4 million years ago.
Steven explains, 'The finding is something we didn't anticipate. It seems the broad global pattern of proboscidean extinction could be reproduced without considering the impacts of early human migration. This contradicts the claims that early humans started wiping out prehistoric elephants.
'However, this isn't to say we conclusively disproved any human involvement. Our research shows modern humans settled on each landmass after proboscideans had started becoming extinct.
'So humans - a highly adaptable, social predator - could have landed the final blow.'
This research presents an invaluable blueprint for evaluating the significance of human impact on near-time megafauna extinctions. It also helps present a case study of why animal groups produce uneven distribution of diversity across evolutionary histories.