by biologists.
There are the early proboscideans (such as Eritherium and Moeritherium), the deinotheres, mastadons, gomphotheres, and elephantidae. Taxonomy is a complex business, though, particularly when dealing with creatures that have been extinct for millions of years and have left only a few fossilised fragments behind to help us. Every time a new fossil or thread of genetic evidence becomes available, scientists end up adjusting relationships between super-families and updating estimated ages, and we’re still a long way off from knowing the full story.
However, from the fossils we do have, we can see that the proboscideans were remarkably successful, having lived on every continent except Antarctica and Australia in their 60 million years on earth, in environments as diverse as deserts, tropical forests, mountain ranges and the Arctic tundra.
Around 20–30 million years ago, the deinotheres (‘terrible beasts’) appeared on the scene. Instead of having tusks in the upper jaw, like elephants, they had downward-curving tusks looping down from their lower jaw. They started small, but some grew quickly (in evolutionary terms) reaching an impressive shoulder height of four metres.
Deinotheres stuck around for about 20 million years; the equivalent group of our human ancestors has been around for only two million, and our species, Homo sapiens, only 300,000 years. Deinotheres were so successful because of their increasing body size, meaning they could tolerate lower quality diets such as fibrous, hard-to-digest plants, which allowed them plenty of flexibility because they could graze more widely.
Around 25 million years ago, the climate began to warm, and big, open grasslands and tundra appeared across the world. Mastodons and the fantastically named gomphotheres marched out from Africa, across Eurasia, and into this favourable new environment in North America, with some species – such as the American mastodon – dominating the landscape until around 10,000 years ago.
Like modern-day elephants, mastodons had tusks that emerged from the upper jaw, and the thick enamel and ridging on their teeth shows that they predominantly browsed on woody plants. The largest known of all these creatures, a mastodon called Mammut borsoni, was well over four metres tall and reached the colossal weight of 18 tonnes. This puts it in the same league as the hornless rhino, Paraceratherium, a monstrous beast that lived across Eurasia 34–20 million years ago and has been officially recorded as the largest mammal species that ever lived.
However, there is some tantalising evidence that a third species may outstrip both these giants. Partial leg bones discovered in the 1800s, from a straight-tusked Asian elephant group called Palaeoloxodon, suggest an animal with a height of over five metres and a body weight of 22 tonnes. They would make today’s elephants look like small fry.
The gomphotheres are the most diverse group of the proboscideans, first appearing in Africa 24 million years ago, then spreading across the globe, before mostly becoming extinct 11,000 years ago. Many gomphothere species had four tusks and while some had trunks that are similar to modern elephants, others had shorter snouts more reminiscent of today’s tapir, which is more closely related to rhinos and horses than it is to elephants.
This brings us to the Elephantidae – the group that includes the three remaining species of proboscidean alive today, as well as the extinct mammoths (Mammuthus) and Palaeoloxodons. Elephantidae emerged in Africa in the late Miocene period, somewhere between 6–8 million years ago. This was truly a time of trunks, when deinotheres, gomphotheres and mastodons all ranged far across the planet from the tropical forests of America to the arid grasslands of Africa and Asia.
We also know a little bit about the behaviour of these early elephants. Scientists recently used aircraft to study the ancient fossilised trackways of an unknown proboscidean species in the deserts of Eastern Arabia, near to modern-day Dubai. Seven million years ago, we know that a single group of around thirteen animals, of different ages and sizes, had moved across the muddy ground in a coordinated fashion, much like elephant families move across the savannah today.
There was also the single track of a large individual, suggesting that the society of these ancient creatures was sexually segregated like today’s elephants, with independent males usually walking separately from the family group. The social patterns seen in today’s elephants stretch back millions of years into evolutionary history.
Around 5 million years ago there was another enormous transition on our planet as the earth became cooler and dryer. The forests began to shrink further, while deserts and grasslands got bigger. As a result, the availability of good quality food for large herbivores declined and those that could not adapt went extinct. Large body size (and perhaps large brain size) was an advantage, because bigger animals can eat more low-quality food to get their daily nutrients and energy.
Then, about 2 million years ago at the start of the Pleistocene epoch, another period of rapid climate change saw the earth cool even further. Elephants were forced to adapt yet again. Glaciation meant that some species became isolated and others were squeezed into areas that were already being inhabited by other animals. Fluctuating sea levels even led to some elephants becoming trapped on islands.
Being large is typically unnecessary on islands without predators, because there is no need to waste time and energy growing big to avoid being eaten, when there is no one to eat you. So, many island elephants underwent rapid and radical shrinking (rapid in evolutionary terms, at least). One such species, the
Cyprus dwarf elephant – a straight-tusked species that became extinct around 13,000 years ago – weighed only 200 kg and was the size of a large dog.
Despite their huge success, with a considerable evolutionary history and the fact that they could be found almost all over the world, there was a sudden and dramatic extinction of proboscideans at the end of the Pleistocene, between 15,000 to 10,000 years ago. Mastodons, gomphotheres and mammoths all died off at an unprecedented rate. Only one small population of woolly mammoths survived much longer. They remained isolated on the Wrangel Island of Siberia until 4,000 years ago, which is remarkable, because by that time the pyramids of Egypt had already been built!
It wasn’t only proboscideans who suffered during this period. Thirty-five groups of large mammal became extinct in North America alone, including the giant sloths, sabre-toothed tiger and the American cheetah. Before these extinctions, the diversity of mammal species in the Americas exceeded that of modern-day Africa.
Two main ideas have been put forward to explain these sudden declines. The first suggests that, as had happened before, rapid climate change altered vegetation to such an extent that large herbivores and their predators could not adapt quickly enough and died out from starvation. The second, equally grim hypothesis suggests that this period saw the emergence of a super-predator, which used its superior intelligence and employment of tools to kill off the big beasts. That predator was, of course, us: modern humans armed with Stone Age technology.
Increasingly, the evidence points an accusatory finger towards us. Either way it’s hard to brush off as coincidence that this cascade of extinctions took place over a remarkably short time period – a few thousand years. As the ancestors of modern humans evolved in Africa and migrated in successive waves across Eurasia, large mammals would have experienced varying levels of exposure to this new predator, depending upon where they lived.
African mammals co-evolved with hominids (the great apes, which includes us) and were slightly better adapted to dealing with our ancestors. Better hearing, smell, and an innate fear of people, perhaps. Species in Australia and the New World, however, were caught completely unawares by our sudden arrival, and were much more vulnerable.
When we look at changes in the distribution of plants and animals over time, the large mammal extinctions correlate almost perfectly with the arrival of modern humans. Those species that had the least time to adapt to the arrival of man were the most likely to die out.
There is even some evidence