Читать книгу The Last Giants - Levison Wood - Страница 11
ОглавлениеAncestors and Evolution
Looking at their body size, where they live, and the kind of environments that they live in, it would be easy to assume that the closest living relatives to elephants would be other megaherbivores, such as rhinos and hippos. But in fact, genetic analysis has revealed that the closest living relative of the elephant is the rock hyrax – a furry, rodent-like creature that looks a bit like a guinea pig and isn’t much bigger.
Elephants, along with hyrax, and, believe it or not, the aquatic manatees and dugongs, belong to the branch of mammals known as Afrotheria, which simply means ‘African beasts’. This ancient group split off from other mammals at a time when Africa was an island continent, probably during the Cretaceous geological period, tens of millions of years before Triceratops and Tyrannosaurus dinosaurs were thundering around the American plains!
Given the long evolutionary history of the Afrotheria, surprisingly few living mammals fall into this group, and those that do are exceptionally diverse. As well as the elephants, hyraxes, and manatees, Afrotheria also includes elephant shrews, golden moles, and tenrecs, which are shrew and hedgehog-like mammals that are mainly found on Madagascar. The group is unique in that is contains one of the smallest living mammals, the long-eared tenrec, which weighs just 5 grams – not much more than a penny coin – as well as the largest, the African savannah elephant. It was only thanks to scientific developments in genetics over the past twenty-five years that we had any idea that these animals were in one related group.
While rock hyrax and the other little creatures are native to the hills, plains and valleys of Africa and the Middle East, manatees and dugongs float around in the tropical waters of Central and South America and off the warm coasts of the Caribbean, Indian and Pacific Oceans. Yet, despite their obvious differences, these seas creatures share a number of traits with elephants: manatees are the only herbivorous marine mammal; they have four small nails at the end of each flipper that are very similar to the toenails of an elephant; and they also have a prehensile upper lip that they use to grasp hold of marine vegetation, much like a stubby trunk. They even have similar teeth, with incisors that resemble tusks, as well as the horizontal molar teeth displacement that also occurs in elephants.
Hyraxes have flattened nails, rather than the claws found on most similar-sized land mammals. They even have small tusks too that develop from incisors. And, like elephants and manatees, their mammary glands (which produce milk for their babies) are near their front legs. In all other mammals, except primates, milk teats are found between the rear legs. A final trait shared with elephants is that their testicles stay inside the abdomen, rather than swinging around like a monkey, bull or human. So, despite the aeons since the common ancestor of these species was alive in the swamps of North Africa, there are still plenty of visible clues to their shared heritage.
Fossil evidence helps us to work out when the anatomical features of living elephants evolved, and why. If we were to journey back over 35 million years ago to Northern Africa, we might be fortunate enough to see a pig-sized animal that looked somewhat like a modern-day tapir, foraging in the soft vegetation around rivers and lakes. This was Moeritherium, one of the earliest proboscideans – the group within Afrotheria that specifically contains elephants and their relatives.
Moeritherium died out without leaving any descendants, so it is not a direct ancestor of today’s elephant species, but they shared features with other proboscideans, like a flexible upper lip, which, like a modern trunk, was used for grasping and handling food. They also had short tusks, although these were more tooth-like than the tusks of a modern-day elephant.
Part of the skull and teeth of another very old proboscidean – Eritherium – was recently discovered in Morocco. It is dated to 60 million years ago, which was a time of rapid evolution and change for mammals following the demise of the dinosaurs. Eritherium was tiny, standing only 20 cm high, no bigger than a well-fed domestic cat, making it a thousand times lighter than a modern African elephant. But, despite appearances, it was its unique teeth that allowed scientists to identify it as the earliest ancestor of the elephant!
Other than humans, the evolutionary history of proboscideans is one of the best-mapped mammalian lineages, with around 175 species identified. They are divided up into five main groups, which are referred to as ‘super-families’ 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 that mammoths were the preferred targets of early human hunters. It makes sense: killing one of these enormous beasts would provide a huge amount of meat for a hunter-gatherer family, which made the effort and risk of hunting them worthwhile. But, as prehistoric cave art depicting them shows, mammoths provided more than simply meat. Ivory is a much better material than deer antler for making the spear and arrow points needed for big-game hunting, so the ivory of slain mammoths was used to hunt more effectively and kill even more mammoths and other megafauna.
Climate change almost certainly played some role in driving a number of extinctions, but this is increasingly believed to be a secondary factor in the collapse of large mammals. South America suffered some of the greatest losses, yet the climate there was far more stable than in Africa, which experienced comparatively few megafauna extinctions over the same period.
Many of the large herbivore species that went extinct were generalist foragers, who could eat grass or browse for food depending on availability. All the evidence suggests that there were still plentiful foraging opportunities, and some species, such as the American mastodon, went extinct even though their preferred plant species have remained abundant up until present day. The only viable culprit for these mass extinctions appears to be the human race.
Human pressure on animal populations. Man’s desire for ivory. Climate change. This all sounds eerily familiar. We – or at least our ancestors – have to hold up our hands for the dramatic decline in the number of proboscidean species existing in the world, a decline that continues, and now encompasses the remaining members of that family.
One thing that can help us to understand elephants better is having an idea of where they lived and how far they ventured – their range and spread. It is difficult to know the exact prehistoric ranges of elephants, but we do know that the ancestors of both the African (Loxodonta) and Asian (Elephas) species lived together in Africa for a very long time.
Even after the latter species spread out of Africa into the Middle East and across Asia, some stayed behind until as recently as 11,000 years ago.
Understanding the movement of elephants over the last few thousand years is a challenge, but we can use rock art – like those etchings found in Libya and Sudan – and descriptions from ancient texts to give us some clues. From these historic sources, one thing is readily apparent: until very recently, elephants roamed over most of the African continent, all the way from the Mediterranean coast in the north to the beaches of South Africa.
They spread from the jungles of Senegal in the west, to the highlands of Ethiopia in the east. They inhabited every kind of African environment, from the arid deserts of Namibia and the fringes of the Sahara, all the way up to the treelines of Africa’s great peaks. Savannah elephants wandered the slopes of Mount Kilimanjaro and the Rwenzoris, while their forest cousins bathed in the Congo river.
Aristotle spoke of elephants wandering the Atlas Mountains near the Strait of Gibraltar in modern-day Morocco, and Pliny the Elder reported that herds of elephants ‘infested’ parts of modern-day Libya and Algeria. But by around AD 500, there were no wild elephants living in Mediterranean north Africa.
It has been predicted that up until the sixteenth century – before European exploration, exploitation and colonisation really took hold – there could have been more than twenty million elephants in Africa. This staggering figure is an estimate, based on our understanding of how much space was available for elephants, and how many elephants each of these areas could have supported, given the vegetation they likely contained.
What is certain is that from the moment Europeans arrived, the number of elephants in Africa began to shrink. Decline was slow at first, but the causes for that decline500 years ago are essentially the same as those today – namely hunting for ivory, and habitat loss due to agriculture.
From the sixteenth century onwards, Europeans introduced new crops to Africa, such as maize and sweet potato, which were grown on land newly claimed for agriculture. Elephants would have been using those same areas as natural feeding grounds before they were turned into farms. This increased the number of encounters between humans and elephants, which would have quickly turned ugly if the elephant was intent on sampling the new food source that had suddenly appeared in their stomping ground.
But as well as crops, Europeans also brought guns. And with them, a seemingly insatiable lust for ivory that would change things forever. These European colonisers were not the first to covet and value ivory, though. Long before history was recorded, Stone Age Europeans used mammoth ivory to carve figurines, toys and religious idols. Tomb inscriptions show that Ancient Egyptians had been collecting tusks since at least 2000 BC, and Tutankhamun’s tomb of 1325 BC was full of ivory trinkets. The Ancient Greeks also discussed the use and beauty of ‘the white gold’, long before any of them had seen an elephant’s tusk.
The Romans used far more ivory than the Greeks or Egyptians before them, and even in the British Isles, Anglo-Saxon women were buried with ivory accessories. But the procurement of ivory in these ancient times, although a highly valuable trade, was not occurring fast enough to dent the millions-strong elephant population at the time.
Commercial ivory trading really took off in the sixteenth century, at the same time as the spread of agriculture in Africa. This loss of habitat and increase in unnatural deaths had a catastrophic impact on the ranges and behaviour of elephants, and by causing local population declines, made any other losses far harder to recover from.
People didn’t only interact with elephants to steal their teeth, of course, and historic accounts show that our fascination with the live animal is not a new phenomenon. The size and strength of elephants has always been a source of wonder for us, and perhaps it is indicative of human nature that it’s something we have always tried to exploit.
We know that elephants have been caught, broken and tamed for use from as early as 3000 BC in India and 1500 BC in Syria. Having encountered them in battles in India, Alexander the Great began using Asian elephants around 330 BC like an ancient precursor to the battle tank, and Ptolemy II – the king of Egypt and Carthage – was the first to capture African elephants for use in war. Ptolemy III apparently had an army of around 300 African elephants by 240 BC.
Most famously, Hannibal used a herd of forty African elephants during his invasion of Italy by way of Spain in 218 BC. Three had died before they even reached the Alps, and the remaining thirty-seven were used mainly as pack animals, although they did join in the combat against the Roman cavalry. In the end, all but one died of cold or starvation; and the fate of the last is unclear. Hannibal did use elephants again in a few subsequent battles, until it was made part of his peace terms with the Romans that he surrendered all his elephants and agreed that he wouldn’t train any more.
The main advantage of using elephants in war was the sheer intimidation factor – it must have been truly terrifying to see these giant monsters charging towards you across the battlefield at speeds of 25 mph, particularly when they were in large numbers, covered in armour with a turret on top. Even battle-hardened Roman soldiers were known to rout under such an onslaught. However, it’s important not to forget that the sights and sounds of battle must have been equally terrifying for the elephants themselves, and if they were wounded or panicked by the enemy, they could end up running amok, trampling men and horses within their own lines.
In fact, because of this risk, elephant riders often carried a large mallet and chisel-shaped tool, which was used to kill the animal if it went berserk and became uncontrollable. Experienced enemy soldiers also learned over time to target the elephants at the outset of the battle – to fire their arrows and direct their lances towards the poor animal’s sensitive trunk – in order to make them panic and flee back into their own ranks.
Perhaps because of their unpredictability, Julius Caesar didn’t rate the use of elephants in battle, so few African elephants ended up being used in European wars after 47 BC. That said, they are still classed as a pack animal in a US Special Forces field manual issued as recently as 2004! The last supposed use of elephants in war occurred in 1987, when Iraq was alleged to have used some to transport heavy weaponry for use in Kirkuk during the Iran–Iraq war.
Of course, other equally exploitative uses have been found for elephants throughout the ages. Elephants were used as bloody entertainers in gladiatorial tournaments, where they were baited and pitted against other wild beasts. They’ve even been used as a gruesome method of torture, where the convicted prisoner is either gored or squashed to death by a tethered beast.
There have been many attempts to domesticate African elephants, too. Whilst African elephants should in theory not be any different to Asian elephants in terms of trainability, the culture has not been as deeply ingrained in Africa, so most efforts have generally ended in failure. That said, there was a moderately successful elephant riding school set up in Victoria Falls in Zimbabwe, where tourists could pay to ride, and local mahoots were trained up in the Asian style. However, these trials inevitably met with disaster, after one of the guides was killed in front of horrified guests.
The other major ‘use’ of elephants has, of course, been as curiosities in zoos and circuses – a trend that sadly continues around the world to this day – and it is estimated that there are up to 20,000 elephants still held in captivity.
A vast range of proboscidean species have walked the earth since their earliest ancestors emerged from the swamps of Africa over 60 million years ago. They have been a remarkably successful group, which dominated the grasslands, forests and tundra of almost every corner of the earth. But only three species survived the dramatic extinctions at the end of the Pleistocene. Humans have been central in the downfall of so many species.
Does the same fate await the last three types of elephants, or can we learn from our history? Before we answer that question, let’s have a look at what makes African elephants so special.
