by Dave Goulson
I was taught that the human brain was the crowning glory of evolution so far, but I think it’s a very poor scheme for survival.
Kurt Vonnegut
Ninety thousand years ago a group of humans living in Africa decided to go for a walk. It was a long and slow walk, fraught with danger. It would take them and their descendants about 80,000 years, but it was perhaps the most significant trek in the Earth’s history. For obvious reasons we don’t know many of the details – they have been pieced together by archaeologists from fragments of bone and shards of rough stone tools excavated from thousands of sites around the globe. There will no doubt be significant new discoveries and endless arguments about the specifics, but what follows is probably somewhere near correct.
Modern humans – apes belonging to the species Homo sapiens – evolved in Africa perhaps 160,000 years ago, at a time when much of Europe, North America and Asia was locked under vast sheets of ice. Our ancestors remained more or less confined to Africa for 70,000 years and then, for reasons at which we can only guess, a group of them left Africa, crossing the mouth of the Red Sea on to the Arabian Peninsula, carrying their stone tools: axes, knives, hammers and arrowheads. They seem to have had a close association with the sea, for they stuck to the coast and spread slowly eastwards, successively occupying the coasts and islands of India, Thailand, Malaysia and Indonesia. It took about 30,000 years before some of them crossed to New Guinea and Australia, approximately 60,000 years ago. Fifty thousand years ago the climate began to warm, and humans were able to spread northwards into the Middle East and Europe. The first humans colonised Britain perhaps 40,000 years ago. At the same time we were also spreading north-east, into Central Asia and China. Twenty-five thousand years ago we had occupied most of the old world, from Britain and Spain in the west to Tasmania and the Bering Straits at the far eastern tip of Siberia. Shortly afterwards a particularly hardy group of humans crossed the eighty kilometres or so from Russia to Alaska, and so colonised the Americas. The climate entered another cold period (the last ‘glacial maximum’, in geologists’ terms) and buried much of northern Europe, Russia and North America under a vast sheet of ice, pushing us southwards for a while and chasing the new arrivals in America down into the southern states, Central America and beyond. We arrived in Chile perhaps 12,000 years ago, so that about 80,000 years after our ancestors first set out from Africa, all of the Earth’s major land masses had been occupied by humans, with the exceptions of inhospitable Greenland and Antarctica.
The final wave of colonisation, of the most remote but habitable places on Earth, took place rather later – perhaps 1,000 years ago, when adventurous Polynesians in dugout canoes chose to sail east from Asia into the unknown vastness of the Pacific. They discovered and colonised New Zealand, the various small archipelagos of Fiji, Samoa and so on, and eventually made it to Hawaii and finally to Easter Island, arguably the most remote inhabited island on Earth.
Homo sapiens were not the first hominids to leave Africa. Other hominids such as Homo erectus and Homo neanderthalensis (Neanderthals) had got a huge head-start, spreading throughout much of the old world long before we arrived – Homo erectus was found all over Europe and Asia well over one million years ago, while Neanderthals spread throughout Europe perhaps 600,000 years ago. At least twelve different species of ape belonging to the genus Homo have so far been described, and it seems very likely that there are many more awaiting discovery. The tiny Homo floresiensis from Indonesia, weighing in at about twenty-five kilos and standing just one metre tall, was not discovered until 2003.
The world that early hominids colonised was dominated by large mammals. Following the extinction of the dinosaurs some sixty-five million years ago, widely thought to have been the result of the devastation caused by an asteroid striking the Yucatan peninsula, the few small mammals that survived went on to proliferate. Many new species arose and filled the vacant niches once occupied by dinosaurs, some of them becoming huge. In the Americas this ‘megafauna’ included giant sloths, camels and llamas, many species of bison, moose and ox, giant beavers, mammoths and mastodons. These herbivores were predated by some of the most formidable predators to walk the Earth in the last sixty-five million years, including huge two-tonne short-faced bears, a species of lion, several species of sabre-toothed cats and massive dire wolves. In Europe we had woolly mammoths and elephants, aurochs, lions, cave bears, cave hyenas, giant elks, several species of rhinoceros (including the ten-tonne indricotherium, the largest land mammal so far discovered) and much, much more. Each continent had its own magnificent selection of giant, hairy beasts.
Imagine being one of those first Homo sapiens to leave Africa 90,000 years ago. They were exploring a world that really did contain monsters. Every river they crossed, every new valley they entered, they could expect to stumble across new and formidable behemoths, many armed with tusks, horns, fangs and claws. As they travelled they would also have come across other hominids – tribes of creatures varying from the large and powerful, beetle-browed Neanderthals to the hobbit-like Homo floresiensis. Our legends are full of dragons and monsters, elves, pixies and trolls. They are not myths; we really did once live in a world that was full of such wonders.
So what happened to all these marvels? Why did they all go extinct during a relatively short period? The answer is almost certainly that we killed and ate them. Our 80,000-year journey was a culinary odyssey. Our ancestors were hunters; by working together in groups with spears and arrows, they could easily kill even the largest of the giant mammals, and a small tribe could have lived off one such kill for weeks. Their motivation for spreading to the more remote and bleak corners of our planet, such as Siberia, was probably that they were following the great herds of mammoth or bison. The animals they encountered as they spread were naïve – they had not encountered humans before and would not have known to run away. Many would have had no effective defence against missiles, so they fell easy prey. The largest creatures in particular would have bred slowly, taking many years to reach maturity (as elephants do today), and so their numbers would soon have been depleted, forcing our ancestors to move on in search of fresh hunting grounds.
Some scientists find this explanation unpalatable, and argue that periods of global cooling or epidemics of disease wiped out the megafauna, but these explanations do not fit the facts at all well, and they smack of wishful thinking. The timing of extinction in different parts of the globe closely follows our arrival, allowing a few thousand years for our populations to expand following colonisation. Humans arrived in Australia about 60,000 years ago, and discovered a land of astonishing and fabulous creatures. As well as the species found there today, there were rhinoceros-sized two-tonne wombats, at least seventeen species of short-faced kangaroo, including one that stood three metres high, and a sheep-sized echidna (spiny anteater), the largest egg-laying mammal ever to have lived. There were terrifying predators, including the leopard-sized marsupial lion, and giant predatory birds with razor-sharp, hooked bills, such as the half-tonne carnivorous ‘thunder bird’, and the slightly smaller but still 2.5-metre-tall Bullockornis, nicknamed ‘the demon duck of doom’ by Australian palaeontologists. Today the komodo dragon is the largest surviving species of lizard in the world, at three metres long and weighing about seventy kilos, and is a formidable beast, but it is a shrimp compared to the seven-metre-long, half-tonne monsters that the early explorers of Australia had to contend with. There was also a land-living, seven-metre-long crocodile, which is thought to have been able to run at speed and chase down its prey, and which must surely have snacked on early humans and been utterly terrifying on first (and presumably often terminal) encounter.
Despite the ferocity of some beasts, within a few thousand years of our arrival all of them were extinct. Even a seven-metre-long crocodile would have been little match for the intelligence of a group of human hunters, able to climb into trees beyond its reach, armed with sharp weapons. We made extensive use of fire to drive animals out of the t
hickets on to our spears, and so completely altered the landscape of Australia by denuding much of it of trees. The large animals of Tasmania had a brief reprieve as the early settlers failed to cross the wild and chilly seas of Bass Strait from southern Australia, but about 43,000 years ago a land bridge formed during a period of lower sea levels, enabling humans to cross; within 2,000 years all of Tasmania’s large fauna had gone, leaving the elusive, dog-sized thylacine (also known as the Tasmanian wolf) as the largest mammal.
Extinction of the megafauna occurred later in North America, a little while after the first humans crossed over from Siberia. In the space of just a few hundred years almost all the large mammals had gone, leaving the bison as the largest survivor. Whether we deliberately hunted the large predators such as sabre-toothed tigers is not known; one might imagine that, as with the Masai of Africa, hunting and killing such fearsome creatures might have been a rite of passage for young hunters. In any case, by eliminating their prey we doomed the sabre-tooths to inevitable extinction. A few hundred years later, as humans moved southwards, the megafauna of South America was similarly extirpated.
New Zealand was colonised much more recently, about 1,000 years ago. As there were no mammals apart from bats, giant birds had evolved there, including at least eleven species of moa, the largest of which stood 3.6 metres high, the tallest bird ever to live. They must have been terribly easy to track and kill, for carbon-dating of Maori middens suggests that all eleven species were driven to extinction within just 100 years of man’s arrival. These studies also suggest that, at least to start with, the Maoris only bothered to take the choicest cuts of meat, slaughtering the helpless beasts and leaving the bulk of their corpses where they fell.1
It is interesting that Africa is the one place left on Earth where quite a bit of the megafauna survives – elephants, giraffes, hippos, lions, and so on – given that Africa is where humans first appeared. We will never know for sure, but there is one likely explanation. Humans didn’t arrive suddenly in Africa. We evolved there slowly, over millions of years, from smaller, tree-dwelling apes. As we became gradually more intelligent, more adept at making weapons, more organised in our hunting, so the wildlife of Africa had time to learn to be scared, to run away at their first sight or scent of the odd, upright apes. In contrast, the ground sloths of South America and the giant wombats of Australia would have been entirely naïve when we arrived, and were probably wiped out so swiftly that they had no time to adapt.
It wasn’t just the animals that disappeared when Homo sapiens spread out of Africa. So too did all other species in the genus Homo – our cousins. There is no clear evidence that we killed them, but it seems certain that often we did. Our record of treatment of more primitive people in recorded history is appalling (think of the Native Americans, the Aboriginal people of Australia – especially in Tasmania – or the slave trade from West Africa). It seems unlikely that our behaviour in prehistory was any better, or that our ancestors were any less violent, aggressive, belligerent and xenophobic than we are today. We probably ate many of them; in West Africa the trade in bush-meat commonly includes our closest relatives, the great apes, despite their severe endangerment, so there is no reason to suppose that we would have turned up our noses at consuming Homo erectus or Homo floresiensis. Those that we didn’t kill were probably driven before us, excluded from the best hunting grounds, so that their populations dwindled. We probably didn’t have it all our own way. Neanderthals were stronger than us, the top predators in Europe before we arrived, and their brains were of similar size to our own, so they would have made formidable opponents. Quite how we overcame them is not known. The Neanderthals survived in remote corners of Europe for several thousand years after our arrival, but the last of them eventually died out about 25,000 years ago. Recent controversial genetic evidence suggests that there may have been very limited interbreeding between Neanderthals and humans, and that most of us carry a few Neanderthal genes, but if so, this is all that remains of them.
Let us move on, finally and depressingly, following the trail of devastation in the wake of human diaspora, to Easter Island. This remote volcanic island, just twenty-five kilometres across, is 2,000 kilometres from its nearest inhabited neighbour, the tiny Pitcairn Island. It has a subtropical climate, and was largely forested when Polynesian settlers first arrived about 800 years ago. There were several species of tree found nowhere else in the world, including the largest known palm tree. There were also at least six species of indigenous, flightless land birds, which must have been easy to catch and good to eat; and there were nesting colonies of seabirds, providing eggs and young. The early settlers thrived, clearing land to grow crops and fishing from dugout canoes. Life was sufficiently comfortable that there was time to carve the huge statues known as Moai for which the island is famous: stylised, shadow-eyed men with jutting jaws, arranged in rows with their backs to the sea, overlooking the human settlements. The statues were carved from a single quarry and were dragged, presumably on log rollers, all over the island to each of the towns that had sprung up. The population eventually grew to about 15,000.
When the first European explorers found Easter Island in 1722, this thriving civilisation had gone.2 There were no trees left anywhere on the island, the indigenous species having been eliminated to make room for crops. Without wood, the islanders had no means to make boats and so could not easily fish, or leave the island. They also had little from which to build houses or to burn for cooking. The indigenous flightless birds were all extinct, having long since been eaten, and the seabirds had ceased to nest, driven away presumably by over-harvesting of their eggs. Most catastrophically, without tree roots to bind the soil, much of it had blown or washed away, so that crop productivity had plummeted. As the food supply dwindled, the population seemed to have abandoned their traditional religious beliefs, perhaps feeling that their gods had deserted them. They pushed over the Moai, and turned to a new and rather violent religion known as the Bird Man Cult. By 1722 the population of 15,000 had dwindled to perhaps 2,000 or 3,000 malnourished individuals, who survived on a very limited diet of chicken, rats and – according to some – cannibalism. Their island paradise had become a bleak prison.
I am sure you can see where I am going with this. The story of Easter Island can be seen as a microcosm, a scaled-down version of what is happening in the world today. We are cutting down our forests, just as they did. Easter Island is so small that it must have been clear to all who lived there that they were rapidly using up their resources, but that did not stop them. The man who cut down the very last tree must have known that it was the last tree, must have known that without trees there would be no boats or fishing, but still he did it, presumably on the basis that his own immediate need was more important than the future of his civilisation. We too know that we are using up our resources at an unsustainable rate. We think nothing of using up fossil fuels that took millions of years to form. We know full well that we are rapidly cutting down the tropical rainforests, and that this is probably going to have a terrible impact on the Earth’s climate, but we carry on doing it nonetheless. Huge areas of agricultural land around the globe have become infertile, just as they did on Easter Island. Ploughing breaks up the soil, making it easy for water to wash it away into the sea, or for wind to blow it away. Removing trees and using herbicides to kill weeds removes the roots that bind the soil together. Globally, about seventy-five billion tonnes of soil are lost every year. Forest clearance and irrigation have led to increasing salt levels in soils all over the world, in the worst cases rendering the land useless; about 320 million hectares of land have been affected by salination so far, and roughly 40 per cent of all agricultural land is now degraded in one way or another. The pesticides, fertilisers and soil particles running off into rivers destroy fresh-water aquatic communities, and when they flow out to sea they can cause tremendous harm to coral reefs and damage fish stocks, which are already under extraordinary pressure from over-fishing.
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sp; Just as the Easter Islanders drove both their native trees and flightless bird species to extinction, so on a global scale we are rapidly losing the Earth’s biodiversity. Of course species have always gone extinct, long before humans came on the scene. The background extinction rate – the average rate at which species have gone extinct in the past – is estimated to be roughly one extinction per ‘million species years’. This means that, if there were one million species, we would expect one to go extinct per year; or if there were only one species, we would expect it to go extinct in one million years, on average. If we estimate the current number of species on the planet to be five million, we would expect five species to go extinct each year. Of course new species also arise over time through evolution, and in the past this generally balanced or exceeded these small losses. Quantifying the current rate of extinction of the Earth’s species is fraught with difficulty, not least because we do not know how many species there are on the planet. We have named one million or so, so far, but there may be anywhere between one and ten million,3 with more remaining to be discovered. Proving beyond doubt that a species is extinct is also tricky; there may always be one or two hiding somewhere no one has thought to look. It is fairly easy to establish that a large creature such as a dodo, living only on one small island, has gone extinct, but for most species the task is much harder, so that only about 875 species have been officially declared extinct since the year 1500. This number is probably just a drop in the ocean compared to the actual number that has gone extinct in recent times. Current estimates, based on the rate of loss of habitat around the globe, suggest that the current extinction rate may be 100,000 times higher than the background extinction rate. Even at the most conservative end of the estimated range of extinction rates, it is likely that several species go extinct on the planet every single day. Most we have not even afforded the dignity of a name, and we will never know for certain that they ever existed. Some scientists predict that as many as two-thirds of all species on Earth will be extinct by the end of this century.
