Next time you look in the mirror, consider how remarkable you really are. Big mammals are normally confined to their continents of origin, yet humans have traversed the oceans on rafts of floating vegetation. Five times heavier than a kori bustard, the largest extant bird capable of flight, humans have also learnt to cross the globe through the air. Humanity has turned itself into a planetary freak, unprecedented in Earth's history, and, in so doing, it has had a profound effect on other animals.
Until very recently, New Zealand had moas, huge eagles, pelicans, swans, giant ravens, as well as flightless ducks, coots and geese; Madagascar had elephant birds, two species of giant tortoise, a dozen lemurs up to the size of a gorilla, an aardvark, a pygmy hippo and a giant mongoose. Within a few centuries of our arrival, all were extinct. Had we not rafted across the Pacific, the world would be about 20 per cent richer in bird species alone.
Much of this devastation has been brought about by hunting and habitat destruction, but just as important - probably more so in the long run - has been the liberation of other organisms from the bonds of the natural order. Once, hitch-hiking across oceans attached to other creatures was the particular talent of seeds and snails. But in the past few thousand years, rats have started doing it, too. And cats. And even cows. Never in Earth's history had a breeding population of cow-sized animals rafted across an ocean; but, in a geological microsecond, humans changed the rules. The ancient geographical barriers that have long restricted migration are breaking down.
Continental humans tend to regard goats, for example, as rather inoffensive. But to island plants, which often lack defences against such animals, they can be lethal. Because these mobile waste- disposal units can survive on almost any type of organic material, they simply continue to eat whatever is available until their adopted landscape begins to resemble a moonscape. When first discovered in 1501, St Helena was densely covered with a luxuriant forest vegetation. Today, thanks to the goats introduced by the Portuguese in 1513, that native vegetation survives only on elevated ridges and in narrow gullies. Of 33 plant species endemic to the island, goats have eliminated ten and endangered a further 18. On Great Island off New Zealand, 73 of 143 plant species were exterminated by goats between 1890 and 1934.
Rabbits and cats can have similar effects. In 1759, Thomas Austin shipped 24 rabbits to Australia from England and released them on to his estate as game; 120 years later, the rabbit population of Australia stood at 700 million, causing, as one commentator put it, "the greatest single tragedy that the economy and the native animals ever suffered". In New Zealand, cats have contributed to the decline of kakapos, bush wrens, bell birds, parakeets, New Zealand shelducks, collared grey fantails, New Zealand stilts, wekas, pheasants, quails, long-billed plovers, kiwis, shearwaters, petrels, penguins, terns, gulls (short-tailed and long-tailed), bats and some lizards. On Stephen's Island off New Zealand, a single cat belonging to the lighthouse keeper eradicated a rare endemic lizard and the Stephen's Island bush wren.
Although displaced mammals provide the most dramatic examples of ecological change, it is only in the past few years that scientists have begun to appreciate that the most insidious threat comes not from particular species, but from the slow stirring of the biosphere as a whole. Botanists, for example, find that oceanic islands across the globe are beginning to look depressingly similar. Over the past few thousand years, the same trees have been planted and the same alien plants introduced. We have taken sacks of our most productive agricultural grains and fruits on our journeys, and weeds and insect pests have hitched a ride, too.
Island ecosystems are particularly vulnerable to aliens, but continental land masses are under siege, too. Of the 6,500 species on American soil that are currently under threat of extinction, it is estimated that half are threatened by alien invaders.
Nor is the problem just a terrestrial one. A study of the ballast tanks of ships arriving in Oregon from Japan found 367 different types of animal and plant lurking inside. The most notorious ballast-tank stowaway of recent years must be the Eurasian zebra mussel, which is currently causing havoc in North America. Zebra mussels reproduce rapidly, carpet lake and river beds, clog up municipal and industrial water inlets, disrupt algal populations and alter the concentration of nutrients in the water. The cost of clearing them from blocked intake pipes alone has been estimated at $2bn.
But the most important result of global mixing is not the economic inconvenience to us, but the homogenisation of the bio-sphere. From a geological perspective, the rate at which we are doing it is truly staggering: we have been stirring life for just a geological microsecond, less than 0.0001 per cent of the Cenozoic era in which we live, which is itself less than 0.02 per cent of the history of life. Some scientists think that this is an even bigger problem than global warming because, as one such scientist has put it, many of the effects of global warming could be eventually reversed, "but once you homogenise the biodiversity of the world, there's really no going back from that".
The biological implications are profound. Just as a globalised economy removes protection from local, specialised jobs and industries, so a globalised biology would remove protection from local, specialised plants and animals. As the biologist Richard Fortey has put it, "if everything could compete, there would be more losers than winners . . . In this respect, a world joined would be a world reduced."
In fact, the long-term impact of our actions on the biosphere may be more profound than anyone has hitherto realised. And we can have some idea of that impact because, as the fossil record shows, something rather like it has happened before.
At the end of the Permian period, 250 million years ago, the geo-graphy of Earth was very different. All the major areas of continental crust had coalesced into a single land mass called Pangea. Either side of the equator, covering much of what is now North America, Europe and South America, were colossal, searing deserts. The Palaeozoic equivalent of rainforests flourished in what is now China, the Malay Peninsula and Venezuela. Girdling the southern pole were forests of the cold-weather, fern-like plant Glossopteris, which must have dominated this part of Pangea in much the same way that conifers dominate northern climes today. A single ocean dubbed Panthalassa surrounded Pangea, except in the east, where a huge embayment called the Tethys Sea separated Eurasia from the southern continents.
Variation in climate maintained a variety of habitats between the equator and the poles; but with all ocean barriers eliminated, very similar types of creature roamed the length and breadth of the Pangean supercontinent. Palaeontologists are certain that tetrapods were much less diverse than they are today. We had global mixing 250 million years before human beings had the bright idea of lashing trees together into rafts.
At the height of this continental amalgamation, the biosphere suffered the greatest mass extinction in history. Many people know that a planet-wide disaster wiped out the dinosaurs during the Cretaceous period, but that was a relatively minor affair compared with the global crisis at the end of the Permian. It has been estimated that 95 per cent of all species on Earth died out. Even insects, traditionally the most resilient of land animals, which rarely suffer extinctions even at the family level, lost seven whole orders. Crinoids (sea-lilies) lost two subclasses. Trilobites disappeared completely, as did blastoid sea urchins and all the characteristic Palaeozoic corals. Reefs did not reappear in the fossil record for another seven to eight million years, one of the longest periods without these distinctive marine communities in Earth's history (corals must have survived somewhere, but we haven't yet found them in rocks of this period). Many brachiopods, the Palaeozoic equivalents of bivalves (the common seashells found on beaches across the world), were either extinguished or reduced from groups containing a hundred or more species to just two or three.
On land, the extinction was just as severe. In southern Pangea, the endless Glossopteris forests were suddenly replaced by a low-diversity assemblage of conifers and clubmosses. Elsewhere, other types of peat-forming trees disappeared, leaving a coal gap in the fossil record lasting several million years.
Tetrapods were particularly badly hit: 21 terrestrial families passed away, a greater proportional loss than in the oceans. The extinction of land animals seems to have consisted of a prolonged decline in diversity over the last few million years of the Permian, culminating in a sharp event that wiped out many small omnivores, all large herbivores, all gliding reptiles and six of nine families of amphibian. The surviving communities were peculiar in being of very low diversity and exceptionally cosmopolitan. In the succeeding Triassic period, over 90 per cent of the earliest tetrapod fossil assemblages from the super- continent of Pangea consist of just one genus of mammal-like reptile, the medium-sized Lystrosaurus.
What could have happened at the boundary of the Permian and Triassic periods to produce such wholesale devastation of life both on land and in the sea? What agent of destruction could have wiped out 95 per cent of all species on our planet? The answer emerging from recent research is disquieting, given our current environmental predicament: it seems that the greatest disaster in the history of life was brought about by the combined effects of global mixing and global warming.
The evidence for elevated global temperatures at the end of the Permian is now overwhelming. Late Permian peats in Antarctica and Australia are similar to those formed at such latitudes today, but they are rapidly replaced in the Triassic by soils indicating much warmer, temperate conditions. Similarly, the Glossopteris forests of southern Pangea were quickly replaced by warm- climate plants. In Australia, the flora flipped from assemblages characteristic of present-day latitudes around 70 degrees south to those of much warmer regions around 40-58 degrees south. In the Karoo Basin of South Africa, the climate switched from temperate to semi-arid. When you also consider that the earliest part of the Triassic is the only geological interval in the past 600 million years for which there is no evidence anywhere of ice, the case for planet-wide warming seems irrefutable.
The warming of tropical regions has been estimated at around 6oC. The increase in temperature at higher latitudes would have been even greater, leading to a flattening of the temperature difference between the poles and the equator. The result was the complete loss of high-latitude floras and the establishment of a uniform warm-to-hot climate across most of the planet. The catastrophic decline in tetrapod diversity and the establishment of a single, low-diversity assemblage of animals may have been partly due to the spread of arid conditions, but the most important factor was probably the homogenisation of habitats across Pangea. Not only were all the land areas joined together, and thus easily navigable by walking animals, but suddenly the pole-to-equator variation in habitat type began to fade away, too. Everywhere started to look the same, and the animals of the planet naturally followed suit.
The oceans were drastically affected, too. The solubility of oxygen in water falls as temperature rises, so global warming would have lowered the concentration of this essential element for life. But the reduction of the pole-to-equator temperature gradient, which drives the circulation of the oceans, probably had a greater impact. Flatten it, and ocean circulation slows down. Flatten it enough, and circulation could stop altogether. This is probably what happened at the end of the Permian. Unstirred, the oceans began to stagnate. Deep waters lost oxygen, and species vanished. Eventually, the oceans stagnated to such an extent that the upper part of the water column ran out of nutrients. Planktonic plants cannot survive without nutrients; without plants, the whole marine ecosystem would have collapsed like a house of cards.
What happened to cause such a severe episode of global warming? Worryingly, the same process that currently afflicts our planet: the release of carbon dioxide into the atmosphere. Chemical analyses of rocks suggest that CO2 enrichment of the air began well before the final cataclysmic extinction. The most likely source of the extra carbon was the Glossopteris coal-bearing deposits of southern Pangea, which were uplifted by tectonic activity and oxidised, thus releasing large volumes of CO2 into the atmosphere. Another big dose of CO2 was vented into the atmosphere over a period of around 900,000 years right at the Permian-Triassic boundary by the eruption in Siberia of between two and five million cubic kilometres of basalt, the largest continental outpouring of volcanic material in the past 600 million years.
The disaster at the end of the Permian teaches many lessons about the processes we have set in train on our Cenozoic earth. If the continental land masses had been separated as they are today, it is inconceivable that the mammal-like reptile Lystrosaurus, the seed-fern Dicroidium and the bivalve Claraia would have come to dominate the early Triassic so completely. Diversity would have fallen sharply without doubt, but there would have been different Lystrosaurus analogues on different continents, and many widely separated ocean basins and areas of continental shelf to protect and foster whatever marine diversity remained. Barriers isolate organisms from each other, and isolation is crucial for maintaining and promoting biodiversity, whatever the extraneous circumstances.
Today, we may not be pushing the world's land masses together in a physical sense, but our global wanderings are having much the same effect. In fact, the rate of biospheric mixing is greater now than at any other time in history: Pangea came together and then broke apart over tens of millions of years, but we have breached the same geographical barriers in just a few thousand years. Everywhere is starting to look the same again, and this time it is happening in a geological microsecond.
The rate at which we are warming the atmosphere through the release of carbon dioxide and other greenhouse gases is just as alarming. Estimates of the increase in average global temperature over the next century range from 1oC to 4.5oC, with most scientists plumping for a figure somewhere in the middle. In almost all cases, the models predict rising temperatures for several hundred years after 2100. If fossil fuel consumption continues to increase, the rise in average temperature could be as much as 5oC to 10oC by the middle of the 22nd century.
A 3oC rise in average global temperature is only half that suggested for the Tethys region at the end of the Permian, but this time we are talking about a timescale of only a hundred years, a period which does not qualify even as a tick of the geological clock. Even the basalt eruptions in Siberia took 900,000 years. It probably took several million years for the Permian world to cook right through.
To contend that we are heading towards a Permian-like environmental crisis would be an irresponsible conceit. The world is a very different place now. For a start, if all the continental landmasses were joined together and surrounded by a single ocean, as they were in the Permian, it would be much simpler to predict climate changes. We also know that Earth has experienced marked changes in climate at other times in the past - most notably during recent Ice Ages - without suffering anything like the wholesale extinctions that characterised the Permian-Triassic transition.
Perhaps the potential for global mixing at the end of the Permian was the key difference. Perhaps it was the sustained nature of the warming over several million years. Perhaps not. We simply do not understand the past or present behaviour of our planet well enough to make trustworthy predictions about the consequences of our current experiments in planetary change. But even if we reject the end-Permian disaster as a valid analogue for our present environmental situation, we should at least have the humility to take it as a warning.
The writer is a lecturer in natural history at the School of Geography, Nottingham University. This article is extracted from his Why Elephants Have Big Ears: nature's engines and the order of life, published by Gollancz this month, £18.99 (£18.03, including p&p, at www.newstatesman.co.uk)