by Tim Flannery
Mid-ocean ridges form where two continents are moving apart, stetching the oceanic crust between them. They resemble a double-crested mountain range, and between the crests, in a sort of rift valley, molten rock from deep in the Earth’s crust comes to the surface. Hydrothermal vents—deep, fluid-filled cracks in the oceanic crust—form, and all of the ocean water in the world eventually circulates through these. It takes between ten million and one hundred million years for all the water to be recycled through the hydrothermal vents, but as it circulates the chemical structure of the seawater is altered by the extreme heat, and the salt is removed. This recycling of the oceans through evaporation, rainfall and rivers every thirty to forty thousand years, and through the crust at the mid-ocean ridges every ten million years or so, keeps the saltiness of the sea constant. And none of it would be possible without continental drift.
What this potted history of Earth tells us is that if we wish to keep our planet fit for life, some of the most routine and humble things we do must change. For as long as we’ve existed our conception of waste disposal has simply been shifting objectionable matter from one of Earth’s organs to another. Whether it’s been a human body or a banana skin, we’ve buried it (returning it to the earth), burned it (returning it to the atmosphere) or tossed it into the sea. On a small scale, this approach to waste disposal works pretty well. But it most decidedly will not do in the twenty-first century, for the very essence of much pollution derives from human actions that weaken the elemental imbalance between Earth’s organs. Over the vastness of geological time Gaia’s housekeeping has put every element in its place. Carbon has been withdrawn from the atmosphere by plants and geological processes, until just a few parts per ten thousand remain. Iron has been stripped by hungry plankton from the seas, as have mercury, lead, zinc, uranium and a great many other elements, all of which have been safely sequestered deep in Earth’s rocks. But now the human burrowers in the Earth have arrived, and, as we tunnel into those buried troves, we undo the work of aeons.
Man the Disrupter
2010
We live in a zoologically impoverished world, from which all of the hugest, fiercest, and strangest forms have recently disappeared.
ALFRED RUSSEL WALLACE 1876
WHAT IS IT, precisely, that makes us humans different? Is our cultural mode of evolution the magic ingredient, as it allows us to evolve far faster than organisms reliant on mere evolution by natural selection? Yet this cannot be the whole story. All higher species have cultures, and their cultures too change and evolve in response to the environment. If you doubt this, consider the deer. Mature white-tailed deer stags have magnificent ten-point antlers that are prized by human hunters, and the deer have developed a cultural means of avoiding a dose of lead in the heart. This was discovered during an experiment in which stags were penned with human hunters in a 2.5-square-kilometre enclosure, surrounded by lookout towers. Observers were surprised to find that many stags survived by lying completely still, so that human hunters stalked right past them.1 Such a strategy would mean instant death for a stag pursued by wolves or cougars—predators that hunt by smell. But somehow stags have learned, then presumably taught each other, that this is the best way to avoid being killed by visually oriented humans.
This, however, falls far short of humanity’s evolutionary strategy. What we have done is combine cultural evolution with technology in a way that allows us to mimic key aspects of evolution by natural selection, and speed it up ten thousand times. Thus we make spears rather than evolve fangs, and weave clothes rather than grow furry coats; and it’s this ability, which has been with us from the very beginning, that makes humans so formidable.
Earth’s history is a story of coevolution, punctuated by disruptions caused by asteroids, abrupt climatic shifts or invasive species that break apart the threads that lie at the heart of ecosystem function. Such disruptions invariably result in an impoverished world, in the short term, at least, because they lower productivity and drive species to extinction. Of all the disruptions Earth has suffered, few can compare with those we have caused, for by virtue of our discovery of a new way to evolve we have become a destructive force par excellence. Over the past fifty thousand years our Medean tendencies have repeatedly been unleashed, leaving in their wake a world of ecological wounds.
Evidence of humanity’s power to disrupt ecosystems dates almost to the moment our species left its ancestral African homeland. We’ve eaten our way through one resource after the other as we’ve spread around the planet, and only after long experience in one place have we acquired the wisdom of managing the land. As a result, it is our misfortune to be only now, perhaps, tentatively emerging from a world in which human genius was so without wisdom that it fractured and disfigured nature’s evolutionary bonds to the point of our own self-destruction.
Our kind arose in Africa, as savannah-living apes, and it’s clear from the rubbish dumps left behind that even 1.8 million years ago our ancestors were capable hunters and ardent carnivores. By around two hundred thousand years ago, when the first humans had evolved, we had become such expert killers that creatures much larger than ourselves, including the largest of all land mammals, were forming part of our diet. From an ecosystem perspective, this ability to hunt the largest and fiercest creatures was destined to become, and until ten thousand years ago to remain, our defining ability. Yet when our species left its natal continent of Africa our hunting skills began to tear apart ecosystems, and the further we wandered, the more god like our control over the fiercest and largest of creatures became.
Genetic studies allow us to read in detail the migrations of our ancestors. The Y-chromosome is passed down solely through fathers, making it an ideal guide to the travels of our male ancestors. Fortuitously it evolves quickly—around 40 per cent of its mutations have occurred since humans began to exhibit regional variation (‘race’ in the old parlance). Studies of the Y-chromosome reveal that all people alive today are descended from a single male who lived in Africa around sixty thousand years ago.2 This genetic Adam, it’s reasonable to deduce, was dark-skinned, tall, slender, and possessed epicanthi, the folds over the corner of the eyes commonly seen in people from Asia.
But what of Eve? The mitochondria can help us here. There are none in the sperm head, so fathers don’t contribute to their offspring’s mitochondria, which makes it ideal for tracing female ancestry. The tale told by mitochondria confirms our African origin. But there is one startling difference between our male and female ancestry: mitochondrial DNA tells us that all people alive today can trace their ancestry to an African woman who lived at least a hundred and fifty thousand years ago. Adam and Eve, it seems, never met, being separated by ninety thousand years.3 The explanation for this lies in the fact that small populations tend to lose genetic lineages faster than large ones. We might expect men and women to have existed in roughly equal numbers in the past, but it is the size of the breeding population that really counts. The population of breeding men, it seems, has long been small relative to that of women, probably because an exclusive group of high-status men has tended to father most children.
Genetic studies reveal that Africa has repeatedly acted as a fountainhead of hominid dispersal. The first successful diaspora of humans occurred around fifty thousand years ago, when a few clans left Africa and began to spread east. A particular genetic marker on the Y-chromosome, known as M130, originated in a man living somewhere north of the Red Sea in the early days of this migration, and this marker is now widely distributed across southern and eastern Asia, as well as in Australia. The American geneticist Spencer Wells postulates that the people bearing this mutation were coastal dwellers adapted to harvesting marine resources, an ecological niche that would have facilitated a rapid spread eastwards.4
The peopling of inland Eurasia, and eventually the Americas, was accomplished by a separate group, whose ancestor was an African man who bore a Y-chromosome marker known as M89. This dispersal involved people who became adapted
to drier, inland conditions. When they reached the Middle East they gave rise to a further mutation, known as M9, and then to three further separate Y-chromosome mutations, the bearers of which went on to settle central Asia, Europe and India. Y-chromosome studies also reveal that it’s only over the last ten thousand years that humans from outside Africa were able to migrate back into their ancestral homeland, presumably because they had developed agriculture and animal husbandry.
It’s ironic that just as we’ve found a way to read the genetic chronicles documenting our ancestors’ travels, increased human mobility is leading to such a mixing of our genes that, within a few dozen generations, the clues to our ancestors’ wanderings will be obliterated. Then, we will have returned to the genetic state that prevailed before we ever journeyed out of Africa: all humans forming a single, genetically uniform population.5
I sometimes wonder what the world would have been like if it had taken our ancestors ten times, or even a hundred times, as long to evolve the technology required to colonise the world. What would it have been like if Columbus, for example, had sailed to the Americas one million years, rather than around thirteen thousand years, after the first Americans had arrived from Eurasia via the ice-age land bridge? A million years is more than enough time for evolution by natural selection to have forged Americans and Europeans into separate species, and if that had happened a smile or a gesture might not have been understood, and sex would not have been a common currency. Had prehistory not unfolded as it did, our global civilisation might not have been possible.
Wherever our ancestors arrived, they fundamentally altered the environment. And as a result, the world we live in today is sadly truncated—all of its largest, fiercest and most striking creatures are no more. Gone are the moa of New Zealand, the mastodons and sabretooths of the Americas, the giant marsupials of Australia and the gorilla-sized lemurs of Madagascar. Only in our species’ homeland of Africa does anything like the bestiary known to our distant ancestors survive. Until recently it was widely disputed whether humanity could have caused such changes. But the evidence is now overwhelming. Indeed, it’s not just humans who, in the right circumstances, can bring about such changes, but even creatures as humble as toads.
The cane toad was introduced into Australia via Hawaii in 1935. Until then, no member of the toad family (Bufonidae) had existed in Australia, meaning that no Australian predator had any experience of the venom that toads produce from the glands on their necks. Originally inhabitants of Central America, the toads were introduced to help eradicate pests in sugarcane, a job they failed dismally at. But they spread quickly, causing environmental destruction wherever they went. In northern Australia it’s easy to know when you are ahead of the toads. You see crocodiles, goannas, frill-necked lizards, birds of prey and countless other native Australian creatures every day, all around you. But if you visit the region behind the moving frontier (which in 2010 lay near the Western Australian border), all is silence and devastation. Unless you were there when the first toads arrived, it’s hard to fathom what happened. A friend of mine was camping on a river in western Queensland when the toads went through. He went out one evening to fish, and was distracted by a nauseating smell. Following it upstream he discovered a logjam of dead crocodiles, their bodies lying so thickly that they clogged the stream. The toads had arrived, and so sensitive were the crocs to the toads’ poison that a single toad was enough to kill even the largest of them.
Because cane toads metamorphose from the tadpole stage while very small, around the size of your little fingernail, they represent a danger even to the tiniest of predators, which accounts for the eerie emptiness of the lands behind the toad frontier. A visitor to the toad’s homeland of Central America would never suspect that cane toads could be so calamitous to so many creatures, for in their ancestral homelands they are relatively rare and live in harmony with the myriad alligators, lizards and other species that share their environment. Coevolution over millions of years has bestowed resistance to toad poison on many. Either that, or they have learned not to eat toads. In Australia no predator knew instinctively that the toad was a threat—a naivety that caused their demise.
The toad pioneers, those at the very front of the invasion wave, have longer legs than the settler toads that populate the land behind, and they are more aggressive. On the frontier, evolution by natural selection favours toads with the drive and physique to invade quickly and take advantage of the bounty of resources awaiting them.
For humans on the frontier, it’s not our legs that respond to evolutionary opportunities, but our culture and our attitude to life. In frontier societies, individuals who can monopolise the greatest bounty, regardless of waste or environmental consequences, are selected as ‘fittest’. And for most of human history the bounty our ancestors sought was big, hairy and dangerous. As our ancestors spread out of Africa, they encountered creatures with no experience of modern humans. But the animals they met did know about upright apes, for a distant relative of ours had left its African homeland nearly two million years earlier. Homo erectus had settled the more temperate portions of Europe and Asia, extending as far north as modern-day Beijing and southern England, and as far south as Java.
I have great difficulty in deciding how to address Homo erectus. What would I say if I met one in the street? Is it an animal, or a man? I use ‘it’ here only because I wish to emphasise the ‘us’ in this story of humanity’s spread. Part of my difficulty comes from the fact that Homo erectus changed over its 1.8-million-year existence, developing several regional types, one of which, that inhabiting Africa, gave rise to us. But it’s the Homo erectus population of Europe and Asia that is important to this story. Homo erectus hunted the large mammals of Eurasia for a million or more years, thus providing everything from mammoth to cave bear with experience of carnivorous apes, and starting an evolutionary arms race that would allow some species to avoid extinction when our kind arrived.
With a brain around 25 per cent smaller than our own, Eurasian Homo erectus doubtless lacked many advantages possessed by our ancestors. One of the best documented concerns stone tools. Those made by Homo erectus are abundant, providing a detailed record of change over time. They slowly improve in quality, and by three hundred thousand years ago Homo erectus was making a greater range of tools than was being produced 1.5 million years earlier. But even at its best, Homo erectus’ toolkit was rudimentary when compared to that of our ancestors. A similar picture emerges with regard to fire, which was haphazard or absent 1.8 million years ago. There is evidence (albeit still disputed) that by three hundred thousand years ago Homo erectus was using fire to some extent. But its control of this vital tool was rudimentary compared to that of our ancestors.
In other ways, however, Homo erectus never approached our achievements. Studies of the skull and neck suggest that it was unable to speak, though it may have communicated in other ways, such as sign language, that have left no fossil record. And of course Homo erectus left us no art, or evidence of care for the dead.
Much is often made of these differences. But what is really important from an environmental perspective is the almost identical ecological niche occupied by Homo erectus and our own ancestors. Like us, Homo erectus was a hunting and gathering ape, and from the beginning it was capable, in some circumstances at least, of hunting prey as large as young elephants. From the perspective of the large hairy beasts of Eurasia’s forests and plains, Homo erectus was simply a less capable version of our ancestors. If you want to imagine what those beasts experienced as Homo erectus gave way to Homo sapiens, imagine playing in the amateur league at whatever sport you prefer, then being suddenly matched against the professional elite in a game where not just egos but life itself is at stake. At least you’d be better off than those who had never played the game at all, which was the position of the megafauna of the Americas and Australia, where no carnivorous ape had ventured prior to our arrival.
There were probably animal victims of Homo erect
us’ expansion into Eurasia. The sabre-tooth cats vanished from the Old World at around the time Homo erectus arrived, yet they survived in the Americas until our species showed up thirteen thousand years ago. Some of these sabretooths specialised in killing young elephants, which would have brought them into direct competition with Homo erectus. A few potential prey species also became extinct at around the time of their spread, but the evidence is so subtle and it occurred so long ago that it’s hard to say whether the extinction resulted from Homo erectus or other factors such as a changing climate. What we do know, however, is that by the time our ancestors arrived in Eurasia around fifty thousand years ago, Homo erectus had struck a balance with the surviving large mammals, for by then they had coexisted with Eurasia’s elephants, rhinos and other great beasts for almost two million years.
Evolution’s Motive Force
2010
There is nothing conscious about life’s lethal activities.
PETER WARD, 2009
WHATEVE REACH DAY held, Charles Darwin tried to set aside time for a stroll around a ‘sand walk’ near his home, Down House, in Kent. Tradition has it that the sand walk was his thinking space—the place where he sharpened his evolutionary theory, as well as the sentences that would so elegantly carry it into print. Consequently, the walk is regarded with reverence by many scientists, and when I made my first pilgrimage to Down House in October 2009 it was this place above all that I wished to see. After paying my respects to the great man’s office and drawing room, I followed the signs to the walk. It’s a little removed from the house and its enclosed gardens, and entering it one feels instantly transported from the ordered human world into the wider world of nature.