Genesis of Eden

Genesis Home

Leakey, Richard; Lewin, Roger, 1996
The Sixth Extinction,
Weidenfield and Nicolson, London

NOTE: This extract is included as an essential reading for transforming the world. You are requested to purchase the book yourself as it is, without question, essential reading material.

Richard Leakey is the worlds most famous living paleo-anthropologist. He was for many vears Director of the National Museums of Kenya and until recently, Director of' the Kenya Wildlife Service. He has written (with Roger Lewin) Origins, Origins Reconsidered and People of the Lake. His most recent book The Origin of Humankind (in the Science Masters series) was called by the Sunday Times an outstanding account of our current understanding of human evolution'. Richard Leakey lives in Kenya. Roger Lewin is Associate of the Peabody Museum, Harvard University. A successful author in his own right, his books include Bones of Contention, for which he won the British Science Book Award in 1988, and Complexity: Life at the Edge of Chaos. He lives in Cambridge, Massachusetts.


Researchers in Denmark recently counted some forty-six thousand small earthworms and their relatives, twelve million roundworms, and forty-six thousand insects under just one square metre of forest floor in their country. One gram of that same soil contained more than a million bacteria of one type, 100,000 yeast cells, and 50,000 fragments of fungus. These numbers cheat ready comprehension, but the numbers themselves are not what is important. We see in them not just a riotous profusion of different life forms, but a rich pattern of interaction, a living network that is the ecosystem. Toward the end of The Origin of Species Charles Darwin conjured up a graphic image of this interconnection as a product of evolution: "It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by the laws acting around us."7 An example of this interaction came as a significant surprise when, not very long ago, biologists realized that the ubiquitous subsoil fungi were essential for the daily survival of higher plants. Countless fungal filaments are in close symbiosis with plant roots, making essential minerals available, without which the plants would perish. In every local ecosystem around the world, microorganisms, higher plants, invertebrates, and vertebrates coexist with labyrinthine interdependence, partners in creating and sustaining the physical environment of atmospheric gases and soil composition and chemicals. Individual ecosystems work as integrated wholes, not as species in the company of but isolated from other species. Homo sapiens is one unit in that pattern of interdependence. Two decades ago, the British chemist and inventor James Lovelock took the notion of ecosystem interdependence a step further; he took it to the global level. Termed the Gaia hypothesis, his suggestion was that all the ecosystems of the planet were essentially interdependent, operating as a whole and inextricably linked to the physical environment. A consequence of that interdependence was the establishment and maintenance of the physical conditions necessary for life. Some Gaia enthusiasts extrapolated the theory to the point of suggesting that the Earth's biota is like a single organism, one with the purpose of maintaining itself. For this reason, the hypothesis was not taken seriously by biologists; it all seemed too mystical.

It was only recently discovered that plant roots are enmeshed in a vast network of fungal filaments, upon which they depend for sustenance. This is an example of interaction among organisms, which is a central aspect of Past and present diversity.

Recently, however, ecologists have cut through the mystical fringe and acknowledge that Lovelock was correct: just as the viability of individual ecosystems is maintained through the interaction of the species that compose them, so the environmental health of the Earth's biota flows from an interaction of all its ecosystems.

The importance of Gaia theory for our appreciation of biodiversity is profound. The theory's author told a major conference on the subject: "No longer do we have to justify the existence of humid tropical forests on the feeble grounds that they might carry plants with drugs that could cure human disease. Gaia theory forces us to see that they offer much more than this. Through their capacity to evapotranspire vast volumes of water vapor, they serve to keep the planet cool by wearing a sunshade of white reflecting cloud. Their replacement by cropland could precipitate a disaster that is global in scale."' Predictions of global disaster aside, the core of Gaia theory has now been tested many times, and vindicated. The balance of many chemical cycles, not just moisture, has been shown to flow directly from the functioning of ecosystems. Although there are many who still talk about the theory in mystical terms, Gaia has become serious science, and we are forced to take note of its implications. Beyond recognizing that the maintenance of physical conditions that sustain life as we know it depends on the Earth's biota working as an integrated whole, what can we say about biodiversity? To reiterate a question I asked earlier, but in a different context: Do we need all fifty million extant species for Gaia to remain operative? Would 100,000 species of plant do the job, rather than the quarter of a million that exist now? Are all 600,000 species of beetle really necessary? The world's ecosystems will operate as a whole so long as each is able to persist, especially in the face of such occasional perturbations as storms and fire. If that persistence-or stability has to rest on a foundation of high diversity of species, then biodiversity as we currently experience it can be judged to have value. If, however, stability does not require a rich species diversity, then we would be unable to value current biodiversity in this way. (There may, of course, be ways other than ecosystem stability in which high biodiversity is essential.) The diversity-stability equation has been debated by ecologists for years, without agreement, particularly between field researchers and theoreticians. Resolution may, however, be close. Field biologists have traditionally believed that the complexity of interaction among species within ecosystems is important for their stability. That belief was based more on intuition than on demonstrated fact, for two reasons. First, doing ecological experiments in nature is notoriously difficult; the scale of things is usually defeating, both physically and temporally. How do you alter a natural ecosystem to order, set up controls, and then wait half a century for results? So, there were few real observations on which to base judgements. Second, as a field biologist, once you are immersed in the ecosystem you study, there is a very real sense that everything has a part to play in the emergence of the whole. This "in the bones" feeling influenced biological thinking for a long time, but in the early 1970s, theoretical models appeared to imply that the fewer species an ecosystem contained, the more stable it would be. The models, developed principally by Robert May, of Oxford, essentially said that the more components there were in the system, the more things could go wrong. And if the components were tightly interconnected, something going wrong in one part of the system could initiate collapse of the whole. Intuition versus powerful mathematical models; the standoff continued for years. As the debate continued, biologists kept asking: If ecosystems don't need a rich diversity, why are they so rich in species? Two views emerged, the rivet-popper hypothesis and the redundancy hypothesis. Developed by the Ehrlichs, the former says that each species plays a small but significant part in the working of the ecosystem, like each of the many rivets that hold an airplane together. The loss of a few species-like the loss of a few rivets weakens the whole, but not necessarily dangerously. Lose some more, and catastrophe looms, especially if the system faces a severe test, like environmental perturbation for the ecosystem or air turbulence for the plane. In the redundancy hypothesis, proposed by the Australian ecologist Brian Walker, most species are seen as superfluous-more like passengers on a plane than rivets holding it together. Then only a few key species are required for a healthy ecosystem. Several of the many species in the system can play the key roles. Which model is correct? James Lovelock stepped into the arena with a theoretical model of his own, in the form of a computer simulation Daisyworld-that built mini-ecosystems. According to the output from Daisyworld, the more species there were in the ecosystem, the greater its stability. This seemed to support a direct relationship between biodiversity and stability, the rivet-popper hypothesis. Theoreticians were reluctant to accept Lovelock's conclusions, but had to listen when results from innovative ecological experiments became known in 1993 and 1994. Researchers in England and the United States independently tested the effect of diversity on the productivity and stability of ecosystems. Productivity is simply the quantity of living material an ecosystem can generate in a given period of time. This is just as important for agricultural systems as it is for natural ecosystems. Michael Swift, a biologist at the United Nations Tropical Soil Biology and Fertility Program in Kenya, has demonstrated convincingly the benefit of species diversity in agricultural systems. The best way to increase productivity in a maize field is by adding melons, trees, and nitrogen-fixing beans, not by squeezing in more maize. In their experiments at Imperial College's field station in England, John Lawton and his colleagues also found that productivity is boosted by species diversity. The result makes immediate sense, once you see it. Individuals of a single species will compete for the same resources, particularly space. Individuals of different species-some small in height, some medium, some tall -can take advantage of different spatial territory. More of the available space is used, so more individuals are supported, giving higher productivity. Does this explain the high species diversity in most natural ecosystems? Only up to a point, beyond which productivity seems to level off. For instance, although the diversity of tree species in East Asian forests is about six times that in North American and eight times that in Europe, productivity of all the forests is similar. Productivity is therefore only part of the answer. Stability may be the rest. In what is surely one of the most important discoveries in a long time, David Tilman, of the University of Minnesota, and John Downing, of the University of Montreal, found a direct link between species diversity and the health of a natural ecosystem. They conducted an eleven-year study of naive grassland ecosystems in Minnesota, which, as luck would have it, included the worst drought experienced in the area for fifty years. What might have been a catastrophe for the experiment turned out to be a blessing, because it highlighted clearly the difference between ecosystems rich in species and those which were poor. The former suffered significantly less loss of species and productivity in the face of the drought, and recovered much more rapidly. "Our results . . . support the diversity-stability hypothesis, and show that ecosystem functioning is sensitive to biodiversity," they stated in Nature in January 1994. "Our results do not support the species-redundancy hypothesis, because we always found a significant effect of biodiversity on drought resistance and recovery."9 Species do appear to be more like rivets in a plane's structure than passengers in its seats. But, again, exactly how many rivets can be lost before the plane becomes endangered is unknown, and no one has a way of finding out. Nevertheless, biologists recently made a strong statement in support of the diversity-stability relationship, which effectively raises a flag in support of high species diversity. The Scientific Committee on Problems of the Environment, a part of the United Nations Environment Program, met in California early in 1994 to review evidence and opinions on the matter. Displaying the courage to admit that no one had a complete answer, the committee concluded that high species diversity is beneficial; there may be some redundancy in many ecosystems; and who could claim to be sufficiently without ignorance to have the confidence to tamper with it?

I've always been passionate about wild and remote places and have had an intense interest in and love of animals. As a teenager I wanted nothing more than to be a game warden, out in the wild, trapping dangerous animals, leading a life of adventure. When I became director of the wildlife service, I not only tackled the practical issues of the conservation of wildlife in the face of development and the daily horrors of seeing the spoils of poaching elephant, but I was also brought close to a deep, visceral passion for nature. By accompanying my parents on their searches for early human relics, as a child I absorbed a deep knowledge of animals and their natural environment. I also learned how to fend for myself in the wild; how to find water and food in what looked like a barren desert; how to track and trap wild animals. Unconsciously I learned how to be part of nature, to respect it, not be afraid of it. Although I didn't realize it at the time, I was extremely fortunate in my childhood experience, because it allowed me to connect with something that is fundamental to the human psyche. Most people are denied this, although, unconsciously, most strive for it. For some 150,000 years, our Homo sapiens ancestors lived as hunter-gatherers, in many different environments. This highly successful mode of existence had its origins with the evolution of the genus Homo, sometime prior to two million years ago. The expansion of the brain that has occurred since that time, and the development of the human psyche that has gone along with it, were in the context of the hunter-gatherer way of life. It was a life of extreme intimacy and dependency upon all of nature. It required keen sensitivity to every aspect of nature. Our ancestors undoubtedly saw the other species in their world as a source of food, of many kinds; they must have witnessed much to wonder at in that world, as we see reflected in the cave and rock shelter paintings of Europe and Africa; and they knew themselves as an integral part of this diverse world. I have written often that, although we occupy a modern technological world, we have the minds of hunter-gatherers. I knew this instinctively, if not explicitly, when, as a boy, I listened, fascinated, to my father's stories; and when I wandered confidently in the wild terrain of Olduvai Gorge. A decade and a half ago, Edward Wilson put a name to this instinct: he called it biophilia. Recently, Wilson defined biophilia as "the innately emotional affiliation of human beings to other living things. Wilson is speaking of something deep within the human psyche, something that has become a part of our very existence through millions of years of evolution. He is speaking of emotional responses that touch the essence of humanity, the very essence of our history. Some of these emotional responses to nature may be negative, as is the aversion many people have to snakes, even to the idea of snakes in the abstract. But many are positive. Why else do people so often seek relief from urban stress by visiting wilderness areas? Why else do people in the United States and Canada more often attend zoos than all major sports combined? And why do people secure a home in the countryside if they have the means to do so? The psychologist Roger Ulrich has shown that, given a visual choice between urban or rural scenes, people overwhelmingly prefer the latter. The preference may go even deeper, however, perhaps reaching back to our ancestral past. Given visual choices among rural scenes, people show an overwhelming preference for rolling landscape vegetated with scattered trees, preferably flat-topped trees. The University of Washington ecologist Gordon Orians interprets this preference as a deep psychic connection to our origins in East Africa, the preferred landscape being reminiscent of woodland savannah. Some find this suggestion far-fetched, but, like people's spontaneous love of some animals and fear of others, it strikes a chord that cannot be ignored. Whether negative or positive, our response to wild nature, according to the biophilia hypothesis, is an ineradicable part of human nature. It is the heritage of eons spent as hunter-gatherers in ancestral times. Western culture, with its high-tech civilization, has come to ignore the essential connection between the human psyche and the world of nature, while emphasizing the promise of worlds beyond our own planet or solar system. It ignores the connection, but the connection is still there. Other cultures do not do this. Half a century ago the Native American Luther Standing Bear wrote: "We are of the soil and the soil is of us. We love birds and beasts that grew with us on this soil. They drank the same water and breathed the same air. We are all one in nature. Believing so, there was in our hearts a great peace and willing kindness for all living, growing things."" Western culture has come to view Homo sapiens as not only special in the world (which we undoubtedly are in many ways), but also separate from that world. It is as if we were set down on the Earth, complete and finished in our present form, to have dominion over Earth's creatures. This is not true, of course, but it is all too easy to think in evolutionary terms, seeing Homo sapiens as the product of a long process, and yet still perceive us as special and separate. There is, after all, nothing like us in the rest of Creation. There is, after all, a tremendous gulf between the mind of Homo sapiens and that of our closest relatives, the African apes. But if one spends one's life reconstructing the path taken by our distant ancestors on their evolutionary journey from ape to human, the gulf disappears. Not only is it possible to find and identify physically the species that connect us to our ancestral roots-Homo erectus and Homo habilis-but we can also build a picture of their behavior. Most important of all, we can see the context in which our evolution took place, the constantly shifting ecosystems of which our ancestors were an integral part. It is this intimacy that impressed itself on the emerging human psyche. It is this intimacy to which Luther Standing Bear instinctively alluded. It is this intimacy each of us experiences today, in different, perhaps muted ways. And it is this intimacy that enables us to place value on the biodiversity of which we are a part today, separate from the direct economic benefits of foods, materials, and medicines, and separate from the ecosystem services upon which our physical survival depends. The value of the species around us now reaches to the human spirit-not an easy thing to say in the context of science, but valid nonetheless. We may value biodiversity because it nurtures the human psyche, the human spirit, the human soul.