A dreadful sentence sneaks through to page 127 of this book. "If biodiversity is depauperated," it runs, "by artificial or natural agency, so too is a fundamental component of human existence." The words are like a lumbering stride by one of Leakey's precious Kenyan elephants, but the passion behind them is clear. It finds a better voice elsewhere: "We risk eroding the human soul if we allow the erosion of the richness of the world of nature around us."
The five mass extinctions of the geological past had elemental causes - climate switch, change in sea level colliding asteroids, whatever. The sixth extinction, now well begun, results from another "environmental abnormality", as Edward Wilson said about human evolution. Leakey, quoting him here, goes on to warn that half the Earth's species may disappear by human agency by the end of the next century.
He seems to think we might behave differently if we felt more in common with our fellow species - if we recognise that our origins were as accidental as theirs. This was the proposition argued so tellingly by Stephen Jay Gould in Wonderful Life, his book on the Burgess Shale fossils. In evolutionary history, humanity's arrival was not at all inevitable, and if the tape was played again we might well be missing.
To accept the chancy origins of human evolution is not difficult. To allow for human extinction, sooner or later seems all of a piece. Both chastening recognitions do, as Leakey says, help to place us "on an equal footing with each and every other species here on earth" - They also strengthen instincts for appreciation and conservation that go beyond the purely aesthetic.
But do they, as he insists, constitute an "ethical imperative", a "duty to protect the rest of nature? Is there not something a bit cock eyed to his assertion that "by this same ethical principle, the fact that one day Homo sapiens will have disappeared from the face of the Earth does not give us licence to do whatever we choose while we are here"?
Why not? Who, as it were, will be watching? Where is the good to be served that stands outside the values of humanity? Is this not the same sort of anthropocentric conceit that bids for the "Christian stewardship" of Earth? James Lovelock, anatomist of Gaia, makes clear the planet's supreme indifference to the presence or absence of people: it will adjust, and carry on.
But of course, since we live as if our presence makes a difference, the impulse is to agree with everything Leakey says There is something quite repellent about the more rational" hubris of the planet managers and eco sceptics on the other side of the debate.
A central theme of The Sixth Extinction may, however, play into their hands. It seems to draw on the special concerns of co author Roger Lewin, who is a Harvard biologist and writer of a book called Complexity: Life at the Edge of Chaos. New (if mainly computer modelled) studies of ecosystems which take into account the precipitate events of chaos theory are changing traditional views of "the balance of nature".
It used to be thought that ecological communities of plants and animals were a stable, harmonious adaption to local conditions. Now we know that, to quote Leakey, "Ecosystems are in a constant state of turmoil, both in space and time, and at any point some populations will be in decline while others may be booming. And constant change is vital as an engine of species diversity."
If this is true, then many of man's more disruptive activities might even be considered beneficial. But bulldozing the Amazon rain forest is probably not to be compared with the way in which, for example, constant and chaotic wave action enriches the diversity of species on the seabed.
Eco sceptics, however, who see current estimates of extinction as wildly overblown, will take comfort from any recognition of change as "an engine of species diversity" and of natural extinctions as "a major creative force in the flow of life". Leakey catches their mood in a quote from the economist Julian Simon: "Recent scientific and technical advances - especially seed banks and genetic engineering - have diminished the importance of maintaining species in their natural habitat."
A further belief traditional to field biologists was that" the sheer number of species in an ecosystem, and the complexity of their interactions, are important for stability. This "in the bones feeling", as Leakey says, influenced biological thinking for a long time. In recent decades, however, theoretical models have suggested that the more components there are in an ecosystem, the more things there are to go wrong.
This, too, could be used by eco sceptics to justify a lack of concern for depletion of species. There may, indeed, be some redundant species in any ecosystem but who would feel confident about deciding which they are? We are as ignorant about that as about the crucial relationships between the size of wild habitats and the ecosystems they sustain.
Leakey warns of "cascades of extinction" - In one experiment with a small island of protected rain forest, three species of frog vanished early on the plot was too small to support the peccaries whose wallowing in mud created the ponds for the frogs.
The recent research insights into the nature and behaviour of ecosystems will prove the chief interest of this book for readers already familiar with the debate. Otherwise, it reiterates a good deal that has already been said, sometimes rather better, by others.
Its overriding value, however, lies in the great concern it voices, and in Leakey's emphatic vote of solidarity with the case for conservation. His perspectives on evolution and nature are derived from a life, as paleontologist, anthropologist, wildlife administrator and field naturalist, that is quite unique in science. It is well worth knowing how he thinks and feels.
