In Search of Deep Time by Henry Gee, Fourth Estate/Free Press, 拢20/$26, ISBN 1857029860
TAKE a complete, illustrated catalogue of London鈥檚 National Gallery. Shred it into tiny pieces and cast them into the wind from the gallery鈥檚 steps above Trafalgar Square. Wait a few weeks, then scour the square for surviving scraps of paper. Now try to reconstruct the history of painting from your haul.
If you manage to produce a coherent story-schools, styles, genres, named painters and all-you are probably a palaeontologist. The task would be a lot easier than what you do for a living. In your everyday work, you are also reconstructing a story from fragmentary clues whose survival depends on the play of chance. But you have to interpret your evidence not in terms of a few centuries or millennia of human culture, but against the backdrop of hundreds of millions of years of geological history. You have to fathom deep time.
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That, says science writer Henry Gee, changes things in ways students of the history of life have tried to deny. 鈥淒eep time鈥�-the term coined by the peerless American writer John McPhee when he was trying to comprehend geology-is qualitatively different from historical time. We have sensed this dimly ever since the Victorians first peered over the brink of a temporal precipice that stretches so far back that the mind reels. But we have not faced up to its intellectual consequences.
How are we to make sense of the fact that any two fossils recovered from the deep past are almost certainly so far apart in time that we can鈥檛 say how they might be connected? Causal stories are simply that-stories. And stories, says Gee, are mere inventions, and so are unscientific. The 鈥渇ossil record鈥� is a hopeful misnomer. A fossil has no birth certificate. It cannot be cast as a missing link in any progressive story about the pageant of life. Rather, fossils are 鈥渋solated tableaux illuminating the measureless corridor of deep time鈥�.
Life, as far as we know, began only once. All organisms are cousins under their outer membrane. How did they get where they are today? There is not much point in cataloguing all the forms that living creatures have taken over the aeons unless we can write some kind of unified history. So what is the conscientious palaeontologist to do? Fortunately, In Search of Deep Time provides the answer. We may not be able to read history directly from the rocks, he says, but we can plot relationships. This is the palaeontological revolution known as cladistics.
Gee was drawn into the cladistics whirlpool as a student in the 1970s. It frees palaeontologists-and biologists-from the causality trap. You can define the characteristics of an organism, living or dead, then plot how closely it resembles any other being we know. The closer the match, the more likely the creatures are to be descended from a recent common ancestor. A diagram based on rigorous use of such data says nothing about cause and effect in time. It is simply a representation of one, testable hypothesis about evolutionary lineages: that if two organisms share a common ancestor, then they share common attributes.
Cladistics has the reputation of being abstruse and severely technical: compiling lists of characteristics and searching for matches in a rigorous way, then testing matches against chance occurrences. The details certainly are abstruse. But Gee explains the basics with exemplary clarity and frequent help from his cat, Fred (they are related, but distantly). Then he shows the implications of this new discipline for our understanding of fossil fishes-where cladistics began-of birds and dinosaurs and, of course, people.
A wider view
His day job for the journal Nature has given him a superb vantage point. He takes the reader inside contemporary palaeontology, from the excitement of a fossil dig with Meave Leakey to the thousands of carefully stored and catalogued specimens in their metal cabinets at the Natural History Museum. He writes engagingly: you are not going to forget an author who tells you that, if the only visible matter in the Universe were nematode worms, you would still be able to make out ghostly human shapes because of the burden of parasitic worms we all carry.
This stylishness, though, has to make up for one or two drawbacks. Some of the points he makes about evolution will be pretty familiar to anyone who reads popular science books. A case in point is his critique of the adaptationist approach, which reads every observable trait as a response to some past problem faced by the creature in question. And Gee鈥檚 intoning of the cladist creed does become repetitious. At times you feel addressed by a dogged teacher wearied by generations of pupils with short attention spans.
More seriously, his insistence that cladism does away with storytelling is not entirely convincing. The temptation to turn cladograms, the branching bundles of lines-looking like railway sidings from the air-that connect living things by their shared characteristics, whether derived from anatomy or, more often these days, from DNA analysis-back into stories is irresistible. And if cosmology and developmental biology can search for meaningful chronologies, why not palaeontology? These disciplines will always create narratives of some kind.
But Gee does drive home the point that the stories we make to explain cladistic links have no moral-in and of themselves-however many we might supply after the fact. Past creatures simply existed in their own right, not as part of a grand scheme that led to higher things. There is no Great Chain of Being. Modern humans are good at being human. Neanderthals were, presumably, good at being Neanderthal. Succession or survival do not imply superiority.
He ends with a reminder that the temporal vistas opened up by the Victorians go in two directions. Deep time extends into the future as well as the past. Like science fiction writer Olaf Stapledon or, more recently, futurologist Freeman Dyson, he sees a far future in which many different species of humanity will have spread through a whole sector of the Galaxy. But however carefully they accumulate traces of their ancestors, we shall all have been dead for so long that they will have no way to trace direct connections into the past worlds of long-extinct relatives. And no more can we.