Probably the first time many people heard of cladistics was in 1979 or 1980. At that time the Natural History Museum in London applied cladistics in two new exhibitions, one on dinosaurs and their living relatives, and the other on man's place in evolution. A palaeontologist called Lambert Beverley Halstead, known to all as Bev, mounted a series of attacks on this innovation. Bev equated cladistics with the advocacy of sudden evolutionary leaps rather than gradual change and linked this with an accusation that the museum wished to introduce a Marxist element into the British educational system. This slightly absurd controversy sparked off a huge amount of correspondence in the Press and in the journal Nature on a wide range of subjects; the general public was left with the impression that the scientists of the Natural History Museum doubted the truth of evolution.
Henry Gee, who is now Senior Editor of Nature, was a witness of this turmoil because he was working at the museum as a student in the 1970s, when he got to know the chief actors in the drama. He remains convinced that the science of cladistics is a vital intellectual tool for our understanding of what he calls Deep Time, to distinguish it from ordinary historical time, which he sees as being qualitatively as well as quantitatively different. In this lively and well-written book he makes a good case for his opinion.
His main argument, which he makes repeatedly throughout the book, is that what might be called the family tree approach to palaeontology, in which fossils are arranged in a developmental sequence that purports to tell a story about how evolution occurred, is unscientific. Fossils are what they are; they don't come with a pedigree and it is impossible to say which are ancestral to which. Even though we may know that fossil A is older than fossil B, because it comes from a deeper geological layer, this does not negate the possibility that species B was really ancestral to species A. Alternatively, there may be no question of ancestry at all; the two organisms may simply have been cousins (all organisms on the planet are, ultimately, related to one another). Claims for developmental sequences are untestable and rest ultimately on the authority of the person who makes the claim, and this, according to Gee, is unscientific.
Another weakness of much writing about palaeontology, Gee says, is that unjustified assumptions are often made about behaviour on the basis of modern models which may be invalid. For example, the ground sloth, which lived mainly in South America, had claws which look like offensive weapons but small weak teeth. Some were huge, much bigger than an elephant, By analogy with modern tree sloths they are usually thought of as vegetarian, but it has also been suggested that they may have been carnivorous, their likely prey being jeep-sized armadillos called glyptodonts. The proposed picture is of the sloth tipping the armadillo over onto its back and opening it up with blows from its fearsome claws. The important, and surely valid, point that Gee makes here is that we simply don't know how extinct animals made a living and it is impossible to decide the question by looking at their fossils.
The cladistic approach to the study of fossils (and indeed to many other subjects, such as the relations between languages) is, Gee insists, the true scientific way to take. For cladistics, what matters is the degree of resemblance, without any presuppositions about ancestry being implied. The idea is to compile lists of characteristics and look for matches using a rigorous procedure; the matches are then tested against chance occurrence. The process has the reputation of being abstruse and technical, and indeed it is difficult to carry out in practice, but the principle is easy to understand and Gee explains it clearly, frequently using himself and his two cats, Fred and Marmite, as examples. This description is very well done, if a bit repetitious at times.
Although quite a lot of what Gee says about evolution will be familiar to most readers of popular science books, he does introduce some new ideas or at least different ways of looking at things. I was particularly struck by his analogy between the Linnaean classification of organisms and the Periodic Table of elements. And yet this analogy does seem to hint at a certain weakness in his argument. The order of the Periodic Table reflects the affinities of the elements, Gee says, but it does not indicate any genealogical connection, just a graded series of properties. This is true, yet we know, or think we know, that the elements were built up in sequence after the Big Bang, starting with hydrogen. Hence there is a sense in which the Periodic Table does represent a family tree.
The resemblances between fossils seem to me to be something like circumstantial evidence in criminology. We know that such evidence may be seriously misleading, yet sometimes it is all we have to go on and we use it to construct what seems to be the most plausible version of what has occurred. Regrettable though it may be, there seems to be an inbuilt human propensity to tell stories. Given a series of seemingly unconnected events, we try to link them together into a narrative. This explains the continuing appeal of novels, plays, and films. It is therefore inevitable that people will not remained satisfied with the austere formality of cladograms but will seek to interpret the resemblances as evidence of historical sequences; indeed, there would be little reason to study fossils if we did not think that they provide an insight into how the past has given rise to the present. But Gee is surely right to warn us that such narratives are interpretations that go beyond the available facts, so that we can never be confident about their validity. In particular, we must resist the ever-present temptation to see the past as a linear progression, with "higher" and "lower" organisms. In this connection, Gee suggests that we might perceive our own position differently if we were not the only surviving human species. However, this is probably over-optimistic, for, as Gee himself points out, the record is not encouraging. When white settlers arrived in Tasmania they regarded the Stone Age inhabitants as animals and hunted them down to extinction.