Graham Budd F ossils have always been a bother. In- itially, natural philosophers were more impressed by their stony composition and where they were found than by what they looked like. Accordingly, they were compared to gemstones as often as to living organisms — perhaps not the best start for palaeobiology. Even when fossils were rec- ognized as the remains of past life, no one knew how to classify them. Dinosaurs, ammonites and trilobites seemed to be quite like other reptiles, cephalopods and arthro- pods. But which ones were they like in particular? Conscientious palaeontologists strained sinews trying to force these groups to behave. Surely trilobites were a type of crustacean? Or did those antennae make them insects? As these efforts at classification often failed, palaeontologists changed tack, creating countless high-level categories for fossils. At best, problematic groups were tagged as, for instance, ‘annelid-like’, given their own class or phylum, and cheerfully connected to the tree of life with dotted lines and question marks. This gave rise to the view that early evolution was different from ‘standard’ microevolution, with living groups of organisms suddenly appearing amid fireworks of excess ‘body plans’. The most popular victim of this muddle has undoubtedly been the origin of animals in the ‘Cambrian explosion’. Yet this amazing pattern — the inspiration for entire books devoted to analyses of its supporting mechanisms — is entirely the consequence of bad systematics. Fossils become awkward if one imagines that our systematic groupings of extant biota somehow reflect the evolutionary process in its entirety — a task that they are ill-equipped to perform. Consider two closely related living groups, birds and crocodiles, neither of which gave rise to the other. By definition, the most recent common ancestor of all living birds was a fully fledged bird, and the last common ancestor of all living crocodiles was, mutatis mutandis , a fully fledged crocodile. But what was the most recent common ancestor of both birds and crocodiles? On the basis of the systematics of ‘living forms’, the possible answers seem to be bird, crocodile, or both. As the first two options are ruled out by definition (neither gave rise to the other), one would have to say that this animal fell at the base of both the bird and crocodile lineages. But from the definitions above, the two animals at the bases of these groups were already completely bird-like and crocodile-like, respectively. If they were identified as the same animal, it would have to have been grotesquely (if memorably) endowed with all the features of both. If your appetite for the bizarre is not sated by such a monster, consider the common ancestor of this grouping and snakes. The ultimate view of evolution under this scheme would be that the ancestor of life somehow appeared with the features of all its descendants, and simply went on to abandon unnecessary lungs or fins as required. This preformationist model is patently absurd, but subconscious acceptance of its premises continues to have a surprisingly widespread and pernicious effect. Our perception of systematics exerts a profound influence on our view of evolution. The division of fossil and living organ- isms into ‘stem’ and ‘crown’ groups, devised by the systematist Willi Hennig and the palaeontologist Dick Jefferies in the 1960s and 1970s, encapsulates the evolutionary process. Surprisingly, it has been applied very little. In this scheme, the most recent common ancestor of living birds and croco- diles represented not both, but neither of the living groups, and as the two lineages diverged from each other, their respective features accumulated in a step-by-step manner. The animals that filled the gap between the most recent common ancestor of the two living groups and that of only one of them form the latter’s ‘stem group’. Con- versely, the most recent common ancestor of one of the living groups plus all of its descen- dants form the ‘crown group’. The nice thing about stem groups for palaeontologists is that they must, by definition, comprise only fossil organisms (living forms are all in crown groups). Of course, stem groups themselves sit within larger crown groups. If you are interested in how a living group evolved its distinctive features, and the principal transitions that led to it, you must look at the fossils of its stem group — the living forms are no help. Dinosaurs and trilobites can now find their rightful homes — in the stem groups of birds and chelicerate arthropods, respectively — without any strain; they also allow the reconstruction of ecological, functional and even develop- mental histories of the living groups in whose stem groups they are situated. The low-tech study of morphology is not currently a popular discipline, but this does not render it unimportant. Morphology represents not just the product of the genes, but the mysterious interface between the genetic material and the environment — a prism through which the glare of selective pressure on the genome is refracted. If we wish, for example, to understand the evolu- tion of the genome, we must also consider what morphology has been doing, probably in ways that we hardly suspect. If so, the secrets of the fossil record should be of inter- est in even the most up-to-date molecular laboratories. The classification of trilobites might yet find its place in the lexicon of the genomic revolution. ■ Graham Budd is in the Department of Earth Sciences, Uppsala University, Norbyvägen 22, Uppsala SE-752 36, Sweden. FURTHER READING Budd, G. E. & Jensen, S. Biol. Rev. 75, 253–295 (2000). Smith, A. Systematics and the Fossil Record: Documenting Evolutionary Pathways (Blackwell Scientific, Oxford, 1994). Climbing life’s tree concepts NATURE | VOL 412 | 2 AUGUST 2001 | www.nature.com 487 Stem groups The division of fossil and living organisms into ‘stem’ and ‘crown’ groups encapsulates the evolutionary process, but has been applied very little. Four-limbed forebear: the tetrapod Acanthostega is thought to be one of the earliest land vertebrates. NATURAL HISTORY MUSEUM, LONDON © 2001 Macmillan Magazines Ltd