Darwin, dinosaurs and dung ducking

Tempo and Mode in Evolution - The Fossil Trail
September 15, 1995

Fossils really are amazing things. It might be that on another planet a species has evolved sufficient intelligence to discover evolution, but conditions are such that fossilisation cannot occur. I suspect that no matter how clever the scientists on that planet, they will know less about the process of evolution than us (even when they think they know it all). Eons of evolution occurred on earth with no one there to document it: how can science prevail where there is no opportunity for observation? Miraculously, however, evolution made her own records and filed some of them in layers of sedimentary rock. Unfortunately, she was somewhat careless in her accounts, for fossils tell a complex story, one we have struggled to understand ever since.

Darwin and his contemporaries promoted fossils from curio to evidence for evolution, but the "synthesis" of the 1930s was the springboard from which modern evolutionary biology was launched. Darwin's original insights concerning natural selection were wedded to Mendel's concerning inheritance and so evolution came to be envisaged as the gradual alteration of frequencies of genes in populations.

Part of this synthesis, but in some ways the forerunner of future developments was George Gaylord Simpson's Tempo and Mode in Evolution (1944), in which the author proposed a problem and a solution. The problem was that the fossil record suggested that evolution had been anything but constant and the solution proposed was the careful documentation of these changes: "paleontology's unique access to theory". However, over the years, paleontology's role in the development of evolutionary theory waned (abetted by Simpson's own recanting) and fossils came to be seen as a purely historical record of the vagaries of evolution on this planet.

Fifty years on, Tempo and Mode in Evolution: Genetics and paleontology 50 Years After Simpson is a motley collection of essays purporting to advance the case that Simpson was right, that is, that aspects of the fossil record sitting uneasily with gradualist theory do not reflect deficiencies in the former but in fact accurately represent fundamental processes in evolution. Irregularities in tempo are now well established, these including not only series of mass extinctions (with up to 96 per cent of species wiped out) but complementary explosions of speciation. The first organisms, primitive bacteria, evolved negligibly in the course of more than three billion years - most of this planet's history. The real big bang of life on earth was not its origin but the Cambrian explosion of (a mere) 600 million years ago which heralded the arrival of multicellular organisms and a frenzied leap in the complexity of species and, correspondingly, their ecology. Did it have to happen this way?

Disappointingly, the book's title promises rather more than is contained within, the contents being a set of rather technical, empirical papers that do not really mesh together. This is a common shortcoming of books originating in conferences but, accepting that its whole is not more than the sum of its parts, there is still merit in the work presented.

Stephen Jay Gould pleases as usual, with a discussion of how whole organisms or populations can be loci of causality in evolution quite independently of the "book-keeping" of gene frequencies that is often considered the totality of evolution. One of the more remarkable papers concerns a fledgling "experimental paleontology" in which bacterial clones were left to evolve in identical novel environments, thus "replicating history". They evolved differently, supporting the role of chance mutations in evolution and undermining radical adaptationism ("if it's useful it'll evolve"). This extended even to fitness achieved (tested by competition with ancestral clones "raised from the dead" out of the freezer), so vindicating the seasoned idea of an "adaptive landscape" in which adaptive peaks are separated by uncrossable troughs of maladaptiveness with species climbing the peaks in the direction that chance mutations steer them.

There is rather little explicit discussion of either tempo or mode and rather more of old chestnuts: adaptive landscapes, micro- versus macro-evolution and the role of random genetic drift. What is forcefully brought home by the choice of essays is a prevailing schism between the molecule manipulators and thefossil finders: exactly the rift that Simpson's original work attempted to bridge. Despite the beautiful simplicity of the notion of natural selection it seems that when we try to instantiate the theory into real organisms its course is impossible to predict. The flow of evolution is the progress of forms over a terrain of selective forces. The complexity of this terrain, interacting with the momentum of the flow created by existing adaptations creates so many eddies in the process that the term "selection" becomes hardly more narrowly defined when found in a biological context than when in general usage.

A negative conclusion perhaps, but the choice of papers in Tempo and Mode can be most favourably described as eclectic; suitable for a library collection but to be read from cover to cover only by (conscientious) book reviewers.

While the theorists wrangle over necessity and grand patterning in evolution, paleoanthropologists face chance and contingency in the evolutionary eyeblink that is human evolution. Yet the author of The Fossil Trail is at pains to drive home that this does not make paleoanthropology a straightforward process of discovery: science may be impartial but scientists are not. We have not yet escaped the urge to create origin myths.

For this reason perhaps, Tattersall employs a strictly chronological and largely descriptive account of "the fossil trail". Paleoanthropologists are a factious bunch but Ian Tattersall largely restricts his analysis to finds rather than finders. The historical approach pays dividends since the result is a unique catalogue of the evolution of hypotheses concerning human origins: this evolutionary tree is certainly a deal more tangled than the real tree it strives to discern. The location of the action shifts across the globe, from the discovery of Java Man, back to South Africa where Robert Broom busied himself dynamiting out bits of Australopithecus from caves before events (and this book) really take off in the Rift Valley. Of course, details of where the diggers laboured in vain are absent, but still, by Tattersall's narration we get a sense of the surprising (almost suspicious) serendipity surrounding many of the great discoveries: Richard Leakey, annoyed at being assigned a duff patch of his father's site in Ethiopia, nips over the Kenyan border to Lake Turkana and happens upon one of the most fertile sites ever found. Another fieldworker, ducking to avoid elephant dung thrown by a colleague, resolves questions surrounding the antiquity of bipedalism as he comes face to face with a set of fossilised footprints. And major revisions will still be made in the future: a hasty footnote appended as the book went to press covers last year's discovery of Australopithecus ramidus: the oldest hominid yet identified.

Commentary on these events is minimal. Tattersall cannot resist noting the racism that underlay early theories; this including the pervasive view that some human races were "more evolved" than others and even one early commentator's ascription of a Neanderthal skull to a Celt since it "resembled a modern Irishman with low mental organisation". But as the subtitle states, this is a book on "how we know" rather than "why we think". The insertion of the chapter "Theory intrudes" sounds almost apologetic.

It is in the last few chapters that the author shows his teeth. His solution to controversies surrounding interpretation is to look at the fossils and describe them purely in terms of similarity. This cladistic approach is eminently sensible, since disputes can be resolved by looking at the primary evidence. Tattersall's chief criticism of work hitherto is its obsession with evolutionary trees and dating the branches of these. This is a tricky enterprise partly because the rate of speciation need not be constant and partly because of the subjective way that fossils have been grouped into species. Reproductive compatibility (the criterion for membership of a species) may have rather little to do with, say, dental morphology. Worse still is the construction of "scenarios" that posit specific interactions of a species and its niche in order to characterise the selective pressures driving human evolution. This should be the final level of analysis but the narrative tendency in ourselves is so strong that we project these stories onto the fossil evidence. Our sense of our own uniqueness, for example, may underlie the long-held assumption that each of our ancestors was the sole representative of the Homo genus, links in an unbroken chain of progress toward sapience. This is almost certainly untrue and Tattersall's second lament is that we have vastly underestimated the number of species represented by existing fossils. Species go extinct at a reasonably constant rate and occupy a reasonably constant geographical range allowing us to estimate the number of species that the hominid niche could have supported. In fact there is room for more than have been traditionally recognised particularly when the geological turmoil of the Ice Ages might have promoted speciation.

You will have to dig fossils to find this book truly absorbing; they adorn every other page and with the absence of much-needed summarising it is easy to get lost. The final chapters are the most readable; the early, dispassionate account of the fossil record is finally abandoned in a (relative) orgy of speculative scenarios. But this is as it should be: sweets come after staple fare and the overall effect is tightly controlled, measured, fair, thoughtful and demands a good deal of respect.

Thomas Sambrook is a member of the Scottish Primate Research Group, University of St Andrews.

Tempo and Mode in Evolution: Genetics and Paleontology 50 Years after Simpson

Editor - Walter M. Fitch and Francisco J. Ayala
ISBN - 0 309 05191 6
Publisher - National Academy Press
Price - £40.95
Pages - 325

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