Up to speed in the human race?

Charles Darwin's combative friend and supporter Thomas Henry Huxley was ever anxious to caution Darwin against too firmly supporting the notion that evolution was a slow, gradual process. This was encapsulated in Huxley's letter imploring Darwin not to burden himself with the dictum natura non facit saltum (nature does not make leaps). By "nature", Huxley was referring to Darwin's theory of evolution by means of natural selection. Central to Darwin's body of work is the principle of extrapolation: that by the accumulation of small changes over long periods of time, major change (such as the origin of new species) can be achieved. Such differences in perception held between such firm allies at the very birth of the Darwinian revolution reflect a dichotomy that can be said to have either dogged or enriched the theory of evolution. Sudden Origins by Jeffrey Schwartz, a professor of anthropology, appears to be an attempt to offer a popular account of this controversial topic and, if we are to believe the blurb, "is a provocative and important book that will change the debate about evolution and challenge a number of popular ideas premised on the foundation of Darwinism". This is, of course, journalistic hype, and suggests that the intended audience is the broadly informed general public. Parts of the book make a good and informative read, such as those concerning the unravelling of human origins (Schwartz's area of expertise) and a long historical section on the contribution of a selection of important scientists to the evolutionary debate.

The book opens with a fast-paced introduction that serves to familiarise the reader with some of the main players in the story of the development of an "uneasy" theory of evolution. The nature of each contribution to the debate is outlined, and some of the key problems associated with the original Darwinian theory of evolution are discussed. For example, difficulties relating to the then unknown mechanism of inheritance with which Darwin wrestled rather unsuccessfully throughout his life, and the mute support from the fossil record for Darwin's gradual mode of evolution. Intermediate types of organisms (missing links) were rare, but Darwin neatly side-stepped this observation by, quite logically, proposing that the fossil record was inevitably full of gaps; he drew an analogy between the stratigraphic column and a battered old book, many of whose pages are missing. Biologists, philosophers and palaeo-biologists have pondered these issues since the 1860s. But it is the saltationists who (naturally) receive the sympathy in a carefully constructed line: Huxley, William Bateson, Hugo de Vries, Haldane (by sleight of hand), Richard Goldschmidt and Otto Schindewolf, culminating with Niles Eldredge and Stephen Jay Gould in the 1970s with their advocacy of punctuated equilibrium.

In recent years, particular attention has been focused on developmental processes, and more particularly on the role of genes that control the pattern of embryonic development (homeobox genes). These latter genes are trailed pretty blatantly as the factor that can be brought into the debate and help to explain the "suddenness" of the origin of species implicit in the title of this book. I suspect that the rather helter-skelter nature of this introduction, while perfectly acceptable to the initiated, would probably be quite daunting to the general interested reader.

By way of a diversion (as it seems to me), the next 150 pages are devoted to what I can only describe as an extended essay on the fascinating history of the scientists and science behind the discovery of human ancestry. This clearly feeds on Schwartz's strengths as an academic as you can tell by the style, which is both comfortable and enjoyable. The purpose of this is to show that it is possible to explain aspects of human evolution (anatomically) in terms of the timing of the process of embryonic development. In simple terms, humans (who share more than 95 per cent of their DNA with chimps) are more similar in appearance to juvenile chimps than to adult chimps. Humans might thus be considered, for the sake of argument, to be apes with slower development and/or accelerated sexual maturity. This latter observation is not original, having been discussed in great detail by Gavin de Beer in the early 20th century.

In 1972 the palaeontologists Eldredge and Gould demonstrated that the fossil record seemed to show an almost universal pattern of species apparently existing for discrete periods of time (the "equilibrium" phase) and then being rapidly replaced by distinctly different species (following a hiatus or "punctuation"). They argued that the fossil record was sufficiently well investigated in some examples (compared with Darwin's time) for this pattern to reflect the genuinely punctuated nature of evolutionary change, as opposed to the gradual nature of change advocated by Darwin and others. In a follow-up to this general set of palaeontological observations Gould set about identifying a mechanism by which such short-term, and by implication rapid, evolutionary change might take place. In 1977 he published an extended argument, Ontogeny and Phylogeny . Gould trawled the same literature as Schwartz and identified the timing of embryonic development (an organism's ontogeny) as the potential solution. In essence what he showed was that if apparently small (outwardly insignificant) changes occur at an early stage in the development of an embryo, the resultant adult can be altered quite dramatically and that, by extrapolation, change in form can result in the appearance, quite suddenly, of new potential species. What Gould could not then know was the revolution in research into the genetics of development, that would occur in the 1980s. This is the point at which Schwartz attempts to enter the debate.

Schwartz's general thesis is that, as Gould and many before had argued, species may indeed appear rapidly because of subtle changes occurring at the level of the genes that control and organise the process of embryonic development - the homeobox genes. This is neither new nor revolutionary.

Although the general nature of his claims would appear to have some basis in logic, given what we now know of the genetics of development, it does seem that Schwartz is indulging in some bluster. Most important he appears to believe that some of the individual genes that he cites as examples operate in isolation during embryonic development. In truth such genes operate in synergistic clusters with a range of other genes, in a very complex and necessarily integrated manner. Experimental developmental biologists and geneticists deal with small abstractions of these complexes in order to understand aspects of the processes involved, but are far from a synthetic view. The simplistic notion of mutating a homeobox gene and letting these altered genes spread (as recessive Mendelian alleles through populations) as Schwartz imagines is far more likely to introduce an element of chaos to the regulated pathway of development than a population of potentially new and "hopeful monsters" (a phrase coined by the geneticist Goldschmidt).

Schwartz's new contribution to the evolutionary debate appears to lie in his acknowledgement of the fact that in order to unravel the process of evolution we need to integrate gradualistic and saltationist views (both may indeed be right). In addition, efforts must be made to integrate the work of morphologists, systematists, developmental biologists and geneticists, if we are to get to the root of the origin of species. There is nothing terribly profound here - I thought that was what we were doing all along.

The book is a bit of a curate's egg. I enjoyed the sections on the origin of human species and parts of the historical review. But I am a little concerned about the overall image it creates for the general reader about progress in the field of biological evolution.

Popularisation of science is incredibly important socially and politically. It is vital that it is done with accuracy and elegance. A book by Schwartz on the topic of human origins might have achieved these ideals in spades. However, lacing this story with information from the fast-moving fields of developmental biology and genetics (fascinating as these fields may be) and then claiming a major step forward in the evolutionary debate was probably not, on balance, a wise move.

David Norman is director, Sedgwick Museum, University of Cambridge.