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1 September 2005 Walking Back through Evolutionary Time
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The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution. Richard Dawkins. Houghton Mifflin, Boston, 2004. 673 pp., illus. $28.00 (ISBN 0618005838 cloth).

Richard Dawkins, the Charles Simonyi Professor of the Public Understanding at Oxford University, has written his best book—a large claim, given its many excellent predecessors. It is also the best book available if one really wants to “get” evolution, in contrast to being lucidly informed about it (which other books accomplish), because Dawkins, using a brilliant structural device, enables his readers to participate in the evolutionary process.

After a few introductory chapters in which Dawkins carefully explains what is meant by “common ancestry” and explores the common ancestry of humankind, we then set out, as a pack of all the humans alive today, on a backward journey through time. After traveling for 6 million years, we meet up with a pack of all the chimps and bonobos alive today (alas, a very much smaller pack than ours). They have also been walking for 6 million years since the present, and we meet at our first rendezvous site, the time of our common ancestor (“concestor”). Here we pause for some commentary on chimps and bonobos, and then the humans, chimps, and bonobos join together and walk for another million years until we meet, at the next concestor rendezvous, all the gorillas alive today. After some gorilla commentary, we all join up until we meet with the pack of orangutans 14 million years ago. They join us, and next we hook up with the marching gibbons at the 18-million-year mark.

The meetings continue with the various living families of the primate order, and then become more coarsely grained, as we join with groups of related mammalian orders, then with birds and reptiles, fish, fungi, plants, and the various unicells. The final (40th) rendezvous, more than three billion years ago, is with the concestor of us all, that DNA-bearing creature with whom bacteria, archeans, and eukaryotes all share common descent. Many of the creatures that join us—like the lungfish, lancets, and sea squirts—have only one group of living representatives; their tenacity reminds us of the countless creatures who fail to join us at all—like the dinosaurs and trilobites—because they are not alive today. Others have radiated since our shared ancestry into forms that startle us with their diversity, like cats, dogs, bears, weasels, hyenas, seals, and walruses, which all evolved from a root concestor that lived 75 million years ago.

This backward chronology is immensely effective. Each chapter opens with the same simple diagram that shows the next group to join, with the relevant dates and geological eras. I found myself turning a page, taking in the diagram, and gasping, “Holy cow! Here come the rodents!” or “Wow, here come the insects!” It's also a most effective antidote to forward chronologies that so readily convey the sense, however unintentionally, that other species in the diagram are lower or more primitive, while humans are the apex and hence the point. Going backward drives home the fact that all present-day creatures are equipositioned in the present and that biological diversity is the consequence of planetary circumstances, and adaptations to those circumstances, in the past.

Going backward also allows us to experience, rather than just know about, the deep time involved. At the outset we are appropriately impressed by concestors encountered 20 million or 75 million years ago, but as we continue to trudge, the time intervals are measured in the hundreds of millions, and the diagram shrinks in scale until the rendezvous points that were once widely spaced are layered on top of one another. By the time we get to the archeal and bacterial concestors, there's the palpable sense of peering into some dark, roiling deep-sea vent on some mysterious planet that is nonetheless this planet. I came away feeling as though I had come to understand the word ancient for the first time.

In less skillful hands, a template like this could become tedious: by the 20th rendezvous, the format could sag. Happily, Dawkins is the consummate tour guide, embellishing the journey at every turn, sometimes pausing to offer a clear and trenchant explanation of cladistics or geological dating, sometimes commenting on the configuration of the continental plates in that period, sometimes telling us of the marvelous ways that particular creatures go about their lives (as a teller of natural history, Dawkins is right up there with Stephen J. Gould). There's a photograph here, an amusing anecdote there, an account of a blistering scientific controversy about phylogeny somewhere else. Dawkins is always with us, letting us know when an idea or a dating is controversial or speculative and how he made the call, sharing his awe with us at everything we encounter. The scholarship involved in pulling this book off quite boggles the mind, yet there's nary a whiff of pedantry. Although I came to this book with a more substantial understanding of evolution than the generic “average reader,” I am confident that it is accessible to anyone with some science background and a sense of adventure.

Those interested in Dawkins's more theoretical perspectives on evolution will find much excellent thinking to consider in his concluding chapters. Since the 1976 publication of The Selfish Gene (Oxford University Press), Dawkins has served as the poster child, not to mention the whipping boy, for a gene-centered view of life and evolution, despite the more inclusive perspectives offered in his later books. In The Ancestor's Tale, he appears to be incorporating the valuable thinking of complexity theorists like Stuart Kauffman into his framework, lifting up the centrality of autocatalytic cycles in biology and mentioning the concept of emergence (“No extra ingredient needs to be added at the micro level to explain the macro level. Rather, an extra level of explanation emerges at the macro level as a consequence of events at the microlevel”; p. 605). He also takes on the now-classic Gould question, sharpened by Kauffman, as to whether, if the clock were rewound, evolution would produce the same outcomes. Dawkins argues persuasively that yes, the same kinds of adaptations—vision, audition, flight, and intelligence—keep popping through, because niches that render such adaptations useful are continuously available.

Particularly important is his consideration, regrettably brief, of the concept of evolvability, a concept he first proposed in a paper in 1987 and one that, to my mind, is likely to have a far more enduring legacy than the concept of the selfish gene. The basic notion is that the capacity to evolve is a feature of a lineage, a capacity that is itself subject to both variation and selection. Organisms with evolvable features are more likely to diversify; hence their lineages are more likely to radiate into new niches; and hence the lineages are more likely to be represented in an inventory of present-day organisms. An example of an evolvable trait is the segmented animal body plan, ab initio a fairly simple scheme involving differential Hox gene expression. Once set up, it is also fairly simple to add or subtract segments in the embryo, and for particular segments to acquire particular features, the outcome being the spectacular diversity of animal forms. Evolvability is fast becoming a central feature of evolutionary theory, and it would be exciting if it were the centerpiece of Dawkins's next book.

URSULA GOODENOUGH "Walking Back through Evolutionary Time," BioScience 55(9), 798-799, (1 September 2005).[0798:WBTET]2.0.CO;2
Published: 1 September 2005

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