Oxford paleobiologist
Martin
Brasier’s new book, Darwin’s
Lost World, is first of all a recounting
of his own research history,
beginning with a 1970 trip to
study the modern reef environments
of Barbuda and continuing
with expeditions to far-flung localities
in China, Mongolia, Siberia,
Oman, Newfoundland, and
Scotland. At the same time, it is a
documentation of Brasier’s role in
investigating one of most intensely
studied episodes in earth history,
the roughly 100-million–year period
that culminated in the appearance
of recognizable animal life,
including such familiar fossils as
brachiopods, trilobites, and snails.
This culmination is the so-called
Cambrian explosion.
At the time of Darwin’s writing of the Origin of Species, there was little or no evidence of fossils prior to the earliest Cambrian strata, making it seem as though complex animal fossils had appeared suddenly worldwide. In the first edition of the Origin, while recounting the difficulties in his theory associated with the imperfections of the geological record, Darwin confessed:
if my theory be true, it is indisputable that before the lowest Silurian was deposited, long periods elapsed as long as, or probably longer than the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.
(At the time, the Silurian encompassed what we now call the Cambrian.)
Readers of RNCSE are aware that this 150-year–old conundrum is still considered state-of-the-art science by many in the creationist community. For example, the acting chair of the Texas state board of education, Don McLeroy, in his failed confirmation hearing before the Texas Senate on May 28, 2009, stated that the sudden appearance of phyla in the Cambrian explosion is evidence from the fossil record against evolution.
But research over the past 50 years has conclusively shown that Darwin’s “lost world” did indeed exist and that the explosion was not really so sudden. The history of life on earth has now been documented for about three billion years prior to the Cambrian. Many of the critical discoveries of Precambrian life and their interpretation are entertainingly recounted in Andrew Knoll’s Life on a Young Planet (Princeton [NJ]: Princeton University Press, 2003), which I highly recommend. The current book focuses on the last part of the Precambrian, the recently established Ediacaran Period (630–542 million years ago) and the succeeding Early Cambrian Epoch (542–513 million years ago). This is the period during which complex multicellular life, including animal life, became established.
Reading Brasier’s book will introduce readers to many of the key localities and discoveries, as well as provide glimpses of many of the major investigators, of Ediacaran and early Cambrian life. The well-known animals of the Burgess Shale — often offered as exemplars of the Cambrian radiation — are about 505 million years old and thus actually postdate the radiation, which was pretty much over by 520 million years ago.
Older still are the Ediacaran fossils, best known from places such as Australia, Newfoundland, Russia, and England, but clearly occurring worldwide. What is not clear is exactly what these forms were; opinions range from the earliest representatives of familiar animal groups to a separate and extinct group of multicellular organisms. Brasier’s own opinion is that they were ancestral to sponges, ctenophores, and jellyfish, living mostly by absorbing nutrients from the water.
One of the ongoing disputes in the field of Precambrian–Cambrian research is when major animal groups first appeared. Paleontologists mostly place originations conservatively at or about their first appearances in the fossil record. Others also use “molecular clocks” based on estimates of the rates of genetic change between groups and calibrated with the fossil record. These clocks have almost always placed the origin of animal groups well before their first appearance, with the lack of fossils being explained as a failure of preservation. Brasier dismisses such explanations as based on what he terms “Lyell’s hunch” — the hope that we lack the fossil ancestors because they have not been found yet. In contrast, Brasier argues that fossil preservation in the late Precambrian was better than it was later in earth history, so that if these early forms were present, we should have found them by now.
The book is illustrated with the author’s own photos and line drawings. It is also enlivened by his sense of humor. I especially liked the “MOFAOTYOF principle”, which stands for “my oldest fossils are older than your oldest fossils” and represents the excitement, attendant publicity, and as Brasier stresses, the necessity for concrete evidence when the oldest member of a fossil group is first discovered and published.
Darwin’s Lost World often assumes too much prior knowledge by the reader. The geologic time scale, for example, is not introduced until p 42. The “Snowball Earth” glaciations are mentioned without explanation on p 96 and are not really discussed until some 90 pages later. I also found his occasional attempts to illustrate a point by arranging the text to resemble a picture or graph to be more irritating than illuminating. A recurring problem is the use of the phrase “Cambrian explosion” to refer to the Cambrian part of this story. As often pointed out by my Berkeley colleague Jere Lipps, the use of the word “explosion” is both a misnomer and misleading. How can something that takes tens of millions of years be an explosion? As a result, you will see many paleontologists using the phrase “Cambrian revolution,” to refer to the profound biological changes occurring during this interval. I prefer the even milder phrase “Cambrian radiation”.
These quibbles notwithstanding, I readily recommend this book as an entertaining introduction to a major field in studying the history of life. It will give you invaluable information for the next time you get asked to explain how evolution explains the Cambrian “explosion”.
ACKNOWLEDGMENTS
Thanks to Jere Lipps and Stephen Dornbos
for their comments on this review.