Dr. Stephen Jay Gould, a Professor of Geology at Harvard University, a Curator of invertebrate paleontology in the Museum of Comparative Zoology, a public figure and spokesperson, and an author of many popular science books, has in a Wonderful Life created a window through which the reader views one of the most important events in natural history: the explosion in the variety of marine animals that took place during the Cambrian Period more than half-a-billion years ago. What could be more thrilling than to witness one of the great steps in the long history story of life that has begun with a microbe and ended with the Homo sapiens?
A favorite theme of Dr. Gould is to overturn commonly accepted notions that have arisen through human prejudices often due to self-centered preconceptions. It is almost as though Gould regards humans as apes with merely enlarged brains and big egos1 who would still believe to this very day that the Earth was the center of the universe had not humanity been enlightened by Galileo, Newton and Copernicus. This is one reason why Gould selected as a subject of a book the Burgess Shale of eastern British Columbia in Canada. Charles Doolittle Walcott, who discovered the remains of tens of thousands of mostly soft-bodied invertebrates in the Burgess Shale in 1909, managed to misinterpret what one day would become one of the greatest discoveries in paleontology. Walcott incorrectly classified these marine animals as ancient versions of modern fauna despite the extraordinary character of some of them such as Opabinia, which had five eyes, and Hallucigenia, whose name describes its bizarre and dream-like appearance (it had seven pairs of toothpick-like spines, a tubular tail, a bulb-like head, seven tentacles along its back and six pods at the base of its tail!). Walcott considered this creature to be a worm. How could the director of the Smithsonian Institution make such an incredible mistake? According to Gould, Walcott was trying to force the Burgess Shale fauna into the classification scheme prevailing at the time. Here was one of the great field paleontologists trying, like an infant child, to squeeze round pegs into square holes.
For more than 60 years, scientists would believe Walcott’s interpretation of the Burgess Shale fauna. However, in 1971, Harry Whittington published a monograph on Marrella in which he expressed doubts that Marrella was a trilobite2 as previously thought. Marrella looked roughly like a fly with wings replaced by a shield consisting of four, pointed prongs sweeping over its back. Eventually, Marrella would be classified as an arthropod unique to the Cambrian Period. Simon Conway Morris and Derek Briggs, who began as graduate students of Whittington, would join him to form a trio of researchers who would radically change Walcott’s view of the Burgess Shale fauna. Through thorough examination of the specimens, the three would in due time arrive at the conclusion that many Burgess Shale specimens were unique arthropods or belonged to new phyla, implying that the Cambrian Period was one of the richest for animal diversity. Thus, this period in geological history marked the appearance of new “wonderful life.” Other paleontologists eventually joined the quest to understand Cambrian marine creatures in what would become a revolution in evolutionary thought — a saga of scientists struggling to discover the truth.3
It is perhaps ironic that Gould, who points out the pitfalls that personal prejudice and ego can play in drawing false conclusions in science, should employ a rather pompous writing style where, in places, his arrogance pervades every other paragraph and where in the Preface of the book he refers to himself as “yours truly.” Nevertheless, Gould is one of the few research scientists who has achieved a heightened mastery of the English sentence and a vocabulary rich enough to make even the most sophisticated writers envious.
|In fact, we are very “lucky” to be here on Earth.|
The Cambrian Period began 570 million years ago. It marked the beginning of the Paleozoic Era (from which henceforth hardened body parts such as exoskeletons, shells and bones would leave a trail of fossil evidence that paleontologists would ponder over to construct the “tree of life”), and it marked the advent of virtually all the major groups of modern animals. Not only were all the basic extant anatomical designs introduced at this great moment in life’s history but also some designs appeared that would be unmatched in modern fauna. The Cambrian was a period during which Nature seemed to be experimenting in an enormous variety of body structures. Many of these “experiments” failed in the sense that they did not survive, but interestingly Gould suggests that somewhat random factors rather than survival-of-the-fittest properties were the reason for the subsequent decimation.
This coincides with a new viewpoint of evolution that Stephen Gould and Niles Eldredge have advocated in which contingency plays a significant role. This is the so-called punctuated equilibrium. During most periods, the environment at most slowly changes and Darwin’s survival-of-the-fittest principle prevails. But there are other times in geologic history when rapidly varying conditions or catastrophes randomly decimate Earth’s creatures. The extinction of the dinosaurs is a good example (See Chapter XVI :Nature’s Holocaust of the Book of the Cretaceous of The Bible According to Einstein). For more than 100 million years, the dinosaurs were the most “successful” land animal, but, when an asteroid struck the Yucatán Peninsula 65 million years ago, large size was a great disadvantage. Only the smallest creatures survived. When 90% of species disappear during an extinction event, it is often random factors that decide which life forms survive. Another example of contingency, Gould argues, is the Cambrian fauna. It is impossible for scientists in examining the remarkable Burgess Shale fauna to predict which ones would survive and be “successful.” Would not Opabinia, for example, with its five eyes have an evolutionary advantage over fauna with two eyes?
These ideas have led to a new view of the tree of life with a richer branching structure. Compared to the old tree, the new tree during brief periods (such as at the beginning of the Cambrian) suddenly broadens in diversification only to suffer a decimation at a later time with many “leaves” terminating in evolutionary dead ends. Only a few lineages survive. Not all scientists believe in the theory of punctuated equilibrium of Gould and Eldredge, but most give the two credit for pointing out that species quite often appear and disappear on what seems to be short geologic time scales. Although this might be an artifact due to the scanty fossil record, it is more likely that evolutionary processes occur much faster than have previously been assumed.
Gould, by his own admission, is unable to provide a convincing explanation of why the explosion in animal diversity occurred during the Cambrian Period. This is often regarded as an unsolved problem in natural history. According to Gould, several evolutionary forces could have contributed. One argument is that life was filling a new, previously uninhabited, ecological niche, thereby providing an “open field of unparalleled opportunity” into which organisms expanded at rapid rates. A second, rather weak, idea argues that genetic systems age and just happened to “mature” at around the time of the Cambrian Period. Some religiously inclined individuals claim that the lack of a good explanation for the Cambrian biological explosion implies divine intervention. This is a Creationist’s point of view applied not at the initial moment of the birth of life but to the middle of its more-than-three-billion-year long development.
The Cambrian explosion can be “explained” by several important developments in evolution that had occurred in the Precambrian. Earth’s very early atmosphere essentially contained no oxygen gas. Indeed, this was “fortunate” for O2 was poisonous for early life. The oxygen in the atmosphere has been largely biologically generated as a waste product of photosynthetic life, and, as a result, oxygen in air has been increasing during the last 3 billion years. Approximately 2 billion years ago, certain organisms uncovered how to use O2 as a more efficient means to generate energy in metabolic processes. This allowed life forms to increase in size. Eukaryotes, which are organisms with cells containing organelles specializing in various biological processes and containing, in particular, a nucleus housing genetic codes, first appeared around 1.4 billion years ago. About a billion years ago, sexual reproduction arose as a more efficient means of biological replication. The potentially rapid increase in evolutionary progress expected from sexual reproduction, however, was retarded by the longest ice age in geologic history. Had Earth been warmer, the Cambrian explosion might have happened several hundred million years earlier. With the eukaryotic cell, respiratory metabolism and sexual reproduction, life forms were able to increase their size: The first multicellular organisms — microscopic metazoa — emerged about 800 million years ago. Then, 200 million years later, the two supercontinents of Earth broke up providing more shallow water habitats for life, and, at around the same time, one of the coldest ice ages, the Varanger, ended. The confluence of all these developments and events provided the “fuel” for the Cambrian explosion. The second chapter of the book of The Cambrian of The Bible According to Einstein summarizes the situation:
Now evolution was proceeding at a rapid pace. The fire on the fuse was burning. The fuse itself was almost gone. And everything was perfect for a biological explosion, an evolutionary radiation. The geology was right; the chemistry was right; the climate was right; the atmosphere was right; the biology was right; and the recent evolutionary past was right, for sea level had risen and the continents had separated, providing new shallow-water habitats; for circulating ocean currents were full of nutrients; for the seas and air had warmed; for the atmosphere had oxygen; for eukaryotic cells had previously appeared and the sexual revolution in evolution had occurred.
|And it was like unto mixing|
gunpowder, gasoline and fire. And in five-hundred-and-seventy-million BC — the evolutionary “big bang” went off!There was a sudden burst in evolution,
the beginning of a bio-revolution. And the diversity of animals on Earth exploded.
Gould is a firm believer that Homo sapiens hold no special status in the scheme of things. It is true — here on Earth, we are the “smartest” creatures. But whether we are superior is a totally different issue. Bacteria outnumber us multiplicatively by the billions. As species, archaea have outlived us by a factor of more than 10,000. Elephants are still bigger, cockroaches more cunning, lions still stronger, rats more numerous and so on. Not only have the seventeenth century astronomers relegated us to an inconspicuous corner of the universe, but modern paleontologists such as Gould have also placed us on a tiny, barely noticeable twig of the tree of life.
In fact, we are very “lucky” (in both senses of the word: “fortunate” and “probabilistically unlikely”) to be here on Earth. We often think of Darwinian evolution as a smooth, almost predicable, process in which the poorly adapted species die out and the “superior” species survive. Such a viewpoint is not valid and neglects some of the truly random events that have shaped the history of life. The asteroid-induced extinction mentioned above is a good example. Had it not struck Earth 65 million years ago, reptiles would probably still be ruling the Earth and the small mammals that survived would not have had the opportunity to radiate. The result would have been no monkeys, no apes and no humans. There are dozens of examples for which a slight change in the history of the Earth would have eliminated one of the essential steps in the long chain that gave rise to Homo sapiens.
Gould, while taking great pleasure in overthrowing false dogma, is probably himself guilty of exaggeration, an effect possibly due to his enthusiasm for his field of research. Has he overemphasized the role of punctuated equilibrium and contingency in the evolution of life? Could the Cambrian explosion be less dramatic, a consequence of a great find of 50,000 soft-bodied specimens in a single location during a particular point in time? Could the introduction for the first time of hard-body parts led to significantly more fossils thereby creating the illusion of a great radiation of species? It is possible that, one day, we shall have more information and a clearer picture as to how the Cambrian fauna fit into the great scheme of things. Perhaps the Cambrian diversity is somewhat exaggerated and future links will be established between divergent phyla. Perhaps some “consolidation of the data” will take place. Indeed, it is now suspected that Hallucigenia is related to the modern velvet worm rather than a new phylum as Conway Morris had proposed. But even if a revision does occur, the Cambrian will remain one of the great periods of experimentation in animal design.
Wonderful Life is detailed, well written, entertaining and informative. It is fun to follow the history of the Burgess Shale story. In the book are some great descriptions and sketches of the main characters of the drama, those diminutive Cambrian marine metazoa of extraordinary design and sophistication. In a Wonderful Life, the reader learns about the thrills and pitfalls of a historically based science such as paleontology. The writing is often intricate, sometimes technical and not always so accessible to the unskilled reader. But enthusiasts of science with an interest in evolution should greatly enjoy reading this classic, best-selling book.