Was the Cambrian Explosion Inevitable

Darwin's theory of evolution describes two of the most important scientific discoveries ever made: (1) that all life has descended from a single common ancestor, and (2) that the various species descending from this ancestral creature have descended with modification. The great advances of physics and chemistry are milestones in human understanding, but they do not themselves describe life. We are life, and we have appeared on this planet through the processes of evolution; it is a central law affecting us. Yet for all its importance, the theory of evolution remains one of the most misunderstood of scientific views. One popular misconception equates evolution with increasing complexity and assumes that evolutionary change (Darwin's "descent with modification") always means an unbroken series of ever more complex organisms or structures within organisms. Although greater complexity often does evolve, it is not an end result of the evolutionary process; modification can occur without increases (or decreases) in complexity. We have only to look at the domains Archaea and Bacteria to see that this is true. From what we can tell from their fossil record, the archaeans and bacteria are no more morphologically complex now than they were 3.5 billion years ago (although, as we have noted, their biochemistry has diversified almost endlessly). They have evolved, to be sure, but that evolution has not involved dramatic increases in morphological complexity.

Of the three domains of life, only one, Eucarya, has undertaken wholesale experimentation in new morphology and body plans. If the process of life's creation were to be repeated innumerable times, it is not at all certain that eucaryan equivalents (lineages exploiting the morphological route of adaptation, rather than the chemical route utilized by the archaeans and bacteria) would appear each time—or even ever again. But on this planet, at least, the eucaryans did arise, and it was from this group that the multicellular animals now dominating planet Earth arose. The pattern and timing of their evolution on Earth may provide major clues to understanding whether, and how often, equivalents of our planet's complex animals could have arisen on other planets.

There are important astrobiological implications in this: Will animal life (or some other type of complex life) inevitably develop on all worlds in a planetary habitable zone? In our estimation, it has always been assumed that forming the first life was the hardest aspect, but that once life originated, it inevitably proceeded "up" gradients of complexity, culminating in very complex animals. Yet the actual history of life on this planet tells a different story. The first life appeared about 4 billion years ago. Eukarytotic organisms did not appear for another 1.5 billion years, and multicellular animals did not appear until more than 3 billion years after the first life. On the basis of this information alone, we would have to conclude that forming animal life is a much more difficult—or at least a more time-consuming—project than the initial formation of nonanimal life. Perhaps the timing observed on Earth was just chance; perhaps on any number of other Earth-like planets with newly evolved prokaryotic equivalents, animals would appear not billions, but millions, of years after life originated. Abundant evidence from our planet's history casts doubt on this possibility, however.

On Earth it is clear that the evolution of animals occurred not as a gradual process but as a series of long periods of little change, punctuated by great advances. This pattern of evolutionary "thresholds" was succinctly described by paleontologists Douglas Erwin, James Valentine, and David Jablonski in a 1997 article in American Scientist: "The fossil record of the last 3.5 billion years shows not a gradual accumulation of biological forms, but a relatively abrupt transition from body plans of single cells to those of a rich diversity of animal phyla." Evolution thus did not gradually create complex metazoans. They evolved quickly, probably in response to a set of environmental conditions quite different from those that allowed the evolution of life in the first place.

There were several of these "great leaps forward." One was the evolution of the eukaryotic cell type with its enclosed nucleus; another was the initial radiation of the animal phyla, described in the last chapter. The most profound, however, was the Cambrian Explosion, that short burst of evolutionary innovation that resulted in the appearance of the larger, complex animals we believe to be so rare in the Universe. In this single, approximately 40-million-year interval, all major animal phyla (all of the basic body plans found on our planet) appeared, each represented by some number of species.

This event has profound implications for the possibility of life on other planets. Is the pattern on Earth—a single, short-lived diversification of larger animals—unique or the standard for all planets? And why did the Cambrian Explosion not take place until 3 billion years after life's first appearance on our planet? Does evolution always require 3 billion years to transform a bacterium into a multicellular animal, or was evolution simply waiting for the environment to become conducive to the proliferation of animal life? This may be one of the most critical questions facing the emerging field of astrobiology.

The Cambrian Explosion signaled a major change in the tempo of evolution then prevailing on Earth. Prior to this, our planet's most complex life

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Figure 7.2 Fluctuations in the number of families, and hence in the level of diversity, of well-skeletonized invertebrates living on the world's continental shelves during the past 530 million years are plotted by geological epoch in this graph. Time proceeds upward.

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Figure 7.2 Fluctuations in the number of families, and hence in the level of diversity, of well-skeletonized invertebrates living on the world's continental shelves during the past 530 million years are plotted by geological epoch in this graph. Time proceeds upward.

consisted of algae, slime molds, and single-celled animals characterized by low rates of evolutionary change. There was little morphological change, and few new species arose over vast stretches of time (see Figure 7.2). The first evolution of metazoans changed all this. The staid tempo of evolutionary change that had characterized the first 3.5 billion years of life's history shifted into a higher gear. New species appeared at a far more rapid rate. They— we—have been diversifying at breakneck speed ever since.

A study of the various animal phyla is thus the study of a few dozen stable and long-lived body plans. This study has resulted in three great surprises. The first was the recognition that evolution has produced only a relatively few body plans. The discovery that the perhaps tens of millions of animal species on Earth today belong to between 28 and 35 phyla was a major surprise to nineteenth- and twentieth-century paleontologists and zoologists. Why this number and not a hundred? Or a thousand? Or five, for that matter? Diversification of the huge number of different species on Earth today (it is estimated to be between 6 and 30 million) has been through elaboration or convolution of simple and conservative structural designs. Astrobiologists seek to discover whether this is how all animals (or their equivalents) evolve. Or is this simply Earth's way, and might there be worlds in space where there are nearly as many body plans as there are species?

A second surprise, and perhaps the most astounding, was that virtually all of the phyla appear to have originated no later than the end of the Cambrian and that none have appeared since. This cannot be proved, for there are some minor phyla (such as the rotifers) that have left no fossil record, and perhaps some phyla originated after the Cambrian. But there are none that we know of. For all the great changes that have occurred in the last 500 million years, with all the evolutionary events and mass extinctions of that long history, it would seem that at least a few new body plans would have appeared. Yet the fact that every phylum with a fossil record is represented in Cambrian strata makes such a supposition problematic.

The third surprise was that there may have been far more phyla on Earth in the Cambrian than there are today. Fewer than 40 extant animal phyla are recognized today. Yet according to some paleontologists, in the Cambrian that number may have been as high as 100! Although the number of species on the Tree of Life has been increasing through time, the number of higher taxa, such as phyla, has been decreasing. Thus the tree keeps adding ever more twigs and leaves on a dwindling number of major branches. Perhaps the Tree of Life on some other planet is quite different, with great new branches appearing continually through time.

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