The Cambrian Explosion and the Expectation of Novelty

The history of life, like any history, has occurred as a vector of time. And as in any history, there is never any going back, at least in any meaningful way. Events and their history create irrevocable changes that make each slice of time unique as it passes through the sequence from future to present to past. In the context of the future of evolution, it appears that there will never again be an Age of Fishes, or Reptiles, or Mammals even approximately similar to those that have occurred in our planet's past. This is a point that conservationists refuse to accept: the Age of Megamammals is over. There will never again be an African veldt with the rich assemblage of mammals now confined to Africa's game parks, and soon enough there may be no game parks at all in Africa. Even if we could somehow remove all humans from the planet in an instant, it is doubtful if things would return to the state they were in 50,000 years ago, at the onset of the end of the Age of Megamammals.

But leaving aside a return to any past era, if humanity suddenly were removed from the planet, could we expect to see new body plans? The reality is that there has been little true evolutionary novelty since the Cambrian Period, 500 million years ago. Although the conquest of land allowed vertebrates and arthropods—the two most successful terrestrial phyla—to evolve and explore new themes of shape, these were only modifications of existing body plans, and even that evolutionary adventure seems far nearer its end than its beginning. The birds are the last class of vertebrates to have evolved, and they did so almost 200 million years ago. Yet there seems to be an expectation that something altogether new will arise. Part of this expectation is raised by what did happen long ago in the past, when evolutionary novelty was cheap, during an event called the Cambrian Explosion.

One future cladogram for the pig (bottom to top): pig, genetically engineered, rhino pig, aquatic pig, pygmy, giraffelike, garbage-eating.

For the first 3.5 billion years of its existence, our planet was without animal life, and it was without animals large enough to leave a visible fossil record for another half billion years after that. But when, 550 million years ago, animals finally burst onto the scene in the oceans, they did so with a figurative bang in a relatively sudden event known as the Cambrian Explosion. Over a relatively short time, all of the animal phyla (the large categories of animal life characterized by unique body plans, such as arthropods, mollusks, and chordates) that exist today either evolved or first appeared in the fossil record. Uncontested fossils of animals have never been found in sedimentary strata more than 600 million years old, no matter where on Earth we look. Yet the fossils of animals are both diverse and abundant in 500-million-year-old rocks, and they include representatives of the majority of the animal phyla still found on Earth. It appears that in a time interval lasting perhaps 20 million years or less, our planet went from a place devoid of animals that could be seen with the naked eye to a planet teeming with invertebrate marine life rivaling almost any species on Earth today in size.

The rates of evolutionary innovation and new species formation during the Cambrian Explosion have never been equaled. It produced both huge numbers of new species and body plans of complete novelty. That all of the animal phyla would appear in one single, short burst of diversification is not an obviously predictable outcome of evolution. From this observation comes the second finding concerning the Cambrian Explosion that is equally puzzling, if far less well known: The Cambrian Explosion marked not only the start, but also the end of evolutionary innovation at the phylum level. Since the Cambrian, not a single new phylum has evolved. The extraordinary fact is that the evolution of new animal hody plans started and ended during the Cambrian Period.

The lack of new phyla and the paucity of new classes after the end of the Cambrian Explosion may simply be an artifact of the fossil record; perhaps many new higher taxa did evolve, and subsequently went extinct. This seems unlikely. It is far more likely that the great surge of innovation marking the Cambrian came to an end as most ecological niches became occupied by the legions of newly evolved marine invertebrates.

Yet there remains a puzzling mystery: why is it that no new phyla evolved after the two great mass extinctions, the Permo-Triassic and Cretaceous-Tertiary disasters? While the Permian mass extinction may have caused the number of species to plummet to levels as low as those found early in the Cambrian, the subsequent diversification in the Mesozoic involved the formation of many new species, but

The Cambrian Era saw an astonishing explosion of diverse new body plans.

very few higher taxonomic categories. The evolutionary events of the Cambrian and the Early Triassic were dramatically different: although both produced a myriad of new species, the Cambrian event resulted in the formation of many new body plans; the Triassic event resulted only in the formation of new species that followed already well established body plans.

Two hypotheses have been proposed to explain this difference. The first supposes that evolutionary novelty comes only when ecological opportunities are truly large. During the Cambrian, for instance, there were many habitats and resources that were not occupied or exploited by marine invertebrate animals, and the great evolutionary burst of new body plans was a response to these opportunities. This situation was not duplicated after the Permo-Triassic mass extinction. Even though most species were exterminated, enough survived to fill most ecological niches. Under this scenario, there was sufficient survival of animals with various body forms to inhabit most of the various ecological niches (even if at low diversity or abundance) and in the process block evolutionary novelty.

The second possibility is that new phyla did not appear after the Permo-Triassic extinction because the genomes of the survivors had changed sufficiently since the early Cambrian to inhibit wholesale innovation. Under this scenario, evolutionary opportunities were available, but evolution was unable to create radically new designs from the available DNA. This is a sobering hypothesis, and one not easily discredited, for we have no way of comparing the DNA we find in living animals with the DNA from the long-extinct forms now preserved only as rock (movies such as Jurassic Park notwithstanding). It could be that genomes gradually become encumbered with ever more information—as they gather more and more genes—and in the process became less susceptible to critical mutations that could open the box of innovation.

One of the central—and currently controversial—aspects of the Cambrian Explosion concerns diversity and disparity. Diversity is usually understood as a measure of the number of species present. Disparity is a measure of the number of body plans, types, or design forms among those species. The controversy centers on the wondrous assemblage of fossils found at the Burgess Shale localities in western Canada, where not only early animals with hard parts, but also early forms without skeletons, are preserved as smears on the rocks.

The Burgess Shale has had an enormous impact on our understanding of the initial diversification of animal life. In large part, it is responsible for showing us that most or all of the various animal phyla (the major body plans) originated rela tively quickly during the Cambrian. But the Burgess Shale may also be telling us that not only were the body plans found on Earth today around in the Cambrian, but so too were other body plans that are now extinct. One of the central messages of Stephen Jay Gould's book Wonderful Life is that the Cambrian was not only a time of great origination, but also a time of great extinction. Gould (and others as well) asserts that there were far more phyla present in the Cambrian than there are today. How many? Some paleontologists have speculated that there may have been as many as a hundred different phyla in the Cambrian, compared with the thirty-five still living today. In observing this pattern, says Gould, "we may acknowledge a central and surprising fact of life's history—marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."

This view—so forcefully and beautifully described in Gould's Wonderful Life—is vigorously disputed in the 1997 book The Crucible of Creation by British paleontologist Simon Conway Morris, also about the Burgess Shale and the Cambrian Explosion. Conway Morris is, ironically enough, a central and sympathetic figure in Gould's book, which portrays him as one of the architects of our new understanding of the Cambrian Explosion. But he is not so sympathetic to Gould. He disputes Gould's assertion that disparity has been decreasing since the Cambrian, citing several cases suggesting just the opposite. Conway Morris also attacks another of Gould's ideas, the metaphor of "re-running the tape." Conway Morris argues that convergent evolution (in which distinct lineages evolve similarly in response to similar environmental conditions) can produce the same types of body plans from quite unrelated evolutionary lineages. He argues that even if the ancestor of the vertebrates had gone extinct during or soon after the Cambrian, it is likely that some other lineage would have then evolved a body plan with a backbone, since this design is optimal for swimming in water.

Simon Conway Morris's point is that convergent evolution will dominate evolutionary processes. He even makes what might be the first academic reference to Dougal Dixon's book After Man, a semi-whimsical prediction of how animals might look in the far future at a time when humankind has mysteriously gone extinct. Conway Morris notes that the animals conjectured by Dixon all seem to resemble animals living on Earth today, even though they are portrayed as evolving from quite novel sources:

In the book he [Dixon] supposes that of all the mammals only a handful of types, mostly rats and rabbits, survived to repopulate the globe. After Man is an exercise rich in imagination in its depiction of the riot of species that quickly radiate to refill the vacant ecological niches left after a time of devastation. All the animals, of course, are hypothetical. Certainly they look strange, sometimes almost alien. When we look more closely at their peculiarities, however, they turn out to be little more than skin deep. In this imaginary bestiary the basic types of mammal, those that trot across the grasslands, burrow in the soil, fly through the air, or swim in the oceans, all re-emerge.

It is thus the physics of environments that decides which shapes are adaptive and which are not, which shapes can allow flight or running ability or the ability to chase down and kill prey. And this assumption points to a central conclusion: no evolutionary novelty. Expectations of exciting and bizarre new life forms—the types seen in any science fiction B movie—are pipe dreams. The animals and plants arising in the future of evolution will in all probability look much like those of the present—except for being far less diverse.

Was this article helpful?

0 0

Post a comment