It’s been likened to the Big Bang. Liberal creationists claim it supports Genesis 1. Researchers at the California Institute of Technology claim it was precipitated by a 90-degree continental flop. Have you guessed what outstanding mystery of the biosphere it is?
It’s the Cambrian explosion.
Half a billion years ago, during this "evolutionary big bang," life evolved at rates of over twenty times the Precambrian rate. From approximately 535 million years ago to 520 million years ago, nearly all the animal phyla in existence today (and many that are no longer with us), save the Bryozoa, first appeared in the fossil record. While this does not necessarily entail that all animal phyla came into existence during the Cambrian explosion – some scientists believe that the "explosion" was a change in climate that produced conditions favorable for the fossilization of preexisting phyla – the evidence for a period of astounding diversification of life is overwhelming. The animals that made their abrupt appearance during the Cambrian explosion are ancestors of virtually all the creatures that swim, fly, and crawl today.
Until recently, scientists believed that phyla evolved over a ridiculously short period of 75 million years. In 1993, a group of researchers from M.I.T. and Harvard did some zircon dating in Siberia, then took the Cambrian period, chopped it in half, and stomped down the evolutionary boom to the first 5 to 10 million years. "We now know how fast fast is," grinned Samuel Bowring of M.I.T. in an interview with Time magazine. "And what I like to ask my biologist friends is, How fast can evolution get before they start feeling uncomfortable?"
The possible causes of the Cambrian explosion are as numerous and whimsical as the animals it created. Predation is a popular explanation. The appearance of multicellular grazers prompted the appearance of multicellular predators. Initial signs of predation appear just before the Cambrian period. The appearance of hard, protective shells in the late Precambrian may indicate the incipience of a biological arms race.3 Another popular explanation is the "empty barrel" hypothesis, which compares Cambrian creatures to settlers and the biosphere of the time to the American West.
Paleobotanist Douglas Erwin of the Smithsonian Institution points to homeotic homeobox genes as the Precambrian triggers of the Cambrian explosion. These genes, also called Hox genes, control embryonic development. Primitive multicellular organisms such as jellyfish have 3 Hox genes. Now scientists believe that around 550 million years ago, some worm-like creature expanded its Hox cluster to six genes. "Boom!" shouted Jablonski in his interview with Time. "Life crossed some sort of critical threshold."4
Researchers at Caltech and the University of Puerto Rico revealed a literally earth-shattering possibility in 1997. The Cambrian explosion might have been detonated by a rollover performed by the crust and mantle of the Earth. Such acrobatics, prompted by a phenomenon known as true polar wander, involved continental drift ten times the normal rate (i.e. 30 centimeters per year, a blatant violation of the "plate tectonic speed limit"5). The dramatic shift, which took place over a mere 15 million years, coincided with the Cambrian explosion.
Continental flip-flop began with the breakup of the supercontinent Rodinia at the North Pole at a time when Gondwanaland stretched from the South Pole to the equator. Because a spinning sphere is most stable when most of its mass lies at the equator, the Earth was thrown off balance by its extensive polar mass. The entire crust and mantle slid over the core, 90 degrees, to return the continents to the equator.
Such a tumultuous change in climate and ocean circulation swiftly fragmented large-scale ecosystems to create smaller, more isolated populations. These populations branched in an eruption of riotous divergent evolution. Groups of species, isolated from each other, evolved rapidly into different species in a process known as speciation and adaptive radiation. Radiation allowed organisms to inhabit new niches and increase diversity, a process that mysteriously left little evidence of transitional forms in its wake. No new phyla, besides the Bryozoans, have emerged since the Cambrian explosion. Scientists speculate that extensive adaptive radiation filled all available niches, leaving no room for additional phyla. Neither has such gargantuan genetic variation occurred since that period. Possibly, once life became as complex as it did in the Cambrian explosion, organisms could no longer risk major changes in physiology. Even miniscule variations in the genetic code could prove fatal, just as one erroneous gene coding for an enzyme might render a creature unable to survive.
The Cambrian Explosion leaves us humans, 500 million years later, with the most puzzling of questions. The Cambrian rocks of the geologic column contain a proliferation of complex life; however, no trace of predecessors to such complex and sometimes offbeat organisms is to be found in Precambrian rocks. For example, the evolution of vertebrate fish from invertebrate animals, which wore exoskeletons and left no traces of turning their exoskeletons inside out to produce vertebrae, remains a gaping hole in the evolutionary timeline. Thus the Cambrian explosion raises questions about Darwin’s grand theory of evolution.
Creationists exploit the Cambrian explosion as evidence that the Biblical record of creation is true. ‘Then God said, "Let the waters abound with an abundance of living creatures."’6 So appeared the abundant fish and marine life that appeared during the Cambrian explosion. These "stationary or slow-moving" creatures were the first to be overwhelmed by the mud and silt of the Deluge, and they were fossilized to be discovered half a billion years later.7
The existence of such a plethora of conflicting hypotheses, all of which are viable, some of which seem fantastic, may instill a sense of doubt in the reader as to whether any of them correctly explain the mystery of the Cambrian explosion. In my opinion, a propitious combination of true polar wander, predation, and an increase in the number of Hox genes provides the most satisfactory and comprehensive explanation. I tend to doubt the creationism hypothesis not because of a prodigious lack of faith, but rather because the creationists seem to embrace it too heartily and ignore the conspicuous incongruities between Genesis and current "scientific" beliefs. However, the true polar wander hypothesis is relatively new and has been challenged in Science, where it was first published. The unstoppable advance of science may eventually reach a firm conclusion as to what triggered the Cambrian explosion. For now we must continue to wonder about and wonder at this remarkable and baffling proliferation of life.