When Did the Cambrian Explosion Take Place

The Cambrian Explosion is marked by the sudden appearance of larger fossils, which can be easily seen at many places around the globe. There is nothing subtle about this evidence, and it was known to even the earliest geologists. In Washington State, for instance, telltale signs of the Cambrian event are readily visible near the small town of Addy, where a slow country road meandering through the Coast Range foothills cuts through low outcrops of quartzite, the lithified remains of what, more than 550 million years ago, was a white, sandy beach. If we could travel back to that time, the beach itself would probably not elicit our wonder, for it would appear completely unremarkable. Beaches are beaches regardless of time. But the appearance of the nearby shore and inland vistas would be remarkable; there would be no plants (or animals) to be seen. On Earth today there are a few areas where plant life is not immediately visible—the harshest deserts, the Arctic and Antarctic regions—but these are exceptions on a planet otherwise carpeted with life. Yet such was not the case 550 million years ago. And it was not only the land that was barren: If we were able to wade through the shallow, warm sea, we would not encounter any shimmering fish or scuttling crabs, no starfish and no sea urchins. There would be no clams burrowed in the sand and almost none of the other animals we see so commonly along the seashores of our world. There might be a few worms or jellyfish, but nothing with a readily visible skeleton. We would conclude that this world boasted little life, or at least little that we would recognize as animals or land plants.

The quartzite in this region is exposed as sedimentary layers stacked one on top of another, and if we were to count the individual layers (or beds), they would number in the thousands. The lowermost beds are devoid of fossils. Yet because these rocks are stratified, they are organized by time. If we wander a short distance farther along the roadside outcrops, moving upward through the succession of these stratified beds (and thus into younger intervals of time preserved in the rocks), we see a wondrous thing. Suddenly, as though by magic, an abundance of fossils appears. We find the remains of shelled creatures called brachiopods, which look like small clams, and a few other types of fossils, such as sponges and a tiny mollusk or two. But by far the most common fossils to be found are also the most spectacular, for the first fossil-bearing beds at Addy are packed with trilobites.

Along with ammonites and dinosaurs, trilobites are perhaps the most iconic of all fossils. At first glance they seem to resemble large bugs or crabs of some kind, but upon closer examination they look like nothing still alive; their closest living relatives are horseshoe crabs and pill bugs, but these are only distant cousins. Trilobite fossils range in size from the microscopic to nearly 3 feet in length. They have numerous spines, great helmet-like heads, and a variety of peculiar eyes, and their undersides housed an array of legs, gills, and other assorted arthropod tools. All in all, they are complicated fossils from complicated creatures—and, for that reason, are unlikely candidates for the honor of being the world's oldest animal fossils. If Darwin's theory of evolution is correct, the first fossils should be far simpler than a trilobite—as indeed they are. Yet at Addy, as at so many other localities around the world with sedimentary rocks of this age, the first obvious fossils are indeed trilobites perched atop thick sequences of strata apparently devoid of fossils. This observation suggests that animals of staggering complexity appeared on Earth without evolutionary precursors. It is as though an orchestra began playing without sounding a single tone to tune up.

This sudden appearance of larger animals in the fossil record is the most dramatic aspect of the Cambrian Explosion. It drove Charles Darwin to distraction and challenged the newly evolving field of geology, which had taken as its guiding principle the idea that important events in Earth history unfolded gradually, not abruptly. Yet even to the earliest geologists, the Cambrian Explosion seemed anything but gradual.

In the early nineteenth century, geology was a newly born scientific discipline established largely for economic motives, such as the search for fuel and metals. It was clear that the discovery of these valuable commodities depended on finding the relative age of rocks. By that time it was also recognized that fossils were the remains of ancient life and that they appeared in a relative, superpositional order and thus could provide a practical and reliable method for determining the relative ages of rock bodies. With the aid of fossils, geologists soon began to subdivide Earth's sedimentary strata into time units.

In 1823, English geologist Adam Sedgewick named one such unit the Cambrian. Sedgewick observed that a thick sequence of sedimentary rocks in

Wales contained a characteristic assemblage of fossils, including numerous trilobites. Overlying these strata were sedimentary rocks with a different suite of fossils that represented a time unit eventually named the Ordovician. Yet as he continued mapping and describing the mineral and fossil content in his field area, Sedgewick encountered something novel: strata without fossils. The Welsh sedimentary rocks studied by Sedgewick were composed of an enormous thickness of unfossiliferous strata, overlain by an equally thick pile of strata containing trilobites and brachiopods. Even more curious, the transition between the unfossiliferous and fossiliferous strata was abrupt, not gradual.

The strata beneath the fossiliferous, Cambrian rocks became known as the Precambrian. The Cambrian period was defined as the block of time during which the fossil-bearing strata recognized by Sedgewick in Wales were deposited. Thanks to modern dating techniques, we now know that this unit of time started about 540 million years ago and ended about 490 million years ago. Although Sedgewick's strata are found only in a part of Wales, we refer to all rocks on Earth that formed between 540 and 490 million years ago as belonging to the Cambrian system.

Sedgewick defined the base of the Cambrian as the stratal level where the first trilobite fossils could be found, and that view prevailed for over a century. Anywhere in the world where trilobite-bearing strata overlay unfossiliferous strata was considered to mark the base of the Cambrian. Recently, however, the way in which the base of the Cambrian is recognized has changed. It is now marked at a level that Sedgewick would have considered below the "base" of the Cambrian. Today geologists use the first occurrence of a particular trace fossil (the fossilized record of animal behavior, rather than the preserved hard parts of the animal itself) as the base of the Cambrian system.

Sedgewick's discovery of the seemingly instantaneous appearance of complex fossils convinced most scientists of his time that life was spontaneously created—put on Earth through the action of some deity; this observation is still cited by creationists as evidence against the theory of evolution. This observation was perhaps the most difficult for Charles Darwin to reconcile with his newly proposed theory of evolution, for the apparently sudden appearance of large, complex animals in the fossil record ran utterly contrary to his expectations. In On the Origin of Species, he speculated that the Precam-brian interval must have been of long duration and "swarmed with living creatures." Yet where were the fossils of these swarms? Surely, if Darwin was correct, a long period of evolutionary change with simpler precursors would have been necessary to produce the complex creatures collected by Sedgewick and others in the lowest strata now known as the Cambrian. Darwin was never able to refute this stringent criticism of his theory. Instead, he railed against the "imperfections" of the fossil record, believing that there must be a missing interval of strata just beneath the first trilobite-bearing beds everywhere on Earth. He was convinced that there must be Precambrian-aged fossils. As it turns out, he was right, but he went to his grave unvindi-cated.

Paleontologists have since proved Darwin correct, for the supposedly "barren" strata beneath those that bear what were thought to be the first fossil skeletons do indeed contain the ancestors Darwin sought and theorized about. They were long overlooked or missed, however, because of their rarity or very small size. Most organisms from the youngest "Precambrian" time both were tiny and lacked skeletons, so they rarely left obvious traces in the fossil record. They are very hard to detect unless special processing techniques are used to extract them from their entombing matrix; Darwin and his contemporaries had not yet dreamed of such methods. The supposedly "sudden" appearance of skeletonized life, more than 540 million years ago, is simply the first appearance of creatures with large skeletons, which produce fossils that are easily noticed. Because of this, the base of the Cambrian has now been "lowered" into the supposedly barren strata beneath the first trilobite-bearing beds. Just as Darwin supposed, trilobites do appear only after a longer period of evolution of simpler forms that rarely fossilize.

The twentieth century has witnessed a revolution in the science of geology. No longer are fossils the sole means of dating rock. Sophisticated laboratory analyses of volcanic and some sedimentary rock give accurate ages in years, and the entire rock record (including the Cambrian) has been far more accurately dated. In the 1960s the base of the Cambrian was deter mined to be 570 million years old, and this date appears on age compilations even into the late 1980s. Recently, however, there have been significant improvements in radiometric dating techniques. The Precambrian/Cambrian boundary is now dated at 543 million years old. The "Middle" Cambrian is dated at about 510 million years ago, whereas the oldest trilobites are no more than 522 million years old, which suggests that the bulk of Cambrian time was "pre-trilobite." Interestingly enough, although the "base" of the Cambrian has gotten younger; its "top" has not changed in age. The Cambrian Explosion remains a relatively sudden and signal outburst of animals— an unleashing of abundant and voracious creatures upon the earlier bacterial world, which continues, unabated, more than half a billion years later. With the exception of life's first formation, it remains the most profound biological event to have occurred on this planet. And we propose that the Cambrian Explosion has an even greater significance than Charles Darwin (or modern scholars of the fossil and evolution record) realized: We believe that it yields crucial evidence for estimating the frequency of animal life in the Universe.

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