Did the Cambrian Really Give Darwin Nightmares?

Considering the fossils of the Cambrian, the oldest fossil-bearing rocks known during his time, Charles Darwin wrote the following in the 6th edition of On the Origin of Species;

... it cannot be doubted that all the Cambrian and Silurian trilobites are descended from some on crustacean, which must have lived long before the Cambrian age, and which probably differed greatly from any known animal. ...

... if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures.

We now know that Darwin was, in a general sense, correct. The longevity of the Precambrian far outstrips the "whole interval of the Cambrian age to the present day", and for at least some of that time the sea did swarm with life. By about 565 million years ago (during the Ediacaran, the period just before the Cambrian) animals were already burrowing beneath the vast bacterial mats that covered the seafloor, and by the beginning of the Cambrian (~542 million years ago) there was a greater diversity of creatures disturbing the mud on the ocean bottom. Other Edicaran fossils have also shown that animals were present and diversifying long before the "Cambrian Explosion" (the famous Burgess Shale is about 505 million years old), even if a number of those forms met with extinction by the end of the Ediacaran. (It should be noted, though, that the identifications of many of these fossils are still controversial. Dickinsonia and Spriggina are good examples of this.)

Despite all we have learned, however, creationists still insist that the Cambrian Explosion was "a moment of geological time [when] complex animals first appeared on earth fully formed, without evidence of any evolutionary ancestors." While it is true that we cannot construct direct lines of descent from Middle Cambrian organisms like Anomalocaris and Hurdia to creatures that lived during the Ediacaran, the fossil record is clear that complex, multicellular life preceded the Cambrian Explosion for tens of millions of years. Darwin did not know of such evidence, nor was he considering the "Cambrian Explosion" as it is discussed today, but his hypothesis has met the test. Trilobites and other famous Cambrian fossils were preceded by a number of creatures that "swarmed" in the prehistoric seas.

Still, Darwin's hypothesis was certainly controversial at the time he proposed it. Geologists like Roderick Murchinson (who engaged Darwin's former mentor Adam Sedgewick in a heated debate about the dividing line between the Cambrian and Silurian, see Secord's Controversy in Victorian Geology) had believed that what was then called the lowest "Silurian system" held the oldest fossil-bearing rocks, only to have even older fossils be found elsewhere in the world. Indeed, many geologists in Darwin's day thought that the fossil strata found in England and continental Europe were representative of what was to be found in the rest of the world, the local fossils thus providing them an almost complete picture of the entire history of life. Taking his cue from Charles Lyell, Darwin cautioned otherwise. The fossil record was not as perfect as some geologists supposed, and who knew what would be found in the prehistoric rock of other nations?

Despite the discovery of early Cambrian fossils, however, Darwin had to admit that large chunks of Precambrian time were still unaccounted for. Perhaps the advance and retreat of the oceans upon the continents over time could explain it, but ultimately the absence of fossil-rich Precambrian deposits would have to be reconciled with his evolutionary ideas. Darwin knew his argument for the imperfection of the fossil record would not appease those who preferred the "sudden appearance" if life to evolution, and Darwin's worries were realized in the reply of Oxford geologist John Phillips in 1860.

Phillips' Life on Earth was one of the first geology books to respond to Darwin's theory. At the time Darwin's hypothesis that the Precambrian seas were full of life had almost no evidence to support it (keep in mind that I used the 6th edition, printed in 1872, for the discussion above). Life seemed to dwindle as one looked further back into the Cambrian until all fossils disappeared. According to Phillips the older Cambrian rocks were of the kind that could have held fossils, yet they did not, therefore meaning that the sea was devoid of life during the earliest parts of the Cambrian.

Phillips also thought that it was impossible to derive later Silurian/Cambrian forms from earlier fossils. Choosing mollusks as an example, Phillips asserted that there was no conceivable way to connect any of the creatures found higher up in the series (like cephalopods) to older ones (like brachiopods). While Phillips recognized that each genus had its own particular range in time, he regarded all as being "equally aboriginal." In other words, even the later forms had sprung from seemingly nowhere. The story seemed the same everywhere geologists looked. How could the later creatures be "descendants" of earlier creatures if the ancestors were not preserved anywhere?

Religion did have at least a small part to play in the geologist's criticisms. Phillips saw the grand unfolding of life as a testament to a Creator. In the conclusion of his work he wrote;

We see the stream of life flowing onward in a determined course, in harmony with the recognized forces of nature, and yielding a great amount of enjoyment, and a wonderful diversity of beautiful and instructive phenomena, in which MIND speaks to mind.

This was far closer to William Paley's conception of nature than to Darwin's, and in fact many naturalists had difficult accepting natural selection for such aesthetic reasons. Darwin's vision of nature seemed chaotic, violent, and was devoid of any spiritual meaning. Surely God would not be so cruel.

I can't help but think of Phillips while reading the assertions modern creationists make about the Cambrian. The arguments made by Disco Institute fellows today are very similar to those the Oxford geologist made nearly a century and a half ago. In all honesty, though, I much prefer the heartfelt and detailed criticisms of Phillips to the dishonest tactics of the Disco Institute fellows and their ilk.

There is still much to learn about Cambrian and Precambrian life, there is no doubt about that, but creationists are often willfully ignorant of what we have learned since Darwin's time. Furthermore, if the Cambrian faced Darwin with a dilemma it was only one specific aspect of the naturalist's concerns over how his theory meshed with the fossil record. Today we know the fossil record to be consistent with the evolutionary theory Darwin described. I am certain that, were he able to speak to us today, Darwin would be thrilled by what we have discovered and the questions that keep driving paleontologists out into the field.

Post script: Blake Stacey has started a poll to determine what we should call the creationist strategy of tricking reputable scientists into appearing in creationist films. I have to say that I like "Switek's Law", but the "Expelled Effect" is probably more fitting.