In honor of Jonathan Wells’ new book, which I haven’t yet seen, I’m recycling from his first book Icons of Evolution, in which he poses 10 unanswerable questions for evolutionists. Here’s my answer to question #2: Darwin’s tree of life.
Wells: Why don’t textbooks discuss the “Cambrian explosion,” in which all major animal groups appear together in the fossil record fully formed instead of branching from a common ancestor — thus contradicting the evolutionary tree of life?
There are a great many premises hidden in this question. Wells claims that 1) textbooks don’t discuss the Cambrian explosion, 2) all major animal groups appeared during the explosion, 3) the groups were “fully formed” when they appeared, and 4) that this all somehow falsifies the idea of common descent. As we will see, none of these premises is true, so the question is pointless. It’s would be surprising if textbooks didn’t discuss the Cambrian explosion, since it’s a major event in the history of life. And in fact they do. Of ten textbooks examined by Wells, he claims that eight don’t even mention the explosion.. In fact all but one does mention it, and four of those give it more than a hundred words. Still, a hundred words isn’t much to deal with such a major event; Wells’ implication is that coverage of the explosion is being deliberately suppressed. Then again, textbooks have limited space to deal with all of the complex field of biology; an alternative explanation is that these books just have limited coverage of the history of life and of evolution in general.
Let’s use the scientific method to test Wells’ hypothesis (cover-up) against an alternative hypothesis (limited space). The biggest event in the history of life since the explosion is undoubtedly the end-Permian (or Permo-Triassic) mass extinction, in which up to 90% of all animal species on earth died, but nobody suggests this extinction is a problem for evolution. Let’s compare how many textbooks cover the Cambrian explosion vs. how many cover the end-Permian extinction. The prediction of the cover-up theory is that more texts would mention the end-Permian than the Cambrian explosion; the prediction of the limited space theory is that the same number or fewer will mention the end-Permian. Result? Of 18 texts examined (including Wells’ ten), 16 mention the Cambrian explosion, while only 10 mention the end-Permian extinction. Of those that mention both events, all but one give more coverage – an average of three times more – to the Cambrian explosion than to the end-Permian extinction. (What does your textbook do?) So both Well’s claim that textbooks don’t mention the explosion and his implication that there is a coverup are refuted.
Of course, Wells doesn’t really mean that textbooks fail to discuss the Cambrian explosion. He means they don’t consider it to be evidence against evolution, and he thinks they should. His reasons, however, don’t hold up under examination. It’s important to note that our knowledge of the Cambrian explosion, and of still earlier life, is fragmentary. Most types of animals are rarely preserved as fossils, so we are limited for much of our information to a few deposits with exceptional preservation, like the famous Chengjiang and Burgess faunas. No deposits like these are known from the crucial period preceding the explosion. And even when we have exceptional fossils, the information we can get from them is limited. Fossils don’t come with labels saying “I’m the common ancestor of mollusks and brachiopods”, or even “I’m an arthropod”. Much information available from study of living animals is missing from even the best fossil. Finally, remember that when we say a phylum “appears” in the Cambrian explosion, we mean that no earlier fossils that we are sure belong to that phylum are known yet. Just a few years ago, there were no known Cambrian vertebrates, but vertebrates have recently been added to the list of groups that originated in the explosion. But maybe that’s just because we haven’t found the still earlier vertebrate yet, or, worse, have failed to recognize it because the fossil is poorly preserved.
Do all major animal groups appear in the Cambrian explosion? No. By “major group” Wells means a phylum (plural: phyla). The Cambrian explosion, which may be as short as 5 million years in duration, saw the first unambiguous appearance of most of the major groups of marine invertebrates with calcified shells, and thus excellent fossil records, as well as several groups of soft-bodied animals: eight phyla in all, out of a total of nearly thirty. A few phyla appear before the explosion; in fact, depending on the debated interpretation of some fragmentary fossils, the number appearing in the explosion may be from one more to three less than eight. Eight more phyla first appear in the fossil record after the explosion, from the middle Cambrian to the Cenozoic, and nine in fact have no fossil records at all. (The last is almost certainly due to lack of preservation rather than a recent origin, because there are a very few ancient fossils of some such groups. For example, the earliest clearly identifiable nematode roundworm is Cretaceous in age; there are claimed earlier examples as early as the Mississippian, but they are all controversial, and nematode fossils of any age are extremely rare.) And note that Wells is entirely ignoring, for no good reason, all plants, fungi, and protists.
Were these first appearances “fully formed”? It’s hard to say what Wells means by that. What would a “partially formed” animal look like? Cambrian animals certainly are not identical with modern ones. They possess some but not all the features that characterize the modern phyla to which they are related. For example, some Cambrian arthropods, such as Anomalocaris and Opabinia, seem to lack jointed legs except for a single pair near the mouth; the rest of the body had lobes, somewhat like the modern Onycophora, which are arthropod relatives but not arthropods. Cambrian relatives of Onycophora are common, but unlike their modern relatives they lived in the ocean and lacked important organs of the modern animals. There are still other Cambrian fossils that do resemble the possible ancestral forms of two or more phyla. Shelled and scaled animals called halkieriids, for example, have characteristics of both mollusks and brachiopods. There were vertebrates in the Cambrian, but they were more primitive than any living vertebrate, resembling most closely modern Amphioxus, a non-vertebrate chordate. There were no bony fish, no sharks, no amphibians, reptiles, or mammals. In fact, there was no life on land at all.).
Wells also plays fast and loose with definitions. The Cambrian explosion is not synonymous with the entire Cambrian period. Even though Wells gives a length for the explosion of 5-10 million years, he also considers groups to have originated in the explosion if they appeared at any time during the Cambrian, a period of over 50 million years. He also counts groups that first appeared in the fossil record “shortly before” the Cambrian, and this is sometimes as much as 30 million years before the beginning of the Cambrian (or more than 40 million years before the beginning of the explosion). Thus the Cambrian explosion by his flexible definition can be as short (when he wants to emphasize its abruptness) as 5 million years or as long (when he wants to emphasize its magnitude) as 80 million years, and these different definitions are never distinguished, leaving the impression that everything that happened during the longer, 80-million year period can be condensed into the shorter, 5-million year period. No wonder he talks about an explosion!
It’s not a matter of contention whether the Cambrian explosion happened. The question is what it was. Did new body plans appear suddenly (if 10 million years can be called sudden)? Or did they just become visible by becoming large and/or gaining hard skeletons? The fossil record is unclear, but there are clues. The Cambrian explosion (defined by the first appearance of trilobites) was preceded by the Tommotian stage of the Cambrian, whose fauna consists of a variety of small, enigmatic shells. Some may have been small mollusks. The Tommotian was preceded by the Late Precambrian, home of the Ediacaran fauna. Some of these may have been relatives of modern phyla – it’s hard to tell because they weren’t preserved in sufficient detail. It’s certain that at least some fairly advanced animals were around, because burrows and tracks made by unknown but necessarily advanced animals became common in the late Precambrian.
Science works by comparing alternative explanations for data, provisionally accepting the alternative that best fits the data. Wells, however, presents no theory to explain the data. Were animal phyla created out of nothing during the Cambrian explosion? If so, does that mean that all modern species within those phyla are each descended from a single phylum ancestor? If that’s not his theory, his question wouldn’t make sense even if his premises were true. The earliest known vertebrates appear at the end of the explosion. They were “fully formed”, meaning that we can tell they were vertebrates. But none of the modern vertebrate classes, let alone orders, families, genera, or species, are known from the Cambrian. Is Wells suggesting the modern vertebrates are all descended from a primitive vertebrate of the Cambrian? Or does he think they were all created, separately, later? But the latter theory would make the Cambrian explosion problematic from his perspective also. What does Wells think?
Budd, G. E., and S. Jensen. 2000. A critical reappraisal of the fossil record of the bilaterian phyla. Biol. Rev. 75:253-295.
Valentine, J. W., D. Jablonski, and D. H. Erwin. 1999. Fossils, molecules and embryos: New perspectives on the Cambrian explosion. Development 126:851-859.
Erwin, D. H., and J. W. Valentine. 2013. The Cambrian explosion: The construction of animal biodiversity. W. H. Freeman.