36 The Truth About Transitional Fossils DONALD R. PROTHERO Department of Geology, Occidental College Los Angeles, CA 90041 [email protected] 1. Of Fossils and Creationists In the United States, the contentious debate about “intelligent design” (ID) has focused on many issues: complexity and its possible explanations; the explanation for biological structures; the naturalistic approach to science; and many other philosophical and biological topics. The books of the “intelligent design” creationists are filled with such examples and discussions, but they pointedly avoid discussing the fossil record or its implications. Behe (1996, p. 27) mentions paleontology only in a few paragraphs in his book-length treatment, and most of the other ID books are similarly silent about the fossil record. Only the ID creationist textbook Of Pandas and People (Davis and Kenyon, 2004) discusses fossils at any length, and only in a single chapter of a 170-page book. The ID creationists have been quoted on numerous occasions as conceding that microevolution occurs, and that the earth may be millions of years old. This distinguishes them from the more extreme fundamentalist “Young-Earth Creationists” (YEC), who believe the earth is only 6000 years old and will not admit that microevolution occurs. The reasons for this lack of interest in fossils are apparent when one scans the contributors and critical reviewers of ID textbooks such as Of Pandas and People (Davis and Kenyon, 2004, p. iii). Although the ID creationists include 833 834 Donald R. Prothero scientists with backgrounds in biology or chemistry, almost none earned an advanced degree in paleontology or geology from a recognized accredited non- creationist institution (with the possible exception of Kurt Wise, who was a student of Stephen J. Gould at Harvard). To my knowledge, not a single ID creationist has ever published a paper on fossils in the peer-reviewed scientific literature. Reading through the little bit that they write about the fossil record, it is very clear that they have no firsthand experience with collecting or interpreting fossils, because they rehash old myths and misconceptions from the YEC literature. Like the YEC, their “research” on fossils consists of reading popular books about paleontology and pulling quotes out of context. ID creationists may impress the uninformed layperson with their Ph.D.’s in biochemistry, but that background has absolutely no relevance to understanding paleontology and fossils. Without the appropriate background or training, they are no more qualified to make statements about the fossil record than they are to discuss music theory or auto mechanics. Thus, their statements about fossils must always be read with the caveat in mind that they don’t actually work on these fossils, and have probably never even looked at the actual specimens (nor do they have the training to tell one bone from another if they did). In a volume such as this, it is useful to examine these myths and misconceptions about the fossil record, and give a short update about the truth about the fossils. For a longer account, the reader is referred to my book Evolution: What the Fossils Say and Why it Matters (2007). 2. Cambrian “Explosion”—or “Slow Fuse”? This is the myth about the fossil record that the ID creationists mention most frequently, including Behe (1996, p. 27), Davis and Kenyon (2004, p. 94-96), and several others. It even popped up when I debated an ID creationist on a radio talk show, but he quickly got in trouble when his ignorance of the actual Cambrian fossil record was exposed. Like all their other arguments, this one was recycled from many of the old YEC books, which have long tried to use it to support their cause. Creationists (both ID and YEC) point to the early appearance of most of the living phyla of animals in the Cambrian as some kind of “creation event.” They hijack the commonly used phrase “Cambrian explosion” as if paleontologists were The Truth About Transitional Fossils 835 suggesting that all life arose instantaneously in the Cambrian, and no fossils were known from rocks older than the Cambrian. Figure 1: Diagram showing the time ranges of the major fossil groups in the late Precambrian (Vendian) and Cambrian. Generic diversity of these fossils shown on the histogram on the right. (After Prothero and Dott, 2004). Such an idea about the fossil record might have been credible in Darwin’s time (and creationists often quote his puzzlement about the lack of Precambrian fossils known in 1859), but it was rendered completely invalid by the discovery of numerous Precambrian fossils since the 1940s. In the past fifty years, the Precambrian fossil record has become enormous, with thousands of localities and tens of thousands of fossils—none of which the creationist literature ever acknowledges. The key difference is that most of these Precambrian fossils were single-celled microscopic organisms for the first 85% of the 3.5 billion years of life’s history, so they can only be found with a microscope and the appropriate techniques in the right kinds of rocks. Nonetheless, there are many extraordinarily well-preserved microscopic fossils (see Schopf, 1999; Knoll, 2004; Prothero and Dott, 2004, for further details). After three billion years of nothing but single-celled fossils, the next stage is the famous soft-bodied Ediacaran fauna, which is found worldwide in rocks as old as 600 million years 836 Donald R. Prothero ago, and vanishes in the early Cambrian (Fig. 1). Following this is the earliest Cambrian, where large shelly invertebrates are rare (and no trilobites occur), but the commonest fossils are tiny forms nicknamed the “small shellies.” These show that the factors that permitted the secretion of mineralized skeletons had finally occurred, but (as would be predicted by evolution) the first skeletons and shells were tiny and simple, not complex invertebrates such as trilobites. It is not until the third stage of the Cambrian (the Atdabanian) that larger more complex invertebrates such as trilobites appear, and the rest of the animal phyla appear even later in the Cambrian. Contrary to the outdated creationist literature, there are several major animal phyla with skeletons (such as the Bryozoa) that do not appear until much later (in the Ordovician). Thus, the fossil record now shows a gradual series of steps from 3 billion years of single-celled organisms, to the multi-cellular but soft-bodied Ediacarans, to the tiny first shelled invertebrates of the early Cambrian, and only by the late early Cambrian do we see the appearance of more complex forms such as trilobites (and much later still the rest of the animal phyla). This is hardly an “explosion” but much more like a “slow fuse,” with multiple steps spread out over thousands of feet of strata (well documented in the rocks of Russia, Namibia, and Canada). This sequence has now been subjected to numerous of the highest-quality radiometric dates by the top labs in the world, so it is now extremely well calibrated. From the first Ediacarans to the first trilobites (600-520 million years ago) is a span of 80 million years! Even the sequence from the first “small shellies” to the first trilobites (545-520) is still 25 million years in duration. Hardly an “instantaneous explosion”! The misleading term “Cambrian explosion” was coined by geologists decades ago to describe the fact that the majority of animal phyla appear in the Cambrian, and not later or earlier. But it was never intended to suggest that the event was instantaneous or ever rapid by human standards. Geologists and paleontologists are so accustomed to dealing with hundreds of millions to billions of years that anything that is only tens of millions of years in duration (compared to the 540 million years since the event) seems relatively rapid. But “geologically rapid” is still thousands to millions of years in duration, not a rapid “explosion” in the sense of human time scales of hours, days, years, and decades. Even if one rejects all of the evidence of radiometric dating (which the YEC do, but the ID creationists apparently don’t), the clear sequences of more and more complex fossils found in many places around the world (single cells to Ediacarans to “small shellies” to trilobites) completely rules out the use of the The Truth About Transitional Fossils 837 “Cambrian explosion” as some sort of support for an instantaneous creation event as described in the Bible. Any time a creationist mentions the “Cambrian explosion” as some sort of geological conundrum possibly supporting their viewpoint, it is clear that they have only read old outdated YEC accounts of the Cambrian, and have not bothered to learn anything about the past 50 years of discoveries which show it was actually a “slow fuse” that fits perfectly with the predictions of evolution. 3. Transitional Forms, “Missing Links”, and the Quality of the Fossil Record Much of the public (including most creationists) has a mistaken notion of what evolution is. Some will ask, “If humans evolved from apes, why are apes still around?” This misconception goes back to pre-Darwinian seventeenth- and eighteenth-century notions of life as a “great chain of being” or a “ladder of life” (scala naturae) up from lowly invertebrates to fish to amphibians to reptiles to mammals to humans to cherubim and seraphim and angels and archangels and ultimately to God at the top. But as Darwin and many other scientists have shown, life is not a “ladder” or “chain” but a branching bush, with many ancestral lineages living alongside their descendants. When humans evolved from apes, they branched out from a lineage that is still around. Apes did not have to become extinct when some of them evolved into the ancestors of humans.