Unlike the authors of Explore Evolution, many elementary school students are well aware that there are mammals which lay eggs, and that there are snakes which have placentas. Scattered among errors so elementary that grade-school students could correct them, this chapter is laced with statements which requiring a level of knowledge far beyond even typical college biology students. Questions are posed to students which cannot be answered without much more background than the book provides, or than the average high school teacher is trained to provide. The book does not offer such resources, nor suggest ways that students could explore those questions through independent inquiry.
The flaws in the chapter go much deeper than such errors and inconsistencies. The practice of science is misrepresented, and common misconceptions about evolution are actively promoted. Far from encouraging scientific inquiry, this chapter blocks and discourages genuine inquiry. Given that the book parrots creationist misrepresentations of biology, then shuts off inquiry into scientific answers to those questions, it is easy to see that the book's aim is to instill exactly those creationist errors.
p. 130: "any transition from a three-chambered heart to a four-chambered heart requires a series of coordinated … changes."
This is a recapitulation of the fallacious "irreducible complexity" argument of intelligent design creationism. Evolutionary and developmental biologists have clear understandings of how morphological structures can and do evolve in parallel.
p. 137: "can there even be a transition between a single-opening and a dual-opening system [of lungs]?"
Scientists have good evidence explaining the evolution of bird lungs – with two openings – from reptilian lungs – with only one opening. None of that evidence is offered.
p. 137: "the ancestors of birds almost certainly had a diaphragm breathing system"
This claim is advanced on the basis of a controversial and widely-criticized paper from 1997, and even that paper does not support this claim. No discussion is offered of the subsequent research offering further evidence against this idea.
p. 129: "the superficial resemblance between skeletons [of early mammals and early reptiles]"
The fossil record of early mammals is one of the most continuous records available to paleontologists, and the chain of similarities is hardly superficial.
p. 130: "Transforming a reptile into a mammal … requires the development of completely new organs like the placenta and mammary glands"
Placentas have evolved dozens of different times within modern reptiles, and mammary glands are highly modified sweat glands, so it is wrong to claim either of these are "completely new organs." More significantly, mammals are not "transformed" from reptiles: they share a common ancestor, but neither descended from the other.
p. 129: "Most reptiles lay eggs, while mammals carry fertilized eggs internally in a placenta and bear live young."
There are mammals which lay eggs, and many other mammals which have live birth but lack a placenta. No discussion is offered of the placentas found in several groups of reptiles.
p. 138: "all we can say for sure is that the internal organs of living reptiles, living birds, and living mammals are very different from one another"
We can do far more than just say that different things are different. The patterns of differences and similarities between living species – and between preserved remains of extinct species – gives us insight into the evolutionary processes driving those differences. Explore Evolution fails to provide students with the background to allow them to explore this fascinating field: evolutionary biology.
Hearts: In discussing the evolution of the four-chambered mammalian heart from the three-chambered reptilian heart, Explore Evolution omits any reference to existing scientific models of that evolutionary path, and makes gross misstatements about the anatomy of hearts in living animals.
Lungs: In discussing the evolution of bird-like lungs from reptilian ancestors, the authors make inaccurate claims about the lungs of bird ancestors, and misrepresent the biology of modern birds and crocodilians. There is no discussion of existing research on the evolution of bird lungs, research answers the very questions posed by Explore Evolution.
Transitional Forms: Explore Evolution states that all mammals give birth to live young and possess a placenta, while all reptiles lay eggs and lack a placenta. In fact, many groups of reptiles have evolved a placenta, and there are mammals which lack a placenta and lay eggs. Each of those counterexamples represents a living transitional form, and it is possible to study the evolution of placentas, lactation and egg-laying using both modern species and fossils. Explore Evolution obfuscates the truth and confuses students with creationist falsehoods.
Worms and other structurally simple animals have simple, single-chambered hearts. Fish have a two-chambered heart. Reptiles and amphibians generally have three-chambered hearts. Mammals and birds have four-chambered hearts, with the four chambers arranged differently in each group. Explore Evolution presents this arrangement as a mystery, unbridgeable by evolutionary means.
Contrary to the account in this chapter, the evolution of the four-chambered heart is no mystery. Many researchers have addressed elements of the question, but rather than addressing these well-tested models, Explore Evolution makes passing reference to their existence and reliance on specific processes in developmental biology, but never gives enough detail for students to understand those models. Having given an inadequate account of mainstream models of heart evolution, the book then criticizes those models, again without offering enough detail for students to evaluate or even fully understand the critique. Rather than encouraging inquiry, this approach forces students to simply learn by rote that certain things cannot evolve, leaving students with no basis for understanding future advances in the evolution of hearts and other anatomical structures.
There are good examples of the functional intermediate stages in the evolution of the mammalian heart. Explore Evolution repeats inaccurate and long-debunked creationist claims about the impossibility of evolving a four-chambered mammalian heart from a three-chambered reptilian heart. Though extensive research exists into exactly how this transition took place, Explore Evolution ignores the research, and abandons intellectual inquiry in favor of creationist talking points.
Explore Evolution claims:
… any transition from a three-chambered heart to a four-chambered heart requires a series of coordinated physiological and anatomical changes, including: 1) lengthening and attaching the existing septum to create a new, separate ventricle chamber, 2) replacing the forked abdominal aorta and two aortic arches with a single aorta, 3) rerouting the pulmonary arteries and veins, and 4) making various secondary structural changes to the walls and valves between chambers.
The need to change so many anatomical features and still maintain function at every step along the way raises some difficult questions about the viability of a series of transitional stages between a three- and four-chambered heart. Would the new chamber arise before the new veins, arteries, and septa? Would it arise before or after the new, single aorta emerged and the new plumbing was rerouted to support it? If the new chamber arose before these other features, how would it work? If it arose after the other features, what are the odd that all these new structures — veins, arteries, septa, and aorta — would fit together properly and function in concert with the new chamber?Explore Evolution, pp. 130-131
Rather than addressing the research that has been done on precisely these questions, Explore Evolution changes subjects, discussing hypotheses from evolutionary developmental biology, and citing a series of papers which do not actually address heart evolution. Yet again, this approach is not inquiry-based, and presents students with an inaccurate view about the way to approach uncertainty in science. The question of whether mutations affecting the early development of the heart might allow multiple changes to happen at once is a question which has to be addressed with evidence in that system, not with arguments about the evolution of novel body-plans, or hand-waving discussions of evolution in the Cambrian. There has been substantial work done on the development of the heart, and it is that literature which the authors of Explore Evolution should have reviewed (for example, Antoon F. M. Moorman and Vincent M. Christoffels. 2003. "Cardiac Chamber Formation: Development, Genes, and Evolution", Physiological Reviews 83:1223-1267).
The shift from three chambers to four chambers is not as large a leap as it might seem. The ventricles of the mammalian heart are separated by a thin muscular septum (wall) which grows out of the muscular sides of the heart. There is a similar septum in the reptilian and amphibian hearts, and as the figure above or even the figure in Explore Evolution illustrates, the transition from three chambers to four would not be at all problematic for the organism. The septum would grow longer, until it entirely separates the two sides. This change prevents oxygenated blood returning from the lungs from mixing with oxygen-depleted blood from the body. In species with slower metabolisms, or which (like amphibians) can exchange gases through their skin, the separation of oxygen-rich and oxygen-depleted blood is less critical, while in species with higher metabolic requirements, there is more intense selection acting on variations which improved oxygen flow to the body. Species with lower metabolisms can go for long periods without breathing, and in those settings, can restrict flow to the lungs, and use the full power of the heart to circulate the blood through the body. For those species, a three chambered heart allows needed flexibility. Mammals with high metabolic requirements cannot go as long without breathing, so there is no need to limit blood flow through the lungs in the same way.
Explore Evolution offers no explanation of why the aortic arch could not have evolved independently of those changes within the heart. Luckily, scientists are not as incurious as the authors of Explore Evolution, and have actually conducted research to better understand the evolution of the aortic arch and other blood vessels. In Functional Anatomy of the Vertebrates, Liem et al. explain:
One of the hallmarks in comparative anatomy is the discovery and broadly based explanation of the evolutionary pattern of the heart and great vessels of vertebrates. It still has profound and pervasive implications in comparative biology.Karel F. Liem, et al. (2001) Functional Anatomy of the Vertebrates, Harcourt College Publishers:Fort Worth, p. 620
An understanding of the details of this evolutionary process require more anatomical background than is appropriate for a high school class, let alone this review, which leaves one wondering why Explore Evolution raises the matter. The contrast between the confusion expressed by Explore Evolution and the detailed explanation offered by professionals in the field is a strong sign that Explore Evolution is not drawing its evidence from the biological literature, and is not interested in encouraging students to undertake genuine inquiry.
In fact, the basis for their claims about the aortic arches is most likely to be Michael Denton's Evolution: A Theory In Crisis (1985, Adler & Adler Publishers: Chevy Chase, MD), which spends several pages arguing the implausibility of evolutionary explanations of heart and aortic arch anatomy. Denton was instrumental in inspiring the intelligent design creationist movement, and Evolution: A Theory in Crisis was cited in Of Pandas and People, the intelligent design textbook ruled too religious for science classrooms as Explore Evolution was being written.
While scientists continue to research the evolution of the heart, producing and testing hypotheses, creationists continued to emphasize whatever uncertainty they could find. An intelligent design creationist group claims the evolution of the heart as an unsolvable problem Until 2005, the group hosting that claim "require[d] that club leaders be Christians," and their discussion of the issue reveals that their claims about the heart are not scientifically grounded. Even while acknowledging that "the 4 chamber mammalian heart probably isn't irreducibly complex" (as Explore Evolution intimates), they express certainty that "it might still be the result of intelligent design and not evolution, and irreducible complexity doesn't have to exist in all instances for it to exist in some." This is not critical thinking, and is not inquiry-based learning.
This argument, like its antecedents in the openly creationist literature, is a religious attack on evolution, not a scientifically grounded investigation. Explore Evolution argues that we know nothing unless we know everything. That attitude is both scientifically and pedagogically inappropriate. The evolutionary process leading to the forms of modern mammalian, reptilian, crocodilian and amphibian hearts remains a subject of research. Students interested in that ongoing research need a solid understanding of evolution, anatomy, physiology, and developmental biology, but more importantly, they need to understand how scientists know what they know, how scientific inquiry works. Explore Evolution will teach students none of this.
The sidebar ["A Heart-to-Heart Connection?"] distinguishing crocodilian hearts from mammalian hearts is a Straw Man fallacy. Biologists do not claim that the crocodilian heart is ancestral to the mammalian heart, nor that the crocodilian heart's four chambers are the same as the mammalian heart's four chambers. For instance, Liem and Walker (2001) explain "Superficially, the mammalian heart resembles those of birds and crocodiles, but the interventricular septum evolved independently and develops embryonically in a slightly different way, so it is not homologous to the interventricular septum of these vertebrates" (Liem, Karel F. and Walker, Warren F. 2001. Functional anatomy of the vertebrates: an evolutionary perspective. Fort Worth, Harcourt College Publishers).
As illustrated in the first figure above (compare hearts C and D of "A detailed view of the heart and aortic arches"), the crocodilian heart is a slight modification of the ancestral reptilian state. So is the mammalian heart, and the bird heart. The bird heart is similar to the crocodilian heart, as Liem and Walker explain: "the ancestors of birds had a heart and pattern of aortic arches similar to that of crocodiles. Because birds evolved endothermy and lungs that are continuously ventilated, shunts bypassing the lungs--as found in crocodiles--would have had no adaptive value. Equal volumes of blood are sent to the lungs and body at all times, which appears to have been simply the result of the loss of the left systemic arch." This hierarchical pattern of similarities is exactly what would be expected from an evolutionary process.
No one portrays the crocodilian heart as an ancestral form of the mammalian heart (or at least Explore Evolution offers no example of anyone making that claim). Either the authors of Explore Evolution are not familiar with the scientific literature on this topic or they are ignoring these studies in their campaign to raise doubts about evolutionary biology.
Bird lungs operate quite differently from mammalian lungs, a fact which Explore Evolution treats as mysterious. In fact, ongoing research by paleontologists, anatomists, and other biologists has rendered this diversity of forms readily explicable. As in so much else of the book, interesting biology is ignored in preference for obscurantism and a lesson that students should learn that certain things cannot be known scientifically.
Birds evolved from a group of dinosaurs, and understanding the anatomy of dinosaurs is an important step in understanding bird lungs. Unfortunately, Explore Evolution offers no background in modern paleontological knowledge about dinosaur lungs or the relationship of dinosaurs to birds. A hands-on exercise for students is suggested, but it offers no meaningful insight into the models of lung evolution proposed by any scientists. The views of scientists who are in the fringes of paleontology are presented uncritically, with no comparable (let alone proportionate) coverage of the views of the overwhelming majority of scientists. This chapter misinforms students about how science works, how lungs work, and, as always, how evolution works.
There are detailed, testable models of the evolution of dual-opening parabronchi in bird lungs from single-opening alveoli found in the reptilian ancestors of birds. Explore Evolution asks a number of questions about this transition, but then fails to offer students any means to answer any of them, or to discuss how a student or scientist might go about finding answers to these questions.
Explore Evolution asks its readers:
What would the intermediate forms between the single openings (in-and-out) reptilian lung and a dual opening (flow through) avian lung look like? How would it happen in small yet advantageous steps? Can there even be a transition between a single-opening and a dual-opening system? How would the balloon-like alveoli transform into the tube-like parabronchi? How would the lung maintain function? Would the lung transformation happen before or after the development of air sacs? Would it be before or after the four stage breathing cycle?EE, p. 137
Having asked its audience of high school biology students these detailed questions about evolutionary biology, EE changes topics, without even suggesting the ways that someone might investigate those questions. These students are unlikely to know anything about the anatomy of bird or reptilian lungs, and little if anything about the anatomy of mammalian lungs. They have no experience forming or testing hypotheses about the evolution of anatomical structures, and Explore Evolution offers no references which might fill in that background. The average teacher is likely to be as stymied by these questions as the students. The authors of Explore Evolution seem to be little better informed, and are apparently comfortable leaving students and teachers with no guidance about how to answer the questions posed by the book.
Fortunately, scientists are not so incurious. The figure above demonstrates one set of hypotheses about the evolution of lungs and their anatomy. By considering not just two sets of lungs, but the full spectrum of variation in lung morphology, scientists can reconstruct the likely evolutionary pathways, and evaluate whether those intermediates might be functional.
Scientists like Steven Perry have proposed detailed models of the evolution of the internal lung morphology, models which answer many of the questions Explore Evolution asks. An inquiry-based textbook might describe this model and invite students to develop ways to test it against new data. Instead, Explore Evolution ignores actual research in order to preserve their creationist argument.
The claim that air sacs in evolving birds would put a hole in the a diaphragm and lead to a nonfunctional, fatal intermediate, is based on selective quoting of a single sentence from a scientific publication from 1997; the conclusions in that publication are more complicated than one might guess from reading that single out-of-context sentence. Furthermore, Explore Evolution ignores more recent findings that have overturned the idea that the dinosaur ancestors of birds even had diaphragms to damage.
Finally, what happens to the diaphragm? The reptiles thought to be the ancestors of birds almost certainly had a diaphragm breathing system (footnote 8). According to many evolutionary biologists, changing from a diaphragm lung system to a flow-through lung would require changing and increasing the musculature of the reptile's chest. At the same time, the diaphragm would need to gradually go away. This poses a fundamental problem. Evolutionary biologist John Ruben points out that the earliest stages of this transformation would have required a hole or hernia in the reptile's diaphragm. This would have immediately compromised the entire system and led to certain death for any animal unfortunate enough to possess this non-functioning intermediate structure.Explore Evolution, p. 137.
Footnote 8 refers to Ruben et al. (1997), Science 278:1268-1269 (actually 1267-1270), and quotes from the article.
8. "Therapod dinosaurs like modern crocodiles, probably possessed a bellows-like septate lung, and that lung was probably ventilated … by a hepatic-piston diaphragm."John A. Ruben, Terry D. Jones, Nicholas R. Geist, W. Jaap Hillenius, (1997) "Lung structure and ventilation in theropod dinosaurs and early birds," Science 278:1268-1269. Explore Evolution, p. 140, note 8, quoting Ruben, et al. 1997.
Note that "Therapod" is a misspelling of "theropod." Also, the correct page numbers for the article are pages 1267-1270. The authors of Explore Evolution managed to cram about five typos into their short quote of Ruben, et al., so for the sake of correctness, as well as including the ellipsed text and the rest of the sentence, here is the exact quote from the original Science article, which might be enough to trigger a question in the mind of a student in a truly inquiry-based activity.
These observations, combined with the occurrence among theropods of a distinct, relatively vertical, crocodile-like, highly elongate pubis (Figs. 4 and 5), as well as well-developed gastralia, provide evidence that theropod dinosaurs, like modern crocodiles, probably possessed a bellows-like septate lung and that the lung was probably ventilated, at least in part, by a hepatic-piston diaphragm that was powered by diaphragmatic muscles that extended between the pubic bones and liver.
The authors of Explore Evolution have miscast the conclusions. Ruben, et al. are basically arguing that the theropod dinosaurs are not the earliest ancestors of birds. This position was highly unpopular in the scientific community in 1997, and is extremely unpopular now — the number of holdouts against the idea that birds are descended from theropod dinosaurs can be counted on one hand. In the 1997 Science paper, Ruben, et al. argued that there is a logical problem with an intermediate form between purported ancestors (theropod dinosaurs, which allegedly possessed hepatic-piston diaphragms) and modern birds. But they also argued that theropod lung physiology was not consistent with endothermy [warm-bloodedness], a character that might be important in creatures (like birds and their ancestors) which are capable of flight. Here is the meat of Ruben, et al.'s conclusion section:
Recently, conventional wisdom has held that birds are direct descendants of theropod dinosaurs. However, the apparently steadfast maintenance of hepatic-piston diaphragmatic lung ventilation in theropods throughout the Mesozoic poses fundamental problems for such a relationship. The earliest stages in the derivation of the avian abdominal air sac system from a diaphragm-ventilating ancestor would have necessitated selection for a diaphragmatic hernia in taxa transitional between theropods and birds. Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage.
In other words, Ruben, et al. are not saying that this poses an insurmountable obstacle for any theory that postulates evolution of the bird lung. They are merely saying that this logic, as well as the arguments against endothermy in putative ancestors, argues against the specific theropod-bird ancestral connection. Birds (with their unique lungs and high oxygen requirements) must, by this logic, be descended from other ancestors. And even that conclusion generated almost immediate controversy. In November of 1998 three critiques of this paper, along with a rebuttal by Ruben, et al., appeared in Science (281(5373):45-48). Interestingly, these focused primarily on the conclusions about endothermy, rather than on the idiosyncratic diaphragm anatomy issue highlighted by the authors of Explore Evolution. Evidence has continued to accumulate against Ruben, et al.'s claim that theropod dinosaurs had diaphragms (see below).
Finally, since 1997 many spectacular fossils (both of birds and dinosaurs thought to be ancestral to birds) have been discovered, but none of these more recent findings are discussed in Explore Evolution, even though many of them (e.g. O'Connor & Claessens, 2005. Nature 436 (7048): 253-256) provide evidence that further argues against the conclusions of the 1997 paper of Ruben, et al.
Paleontologist Matt Wedel explains:
Non-avian dinosaurs did not necessarily have the same pulmonary anatomy as crocodilians or extant birds. As hypotheses of pulmonary anatomy in dinosaurs, "croc lungs" versus "bird lungs" is a false dichotomy. It is more informative to identify the derived features that non-avian dinosaurs share with their extant relatives, and to determine the hierarchical distribution of these characters in archosaurian phylogeny.Wedel, Matt (2007) Postcranial pneumaticity in dinosaurs and the origin of the avian lung, Ph.D. dissertation, University of California, Berkeley, p. 112.
That is exactly the approach that evolutionary biologists and textbooks about evolutionary biology take in addressing the evolution of organisms and particular parts of organisms. An inquiry-based textbook could include exercises allowing students to undertake the same process of investigation. Explore Evolution does not use this comparative approach, and discourages students from further investigation in areas of ongoing biological research, or even areas where the research has already been conducted.
By comparing fossils of dinosaurs to modern birds, it is possible for paleontologists to produce and test hypotheses about the evolution of the dinosaur lung. Wedel describes his approach:
Instead of focusing on particular [traits] that are either not present in all birds (large sternum, uncinate processes of the ribs) or not clearly necessary for air sac ventilation (ossified rather than cartilaginous sternal ribs), it may be more productive to identify the skeletal movements that take place during avian respiration and the effects of these movements on the shape and volume of the thoracic cage, and then to ask whether the skeletons of non-avian dinosaurs were able to produce similar movements.Wedel (2007), p. 119
He concludes that "the respiratory movements in non-avian dinosaurs would have had a similar effect on the volume of the thorax as those of extant birds. … there is no basis for inferring that non-avian dinosaurs could not have ventilated an air sac system, based simply on the absence of some avian features" (pp. 120-121).
Wedel's analysis of the fossils shows that a functional intermediate could have existed without the need for the full suite of avian (bird-like) adaptations. Those adaptations may improve the efficiency of bird breathing, but their absence would not be fatal, despite Explore Evolution's claims. Explore Evolution misrepresents evolution as a linear path from reptilian anatomy to avian anatomy, and considers it problematic if a straight line cannot be drawn from ancestral to modern conditions. This is not how evolution works.
As to the particular issue of a diaphragm in the common ancestor of birds and dinosaurs, paleontologists are skeptical. The paleontological evidence used by Ruben to support the claim that the ancestors of dinosaurs had a diaphragm is weak at best. He takes the coloration of rock within a fossil to reflect the location of the liver in the living organism, and then suggests that a liver in that position requires the sort of diaphragmatic breathing found in crocodiles. Even if he were correct that the color in the rock originated in the liver, and if that liver hadn't shifted as the organism decayed, it would still not support his final claim, since living birds have livers in exactly the same position, and do not have diaphragms (see discussion in Wedel, 2007, p. 128).
While crocodilians do breath using a diaphragm, many reptiles do not use a diaphragm, and neither do the amphibians which are ancestral to reptiles, mammals, dinosaurs and birds. By examining the full range of paleontological evidence, scientists can reconstruct probable anatomies not seen in modern species, but which would provide the sort of functional intermediates which evolutionary theory predicts should exist. By contrast, Explore Evolution and the sources it cites take an "approach to inferring soft tissue anatomy, function, behavior and physiology [which] tends to force extinct animals into the reduced spectrum of animals available to us today, without considering substantial evidence of mosaic change in related extinct forms. It lacks an evolutionary component, produces only conundrums, and explains very little" (Kevin Padian and John R. Horner. 2002. "Typology versus transformation in the origin of birds," Trends in Ecology and Evolution, 17(3):120-124).
The precise meaning of an evolutionary transition can confuse students. As NCSE's Louise Mead notes: "A common misconception of evolutionary biology is that it involves a search for 'missing links' in the history of life. Relying on this misconception, antievolutionists present the supposed absence of transitional forms from the fossil record as evidence against evolution." This is the precise strategy employed throughout Explore Evolution.
In this chapter, Explore Evolution obscures the scientific knowledge behind certain significant transitions in the history of mammals, birds, and reptiles. Rather than presenting students with a clear background on the underlying biology, and the fossil evidence we have in hand, the chapter presents every instance of ambiguity or uncertainty in our knowledge as an insurmountable problem for evolution, and indeed for science. Along the way, the book makes trivial errors regarding the biology of mammalian and reptile reproduction, obscures fascinating and well-established science about the evolution of lactation and live-birth in mammals, and how fossilized bones can tell us about the soft tissues of ancient animals.
Modern species and fossil evidence all give us insight into evolutionary history, and of the sequence of evolutionary changes leading from the common ancestors of modern species to the modern forms.
Explore Evolution is flatly wrong to tell students "all we can say for sure is that the internal organs of living reptiles, living birds, and living mammals are very different from one another" (p. 139). We can say a good deal more than that, and it is irresponsible to so grossly misinform students. To justify the claim that there is not "much fossil evidence of the internal organs of any 'intermediates,'" they cite a 30 year old paper which provides no empirical justification of its claim. Even if the argument were true 30 years ago, it is decisively not true now.
The claim that "all we can say" is that various living species "are very different from one another" is laughably wrong. It is equivalent to saying that we know nothing about a classroom of students except that all people are different from one another. Everyone certainly differs, and it is definitional that all species are different. But science does not stop with a simple statement that differences exist. What differences (and what similarities) exist? What are the patterns of similarity and difference?
Biologists ask those sorts of questions, and evolution provides a framework for suggesting testable hypotheses to answer them. The variation among living species helps to show what anatomical structures can persist, and what forms might have existed as intermediate stages in the evolution of other living forms. The evolutionary branching process gives us testable hypotheses also about the connections between structures, so that we can make testable predictions about parts of a fossilized organism which weren't preserved. In that way, fossils can tell us much more than just what came first. They allow us to test predictions about what life was like millions of years ago, and how different modern forms came to be.
Few other parts of Explore Evolution are so clear in their contradiction of the book's stated aim of "inquiry-based learning." This passage actively discourages inquiry into the connections between living things, their fossil ancestors, and the processes influencing the evolution of life today and in the past.
Fossils are not the only evidence that mammals have a common ancestor with reptiles, and living transitional forms exist illustrating the evolution of the organ systems they cite as examples.
Explore Evolution acknowledges the evidence of fossilized forms transitional between reptiles and mammals, but asserts that:
critics say that the superficial resemblance between skeletons is not all there is to the story. Transforming a reptile into a mammal would involve more than simply changing some bones along the way. It would also involve major changes in organs and organ systems. … Transforming the reproductive system, for example, is not just a question of changing where the eggs grow. It also requires the development of completely new organs like the placenta and mammary glands.EE, pp. 129-130
This argument repeats a claim by creationist Duane Gish (Evolution? The Fossils Say No!, 1972) and, more recently, the "godfather of intelligent design," Phillip E. Johnson:
We may concede Gould's narrow point [about transitional fossils for the mammalian skull], but his more general claim that the mammal-reptile transition is thereby established is another matter. Creatures have existed with skull bone structure intermediate between that of reptiles and mammals, and so the transition with respect to this feature is possible. On the other hand, there are many important features by which mammals differ from reptiles besides the jaw and ear bones, including the all-important reproductive systems.Phillip E. Johnson (1991) Darwin On Trial. Regnery Gateway, Washington, D.C.
It is particularly noteworthy that the authors of Explore Evolution did not even bother correcting the erroneous terminology used by Johnson. Modern evolutionary biologists have found that modern mammals and modern reptiles share a common ancestor, and that ancestral group of organisms is referred to as the amniotes (a reference to a set of membranes found in the eggs of all members of this group, but not in the eggs of amphibians or fish). Johnson's reference to the ancestors of mammals as "reptiles" reflected the accepted usage of his time, but EE's usage of the same term 16 years later does not reflect current scientific thinking. The claim that no transitional forms exist for the mammalian reproductive system was not accurate in 1991, and is even less accurate now.
While fossils can provide only incomplete information about the form of soft tissue, fossils are not the only evidence available in examining the evolution of such organ systems. If those systems could evolve through a series of viable intermediate steps, we might expect that at least some exemplars of transitional forms would persist today.
In the cases of placentas and lactation, examples of transitional forms are not difficult to find. Most mammals, for instance, have distinct nipples which the young use to drink milk. Like all mammals, monotremes produce a form of milk. They do not, however, have nipples. Instead, young monotremes suck milk either from fur on the mother's belly, or from lobules in a pouch as with echidnas. The glands which produce milk are morphologically and developmentally very similar to the glands which produce sweat and oil on the skin.
Studies of the anatomy, development and molecular sequences of mammary glands have given scientists clear insights into the evolution of lactation. Mammary glands are similar in many ways to sweat glands and to the glands which produce oil on our skin. One major component of milk — alpha-lactalbumin — is chemically very similar to an immune protein — lysozyme — found in many species. Support for the idea that the gene for this protein was duplicated and then evolved into a nutrient in milk comes from the observation that some monotremes have "a protein which is a structural and functional intermediate between that of lysozyme and alpha-lactalbumin" (V. Hayssen and D. Blackburn, 1985. "alpha-Lactalbumin and the Origins of Lactation." Evolution, 39(5):1147--1149). This observation suggests that lactation originated as a means to prevent unhatched eggs from becoming infected, a testable hypothesis confirmed by various studies. Other authors have suggested that the mammary glands originated as glands which helped prevent eggs from dehydrating, and that the immune aspects of the mammary glands evolved subsequently. For recent reviews, see Vorbach, C., M. R. Capecchi, and J. M. Penninger (2006) "Evolution of the mammary gland from the innate immune system?" BioEssays 28(6):606–616, Oftedahl, Olav (2002) "The Origin of Lactation as a Water Source for Parchment-Shelled Eggs," and "The Mammary Gland and Its Origin During Synapsid Evolution," Journal of Mammary Gland Biology and Neoplasia, Vol. 7, No. 3, pp. 225-266, and Blackburn, D. G., V. Hayssen, and C. J. Murphy (1989) "The origin of lactation and the evolution of milk: A review with new hypotheses." Mammal Review, 19:1–26.
The transition from eggs to live birth (vivipary) can also be traced using evidence from living species. Amphibians and fish lay eggs which require that they be in water to allow oxygen exchange and to prevent the egg from dehydrating. The hard eggs laid by birds and reptiles contain a number of additional membrane layers which allow gas exchange without excessive loss of water; layers shared by mammalian eggs. This innovation allowed the ancestors of mammals, birds and reptiles to move away from the water.
A few mammals still lay eggs. This group, called monotremes, do not supply their eggs with enough yolk to develop fully inside the egg. The embryos hatch from the egg at an early stage in development and suckle from the mother until they are fully developed. In the platypus, they suck milk from a patch of skin through the hairs growing on it. Young echidnas suckle from lobules containing milk glands; a precursor to the nipples found in other groups of mammals.
This system of external gestation is also found in marsupials. The marsupial mother gives birth to live young very early in development, before all the cranial nerves are complete, before the heart is fully developed, and before the lungs have a complete blood supply. These offspring then crawl to a nipple, where they attach themselves and develop until the stage at which they can survive independently.
Instead of the egg and yolk produced by monotremes, marsupials supply food and oxygen to their gestating offspring through a form of placenta. The marsupial placenta is a modified yolk sac, similar to the one produced by monotremes and reptiles, and derived from the yolk found in amphibian eggs. In marsupials, when this yolk sac contacts the wall of the uterus, the uterine wall secretes a nutritive liquid, which the modified yolk sac absorbs. In general, the diffusion from maternal blood supply to the embryo is much less efficient than is found in truly placental mammals. This system is similar to that found in some viviparous reptiles, which secrete a liquid from the walls of the uterus which is absorbed by the developing embryos.
In two families of bandicoots (marsupials), a different arrangement exists, essentially the same as that found in placental mammals like humans, known as eutherians. In this form, the yolk sac does not have direct contact with the uterine wall, but the fetal blood stream and the maternal blood stream pass close to one another. This allows more efficient flow of nutrients and gases through diffusion. In eutherians, hairlike extensions from the placenta increase the surface area, further improving flow of nutrients.
It is not clear why marsupials have such brief internal gestations, rather than carrying offspring internally until they are more fully developed. One important hypothesis suggests that the maternal immune system may attack the fetus after it reaches a certain stage of development, blocking the flow of nutrients and requiring the mother to expel the embryo before it is fully formed. Eutherian mammals have developed a range of adaptations in the placenta which help prevent the maternal immune system from detecting the foreign antigens produced by the fetus.
Thus, we can observe various transitional forms related to the placenta and lactation in living mammals, and can trace their descent from common ancestors using fossils. We can trace a similar evolution in snakes and lizards. Viviparity has evolved over 100 times in reptiles, mostly through eggs simply being held internally with small amounts of water and nutrients passing through the shell. At least 4 of those lineages have evolved a placenta capable of more significant nutrient transfer. (Recently reviewed in Blackburn, D. G., 2006. "Squamate Reptiles as Model Organisms for the Evolution of Viviparity," Herpetological Monographs 20:131–146)
Researchers Michael B. Thompson and Brian K. Speake explain:
Within the Squamata [lizards and snakes], significant placentotrophy [feeding of offspring through the placenta] has evolved only in the lizard family Scincidae [skinks], and within this family there is a range of modes of nutrient provision from lecithotrophy [nutrition via an isolated yolk] through to placentotrophy, with several lineages showing intermediate conditions. Not surprisingly, therefore, skinks have been the major focus for research into the evolution of complex placentae.Thompson M. B., B. K. Speake (2006) "A review of the evolution of viviparity in lizards: structure, function and physiology of the placenta." Journal of Comparative Physiology. 176:170–189.
Also unsurprisingly, Explore Evolution ignores that research, pretending that the evolution of a placenta occurred only once, and that no evidence exists to explain how it evolved. The reality is far different. The evolution of the placenta is an area under active research, not just within mammals, but within the herpetological community, with several conferences dedicated to new research in the field in the last few years (see, for instance, Volume 20, Issue 6 of Herpetological Monographs, 2006.
Understanding the likely evolutionary trajectory which led to live birth and lactation, we can even detect evidence of these phenomena in fossils. Dr. Olav T. Oftedal argues that the epipubic bones found in marsupials and monotremes probably evolved to support developing young in an external pouch. While the pouch and mammary glands on which those young would have suckled do not fossilize, the epipubic bones do, and can be found in some of the earliest ancestors of mammals (Olav T. Oftedal, 2002. "The Mammary Gland and Its Origin During Synapsid Evolution," Journal of Mammary Gland Biology and Neoplasia, 7(3):225-252). Oftedal also argues that the shift from continuous replacement of teeth (seen in many reptiles) to a single replacement (as seen in most mammals), reflects a physiological constraint which could only be overcome through lactation. Fossils show the shift in tooth replacement occurring around the same time as the emergence of epipubic bones.
A truly inquiry-based textbook might take this and other evidence and invite students to consider what evidence might allow them to test various hypotheses about the evolution of lactation, placentas, and other mammalian traits. Doing so would encourage students to think scientifically, proposing hypotheses and experiments to test them. Instead, Explore Evolution ignores important lines of evidence and instructs the student to give up when biology gets complicated.
Reptiles and mammals reproduce very differently. Most reptiles lay eggs, while mammals carry fertilized eggs internally in a placenta and bear live young.EE, p. 129
An errata sheet, dated May, 2007, came with the first releases of EE. It made a correction to this statement (correction in bold).
Reptiles and mammals reproduce very differently. Most reptiles lay eggs, while mammals carry fertilized eggs internally, which they nourish through a placenta, and bear live young.EE, errata sheet of May 2007
These two sentences in EE, and the botched correction, provide perhaps the clearest evidence of the distance between the authors of this book and mainstream science, basic biological knowledge, and science pedagogy.
This correction addressed only one of the many things wrong with this statement. Indeed, the uterus (not the placenta) is the organ in which mammals carry their fertilized eggs; the placenta is the source of nourishment for these internally carried young in most mammals. It is gratifying to note that this basic biological fact is now correct in the textbook, but it is actually quite puzzling that such an error could have escaped the attention of even the most cursory reviewer.
Three other errors, however, remain uncorrected. The sentences (1) ignore other reproductive modes of mammals, (2) downplay the rich diversity in reptilian reproduction, and (3) imply the nonexistence of intermediate reproductive modalities.
The marsupial mammals have a rudimentary and short-lived placenta which is, in most marsupials, structurally and functionally different from the typical eutherian placenta. Placental nourishment of marsupial young is negligible compared to nourishment from the milk obtained in the pouch. Furthermore, there are mammals which lay eggs and have no placenta. These creatures, the monotremes, share with other mammals the characteristics of fur and the ability to lactate, but they lay eggs with leathery shells, which the females then incubate in a pouch.
Some reptiles (e.g. garter snakes) are viviparous and develop a rudimentary placenta (see Stewart, JR, American Zoologist 1992 32(2):303-312, "Placental Structure and Nutritional Provision to Embryos in Predominantly Lecithotrophic Viviparous Reptiles" for a not-so-recent discussion of these facts).
It is also quite telling that these variations on reproductive physiology in both reptiles and mammals are more evidence that there are identifiable transitions in form and function among living organisms. In fact the placenta, in all of its variants, is functionally and structurally derived from membranes found in eggs; the transition between reptiles and mammals becomes obvious when all of the data are considered. A "lack of transitional forms", as noted elsewhere in this review, is a standard creationist canard, but in this book transitional forms are lacking only because the authors choose to ignore them.
In fact, the living mammals illustrate a clear transition from egg-laying through various transitional stages along the way to live birth as seen in humans. In monotremes, which consist of the platypus and spiny anteaters called echidnas, the reproductive system is, as James Vaughan explains in his textbook Mammalogy, "a mix, including primitive features shared with amniotes and unique specializations." Monotreme eggs are structurally more similar to reptile and bird eggs than to eggs of other mammals, and have texture similar to reptile eggs as well. Monotremes, like reptiles, have a structure called a cloaca, in which the reproductive, urinary and digestive tracts all exit through a single opening, rather than through two, a trait shared with birds, reptiles and marsupials, but not with the rest of the mammals. It is a small step from laying these sorts of eggs to the marsupial system of briefly holding the developing embryo internally and nursing the partially developed embryo externally, then successively modifying the placental interface between fetus and mother until we see the sort of live birth found in humans, other eutherian mammals, and even a few transitional marsupials.
This sequence illustrates several important errors in Explore Evolution. Clearly, EE badly misrepresents mammalian reproduction. More fundamental, and more widespread, is its tendency to treat a large taxonomic group of species as if they are all practically identical. Species do differ one from another, and in important ways. Those differences are essential to evolutionary processes, and understanding such variation is vital to a student's understanding of evolution. By teaching students that it is acceptable to treat mammals or reptiles or other groups as if all members of the group were interchangeable does students a disservice and misinforms them about the range of adaptations which exist in the natural world, and how the variation among living things reveals evolutionary processes. Only by obscuring legitimate science can Explore Evolution create the false impression that there are unbridgeable gaps between major taxonomic groups; gaps which bolster their preconceptions against common descent.
These three errors — and the original eggs-in-placenta error — could have been avoided if the authors consulted a standard college-level introductory textbook, or if they were familiar with basic biological literature, or even if they had used used basic resources like Wikipedia, Google, or even a smart 5th grader.