Summary of problems with claim:
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.