Embryos in Deep Time: The Rock Record of Biological Development

9. Mammalian and Human Development

158 There are about 5,300 species of extant mammals. They represent only a fraction of the number of species that ever existed since the separation of the evolutionary line leading to them, at least 315 million years ago. Then the reptilian and the mammalian lineages split. The estimate based on fossils is that about 100 million years passed until the appearance of the last common ancestor of all living mammals in the Jurassic.1 Although 5,300 extant species sounds like a large number, it is not so impressive when compared with other groups: there are twice as many species of birds and five times as many bony fishes. But mammals are special when we consider their great ecomorphological diversity—locomotion in humans, kangaroos, whales, and bats; the size range between a shrew and the blue whale. What all mammals have in common is a unique life history and physiology among vertebrates, including an energetically expensive endothermy, parental care, milk production, and determinate growth. These and many other features must have characterized the last common ancestor of the living mammals. When nine Mammalian and Human Development Mammalian and Human Development / 159 did these features arise since the split from reptiles? Did they do so simultaneously? If not, in what sequence? Whatever our ancestors did, at two critical times in geologic history they had good luck and good genes. I have already discussed how Lystrosaurus and its relatives survived the Permian-Triassic boundary, a time when many groups of organisms were completely wiped out. Mammals also survived the end of the Cretaceous event, when most dinosaurs became extinct. Figure 43. After the divergence from reptiles no later than 315 million years ago, there was a long history of “stem mammals” until the last common ancestor of living species originated at least 220 million years ago. Representing living mammals are the echidna, a monotreme; the kangaroo, a marsupial; and the human, a placental. The stem species depicted here are all important ones mentioned in the text but are just a miniscule portion of the diversity of parallel paths of evolution that were followed in the deep time of mammalian evolution. Morganucodon Edaphosaurus Lystrosaurus Thrinaxodon 160 / Mammalian and Human Development Several unique anatomical features of mammals are not coupled directly with life history traits, for example, the presence of three little ear ossicles as opposed to just the one in reptiles and birds. The way embryological and paleontological data document the origin of the mammalian ossicles fits wonderfully well together, their discovery being one of the greatest triumphs of comparative anatomy. As I discussed in chapter 8, Goldschmidt had stressed the lack of intermediate forms in evolution and the importance of epigenetics, and, in addition to the the eyes of flatfishes , his major example was the middle ear of mammals. Both were bad examples of an otherwise good idea. In fact, Goldschmidt would have welcomed the newest evidence that the origin of the mammalian middle ear was coupled developmentally with an increase in brain size and most likely with many other features, including changes in the masticatory apparatus. The concomitant developmental changes show that these features cannot be understood in isolation. During the ontogeny of living mammals, the growth of the neocortex is coupled with the gradual detachment of elements associated with the jaws. Some of the old jaw elements become relatively smaller and smaller as they detach and begin to form the future elements of the middle ear, an example of negative allometric growth. A gradual anatomical change similar to that of embryos is what fossils document in the stem line leading to the living mammals. These changes have been documented by Tim Rowe at the University of Texas based on the study of computer tomographic images of fossils and reconstructions of developing marsupial embryos of living species.2 Coupling, or nonindependence, during the evolutionary origin of mammals also characterizes life history features. The documentation of those evolutionary changes is restricted to the skeleton, as soft tissues, including organs such as the heart or the Mammalian and Human Development / 161 stomach, are rarely if ever preserved. Not surprisingly, most evidence derives from teeth and the bone histology. The complicated pattern of tooth occlusion in mammals, with the upper and lower molars possessing several cusps occluding with each other in a specialized way, is surely correlated with a high metabolic rate, coupled in turn with the life history traits. Whereas...