Embryos in Deep Time: The Rock Record of Biological Development

8. “Missing Links” and the Evolution of Development

141 Many people are accustomed to thinking of the evolution of life in terms of a ladderlike progression, with a different animal on each rung. In the case of vertebrate evolution, they may envisage a fish on the bottom rung, a salamander on the next, then a lizard , a mouse, and finally a human on top. Following this medieval myth, it seemed natural to suppose that “lower” animals evolved into “higher” animals. And if this were so, we should expect to see “links” between them, all the way up and down the “ladder.” This idea of the Great Chain of Being, or Scala naturae, is an image of evolution that remains common in the popular media, although scientists have long realized that such a concept is inaccurate. The ladder myth is misconceived and does not fit with evolution. Instead of resembling a ladder, the evolution of life is more similar to a branching bush. Each branch of the bush represents a distinct lineage of organisms. Places where two or more branches diverge from a single point on the bush indicate that the lineages represented by the branches must have shared a common ancestor at a particular point in their history. Each liveight “Missing Links” and the Evolution of Development 142 / “Missing Links” ing species is a “missing link,” as each species is part of the tree of life, with a mosaic of both “primitive” retentions and derived features, which document transformations. From each species there is much to learn about these changes. Every one of the millions of species of animals shares an ancestor with every other one. The missing link is the ancestral species that gave rise to two modern groups, for example, humans on the one hand and chimpanzees on the other. But it is not reasonable to expect to find that critical species, for its identification would require a complete series of ancestor-descendant fossils. We can certainly trace the evolution of birds from feathered dinosaurs, but we are not sure exactly which fossil species were the direct ancestors of modern birds. Our focus in reconstructing the history of life is not to try to imagine how a chimpanzee could transform into a human, as this of course never happened. Rather, we try to discover which features chimps and humans inherited from their common ancestor and which features the lineages evolved after they diverged. Some authors correctly argue that the concept of the missing link is not only archaic, but its search is also an outmoded approach to the study of macroevolution. In current evolutionary biology, the focus of investigation has shifted from finding transitional taxa to finding transitional features shared by closely related forms with common ancestry. Many examples are well known today, and they provide information not only about anatomical transformations but also about the functional and ecological contexts of those transitions. Particularly important in this context are stem species, those that document the time and mode in which the acquisition of diagnostic features of living species evolved.1 An example is provided in our mammalian lineage. According to molecular esti- “Missing Links” / 143 mates, supported by fossil evidence, the last common ancestor of all living mammals, that is, the last common ancestor of a human (a placental), a kangaroo (a marsupial), and a platypus (a monotreme), lived at least 220 million years ago. But the history of mammalian origins from our last common ancestor with reptiles goes even farther back in time, until at least 315 million years ago. These earliest 100 million years of evolutionary history are documented by the stem species of mammals. They are placed in the evolutionary tree between the last common ancestor of the living species and the last common ancestor of the next living relative, in this case the reptiles. Fossils provide for several major taxa, a fairly complete picture of the morphological transformations involved in their origin . The fossil record of many groups has changed from almost nonexistent to fairly diverse in a few decades, as in the case of Mesozoic mammals. The continuous reports on new discoveries of “transitional forms” show the large advances that have been made and are being made in the documentation of macroevolutionary steps of vertebrates since Darwin’s time. These reports also show that purpose and plan are not characteristic of organic evolution. Organisms fulfill their “conditions for existence” at each particular time in which they live—that is...