The eggs, the single celled starting points of the embryos of all animals, are vastly different. The bird and reptile eggs are of very great size. Fish eggs are usually smaller, but still easily visible to human eyes. The human egg, on the other hand, is of microscopic size.
The first stage of embryonic development is cleavage, the division of the egg into cells. Each group of vertebrate animals has its own cleavage pattern, very different from the others. During the cleavage stage, the basic anterior to posterior (front to rear) direction of the body is established. Next comes the gastrula phase, during which the basic body plan of the animal is elaborated. During gastrulation, the cells begin to differentiate into the various tissues. As in the case of cleavage patterns, gastrulation patterns display a great deal of variation among the different kinds of animals. At this stage in development, the embryos therefore look quite different from each other (Nelson 1998, p. 154; Wells 1998, p. 59; Elinson 1987).
It is only in the next stage of embryonic development, the pharyngula stage, that the embryos of fish, reptiles, birds, and mammals come to temporarily resemble each other, looking somewhat like little fishes. In the pharyngula stage, all the embryos have little folds of tissue in the throat region that look like gills. In fish, they do become gills, but in other animals they form the inner ear and thyroid glands. So the embryos of humans and other mammals never have gills, nor do the embryos of birds and reptiles (Wells 1998, p. 59). After pharnygula stage, the embryos again diverge in appearance.
Considered in its entirety, the embryonic development of the vertebrates, rather than supporting evolution, tends to pose a strong challenge to it. According to evolutionists’ theory, all metazoans (multicelled creatures) must have come from a common ancestor. This creature would have had a certain body plan. To change that basic body plan would require changes in the genes that control the early embryonic stages of that body plan’s development. But according to evolutionary theory, the genes controlling the early stages of development should not be subject to very much change. Any such changes could cause massive disruptions in the development of the organism, causing its death or serious malformation.
That is what we see today. As Nelson (1998, p. 159) says, “All experimental evidence suggests that development, when perturbed, either shuts down, or returns via alternate and redundant pathways to its primary trajectory.” Therefore, according to most evolutionary biologists, positive mutations should occur only in genes responsible for details of later phases of development of an organism.
According to evolutionary theory, we should expect the earliest phases of development in living things to be quite similar. But, as we have seen, the early developmental stages of living things are vastly different from each other (Nelson 1998, p. 154). For example, after the egg begins to divide, there are several pathways by which the embryos of different animals reach the gastrula stage. Eric Davidson (1991, p. 1), a developmental biologist, has called this variety of cleavage patterns “intellectually disturbing.” It is somewhat of a mystery how all these very different patterns of early development came from some common ancestor. Richard Elinson (1987, p. 3) asked: “If early embryogenesis is conservative, how did such major changes in the earliest events of embryogenesis occur?” He calls it “a conundrum.”
Some (Thomson 1988, pp. 121–122) have proposed that early changes in development are obviously possible, simply because they have obviously occurred. This is a typical example of blind faith in evolutionary doctrine. Nelson (1998, p. 158) says: “Note that this position rests entirely on the assumption of common descent. There is little if any experimental evidence that ‘changes in early development are possible.’ I know of only a single example of heritable changes in metazoan cleavage patterns.” In other words, there is only a single experimentally verified example of a genetic change in the early development of an animal that has been passed on to its descendants. The change involves a mutation in the early development of the snail Limenaea peregra, which causes only the direction of the coiling of its shell to switch from right to left (Nelson
1998, p. 170, citing Gilbert 1991, p. 86) This is not a very significant change. It represents no new biological feature.
So today there is practically no experimental evidence that early changes in development can result in viable organisms with new features. Some scientists propose that although such changes are not possible in today’s organisms, they were possible early in the history of evolution, resulting in major changes in body plans. Foote and Gould (1992, p. 1816) suggest that this proposed early period of developmental flexibility was closed off hundreds of millions of years ago at the end of the “Cambrian explosion,” during which all major body plans now seen in living things supposedly emerged. After the Cambrian explosion there was “some form of genetic and developmental locking.” The proof of this, say Foote and Gould, is that no new major body plans have emerged since the Cambrian. Further, they say that we do not see today that creatures with major mutations in genes that control early development survive (Foote and Gould 1992, p. 1816). But this era of early plasticity of body plans, generated by changes in early developmental stages of the embryo, is purely speculative. Scientists cannot point to any specific reason, on the biomolecular level, exactly why Cambrian creatures could survive such major mutations.
Nelson (1998, p. 168) says: “Golden ages of evolution are postulated (e.g., the Cambrian explosion), in the complete absence of any mechanistic understanding, to accommodate the demands of a philosophy of nature that holds, in the face of abundant disconfirming evidence, that complex things come into existence by undirected mutation and selection from simpler things. Yet, however unlikely they may be, these golden ages of macroevolution are preferable by neo-Darwinists to taking at face value the demonstrable limits of organismal structure and function—for those limits imply the primary discontinuity of organisms one from another.” Discontinuity implies intelligent design of the separate species.
Scientists find it difficult to explain in any detailed way how these body plans (or Bauplans) came about from some common ancestor by evolutionary processes. Bruce Wallace (1984, cited in Nelson 1998, p. 160) tells of some of the problems involved in modifying a body plan: “The Bauplan of an organism . . . can be thought of as the arrangement of genetic switches that control the course of the embryonic and subsequent development of the individual; such control must operate properly both in time generally and sequentially in the separately differentiated tissues. Selection, both natural and artificial, that leads to morphological change and other developmental modification does so by altering the settings and triggerings of these switches . . . The extreme difficulty encountered when attempting to transform one organism into another but still functional one lies in the difficulty in resetting a number of the many controlling switches in a manner that still allows for the individual’s orderly (somatic) development.” It is like trying to transform a six cylinder engine into an eight cylinder engine while keeping the engine running through all the changes. Arthur (1987, cited in Nelson 1998, p. 170) says that “in the end we have to admit that we do not really know how body plans originate.”
What to speak of understanding how genes can govern major changes in body plans, to produce new organisms, scientists do not yet fully understand how genes direct the development of the body plan of any par-ticular species. R. Raff and T. Kaufman (1991, p. 336) speak of science’s “currently poor understanding of the way in which genes direct the morphogenesis of even simple metazoan structures.” Each human being starts as a single cell—a fertilized egg. The egg begins to divide into more cells. Each cell contains the exact same DNA, but the cells differentiate into various tissues and structures. How exactly this happens is not currently understood, even in very small multicellular organisms.