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|EMBRYOLOGY AND EVOLUTION|
Embryos and fetuses are not little adults. In humans, the embryonic brain does not form as a miniature adult brain; the occipital bone does not form as a miniature occipital bone; the kidneys do not form as miniature adult kidneys. Many of the characteristics of human embryos are more similar to the characteristics the adults of more primitive vertebrates rather than those of adult humans. This observation long preceded Haekel’s notion that an embryo actually recapitulated evolutionary ancestry in order to achieve its adult form. Geoffrey argued that some embryonic structures were vestigial since 1829 and the notion that embryos recapitulate stages in which they are equivalent to other organisms since 1811. In 1793, Kielmeyer, one of Cuvier’s teachers, compared the development of life on earth to the stages of fetal development (Hall, p. 42-52). While embryological development does not recapitulate phylogenetic history, embryos can retain structures from ancestral embryos even though these structures may be lost or highly modified in the adult forms. The following chart depicts some of these similarities.
The similarities of embryological features provides overwhelming evidence for evolution since there is no reason to predict that these similarities would exist if evolution had not occurred. Of all of the ways that an occipital bone could form, there is no reason that it would form from the fusion of the same elements present in adult synapsid reptiles if humans were completely unrelated to synapsid reptiles. Of all of the ways that a pharynx could be formed, there is no reason that land living humans should begin their pharyngeal development in the same way as fish with gills do. There is no reason that the primitive pronephros and mesonephros should form in the same way as they do in fish if adult humans rely only on a metanephric kidney. There is no reason that blood vessels equivalent to those in fish should form which would have to be completely reworked to form the adult vasculature. There is no reason that human embryos should have a yolk sac or allantois given that nutrient and waste exchange occurs at the placenta. There is no reason that an uninterrupted notochord should form in an embryo which is about to form a vertebral column.
Some have argued that these structures cannot be considered as vestigial evolutionary remnants if there is a function that they perform. Does the human embryonic notochord help to induce the nervous system? Yes. Do remnants of the embryonic notochord still exist as the nucleus pulposus in the intervertebral disks which help cushion human steps? Yes. BUT IT’S A NOTOCHORD! There are many ways that a nerve chord could be induced to development—all bilateral animals which are not chordates do so, in fact. There are many possible sources of tissue which could form between neighboring vertebrae. All humans once had a notochord as an embryo stretching from head to tail, just like in the most primitive chordates. Human vertebrae formed as a series of small bones around the notochord (a pair of neural arches, a pair of pleurocentra, perhaps remains of intercentra) that gradually replaced it—in the same way that is evident in the fossil record. In the modern evolutionary model, higher vertebrates which lack notochords as adults evolved from vertebrates which had notochords as their primary support rods. The observation that human embryos have notochords supports this--whether or not these embryonic notochords have other functions. The question is not “Do embryonic notochords have a function?” but “Why is it a notochord which performs this function?”
The same can be said of other structures. If you look in the head region of an early vertebrate embryo, you can’t miss the prominent pharyngeal arches. Do these arches have a function? Yes. Pharyngeal arches contribute cartilage to the larynx and hyoid, small segments of embryonic blood vessels to adult vessels, tissue to the thymus and thyroid, etc.—BUT THEY ARE PHARYNGEAL ARCHES! There are many ways a small hyoid bone could develop or blood vessels to enter the head could develop or a thymus could develop, etc. One of the first things I did as an embryo was to dedicate a large number of my embryonic cells into making a yolk sac. Does the human yolk sac have a function? Yes--my first blood cells and spermatogonia and oogonia are made there—BUT IT IS A YOLK SAC!! Blood cells can be made from lots of places, especially in fetal development, a yolk sac isn’t necessary. The reproductive system would make much more sense if spermatogonia and oogonia actually formed in the gonads where they will exist later in development rather than being formed outside the embryonic body and having to travel to the gonads. Regardless of these odd functions in the production of blood and reproductive cells—it is a yolk sac. The question is not “Do embryonic pharyngeal arches and yolk sacs have a function?” but “Why is it pharyngeal arches and a yolk sac which performs these functions?”
For each step which should occur in normal embryonic development, there is the possibility that a mistake occurs. Because human embryos develop characteristics unlike those of human adults, they must undergo extra developmental steps which must modify these early features. With these extra steps come additional possibilities for birth defects. The development of the yolk sac increases an embryo’s susceptibility to herniation of the intestines. The development of the thymus in the neck of the embryo creates the possibility that its migration to the thorax will not occur normally and will result in remnants of the thymic tissue left in the neck. The formation of the primordial germ cells which will produce gametes outside the gonads (in the yolk sac and base of the allantois) necessitates that they migrate to the gonads, although this does not always occurs successfully. The formation of the testes near the thorax requires that they descend into the scrotum but this process is not completed in many males, resulting in sterility. The kidneys begin development in the pelvis and in some individuals, there are abnormalities resulting from their ascent. A large number of abnormalities can be traced to a problem in converting an early developmental stage into the normal adult morphology such as pharyngeal slits which do not fully close, a palate which remains cleft, notochord remnants which persist (and may even cause a cancer known as a chordoma), composite bones which do not fuse normally, heart chambers which are not fully separate, persistent embryonic vessels such as the ductus arteriosus, and many others. Thus, an understanding of the evolutionarily retained features of the embryo are important in understanding human birth defects.
Comparative embryology reveals unexpected similarities in the development of animals and provides great evidence for their shared, evolutionary ancestry.