Darwin recognized that embryonic resemblances were a very strong argument in favor of the common ancestry of different animal groups (see Oppenheimer 1959; Ospovat 1981). “Community of embryonic structure reveals community of descent,” Darwin said in Origin of Species. Thus, he looked to embryonic and larval stages for homologies that would be obscured in the adult. In the Origin, Darwin celebrated the case of the barnacle, whose larvae showed it was a shrimplike crustacean rather than a clamlike mollusk, and in The Descent of Man (1874), he gloried in Alexander Kowalevsky’s (1866) discovery that the tunicate—hitherto also classified as a mollusk—was actually a chordate (Figure 9.6). Tunicate larvae have a notochord and pharyngeal slits arising from the same cell layers that give rise to those structures in fishes and chicks. Thus, the “great divide” between the invertebrates and the vertebrates was bridged by the discovery of larval homologies.
Comparative embryology became evolutionary morphology as new information about the homologies of the germ layers in various animals became paramount in answering questions about phylogeny (e.g., Lankester 1877; Balfour 1880–1881; Oppenheimer 1940). Mùˆller (1869) united the Crustacea (including in this phylum not only the barnacles but a whole range of parasites) through their possession of a larval type called the nauplius, and E. B. Wilson (1898) proposed a superphyletic taxon that included all those animals with spiral cleavage and possessing a particular mesoderm-forming cell. A century later, Wilson’s proposed taxon was recognized by molecular and cladistic techniques and named the Lophotrochozoa (see Halanych et al. 1995; Halanych 2004). The paper by Wilson is a beautiful example of turn-of-the century embryology, and can be accessed by clicking here. In the past five years, new techniques of cell lineage tracing have enabled us to return to these questions of cell lineage.
Balfour, F. M. 1880–1881. A Treatise on Comparative Embryology. (2 vols.) Macmillan, London.
Darwin, C. 1874. The Descent of Man, and Selection in Relation to Sex, 2nd Ed. (2 vols.) John Murray, London.
Halanych, K. M. 2004. The new animal phylogeny. Annu. Rev. Ecol. Evol. Syst. 35: 229–256.
Halanych, K. M., J. D. Bachelier, A. M. Aguinaldo, S. M. Liva, D. M. Hillis and J. A. Lake. 1995. Evidence from 18S ribosomal DNA that the lophophorates are protostome animals. Science 267: 1641–1643.
Kowalevsky, A. 1866. Entwickelungsgeschichte der einfachen Ascidien. Mémoires de L’Académie Impériale des Sciences de St.- Pétersbourg, VII Série. Tome X: 1–22.
Lankester, E. R. 1877. Notes on the embryology and classification of the animal kingdom: comprising a revision of speculations relative to the origin and significance of the germ layers. Quart. Rev. Micr. Sci. 2/17: 399–454.
Müller F. 1869. For Darwin. (W. S. Dallas, transl.) John Murray, London.
Oppenheimer, J. M. 1940. The nonspecificity of the germ-layers. Q. Rev. Biol. 15: 1–27.
Oppenheimer, J. M. 1959. An embryological enigma in The Origin of Species. In B. Glass, O. Temkin and W. L. Straus, Jr. (eds.), Forerunners of Darwin 1745–1859. Johns Hopkins University Press, Baltimore, pp. 292–322.
Ospovat, D. 1981. The Development of Darwin’s Theory: Natural History, Natural Theology, and Natural Selection, 1838–1859. Cambridge University Press, Cambridge.
Wilson, E. B. 1898. Cell lineage and ancestral reminiscence. In Biological Lectures from the Marine Biological Laboratories, Woods Hole, Massachusetts. Ginn, Boston, pp. 21–42.