BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.7. Evidence for Evolution

26.7.7. Comparative Embryology

Karl Ernst von Baer (1792–1867), studying the embryonic development of various vertebrate groups, discovered a striking structural resemblance among them, especially during the Cleavage, Gastrulation, and early differentiation stages. Ernst Haeckel (1834–1919) proposed that this similarity has evolutionary significance. He formulated the biogenetic law, according to which "ontogeny recapitulates phylogeny"—that is, the stages an Organism goes through during its development mirror the evolutionary history of its taxonomic group. Although this principle oversimplifies the actual state of affairs, it remains compelling and valid to a certain extent. Looking solely at the early embryonic stages of any vertebrate group, it is impossible to determine to which specific group they belong.

As seen in Fig. 26.15, an embryo begins to acquire recognizable similarities to its respective adult form only at a relatively late stage of development. At specific comparable stages, the embryos of all vertebrates share the following structures:

1. Pouch-like invaginations of the ectoderm and corresponding outgrowths of the pharyngeal walls growing toward each other. In fish, these meet and fuse, subsequently rupturing to form true gill slits that facilitate gas exchange. In other vertebrate groups, a single such slit persists, giving rise to the Eustachian tube and the auditory canal.

2. Segmented myotomes (Skeletal Muscle precursors), which are visible in the tail-like Structure. These persist only in certain species.

3. A single circulatory loop with a two-chambered Heart that is not divided into right and left halves; in adults, this heart structure is retained only in fish.

As a vertebrate embryo develops, it undergoes changes that impart the characteristics of a fish, amphibian, reptile, bird, or mammal, depending on the Lineage of its parents. This initial embryonic resemblance is explained by the fact that all of them—and consequently the classes they represent—shared a common ancestor. If an organism develops embryonic rudiments that never mature into functional Organs, they can only be interpreted as remnants of structures present in its ancestors. The recapitulation law, however, cannot be accepted unconditionally, since no living organism exhibits all the features of its putative ancestors. Nevertheless, it is likely that organisms retain developmental mechanisms inherited from their forebears. Therefore, at various stages of Embryogenesis, an organism may display structural similarities to the embryos of ancestral forms. Subsequent adaptations to different environments and lifestyles alter the subsequent course of development. Observations show that the more closely related two organisms are based on shared homologous definitive structures, the longer their developmental similarity persists during embryonic stages. Organisms adapted to a specific lifestyle or habitat atypical of the broader group to which they belong show fewer developmental similarities with other members of that group. This is clearly demonstrated by The Development of two parasitic Flatworms, Fasciola and Taenia: they possess a series of larval stages adapted to secondary hosts, which are absent in The life cycle of free-living flatworms like Planaria. Similarly, the terrestrial earthworm Lumbricus lacks the trochophore larva typical of more primitive Annelids. These Examples clearly illustrate the limitations of Haeckel's recapitulation law.

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Fig. 26.15. Comparison of embryonic development stages across representatives of three vertebrate classes.

The Study of embryogenesis in Major Groups of organisms reveals structural similarities that are evident at embryonic and larval stages but absent in adults. These observations are interpreted as signs of phylogenetic relationships between different groups, implying an evolutionary process. Based on the pattern of zygote cleavage and The Fate of the blastopore, triploblastic organisms can be divided into two groups: protostomes and deuterostomes. Protostomes are characterized by spiral cleavage, and the blastopore develops into the Mouth of the adult. This developmental pattern is typical of annelids, Mollusks, and Arthropods. Deuterostomes are characterized by radial cleavage, with the blastopore developing into the anus of the adult. This occurs in Echinoderms and Chordates. The differences between these two groups are illustrated in Fig. 26.16. Data of this nature have helped clarify the phylogenetic affinities of echinoderms. The anatomy of adult echinoderms suggests they are a distinct invertebrate phylum, yet their deuterostome-like embryonic development confirms their close affinity to the chordate evolutionary line. This example demonstrates that phylogenetic relationships cannot be resolved based solely on data from adult homologous structures.

Fig. 26.16. Early Developmental Stages of deuterostomes and protostomes.

Studies of plant embryogenesis also provide evidence pointing to the progressive evolution of various groups, though the documented examples are not as robust as those in the animal kingdom. In mosses and ferns, the gametophyte—represented in early developmental stages by a protonema formed from spore germination—resembles filamentous green Algae in structure, physiology, and growth pattern; it is therefore widely accepted that both mosses and ferns originate from such algae. The Morphology/12.html">ALTERNATION OF GENERATIONS in plant life cycles, along with its various adaptations to changing environments, can be interpreted as further evidence supporting evolutionary links among different plant groups.

Gymnosperms represent an intermediate group between forms adapted to terrestrial life and those whose Gametes must meet in Water. In cycads, the male gametophyte resembles the lightweight, dry microspore (pollen grain) of angiosperms in being wind-dispersed. During the Development of the male gametophyte, one of its Cells forms a pollen tube, much like in angiosperms; however, rather than delivering a non-motile male gamete to the archegonium, the pollen tube produces two flagellated sperm cells that swim toward the ovule to effect Fertilization. Consequently, cycads may be viewed as an intermediate group between lower plants and angiosperms, which in turn suggests phylogenetic continuity within the plant kingdom.

The existence of organisms sharing traits common to two different groups—representing different levels of complexity or adaptations to distinct habitats—can be interpreted as an indication of phylogenetic continuity among all three groups, with one group (such as angiosperms) deriving from another (such as lower plants) through an intermediate group (such as cycads). Many such intermediate forms are now extinct, and tracing the evolutionary sequence relies heavily on the fossil record. In many cases, intermediate forms remain undiscovered; these correspond to the "missing links" in the fossil record. Alternatively, such gaps may reflect reality, aligning with the punctuated equilibrium hypothesis described in Section 26.7.1. However, the absence of these links can also be attributed to poor fossil preservation or failure to find them yet. An example of an intermediate link between annelids and arthropods is the phylum Onychophora, represented by the genus Peripatus, which exhibits characteristics of both annelids and arthropods (Fig. 26.17). Its annelid affinities include a body wall structure featuring circular and longitudinal Muscles, unjointed parapodia-like limbs, nephridia-like excretory tubules in each segment, and a double ventral nerve cord. Features shared with arthropods include a chitinous cuticle, tracheae, spiracles, and an open Circulatory system. Another group of "living fossils" serving as a link between fish and amphibians is represented by lungfish.

Although the bulk of this evidence points toward some form of evolutionary process, we must bear in mind that definitive, direct proof remains elusive.

Fig. 26.17. The primitive arthropod Peripatus.