Ancestry and Evolution
Ancestry and
Evolution
Since the mid-nineteenth century when the theory of organic evolution became the focal point for ferreting out relationships among groups of living organisms, zoologists have debated the question of chordate origins. It has been very difficult to reconstruct lines of descent because the earliest protochordates were in all probability softbodied creatures that stood little chance of being preserved as fossils even under the most ideal conditions. Consequently, such reconstructions largely come from the study of living organisms, especially from an analysis of early developmental stages, which tend to be more evolutionarily conserved than the differentiated adult forms that they become.
Zoologists at first speculated that chordates evolved within the protostome lineage (annelids and arthropods) but discarded such ideas when they realized that supposed morphological similarities had no developmental basis. Early in this century when further theorizing became rooted in developmental patterns of animals, it became apparent that the chordates must have originated within the deuterostome branch of the animal kingdom. As explained earlier (Figure 8-9), the Deuterostomia, a grouping that includes the echinoderms, hemichordates, lophophorates, and chordates, has several important embryological features that clearly separate it from the Protostomia and establish its monophyly. Thus the deuterostomes are almost certainly a natural grouping of interrelated animals that have their common origin in ancient Precambrian seas. Several lines of anatomical, developmental, and molecular evidence suggest that somewhat later, at the base of the Cambrian period some 570 million years ago, the first distinctive chordates arose from a lineage related to echinoderms and hemichordates (Figure 25-2).
While modern echinoderms look nothing at all like modern chordates, evolutionary affinity between chordates and echinoderms gains support from fossil evidence. One curious group of fossil echinoderms, the Calcichordata, have pharyngeal slits and possibly other chordate attributes (Figure 25-4). These small, nonsymmetrical forms have a head resembling a long-toed medieval boot, a series of pharyngeal slits covered with flaps much like the gill openings of sharks, a postanal tail, and structures that are doubtfully interpreted as notochord and muscle blocks. These creatures apparently used their pharyngeal slits for filter feeding, as do protochordates today. Although calcichordates seem to have some of the right chordate characters based on soft anatomy, there is no convincing similarity between the hard skeleton of calcichordates (which was calcium carbonate) and that of vertebrates (which is composed of a complex of calcium and phosphate). Thus, while we do not yet understand the precise structure of the long-sought chordate ancestor, we do know two living protochordate groups that descended from it. These we will now consider.
Since the mid-nineteenth century when the theory of organic evolution became the focal point for ferreting out relationships among groups of living organisms, zoologists have debated the question of chordate origins. It has been very difficult to reconstruct lines of descent because the earliest protochordates were in all probability softbodied creatures that stood little chance of being preserved as fossils even under the most ideal conditions. Consequently, such reconstructions largely come from the study of living organisms, especially from an analysis of early developmental stages, which tend to be more evolutionarily conserved than the differentiated adult forms that they become.
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| Figure 25-4 Fossil of an early echinoderm, a calcichordate, that lived during the Ordovician period (450 million years BP). It shows affinities with both echinoderms and chordates and may belong to a lineage that was ancestral to chordates. |
Zoologists at first speculated that chordates evolved within the protostome lineage (annelids and arthropods) but discarded such ideas when they realized that supposed morphological similarities had no developmental basis. Early in this century when further theorizing became rooted in developmental patterns of animals, it became apparent that the chordates must have originated within the deuterostome branch of the animal kingdom. As explained earlier (Figure 8-9), the Deuterostomia, a grouping that includes the echinoderms, hemichordates, lophophorates, and chordates, has several important embryological features that clearly separate it from the Protostomia and establish its monophyly. Thus the deuterostomes are almost certainly a natural grouping of interrelated animals that have their common origin in ancient Precambrian seas. Several lines of anatomical, developmental, and molecular evidence suggest that somewhat later, at the base of the Cambrian period some 570 million years ago, the first distinctive chordates arose from a lineage related to echinoderms and hemichordates (Figure 25-2).
While modern echinoderms look nothing at all like modern chordates, evolutionary affinity between chordates and echinoderms gains support from fossil evidence. One curious group of fossil echinoderms, the Calcichordata, have pharyngeal slits and possibly other chordate attributes (Figure 25-4). These small, nonsymmetrical forms have a head resembling a long-toed medieval boot, a series of pharyngeal slits covered with flaps much like the gill openings of sharks, a postanal tail, and structures that are doubtfully interpreted as notochord and muscle blocks. These creatures apparently used their pharyngeal slits for filter feeding, as do protochordates today. Although calcichordates seem to have some of the right chordate characters based on soft anatomy, there is no convincing similarity between the hard skeleton of calcichordates (which was calcium carbonate) and that of vertebrates (which is composed of a complex of calcium and phosphate). Thus, while we do not yet understand the precise structure of the long-sought chordate ancestor, we do know two living protochordate groups that descended from it. These we will now consider.
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