The origin and evolution of chordates
The lowest living chordates, such as sea squirts and amphioxus, have not found their fossil ancestors because they have no hard bones in their bodies. Therefore, the origin of chordates can only be inferred from the evidence of comparative anatomy and embryology.
Among all kinds of invertebrates, which one is the ancestor of chordates? In the past hundred years, many zoologists have put forward various hypotheses. There are two important assumptions:
One is annelid theory, which holds that chordates originated from annelids, and points out that these two animals have symmetrical segments on both sides, segmented excretory organs and developed body cavities, which are closed circulatory systems. If the dorsal abdomen of annelids is inverted, the ventral nerve cord becomes the same as the dorsal nerve canal of chordates; The position of the heart and the direction of blood flow are the same as those of chordates. However, such an upside-down argument cannot be established. For example, the mouth becomes dorsal and the brain is ventral. Unlike chordates, the differences in chordates, branchial cleft and embryonic development cannot be explained. So this assumption has been abandoned at present.
The second is echinoderm theory, which holds that chordates originated from echinoderms. This is a study based on embryonic development. Echinoderm belongs to the class Posterior Stomata during embryonic development, and at the same time, it forms a body cavity by means of cyst, which is different from ordinary invertebrates, but similar to chordates. In addition, Auricularia auricula, the larva of echinoderms, and Tornaria, the larva of Hemiechinoderms, are very similar in morphology and structure (below). The status of vertebrates in the animal kingdom is a transitional position between invertebrates and chordates. Biochemical studies have also proved that echinoderms and semi-echinoderms are closely related. Both animals contain creatine and arginine at the same time, which shows that the two animals are closely related, and on the other hand, they are in a transitional position between invertebrates (only arginine) and chordates (only creatine). For the above reasons, echinoderms believe that echinoderms and chordates come from the same ancestor.
Of the two hypotheses, the latter hypothesis is more satisfactory and may be correct, although there is no direct fossil evidence yet.
As for the ancestor of Chordata, it is speculated that it is a worm-like hind jaw animal with a notochord, dorsal neural tube and branchial fissure. This imaginary ancestor can be called a primitive headless class. Primitive Decapoda has two specialized branches, namely, urochordata and cephalochordata. Primitive cephalopods evolved from the spine of primitive headless animals, that is, the ancestors of vertebrates.
Primitive cephalopods later developed in two directions: one evolved into a relatively primitive jawless class (soft-shelled turtle and roundmouth) without upper and lower jaws; The other branch evolved into a jaw with upper and lower jaws, which is the ancestor of fish.
The evolution of vertebrates can be divided into three stages: the first stage is the evolution in water, that is, the evolution of fish (cartilaginous fish and bony fish); The second stage is the evolution from water to land, namely the evolution of amphibians and reptiles; The third stage is the evolution of two higher vertebrates, birds and mammals, which evolved from reptiles.
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