| $Diary January 26, 2008$ "Why do birds resemble mammals in embryo development?" |
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Birds and mammals are said to have evolved from reptiles. But, why is there an existence of such resemblance between birds and mammals in embryo development? "The more yolk there is, cleaving of cells would less likely to occur". Everyone who took up biology may have encountered this law in high school. An egg with extremely large amount of yolk does the cleavage only at the animal pole. It is because yolk restrains a cleavage and this is called "meroblastic cleavage". And, mammals do an equal segmentation because there is little yolk. However, there is a common point where the birds resemble mammals in embryo development. It is called the gastrula period. The discodial cleavage in aves and the equal segmentation in mammals are the a first steps of embryo development. These are also the basic laws in biological entrance examination. On the other hand, the "Haeckel's Biogenetic Law" is a theory, in which, ontogeny repeats phylogeny. This is also a theory that has a tendency to occur but doesn't apply strictly in all cases. Yet, I think this could be applicable to humans. The two (mammals and birds) may share a close resemblance in embryonic development, but there could be many doubts especially in the course of one's self analysis. There are two things that may cause such change in one's belief of such resemblance; the diffusion and the convergence in evolution. Diffusion is the adaptive radiation of mammals which grasped the world after the reptiles declined while convergence is the functional evolution in the same direction that maybe caused by closely resembled environments like the birds and the bats, for instance. Because of such occurrence, therefore, the close resemblance of birds and mammals is due to convergent-evolution. However, an answer was found recently. "Reptiles do a discodial cleavage." Yolk seemed to be a decreased form of a human egg rather than the convergent-evolution in the process of the evolution. Nevertheless, current findings are still insufficient to draw to a concrete conclusion. The natural selection should have been more sensitive to advantages and disadvantages rather than necessity, may there be a large amount of yolk or not. Doesn't a placenta need a yolk, too? Well, this question shall only remain in one's head. In the end, only nature knows the answer for there are no strict laws in evolution. |