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Politics : Formerly About Advanced Micro Devices -- Ignore unavailable to you. Want to Upgrade?

To: tejek who wrote (246339)	8/17/2005 10:07:49 AM
From: neolib	Read Replies (1) \| Respond to of 1571983

When you say related species......do you mean that all mammals go through similar stages; all birds go through similar stages; all insects go through similar stages and so on......no matter whether the insect is a tarantula or a lady bug Mammals will go through more similar stages than say humans & birds. This is because all mammals share more common development than mammals & birds. But mammals, birds & reptiles share a lot of more basic (read ancient) common development, so the earliest stages of how a body plan starts to develop will be similar. If you go all the way back to very primitive small worm like organisms, most all living "animals" are all bilaterans, that is animals that have bilateral symmetry. The genetic mechanisms for laying this down in all modern species is similar, hence the early embryo development reflects this. At a somewhat later stage, the gill structure of fish, and parts of our face & neck develop from a common portion of the embyro that looks rather like gills in all embryos. So on it goes... Its a little bit like the fact that all humans start as female, it is only around weeks 12-16 that developmental switches trigger to make some of us male. Prior to that the embryo is either sexless or female, depending on your POV, so it is proto-sexual? You could loosely generalise that and say that prior to weeks xx the human embryo is not human, but proto-ape, and so on back to proto-mammal, proto-fish, and finally just proto-bilateran. At each one of these stages you identify, the embryos across a wider and wider swath of spieces will appear earily common (as one goes back towards conception, from species by itself near birth, to common bilateran near the egg stage). Ernest Haeckel's contribution should be correctly understood as the first serious stab at developmental biology & understanding its relationship to evolution. Today this is an entire field of its own, known as Evolutionary Development or Evo-Devo. His detailed explanation, that the embryo was marching through a succession of forms that look like the adult forms of our ancient ancestors was incorrect as we now know. The problem is that Haeckel did fudge his drawings a bit, exaggerating the similarities to make his point. Those that don't like evolution have latched onto this point, while failing to understand his actual contribution in the light of modern biology.