Later, in adults these springs rudimentary causes a number of tumors).I hope someone give me a good explanation
Humans are vertebrates, as are fish. Fish and human beings share a common evolutionary ancestor. Our foetal development at the early stages is somewhat similar.
While some examples of embryonic stages showing superficial features of ancestral organisms exist, the theory of recapitulation itself has been completely disproven within the field of biology.
I must say though that within the first week, any vertabrate shows remarkable similarities... I do know that every land creature descended from a shrewlike creature about 4 million years ago. http://en.wikipedia.org/wiki/File:Haeckel_drawings.jpg In this rough schematic, the similarities are profound. How has it been ''completely disproven''...? Through what techniques? I'd say there is quite a lot of genetic evidence going on there... the way life starts out seems to be more or less the same for most vertabrates as an evidence of some genetically-linked past. I mean... only genetics can answer why any embryo would begin the way these had, wouldn't it? Or is there something I am not aware about?
You need to recheck the data on that. It is not even true that all *mammals* have a common ancestor 4 million years ago, let alone all land animals. Birds. for example, are likely descended from dinosaurs, and they died out 65 million years ago. Terrestrial insects broke off even earlier than that. Mammals ancestry likely started to divide sometime after the evolution of cynodonts (250 million years ago). Four million years ago, primates were already living in Africa, and the line that would produce humans had already diverged from that of the chimps. As for recapitulation theory, just google "ontogeny recapitulates phylogeny" and you will see that that theory was discredited long ago. It's flaws were pointed out even in the 19th century. In fact, here's one site: http://evolution.berkeley.edu/evosite/evo101/IIIC6aOntogeny.shtml Here's another: http://en.wikipedia.org/wiki/Recapitulation_theory. Note that footnote 5 includes the quote, "The so-called basic law of biogenetics is wrong. No buts or ifs can mitigate this fact. It is not even a tiny bit correct or correct in a different form, making it valid in a certain percentage. It is totally wrong."
It is part of a scientific theory, previously largely accepted, which has since been proved false. And not that long ago. Have you been reading an old Biology manual?
Most (all?) vertebrate embryos go through a similar tailbud stage, with somites, a neural tube, optic anlagen, notochord, and pharyngeal arches/pouches. The pharyngeal arches are also called gill arches, because in fish some of the pharyngeal pouches (between the arches) later develop into gills. I.e. it just so happens that fish have an adult structure that superficially resembles the early embryonic structure. The Wikipedia link above has a table listing the structures that develop from the pharyngeal arches/pouches.
Please Register or Log in to view the hidden image! This question involves stem cell differentiation. At the site established for gills in a fish, the land animals undergo a switch in the embryonic stage, which changes the way the stem cells develop. Organs and tissues needed by land animals develop from these stem cells instead of gills. For more information: Pharyngeal arch
This is called the Recapitulation Theory . . . . ."Phylogeny recapitulates ontogeny" (or is that visa versa?)
No. Haeckel's Recapitulation Theory was that an embryo resembles the adult forms ancestral organisms as it develops, eg that human embryos at a particular stage are similar to adult fish. Which isn't true. What James said is that vertebrates have similar early embryonic development, eg that human embryos at a particular stage are similar to fish embryos at a particular stage. Which is true.
Firstly, some contributors to this thread need to make the distinction between embryonic development and foetal development. The latter is largely just a question of growth. It is embryonic development that transforms a zygote into an organism identifiably belonging to a particular species. Secondly, in humans, the embryonic developmental process takes about eight weeks. So the developmental stages that offer such striking similarities between, actually all vertebrate species, comes somewhat earlier than two months. Thirdly, I know nothing whatever of any ‘theory of recapitulation’ discredited or otherwise. But I know that nowhere is greater nonsense talked on this thread than when people try to deny the patently obvious generic elements of the embryonic developmental process across species. And that even goes beyond vertebrate species. There are striking similarities in the embryonic developmental processes of human beings and fruit flies. And the evidence for their common origins does not rely purely on the superficially observable similarities. Modern science understands a great deal about what drives the embryonic developmental process. It seems a quite important point to me to understand that, in point of fact, it is not so much the morphological features or behavioural traits that evolve. Or at least, those are really just secondary consequences of it. What actually evolves is the embryonic developmental sequence. And in some ways, what is remarkable is just how constant it has been across 4 billion years of evolution.
Recapitulation theory was the notion that each successive evolutionary change in a genus or species is represented by a new stage of embryonic development, and that each embryo passes through all of the embryonic stages of each genus or species from which the organism arose. As such, if the theory were correct, you could see evidence of the entire phylogenetic history of humanity by looking at the stages through which a human embryo develops. So, for example, what would have been the final stage of embryonic development for an australopithecine (prior to its becoming a fetus) would have been a late, but not final stage of embryonic development for a homo sapiens. I wonder if this isn't want wellwisher is suggesting when he says: (Though I must admit that I could be misunderstanding that sentence.)
The reason is that those genes which determine early stages of development [by controlling differential through gene-expressed chemical concentrations and gradients] are still around similiar to our ancestors 500-600 million years ago, as a result, before specific gene cause the differentiation of distinct human features [like opposable thumbs and pevlic bones for upright posture], the developmental series goes from early and generalised groundworks of early life-forms [this embryology has no effect on the final morphology or other charateristics of the animal, hence it is still around]. This is why a somewhat evolutionary sequence can be seen in the development of the embryo.
Haeckel was an amazing artist and naturalist overall, but his embryo etchings are bogus. They were called out as wrong back in the 1800's by others who had studies animal embryos, but despite that, they were still included in my High school and college biology textbooks (1990's). Please Register or Log in to view the hidden image! see: "Referate" L. Rutimeyer, pp 301-302 Archives of Anthropology 1868
@river-wind -- That's most likely because the people who put your biology textbook together weren't biologists.
