The processes of evolution

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Originally posted by Hanno:
Thanks for your reply.

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I cannot say I understood exactly what you said as I don't have any tertiary education in science. I think I might have asked the question wrong. What I meant was WHY did they develop lungs. I think this is a question rather like "What came first: the chicken or the egg?". A fish swimming merrily in the ocean will have no disire to walk around on the surface, since it can't breath there. If it did, somehow, manage to get out to the surface, it would die within minutes. Therefore, before the fish got out on land, it had to develop lungs first. But why would it develop lungs, if its gills work just fine in its living environment?

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It's not hard to visualise advantages of being able to exist out of water for extended periods. Extra feeding opportunities, predator evasion etc. Lungfish do exactly this.Many fish have swim bladders which contain gas, uasually used to control bouyancy, but note that monkensticks abstract shows that the surfactant proteins that we have in our lungs are the same as fishes (or the same family, at least). The fishes have a ready made (but for another purpose) organ that could potentially double as a primitive lung, as is the case with lungfish. Consider a lineage which spends more time on land than in water, eventually gills will not be needed & will atrophy. In fact humans still possess gill slits at a particular stage of embryonic developement, they eventually go on to become part of the ear & pharynx.There are numerous examples of fish adaptions to terrestrial environments, lungfish being the most quoted. Mudskipper species are able to drag themselves around mudflats, in this case gaseous exchange is provided by gills.------------------
Occam's razor is not for shaving with.

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Originally posted by Dr_Tazimus_maximus:

Not to mention the snakehead fish in the news in the Washinton DC metro area. This fish can survive for about three days on dry land and can use this to migrate from one body of water to another.

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The better known common eel (of Sargasso Sea fame) is able to travel overland between waterways, too.Mark------------------
Occam's razor is not for shaving with.

Quetzal,Interesting, do you have a cite? I came across this once before but dismissed it as a teachers thought experiment, & investigated no further.Mark------------------
Occam's razor is not for shaving with.

Hanno,I understand what you mean, but as has been pointed out already, only things that pre-existed already, were used. Let me explain. Getting a Mackeral to walk is looking like a poor proposition. It has very flimsy ray like fins. BUT, not all fish had ray like fins. Some were "lobe finned", that is, they already had an adaption (for something else) that predisposed them to having stronger limbs than ray finned fishes. Anything that provides a selective advantage for stronger fins will mean stronger fins/limbs over time. Absorbtion of oxygen simply requires a wet membrane, as has been pointed out. A fish, yesterday. "The evolution of the skeletons of the earliest amphibians (top) from Devonian lobe-finned lungfish (bottom) did not require major modifications in structure and the skulls have a great many similarities as well."You can see the extreme similarity in morphology between lungfishes & amphibians in the image above.All of that said, modern fishes (even ray finned) DO have terrestrial adaptions, so perhaps you would do better by looking at extant fish that are able to go "feet dry", to answer your question "why".Mark------------------
Occam's razor is not for shaving with.

Quetzal,Interesting stuff, heres a cladogram providing evidence that swim bladders did indeed evolve from lungs. Learn something new every day Thanks for the heads up. Mark------------------
Occam's razor is not for shaving with.

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Originally posted by Hanno:
What I mean to ask, is this. We are often told that, if evolution would've happend all over again, live would've looked totally different. Thus, if more than one organism evolved from the primordial soup, then they would've been radically different from each other, and unable to reproduce with each other. They would all have formed seperate chains of very distinct liveforms, with no similarities between them. Yet, if we study evolution, it seems as if all animals comes from the same evolutionary tree. The very makeup of cells, indicate a common origin, does it not?

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If this is not the case, how is it possible that a cell that is totally historically unrelated to another, can have the same make-up?
Why don't we find lots of independant evolutionary trees, instead of just one?

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Surely, a single evolutionary tree indicates a single origin, doesn't it?

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Hanno,I agree, given the genetic code is universal (in a nucleotide-codon sense), indicates to me that there was one common ancestor. There may well have been more "abiogenesis events", but most likely only one lineage survived.I disagree with Joe on this, although I expect we are talking at cross-purposes. Most likely I'm in for more educating at the hands of my evo compadres We also have to entertain the likelyhood that there was up to 1-2 (give or take) billion years of non-cellular evolution, that is, purely molecular interactions (that probably were no more alive than a virus), in which multiple abiogenetic events may have been able to interact, which I'm guessing Joe may be hinting at. This may well have occurred BEFORE a code existed.Mark------------------
Occam's razor is not for shaving with.

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Originally posted by Hanno:
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The biggest problem we have once you get to the bacterial stage (which lasted about 2 billion years) of the development of life, is that the lineages start getting very, hmm, promiscuous. A whole lot of gene swapping between wildly different organisms (such as between archaea and bacteria) and a lot of serial endosymbiotic events took place.
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Are you saying what i think you're saying?

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If animals of two different species were to mate, there will be no ofspring, because their DNA code is not compatible. (With the notable exception of a horse and a donkey)

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Are you saying that this rule do not apply to micro life forms?

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In other words, if monkeys and lions were bacteria, would they be able to produce offspring?

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No, single celled prokaryotes (cells without nuclei, & other cell organelles like mitochondria- bacteria are prokaryotes) were able to receive genetic mateial from other species. The tuberculosis bacteria gets its antibiotic resistance in just this way, from another species. Given that the genetic code is universal (well, mostly), a gene that produces a protein in one bacteria will do the same in another.You are thinking of incompatibilities between sexually reproducing species that actually prevent such gene transfer. Interestingly, there is good evidence that we harbour both viral & bacterial DNA in our own genomes that got inserted accidentally.Mark------------------
Occam's razor is not for shaving with.