The processes of evolution

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All the examples shown here are of fish that already possesses the capability to live outside water. Their breathing system is already adapted for it. I am aware of their existance. However, evolution dictates that this capability is not automatic, but that it must have been develop. Therefore, I wish to concentrate on fishes that does not have this capability. Lets take a fish that cannot survive outside water, and cannot use its fins to move around outside water. Acording to evolution, this is what the precursors of land animals must have been like. The question is, how do these fishes get the capability to move on land? Remember, the genetic code to create lungs does not exist yet, so this process cannot be compared with the live cicle of an amphibian, whos genetic code already contain the information to form lungs.

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That's not entirely accurate. If you'll re-read my post, you'll note that by the mid-Devonian, early lungs had already developed in fish. It's the ones whose fossils are found in fresh water (rather, formerly fresh water) that were the ancestors of most of the modern fish, as well as amphibians, and all terrestrial vertebrates (basically every vertebrate except the Chondrichthyes (sharks, skates, rays, etc). Almost no amphibian has lungs. Many of them have lung-like structures attached to their gut - as do many fish (swim bladders are derived from lungs).Beyond that, are you asking for information on the development of the tetrapod limb (necessary for locomotion), or lungs? It appears you're changing the question.

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I do not agree with the arguement that it was food that lured them out to land. When a whale beaches itself, only animals that can move on land will come out from the water to feed on it. You do not see fishes (such as sharks) that cannot live above water, struggling out to join the feast. If this was the case, evolution is feasable: The fish that can crawl out better, is more likely to survive and pass on their genes. But this is crearly not the case. They simply don't come out. Similarly, the lush plants of the tropics do not lure fish that can not move on land. Science assume that things worked in the past as they work today. Therefore, we can savely descard the theory that propose that fish were "lured by the advantages of walking on land". This advantage, however great it might be, cannot change the genetic makeup of animals, because genetics is chemistry, and habitat advantiges is not.

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Actually, sharks are one of the few fish not descended from the Actinopterygians of the Devonian. They really CAN'T use air because their ancestors (the Chondrichthyes) never had lungs. This is also why sharks don't have swim bladders like most modern fish.As far as the selection pressures that led to the terrestrial radiation of the tetrapods, well, the jury is still out. We have quite compelling evidence that it happened, but not quite so much as to the "why". There are two major hypotheses. Romer, among others, postulated that the shallow seas and lakes of the Devonian supercontinent were subject to repeated and severe droughts - like southern Africa today. With water supplies being uncertain, natural selection would favor those organisms with a greater or lesser capability to use air directly. Watching a blenny - a salt-water goby - sitting on a stone gulping air is an amazing example of this. Blennys have no lungs - their pharyngeal pouches have evolved into swim bladders like most other teleosts - so they hold a bubble in their mouths and absorb oxygen through the epithelium. They can sit there for about ten minutes before having to dive back in their pools. As an aside, the lungfish and mudskippers mentioned are NOT the likely ancestors of tetrapods, although the Dipnoi have been around since the Devonian. They are illustrative of the lifestyle, but not necessarily in the line of descent.Others have postulated that tetrapods evolved from shallow sea/coastal dwellers. Being able to sit on the bottom and snag passing arthropods was a great way to make a living. This is a very similar to modern Ambystomatidae (salamanders with both lungs and, at least in the neotenic versions, external gills), among others. The ones who were able to move further up the beach, or stay out of the water longer, had better chances to get food and hence reproduced more. Remember, this represents a LOOOONG slow, millions-of-years process. It wasn't just some Icthyostega that woke up one morning and decided to take a stroll. In any event, this idea is not as unlikely as it might seem: the one thing the study of biodiversity shows us more than anything else is that life will radiate to fill any available niche given half a chance. And it's often caused by some small adaptation that allows organisms to exploit new ecosystems.

Thanks. The theory seems plausible.There is something else that is bothering me: the origens in live. Is it true that all (cell based) live forms has a common ancestor?

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Originally posted by Hanno:
Thanks. The theory seems plausible.

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There is something else that is bothering me: the origens in live. Is it true that all (cell based) live forms has a common ancestor?

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Probably not, life likely arose in several places and times. The orIgin of life is an interesting subject in its own right independent of evolution.CheersJoe Meert

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.

For starters, the evidence that convince me of the common descent of all life is the universality of the genetic code.Glad you want to open your mind

My pleasure, amigo. One of the great things about hanging out on this board is that I literally learn something new every day.

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

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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 mark24:
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.

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This is very much along the line that I would argue.------------------
http://www.hells-handmaiden.com

Just so I am 100% percent clear on this. Am I then correct in saying the current theory is that all dna based live forms evolved from a single organism?

Hanno,Basically that's correct, although I would say that there was one surviving lineage that led up the long eons to the diversity of life on the planet. However, I'm not sure that saying that a single cell was the last common ancestor is very informative. 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. Meaning that the "root of the tree of life" more resembles a tangled mangrove than a single root. I'm not sure you can even say (at least when you get back around 3.8-4 gya) that we're even talking "DNA lifeforms" were the last common ancestor. What the theory DOES say is that, once DNA was adopted as RNA's baggage carrier, everything else in history was based on it.If you're asking whether there was a particular DNA or RNA "code" that was the last common ancestor of all life, I think the answer is probably "no". There were a bunch of them - but all based on the same kind of nucleic acids - which is why we say "all life on Earth is related".

Here are some good links on the subject:My Name is LUCA -- The Last Universal Common Ancestor:
http://www.actionbioscience.org/newfrontiers/poolepaper.htmlWhat is the Last Universal Common Ancestor:
http://www.actionbioscience.org/...ontiers/poolearticle.htmlWere Bacteria the First Forms of Life on Earth?:
http://www.actionbioscience.org/...tiers/jeffares_poole.htmland this paper traces the origin of all RNA metabolism back to LUCA:Comparative genomics and evolution of proteins involved in RNA metabolism. Anantharaman V, Koonin EV, Aravind L. Nucleic Acids Res 2002 Apr 1;30(7):1427-64. PMID: 11917006maybe it deserves its own thread in the origin of life forum?

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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?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)Are you saying that this rule do not apply to micro life forms?In other words, if monkeys and lions were bacteria, would they be able to produce offspring?

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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.

Hanno: Mark has it right on the money. It's pretty weird when you get to the prokaryotes. Basically, lateral gene transfer is pure larmarckism: inheritance of acquired traits. It would be sort of like a monkey and a bird mating, producing a primate with wings in one generation. Really, though, they're just exchanging a few bits of DNA. Prokaryotes don't have a nucleus, so their DNA is just hanging around inside their cell membrane. It's quite fascinating, IMO.