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Thread: Without a fossil representation, how do we know that mammals share a common ancestor?

  1. #11
    It should be noted, first off, that "mammal" is a term that has a bit of fuzziness around the edges of it. Marsupials, for example, are classified as mammals despite the fact that some of them lay eggs instead of using a womb for incubation.

    Secondly, fossils are only one kind of record and in fact are not nearly as good as the DNA evidence is. Many animals, not just mammals, are vertebrates, and the way the body builds bone structure looks pretty much the same regardless of whether an animal is cold blooded or warm blooded for example, and for a long time people made the mistake of thinking Dinosaurs were cold-blooded lizard ancestors. The point is that the definitional things that differentiate mammals from other groups are all tied to the soft tissues - mammary glands, hair, warm-blooded, etc - Most of the time these are not preserved in a fossil and all you see is the skeleton. The skeleton of a cold-blooded tetrapod and a warm-blooded tetrapod don't look that different. We share the same basic body plan with lizards - 2 arms, 2 legs, optional tail, 1 head, food goes in the head and waste comes out the back near the tail, and internal organs consisting of lungs, heart, stomach, intestines, throat. The basic body plan for the torso is therefore the same.

    The fossil record is really the hardest place to look for the dividing line between mammals and what came before. What is a lot easier to see is the DNA record in modern animals. You can identify genes that are associated with those soft-tissue traits that are missing from fossils, and compare them in modern animals to check how similar or different they are from each other, and analyzing that is the best place to get that sort of evidence.

  2. #12
    Senior Rocket Scientist Mr Shifty's Avatar
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    Quote Originally Posted by Steven Mading View Post
    It should be noted, first off, that "mammal" is a term that has a bit of fuzziness around the edges of it. Marsupials, for example, are classified as mammals despite the fact that some of them lay eggs instead of using a womb for incubation.

    Secondly, fossils are only one kind of record and in fact are not nearly as good as the DNA evidence is. Many animals, not just mammals, are vertebrates, and the way the body builds bone structure looks pretty much the same regardless of whether an animal is cold blooded or warm blooded for example, and for a long time people made the mistake of thinking Dinosaurs were cold-blooded lizard ancestors. The point is that the definitional things that differentiate mammals from other groups are all tied to the soft tissues - mammary glands, hair, warm-blooded, etc - Most of the time these are not preserved in a fossil and all you see is the skeleton. The skeleton of a cold-blooded tetrapod and a warm-blooded tetrapod don't look that different. We share the same basic body plan with lizards - 2 arms, 2 legs, optional tail, 1 head, food goes in the head and waste comes out the back near the tail, and internal organs consisting of lungs, heart, stomach, intestines, throat. The basic body plan for the torso is therefore the same.

    The fossil record is really the hardest place to look for the dividing line between mammals and what came before. What is a lot easier to see is the DNA record in modern animals. You can identify genes that are associated with those soft-tissue traits that are missing from fossils, and compare them in modern animals to check how similar or different they are from each other, and analyzing that is the best place to get that sort of evidence.
    1) There is no fuzziness for living species: mammals have sweat glands, including special ones modified to produce milk. (Mammal => mammary.)
    2) A trained zoologist/paleontologist can recognize mammals from fossils. Distinguishing features include the dentary-squamosal jaw joint, the presence of three bones in the middle ear, prismatic enamel on the teeth, and the presence of a second knob on the underside of the occipital bone.
    3) DNA evidence is harder to use than you suggest. We typically do not have DNA samples from extinct species, so you can't just compare. Nor do we usually know what phenotypic characteristics a particular gene gives rise to (i.e. we don't know all of the genes responsible for mammary gland development.) Dawkins gives a really good description of how DNA is used to construct phylogenetic family trees in The Ancestor's Tale. But IIRC, it involves melting DNA strands and allowing them to re-anneal with the strand they're being compared to, then observing how well the hybrid strands bind to each other. This allows you to find the genetic distance between living species, but not between living species and fossils. You can find more here: https://en.wikipedia.org/wiki/DNA-DNA_hybridization Note that DNA is not compared gene-by-gene to determine differences; in fact, the non-coding sections of DNA are what is useful for determining genetic difference because they're not subject to selection pressure and thus have a statistically constant mutational drift over time.
    4) For extinct species, cladistics is used to form family trees, and since fossils are usually all we have, fossil features are what is used. Cladistics is an optimization analysis, where distinguishing fossil traits are grouped, using computer programs for optimization, to produce the most likely distribution of traits along a lineage tree. Here is more information: http://en.wikipedia.org/wiki/Cladistics
    Last edited by Mr Shifty; 25th June 2013 at 09:31.
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  3. #13
    Quote Originally Posted by Steven Mading View Post
    It should be noted, first off, that "mammal" is a term that has a bit of fuzziness around the edges of it. Marsupials, for example, are classified as mammals despite the fact that some of them lay eggs instead of using a womb for incubation.
    No they don't, marsupials don't have a placenta but they do give live bearth. Monotremes are mammals that lay eggs, but they are no more related to marsupials than placental mammals (despite both being largley confined to australia in modern times).

    Mammals aren't really a fuzzy clasification, at least among non-extinct species (and classifying extinct species is always fuzzy if you go back far enough). Monotremes seem to have split off from the rest of mammals about 200 million years ago, in the late Triassic (or possibly early Jurassic). However synapsids, the reptiles which mammals are descended from, split off from diapsids (ancestors of lizards, crocodiles, birds, dinosaurs and probably turtles) at least 320 million years ago.

  4. #14
    If we had no fossils, because, for example, we've just found an interstellar ark containing all the extant species of another world, we would still be able to compare their DNA ( or equivalent ) working out the familiar branching pattern of relationship that arises from divergence trough continuous small changes.
    If we even hadn't any DNA because for example, our interstellar probe can't take samples, just high-res pictures from orbit, we could do the same with anatomical features. create standard sets of them and compare them obtaining the same ( though slightly less exact ) result as with DNA.
    .
    But here on earth we DO have a lot of fossils, so what is the point ?

  5. #15
    Pug On Leave Pugspaceprogram's Avatar
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    Thanks for your submissions; keep 'em coming! I am having fun reading your theories on this (technically) unanswered science question. I probably should edit the question explaining that the theory is thought correct, but there is a debate as to whether the ancestor actually existed.
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  6. #16
    Quote Originally Posted by Pugspaceprogram View Post
    the theory is thought correct, but there is a debate as to whether the ancestor actually existed.
    .
    What does that even mean ?

  7. #17
    Capsule Communicator Kryten's Avatar
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    A debate between who exactly? Who seriously argues that mammals aren't monophyletic?

  8. #18
    Quote Originally Posted by MBobrik View Post
    What does that even mean ?
    It means aliens did it. That's the only option I can think of where all mammals share an ancestor, but the ancestor did not exist.

  9. #19
    Quote Originally Posted by Kryten View Post
    A debate between who exactly? Who seriously argues that mammals aren't monophyletic?
    AFAIK only creationists & other cranks ...

  10. #20
    So the other theory is... some mammals are a result of convergent evolution? Oh wait, they would still have a non-mammalian common ancestor... so the other theory is... some mammals evolved from life that came into existence independently of the rest of Earth based life?

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