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

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The title gives it away, this is a unanswered question in science, and i want to know your thoughts. Have fun with your creative ideas and theories.

Remember, there is (currently) no right or wrong answer, just theories. :)

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Common ancestry with what? Each other? Or with something else?

We know a lot about what animals are closely related to each other by looking at their DNA and seeing which sequences in it (including faulty parts) are in there. The "family tree" derived this way matches well with what we derive from the fossil record.

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I'm not sure I even understand the question, why would we even assume that similar species didn't have common ancestor without evidence that said they didn't?

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I'm not sure I even understand the question, why would we even assume that similar species didn't have common ancestor without evidence that said they didn't?

Mammalian Diversification

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Because as we discover more fossils, it appears that the trend is moving in that direction. The fossil record will never be complete, but our theory should explain the evidence in the simplest way possible without introducing the most new assumptions. The single common ancestor idea meets that criteria.

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Remember, there is (currently) no right or wrong answer, just theories.

Incorrect, this is not an unanswered question in science. And while I suspect your question is loaded, here is a reasonable answer:

- Any group of living things that you could care to define has a common ancestor. You can define a group consisting of me and a random bacteria; we have a common ancestor.

- I'm not actually sure what else you're trying to suggest. Are you asking a question about cladistics? On top of the overwhelming DNA evidence, there are many anatomical features that point to mammals-as-clade, but really the specialized scales we call hair or fur are enough. Only mammals have fur. Likewise feathers and dinosaurs; only dinosaurs have feathers (though some extinct species of dinosaurs did not), and all feathered dinosaurs share a common ancestor that is more recent than any ancestor they share with non-feathereds and/or non-dinosaurs.

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One main way you can see a common relationship is skulls. All mammals have one hole in the sides of their heads - the temporal fenestra. All reptiles have two (except turtles, but turtles are complicated)

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Theory of evolution predates most of the fossil evidence. People started digging through the fossils primarily in an attempt to confirm or disprove evolution. There are many similarities and differences in the various organs, tissues, and bones of animals that let us reconstruct biological relation, and this is what taxonomy was originally based on. And this already predicted a common ancestor for all mammals. Then we found the fossil evidence that fits with that. And then later we have learned how to work with DNA, and that opened many new doors for finding relations between species.

So in terms of most conclusive and error-proof test of whether mammals share common ancestor, look at DNA evidence. It is very conclusive.

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Just thought I'd point out that this is the same guy who started the How does zero-G work in planes thread and then concluded it with this:

Thanks for your submissions. Surprisingly, this was a test! I already knew but i wanted to test your knowledge. You were all correct! I congratulate you all. Why don't you try to simulate this in KSP. Do it. DO IT NOW!

Once bitten, twice shy...

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Is he looking for a common ancestor of all Kerbals? :wink: No problem - look at cucumber :sticktongue:

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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.

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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

Edited by Mr Shifty

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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.

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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 ?

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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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the theory is thought correct, but there is a debate as to whether the ancestor actually existed.

.

What does that even mean ?

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A debate between who exactly? Who seriously argues that mammals aren't monophyletic?

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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.

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A debate between who exactly? Who seriously argues that mammals aren't monophyletic?

AFAIK only creationists & other cranks ...

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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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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?

.

Convergent evolution works only on things under strong selection pressures. It doesn't make things like junk DNA or vestigial features, which aren't under any reasonable selection pressure, converge.

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.

Convergent evolution works only on things under strong selection pressures. It doesn't make things like junk DNA or vestigial features, which aren't under any reasonable selection pressure, converge.

To clarify, I was not seriously suggesting it, I was just trying to imagine what possible scientific alternative the OP was suggesting there could be to mammals having a common ancestor.

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To clarify, I was not seriously suggesting it, I was just trying to imagine what possible scientific alternative the OP was suggesting there could be to mammals having a common ancestor.

Yeah, I know. but there just aren't any scientific alternatives. A plenty of un-scientific, though.

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Ah, but you are forgetting that some mammals may have come over from parallel universes, and therefore not be evolved from the same common ancestor as the mammals in this universe.

OK, that's as wacky as I can get.

Goofy thread is goofy. I'm outta here.

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Yeah, I know. but there just aren't any scientific alternatives. A plenty of un-scientific, though.

What the OP might conceivably consider scientific, regardless of it's actual potetial validity.

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