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Snafuperman
Post  Post subject: Common ancestor . . . why no examples?  |  Posted: Sun Sep 27, 2015 3:47 pm

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When I look at a picture of the human tree of evolution or the great ape tree of evolution, there are many fossil examples of species that have branched off the tree but I never see a fossil species of a "common ancestor". In other words, why don't we see an example of a species at the branch intersection of the main tree? Is it that we just haven't found a fossil example yet? I know the fossil record is very, very slim if we go back more than a million years or so.


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iNow
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Sun Sep 27, 2015 5:34 pm
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Which one of these do you mean?

https://en.m.wikipedia.org/wiki/Tree_of_life_(biology)
https://en.m.wikipedia.org/wiki/Phylogenetic_tree

Also, have you ever seen this one?

http://visual.ly/great-tree-life

Image
<click to enbiggenfy>

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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Sun Sep 27, 2015 5:42 pm

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Here's an example of a human family tree:

Image

But I notice that there are a couple of "common ancestors" listed in this picture. One is Homo heidelbergensis.


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iNow
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Sun Sep 27, 2015 6:39 pm
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If you go back far enough,mothers aleays a common ancestor. I guess you'd need to be more specific to the get the answer you seek. Common between which two specific branches?

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SkinWalker
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:36 am
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If I understand the question correctly, you shouldn't ever find a specific species that stands at the crossroads, as it were, for one or more other species. This is because of the nature of speciation itself. There is no hard, fast change. Changes are on a continuum and never abrupt. In other words, there isn't a person who was considered a H. sapiens and another who was considered H. neanderthalensis who would both say their mother is H. heidelbergensis. There would have been generations where the distinction was quite vague, but differentiation occurred over time as populations separated.

This, of course is the key to understanding how it all fits: populations. Evolution works on populations, not individuals.

So, in looking at the evolutionary tree, we should expect to see significant variety and, from one end of the tree to the other, variation that can only be said to be that of differing species. In the middle however, the point of divergence should not be expected to be well defined. An analogy would be that multicolored ball or yarn in which colors change from red to blue to yellow with all the secondary and tertiary colors in between on a spectrum that ranges widely. We don't know at what point red becomes blue, but we can definitely say that there's some purple in the middle. Even this analogy is flawed in that it is too abrupt in the color changes compared to how evolution changes.

In closing, I'll leave you with a quick fact about speciation: many, many folk like defining new species, but there are those that also argue that some of the "species" defined are best removed from the hominid continuum. Particularly in the Homo lines.

I hope that answered it. Or, if not, gave you more to question! :)


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:41 am

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Thanks for that. It is much clearer now.

Thank you too, iNow.


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:46 am

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I often wonder . . . If a paleoanthropologist would look at hundreds of 1-year old skeletons from all over the Earth, he/she would see so much variety. Some over 7 ft tall. Some under 4 ft tall. Some heavy boned, some light boned. Large skulls, small skulls. Long arms, short arms. Prominent frontal lobes on some skulls. How would he/she ever label all these skeletons as Homo sapiens?


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paleoichneum
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 4:51 am
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Snafuperman wrote:
I often wonder . . . If a paleoanthropologist would look at hundreds of 1-year old skeletons from all over the Earth, he/she would see so much variety. Some over 7 ft tall. Some under 4 ft tall. Some heavy boned, some light boned. Large skulls, small skulls. Long arms, short arms. Prominent frontal lobes on some skulls. How would he/she ever label all these skeletons as Homo sapiens?

By looking at the gradation between the phenotypes. There are't blocks of only tall separated from blocks of only small separated from blocks of only fine boned etc... Looking at hundreds of adult skeletons shows a gradation between all the phenotypes, with all the derived features of the species morphology present.

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SkinWalker
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 5:00 am
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While there is incredible diversity, there are are also key characters for individual species.

For instance, while there are degrees of prognathism (the extent to which a person's jaw (maxilla and mandible) protrude from the rest of the skull), there are also characteristics that are unique to H. sapians: position of the foramen magnum, dimensions of the skull, cranial capacity, dentition, and so on. There might be characters that are on the edge of the range expected with H. sapiens, but there will be others that are well within the range. There are always exceptions as well. Which is why there is considerable debate among paleoanthropologists when it comes to speciation.

I recommend becominghuman.org, which i recently discovered. I found it to be an interesting and well put-together site with info on the "human career."


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:02 pm

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Since it has been found that some people today (those of European and West Asian ancestry) have a small percentage of Neanderthal DNA, it has been surmised that sapiens and neanderthalensis interbred. Will the Neanderthal classification of a subspecies of sapiens be accepted? Homo sapiens neanderthalensis vs Homo neanderthalensis?


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paleoichneum
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:26 pm
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I all depends on if your talking to a taxonomic lumper or taxonomic splitter to be honest.

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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:33 pm

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I thought that if one animal can mate with another animal and have viable offspring, then those two animals are the same species. Is that correct or too simplistic?


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:33 pm

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SkinWalker wrote:
While there is incredible diversity, there are are also key characters for individual species.

For instance, while there are degrees of prognathism (the extent to which a person's jaw (maxilla and mandible) protrude from the rest of the skull), there are also characteristics that are unique to H. sapians: position of the foramen magnum, dimensions of the skull, cranial capacity, dentition, and so on. There might be characters that are on the edge of the range expected with H. sapiens, but there will be others that are well within the range. There are always exceptions as well. Which is why there is considerable debate among paleoanthropologists when it comes to speciation.

I recommend becominghuman.org, which i recently discovered. I found it to be an interesting and well put-together site with info on the "human career."

Thanks for the info and link.


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:34 pm

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paleoichneum wrote:
Snafuperman wrote:
I often wonder . . . If a paleoanthropologist would look at hundreds of 1-year old skeletons from all over the Earth, he/she would see so much variety. Some over 7 ft tall. Some under 4 ft tall. Some heavy boned, some light boned. Large skulls, small skulls. Long arms, short arms. Prominent frontal lobes on some skulls. How would he/she ever label all these skeletons as Homo sapiens?

By looking at the gradation between the phenotypes. There are't blocks of only tall separated from blocks of only small separated from blocks of only fine boned etc... Looking at hundreds of adult skeletons shows a gradation between all the phenotypes, with all the derived features of the species morphology present.

Thank you.


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paleoichneum
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 2:54 pm
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Snafuperman wrote:
I thought that if one animal can mate with another animal and have viable offspring, then those two animals are the same species. Is that correct or too simplistic?

Unfortunately is is indeed to simplistic. Technically great Danes and chihuahua s can produce viable offspring, but they are highly unlikely too.

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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 3:48 pm

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paleoichneum wrote:
Snafuperman wrote:
I thought that if one animal can mate with another animal and have viable offspring, then those two animals are the same species. Is that correct or too simplistic?

Unfortunately is is indeed to simplistic. Technically great Danes and chihuahua s can produce viable offspring, but they are highly unlikely too.

Yes, but aren't all dogs the same species, Canis familiaris, with different varieties or breeds. This is why they can reproduce. They may not want to or even have any desire to.


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Snafuperman
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 4:16 pm

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Can all species of the same genus interbreed? I read that if the mating is of two distinct species then the offspring will be sterile, but is this always the case?


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marnixR
Post  Post subject: Re: Common ancestor . . . why no examples?  |  Posted: Mon Sep 28, 2015 4:28 pm
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most of the time that's the case, either sterile, or in some fashion less successful - there are rare exceptions (i seem to remember an instance of Darwin's finches where a hybrid proved to do better than either of the parent species), but as i said you don't see them quite often

sometimes species can interbreed but just don't because their recognition signals are out of synch, or they frequent different host plants, or because their breeding seasons don't overlap

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