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View Diary: "Y-Chromosomal Adam" possibly farther back in time than thought (237 comments)

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  •  Lucy's chromosome number (3+ / 0-)
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    milton333, Gordon20024, raincrow

    You may recall that the Houston Museum of Natural Science carried the Lucy exhibit a few years ago. I called up the museum and asked whether it was feasible to determine Lucy's chromosome number. The answer I got was that the bones were too mineralized.
    You see, H. sapiens has 46 chromosomes. Chimpanzee, gorilla, and orangutan have 48. There is clear evidence that 2 chromosome pairs at some time fused to become our chromosome pair # 2. It was first inferred from chromosome band markings, then confirmed from DNA sequencing.
    There is no direct evidence on when this chromosome fusion happened. It may have been long before H. sapiens or H. neanderthalensis originated.

    Censorship is rogue government.

    by scott5js on Tue Mar 12, 2013 at 10:00:01 PM PDT

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    •  Fused??? (1+ / 0-)
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      My, admittedly quite limited, understanding of what happens to chromosomes within the cell makes splitting much likelier than fusing.

      How do we know that the 48-chromosome line is older than the 46-chromosome line? We're 46, but why are we certain that the mutual ancestors were 48?

      •  One test (0+ / 0-)

        Are chimpanzee, gorilla, and orangutan DNA alike at the boundaries of the chromosomes in question? If not you would have a point.
        I do not know, but I suspect the question has been investigated. I believe complete genome sequences have been found for chimpanzee, gorilla, and orangutan.
        H. neanderthalensis could be investigated - its mitochondrial genome has been - but I would be surprised if it did not have 46 chromosomes.
        How about the splitting of the genus Homo from Australopithecus? In that case Lucy would likely have 48.

        Censorship is rogue government.

        by scott5js on Wed Mar 13, 2013 at 11:37:02 AM PDT

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      •  Wikipedia article ... (0+ / 0-)

        on chromosome 2 (human). It gives further justification for thinking there was a chromosome fusion rather than a splitting.

        Censorship is rogue government.

        by scott5js on Wed Mar 13, 2013 at 01:28:24 PM PDT

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      •  Fusions are quite common actually (1+ / 0-)
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        Evolutionary biologists look at something called synteny to infer where chromosomes came from.  A simple example:

        Chromosome 1 from humans has genes A, B, C
        Chromosome 1 from chimps is also A, B, C
        this would suggest these two chromosomes have not undergone any large change since our last common ancestor

        Chromsome 2 from humans has genes D, E, F, G, H
        Chromosome 2 from chimps has D, E, F
        Chromosome  3 from chimps has G, H

        this would suggest that either our last common ancestor had 1 chromosome that split (we were ancestral) our 2 chromosomes that fused (chimps were ancestral).  The reason its believed to be a fusion is that our chromosome 2 has remnants of a second centromere (should only be 1 per chromosomes), as well as remnants of telomere repeats (which normally are on the ends of chromosomes).  

        These large changes of structure happen a lot over evolutionary time, and are called gross chromosomal rearrangements (GCRs).  

        PZ Myers at Pharyngula has a really nice discussion on synteny, and has a picture at the bottom of the post of how human chromosomes match to puffer fish; large blocks of the puffer fish chromosomes are still intact, but in human chromosomes, they've been scattered all over.

    •  Horse and donkey (0+ / 0-)

      We all know that these 2 species interbreed but mules are usually sterile. The 2 parent species differ by one chromosome pair. A mule has an odd chromosome number and that is a big problem for its reproducing.
      btw the gender of the parents makes a difference. The opposite of a mule is called a hinny. A mule concentrates the more useful traits of both species and a hinny concentrates the less useful ones.
      Now back to our line. One reason to suspect the Australopithecus/Homo juncture as the point of chromosome fusion is that some strong-survival traits would greatly help to compensate for a change of chromosome number in a new species.

      Censorship is rogue government.

      by scott5js on Wed Mar 13, 2013 at 11:59:18 AM PDT

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