Implications of S.tchadensis

From: Glenn Morton (glenn.morton@btinternet.com)
Date: Sat Jul 13 2002 - 18:28:57 EDT

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    The most ancient fossil hominid, announced last week, causes several changes
    in the way we view human origins and these changes will have theological
    implications. First, the issue of is it a hominid. Usually at these
    announcments some creationist organization or apologetical organization will
    claim it isn't a hominid. It is. Bipedality is a key criteria of being a
    hominid. Indeed, some call it the 'hallmark' of hominid status (Noel Boaz,
    Eco Homo, (New York: Basic Books, 1997), p. 139) From what I can see in the
    pictures, this creature does show evidence of bipedalism in the placement of
    the foramen magnum, the opening at the base of the skull where the spinal
    cord enters the brain. In chimps, who walk on all fours, the foramen magnum
    is at the back of the skull. If it were underneath the skull, the chimp
    would be looking at the ground all the time. If we had a foramen magnus
    placed in a chimp-like position, we would be looking at the zenith of the
    sky all day. Figure 1:d of the article clearly shows the foramen magnum
    underneath the skull. An associated article notes: "And the point at the
    back of skull where neck muscles attach suggests that Touma‘ walked
    upright." http://www.nature.com/nsu/020708/020708-12.html. Also supporting
    its hominid status are the shape of the foramen magnum, which is oval
    (http://www.bartleby.com/107/31.html). Chimp foramen magna are more rounded.

    The article lists all the hominid traits:
    žSahelanthropus has several derived hominid features, including small,
    apically worn caninesůwhich indicate a probable non-honing CŮP3 complexůand
    intermediate postcanine enamel thickness. Several aspects of the basicranium
    (length, horizontal orientation, anterior position of the foramen magnum)
    and face (markedly reduced subnasal prognathism with no canine diastema,
    large continuous supraorbital torus) are similar to later hominids including
    Kenyanthropus and Homo. All these anatomical features indicate that
    Sahelanthropus belongs to the hominid clade.ÓMichel Brunett, et al, žA New
    Hominid from the Upper Miocene of Chad, Central Africa,Ó Nature,
    481(2002):145-152, p. 151

    Another associated article notes:

    "Touma‘ has prominent brow ridges of the kind seen only in our own genus,
    Homo. " http://www.nature.com/nsu/020708/020708-11.html

    There is little doubt of the hominid status of this creature.

    Secondly, this skull has implications for the age of mitochondrial Eve. This
    is because the rate of mutation is calibrated by the date of the
    chimp-hominid split. Given that tchadensis moves the date of that split
    further back in time, it means that mitochondrial Eve is now older. It can
    be illustrated by the following:

    "Hasegawa and Horai (1991) used a different analytical approach to estimate
    the time of coalescence from the mitochondrial DNA sequence data. Using a
    four-million-year calibration point for the human-chimpanzee divergence,
    they estimated a divergence time of 280,000 years ago. Using their variance
    estimate in a gamma distribution, the resulting 95% confidence interval is
    191,000 to 386,000 years ago. Using a 9.2-million-year calibration point for
    the human-chimpanzee divergence, the resulting 95% confidence interval using
    the approach of Hasegawa and Horai (1991) is 439,000 to 844,000 years ago.
    Consequently, the approach of Hasegawa and Horai (1991) indicates that the
    coalescent time is even older than that implied by Tajima's equations." ~
    Alan R. Templeton, "The 'Eve" Hypothesis: A Genetic Critique and
    Reanalysis," American Anthropologist 95(1): 51-72. p. 59

    Sahelanthropus tchadensis pushes the calibration point back towards the 7-9
    million year range and that means then that the mitochondrial Eve gets
    older, meaning that she is from a time when there were NO anatomically
    modern humans on earth. Our Eve is either an archaic Homo sapiens or Homo
    Erectus.

    "Assuming a human-chimpanzee divergence at 6 my, this corresponds
    to a molecular divergence time among modern humans of 222,000
    years.
            "We also used the combined mitochondrial sequence data from
    COII and ND4-5 to estimate coalescence times for human alleles. We
    used a variety of mutation rates, which is equivalent to
    calibrating with different divergence times between humans and
    chimpanzees. Using 6 my for the human-chimpanzee divergence, the
    mean coalescence time for human alleles is 298,000 years (somewhat
    older than the date derived above using a relative branch length
    approach) with 95% coalescent error bars of 129,000 to 536,000
    years." MaryEllen Ruvolo, "A New Approach to Studying Modern Human
    Origins: Hypothesis Testing with Coalescence Time Distributions,"
    Molecular Phylogenetics and Evolution 5(1996):1, p. 202-219, p.
    208

    Using 7 million years for the split we have the error bars between 150 and
    625 kyr with the mean at 347 kyr.
    Using 9 million years for the split we have the error bars between 193 and
    804 kyr with the mean at 447 kyr.

    The oldest fossil of what is called anatomically modern man(they have
    archaic features) is found 130,000 years ago at Omo Kibish and Klasies River
    Mouth Cave (Chris Stringer and Clive Gamble, In Search of the Neanderthals,
    (New York: Thames and Hudson, 1993), p.218; Bernard G. Campbell and James D.
    Loy, Humankind Emerging (New York: HarperCollins, 1996), p.466).

    With the discovery of Sahelanthropus, Eve just got much older than that.
    Thus, Mitochondrial Eve didn't look like us and she wasn't within the past
    60,000 years.
    glenn

    see http://www.glenn.morton.btinternet.co.uk/dmd.htm
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