On Mon, 10 Feb 1997 20:17:54 -0500 (EST), Gordon Simons wrote:
GS>Several months ago I asked this forum for comments on the extent
>of evidence that human mitochondrial DNA is solely passed along
>maternal lines. As I recall, Terry Gray responded with some positive
>evidence based on mouse, or rat, studies.
>GS>Since then, my cousin's wife, Friderun Ankel-Simons, has written a
>very interested paper directly concerned with this subject, jointly
>with Jim M. Cummins. My cousin tells me that the article has
>received a tremendous number of requests for reprints. I will simply
>quote the title and abstract, which should explain the excitement.
>Those interested can locate the article in the Proceeding of the
>National Academy of Science, USA, Nov. 1996, pp 13859-13863.
It should be pointed out to newer Reflectorites that Gordie's
"cousin's wife, Friderun Ankel-Simons" is the wife of noted
anthopologist E.L. Simons, whose co-discovery of Ramapithecus was
adversely affected as a human ancestor by the use molecular clock
data:
"The debate raged for more than a decade, during which time more and
more molecular evidence was produced by Wilson and Sarich and also
independently by other researchers. The great majority of these new
data supported Wilson and Sarich's original contention. The weight
of this evidence began to shift anthropological opinion, but the
change was slow. Finally, in the early 1980s, discoveries of much
more complete specimens of Ramapithecus-like fossils, by Pilbeam and
his team in Pakistan and by Peter Andrews, of London's Natural
History Museum, and his colleagues in Turkey, settled the issue (see
figure 1.1). The original Ramapithecus fossils are indeed human-
like in some ways, but the species was not human. The task of
inferring an evolutionary link based on extremely fragmentary
evidence is more difficult than most people realize, and there are
many traps for the unwary. Simons and Pilbeam had been ensnared in
one of those traps: anatomical similarity does not unequivocally
imply evolutionary relatedness. The more complete specimens from
Pakistan and Turkey revealed that the putative humanlike features
were superficial. The jaw of Ramapithecus was V- shaped. not an
arch; this and other features indicated that it was a species of
primitive ape (the jaw of modern apes is U-shaped). Ramapithecus had
lived a life in the trees, like its later relative the orangutan, and
was not a bipedal ape, still less a primitive hunter-gatherer. Even
diehard Ramapithecus-as- hominid anthropologists were persuaded by
the new evidence that they had been wrong and Wilson and Sarich had
been right: the first species of bipedal ape, the founding member of
the human family, had evolved relatively recently and not in the deep
past." (Leakey R., "The Origin of Humankind", Phoenix: London,
1994, pp7-8)
This does not mean that Friderun Ankel-Simons is wrong (it is
always possible that her `vested interest' gives her a insight into
the truth that others might miss) but it does mean that it should be
noted.
>GS>"Misconceptions about mitochondria and mammalian fertilization:
>Implications for theories on human evolution"
>
>>ABSTRACT
>
>>In vertebrates, inheritance of mitochodria is thought to be
>predominantly maternal, and mitochondrial DNA analysis has become a
>standard taxonomic tool. In accordance with the prevailing view of
>strict maternal inheritance, many sources assert that during
>fertilization, the sperm tail, with its mitochondria, gets excluded
>from the embryo. This is incorrect. In the majority of mammals --
>including humans -- the midpiece mitochondria can be identified in
>the embryo even thought their ultimate fate is unknown. The "missing
>mitochondria" story seems to have survived -- and proliferated --
>unchallenged in a time of contention between hypotheses of human
>origins, because it supports the "African Eve" model of recent
>radiation of Homo sapiens out of Africa. We will discuss the
>infiltration of this mistake into concepts of mitochondrial
>inheritance and human evolution.
The above abstract notes that "the midpiece mitochondria can be
identified in the embryo even thought their ultimate fate is
unknown". As I understand it, it is acknowledged that some male
mitochondria might get into the egg:
"A few of the mitochondria, too, can escape the doom awaiting the
great majority of their companions by being passed on to the next
generation, carrying with them their little loops of DNA. The
mitochondria, however, are confined to the cytoplasm of the cells,
not the nuclei. Only if a little of this cytoplasm is passed on to the
next generation can a few of the mitochondria be passed on as well.
Here is where the unusual nature of mitochondrial inheritance
becomes obvious. The egg passes cytoplasm filled with mitochondria
to the next generation, while the sperm does not. Sperm start out as
normal cells, but they quickly lose most of their. cytoplasm and shrink
down to become little more than a package of chromosomes with a
frantically waving tail attached. Most of the mitochondria are lost as
well, and the few that are left are located at the base of the sperm's
tail where they supply the ATP needed to drive the sperm in its
single-minded quest for the egg. Eggs are huge balloonlike cells, with
a voluminous cytoplasm richly endowed with all the materials needed
to start the embryos development after fertilization. When sperm and
egg fuse, the ordinary nuclear chromosomes carried by the sperm are
passed safely to the egg. But the shreds of mitochondria in the sperm
are usually left outside the egg, along with their tiny mitochondrial
chromosomes Almost always only the plentiful mitochondria of the
egg, carrying their little chromosomes with them, are passed on to the
child. If the child is a girl and has children of her own, her
mitochondrial chromosomes will be passed on-at least to the
following generation. If the child is a boy, then whether or not he has
children, his mitochondrial chromosomes will be lost." (Wills C.,
"The Runaway Brain", 1994, pp22-23)
But it apparently doesn't make any difference. Even if male
mitochondria did get into the embryo of the odd individual, it is
hard to see it becoming fixed in a population, being overwhelmed by
female mitochondria in the next and succeeding generations.
>GS>A disclaimer: While I think I understand the importance of the
>issues surrounding the subject of this paper, I am not a biologist,
>and only superficially understand the biological details under
>discussion. The paper does not claim to have settled the inheritance
>issue. But it seems to have stirred up a hornet's nest.
I am not a biologist either, so I will have to await the outcome of what
the biologists decide. But I doubt if it will change the basic outline of the
Mitochondrial Eve theory.
God bless.
Steve
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