[escepticos] RV: NEW HUMAN EVOLUTION FIND

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-----Original Message-----
De: Skeptic Mag Hotline <skeptic-admin en lyris.net>
Para: Skeptics Society <skeptics en lyris.net>
Fecha: miércoles 17 de marzo de 1999 1:57
Asunto: NEW HUMAN EVOLUTION FIND


>IMPORTANT NEW HUMAN EVOLUTION FIND
>
>If you haven't seen this yet it is an extremely important discovery about
>human evolution that will (once again) reshuffle the deck on such matters
as
>when humans left Africa, when the major racial groups developed,
within-group
>v. between-group population differences, and the relationship of genetics
to
>the fossil evidence. For awhile now paleoanthropologists have been leaning
>toward a recent split between non-African and African populations, but now
it
>appears the split is considerably older, which would imply that racial
group
>differences are not as recent as we thought, that there is greater
diversity
>with populations than between populations (with implications for THE BELL
>CURVE arguments of racial differences in IQ and other traits), but that
this
>new genetic evidence needs to be tempered with fossil and archaeological
>evidence.
>
>And so it goes in science . . . provisional and always changing. What fun!
>
>Michael Shermer
>
>Study Alters Time Line for the Splitting of Human Populations
>1999 March 16, New York Times
>By NICHOLAS WADE
>
>The ancestors of Africans and non-Africans split apart
>almost 200,000 years ago, much earlier than previously
>thought, a study of DNA in living populations has found.
>
>The estimate is so early that it precedes by some 70,000
>years the first known fossils of modern humans.
>
>The authors of the study say they cannot tell in which
>branch of the split population the genes that shape modern
>humans first developed, but they believe the genes spread
>quickly throughout the other branch because of the
>advantage they conferred.
>
>The study, published in Proceedings of the National Academy
>of Sciences, also reports the first fixed DNA difference
>between African and non-African populations. The
>difference, which may be rendered less absolute when more
>people are tested, is in a single chemical unit of DNA that
>has no functional significance. Several geneticists said
>the finding did not alter the well-established fact that
>different racial groups differ very little at the genetic
>level.
>
>The new finding, if sustained, is surprising in the light
>of two widely held assumptions about human origins. One is
>the view that modern humans evolved in sub-Saharan Africa
>and, in a later split, a small band then emigrated to
>populate the rest of the world.
>
>The other is the expectation that only a handful of minor
>genes underlying superficial characteristics like hair and
>skin color would show sharp differences between major
>population groups. The gene under study is part of an
>enzyme, known as pyruvate dehydrogenase, which controls an
>important step in glucose metabolism.
>
>The report, by Dr. Eugene E. Harris and Dr. Jody Hey of
>Rutgers University in New Jersey, suggests that the archaic
>human population split first, probably in Africa, and that
>the modernity typical of people today developed later.
>
>Dr. Hey said that he and Dr. Harris selected the pyruvate
>gene purely for reasons of convenience: it lies on the X
>chromosome, and by studying the gene in men, who have only
>one copy of the X chromosome, they had to determine the
>sequence of DNA units only once in each individual.
>
>They sequenced a 4,200-unit-long segment of DNA from the
>pyruvate gene in 16 Africans, 19 non-Africans and 2 male
>chimpanzees. The forebears of chimps and of humans are
>believed to have split into separate species some five
>million years ago, which yielded a time scale for measuring
>the rate of mutation in the DNA segment. Though the number
>of DNA samples was small, the researchers said their
>statistical analysis produced a meaningful result.
>
>The Rutgers biologists noted 25 positions in the DNA
>segment at which their human subjects tended to have
>different chemical letters from the chimp sequence. The
>differences are caused by successive changes or mutations
>in units of DNA, allowing a family tree of the changes to
>be constructed.
>
>Assuming the mutations occur at regular intervals, like a
>steadily ticking molecular clock, the Rutgers biologists
>concluded that the ancestral hominid sequence is 1.86
>million years old. This is about the time of an archaic
>human species known as Homo habilis, and confirms the idea
>that much of the genetic variation in living populations is
>very old.
>
>Much later, at about 189,000 years ago, the researchers
>said, the gene tree split into African and non-African
>branches, a point marked by a new mutation found only in
>the non-African subjects. At a place in the gene where
>Africans have the chemical sequence GCG, non-Africans have
>GCA, the letters standing for different units in DNA. In
>the genetic code, both GCG and GCA specify the same amino
>acid unit of a protein, so the difference has no practical
>effect.
>
>However, the date it implies for an African/non-African
>split is substantially earlier than estimates so far
>derived from other parts of the human genome, which range
>from 100,000 to 156,000 years ago.
>
>Dr. Hey said these estimates did not take into account, as
>his does, the subsequent flow of genes between the separate
>branches, an omission which lowers the apparent splitting
>date.
>
>The Rutgers biologists cannot say from their study where
>the split took place, though presumably it was somewhere in
>Africa.
>
>But from the amount of variability seen in their DNA
>sequences, they can estimate the effective size of the
>ancestral human population, which they put at a mere 18,000
>people. Estimates from other genes range from 11,000 to
>18,000, numbers thought not to have been greatly exceeded
>until a population expansion began some 50,000 years ago.
>
>Dr. Hey said that 18,000 people, even hunter-gatherers, did
>not take up a lot of room, and that the African/non-African
>population split could still have left the two groups close
>to each other, even if geographically separated.
>
>When the gene changes that led to modernity evolved, they
>would have reached and quickly spread through the other
>because of the powerful selective advantage they conferred.
>
>Dr. Hey said his genetic data gave no clue on whether the
>genes for modernity first evolved in the African or
>non-African sub-population. Given that the transitional
>fossils and many of the earliest modern fossils appear in
>Africa, the African population seemed to him the likelier
>candidate, he said.
>
>Defenders of the established view that modern humans arose
>from archaic hominids in sub-Saharan Africa may wait for
>confirming evidence before changing their minds. In a
>commentary on the new report, Dr. Rosalind M. Harding, a
>geneticist at the John Radcliffe Hospital in Oxford, writes
>that true dates could be twice or half the estimates
>derived at present from genetic data, such is the range of
>uncertainty in genetic dating methods.
>
>Population genetics is a rigorous science but is
>statistical in nature and depends on several general
>assumptions that may not always perfectly mirror actual
>events. Those who study human origins from the fossil
>evidence believe their dates, usually based on radiocarbon,
>are more firmly grounded than the geneticists' molecular
>clocks. Richard Klein, an archeologist at Stanford
>University, said he gave even odds on whether the
>African/non-African split occurred before human modernity,
>as the new study says, or after.
>
>"In the end the genetics will have to accommodate the
>fossil and archeological records, not the reverse," he
>said.
>
>Dr. Ofer Bar-Yosef, an archeologist at Harvard University,
>said the archeological record supported the geneticists'
>calculation that the early human population was very small.
>But the archeological evidence is silent, he said, on
>whether there was a population split 200,000 years ago.
>
>But Dr. Henry C. Harpending, a geneticist at the University
>of Utah, said he too had seen signs, from study of a
>different genetic region, of splits in the pre-modern human
>population. "I like the idea," he said.
>
>The second surprise in the Rutgers report relates to the
>well-established finding that humans as a species show very
>little genetic difference between their various
>subpopulations. All people have exactly the same set of
>genes, as far as is known, but the genes come in slightly
>different flavors, embodied by minor variations in the DNA
>sequence.
>
>Overall, there is much more variation among people within a
>subpopulation than between populations.
>
>The gene fragment studied by Dr. Hey and Dr. Harris is
>unusual in that the versions of the gene found in Africans
>and non-Africans are quite different. Dr. Hey said he
>viewed the gene as an anomaly and that "our data cannot be
>interpreted as supporting the idea of very distinct races
>or populations."
>
>"Human biologists have always known that would be some
>genes out there that would show this pattern, such as those
>for hair color, so finding this gene doesn't change the
>overall story that most genes do not show this kind of
>pattern," he said.
>
>The pyruvate gene has no obvious correlation with any known
>difference among major population groups.
>
>"It is surprising to find this all-or-none difference when
>previously we would have expected just frequency
>differences," said Dr. James F. Crow, a geneticist at the
>University of Wisconsin. He described the pyruvate gene
>finding as a "striking exception" to the rule that
>different versions of a gene tend to be present to varying
>extents in all populations.
>
>Dr. Hey believes that Africans and non-Africans may have
>distinctly different versions of the gene because of a
>"genetic sweep" that erased much of the variability in the
>non-African version of the gene. A genetic sweep occurs
>when some gene or element of a gene confers so large a
>survival advantage that a wide region of DNA on either side
>of it becomes embedded in the whole population.
>
>Since none of the mutations found by the Rutgers group make
>any practical difference, Dr. Hey believes the point of
>selective advantage may lie in another part of the gene or
>even in a neighboring gene.
>
>The selective advantage, whatever it was, may have
>conferred an edge against some threat -- perhaps a disease
>or climatic change -- confronted only by non-Africans. "It
>was some kind of environmental difference but I don't have
>a clue as to what," Dr. Hey said.
>
>--------------------------------
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