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07/18/2000
Roots
To my way of thinking, when it comes to determining our roots
there are three classes of researchers - the diggers, the daters and
the DNAers. The diggers are those who go out to these remote
places and painstakingly chip and brush and carefully retrieve the
remains of various species, as well as any tools or other artifacts.
In many cases, the diggers will have some ideas as to the dates of
the findings just from the location in the layers of rock laid down
over the ages or from the fact that particular objects are retrieved
from soil lying under an earlier civilization.
Today, there are other methods of dating, carbon dating being
perhaps the best known of the techniques. Other methods may
employ the amounts of certain radioactive elements or the ratios
of various isotopes of certain elements. The "daters" generally
can operate in the comfort of their laboratories by analyzing the
samples obtained by the diggers. Of course there are those who
are both daters and diggers. An example is Sue, one of my
nieces. Sue has been involved in the dating of the Shroud of
Turin but also has traveled on ships in the Southern Hemisphere
collecting samples of coral from the sea bottom.
The third group, the DNAers, uses DNA to trace the evolution
and wanderings of various species. Back in the late 1980s one
group of DNAers published a study that created quite a fuss. The
study, by Rebecca Cann and her coworkers, involved comparing
the mitochondrial DNA, mtDNA for short, in women having
ancestries that spanned the world. Their results led them to
postulate that we Homo sapiens could trace our ancestry back to
an African "Eve" who lived about 140,000 to 290,000 years ago.
Let''s digress a bit for those of us who don''t or didn''t know (that''s
me) what mtDNA is and why one might be presumptuous enough
to think we could use it to determine our ultimate grandmother.
First, what are mitochondria? And, didn''t we talk just a couple
weeks ago about DNA being in the nucleus of a cell? The
mitochondria are little structures that lie in cells outside the
nucleus, but inside the cell wall or membrane. These
mitochondria are pretty important since they handle the energy
processes going on in our cells. Mitochondria that aren''t properly
constructed can also cause various diseases. It turns out that the
mitochondria contain some DNA that lies outside the nucleus and
hence, as I understand it, is not part of the 23 chromosomes that
have been decoded in the human genome.
The history of mitochondria is rather strange, too. You''ve all
seen or heard of cases where large predatory land or sea animals
allow much smaller creatures such as birds or small fish to
perform tasks that benefit the larger animal. For example, small
birds might pick off insects or otherwise tidy up material that
annoys a rhinoceros or a crocodile. Other smaller creatures might
provide or attract food for their host. Well, current thinking is
that billions of years ago, some bacteria managed to either invade
or be engulfed by some cells. It is presumed that the bacteria
turned out to perform some kind of useful function and the
symbiotic arrangement between the cells and the bacteria
endured. The bacteria, in a modified form, are now the
mitochondria containing the DNA originating from the bacteria.
What makes the mtDNA special is that, unlike the DNA in the
nucleus, the mtDNA does not have a contribution from the father
but only the mother (however, see later). Another feature of the
mtDNA is that the DNA tends to change or mutate at a faster rate
than the nuclear DNA. It is the rate of mutation that was used in
the mtDNA work to essentially extrapolate backwards to our
ancestral "Eve". I''m not privy to exactly how this extrapolation
was done.
Now, anthropologists and archaeologists are quite a contentious
lot and immediately there was a lot of skepticism about the
mtDNA work. The criticisms ranged from concerns about such
things as the sampling and the computerized treatment of the
data. In the intervening decade or so, the controversy has not
abated and now there is a new factor that has been thrown into
the pot. There is now evidence that some paternal DNA can
enter the mitochondria, at least on rare occasions. This of course
throws a monkey wrench into the assumption of a certain
mutation rate extending back in time. At the same time, there are
those who claim not to have found any evidence of any paternal
contribution to the mtDNA in families, where you might expect
evidence of paternity entering in. Studies are underway to find
the original "Adam" by studying the Y-chromosome, which has
only contributions from the father.
While all this controversy goes on, the diggers keep digging and
the daters keep dating. I happen to be one of those individuals
funding some of these digs. Ok, I confess that it''s probably only a
few cents, if that, per year through my subscription to National
Geographic. (Do you also have the same problem we have of
hating to throw out any of decades'' worth of Geographics?) The
National Geographic Society funds many archaeological digs and
always has the greatest pictures in their articles. In the July issue,
I was taken with an article by Rick Gore entitled "People Like
Us", dealing with the dawn of the thinking and creative Homo
sapiens, us modern types.
Over the years I''ve read all kinds of articles on the evolution of
Homo sapiens. The prevailing theories of our roots and the
spread of our modern brand of primate around the world have
undergone many revisions. For the most part, the revisions have
come about thanks to the work of the diggers and daters. For
example, up until recently, the prevailing opinion was that the
settling of North and South America was accomplished by
adventurous characters from Siberia walking across the frozen
Bering Strait somewhere around 12,000 years or so ago. These
people and their heirs then were presumed to gradually penetrate
inland and migrate over generations down into South America
eventually populate the Western Hemisphere.
Now the diggers have unearthed sites that put humans in Chile
over 12,500 years ago. This led to the proposal that there was a
colonization parallel to the walkers that took place by people who
used boats to skirt and settle along the coasts of North and South
America. Furthermore, digs in the eastern United States, even
near Pittsburgh where I did my graduate work, has yielded what
could be important information. These sites show signs of a
possible European influence way before the Vikings are now
thought to have made their expeditions to North America. The
same sort of thing has happened in Brazil, where reconstructed
facial features of fossil remains don''t appear to resemble the
American Indian but may indicate an Australian connection. Thor
Heyerdahl, of Kon Tiki fame, postulated such a migration across
the South Pacific many years ago but it has apparently been pretty
much ignored as a viable hypothesis.
The above paragraph is based mostly on an article in the New
York Times Science Times of November 9, 1999. The National
Geographic article is broader in scope, trying to track down
Homo sapiens'' earliest origins. Our Western Hemisphere
colonizers were relative newcomers to the scene compared to the
original settlers of Australia, for example. Take the case of the
Mungo Lady. This poor gal died in her 20s. After her death, her
body was placed on a funeral pyre and her charred bones were
smashed before being placed in a hole in a dune field. She
remained there until discovered in 1968. Initially, her bones were
dated at 24,000 years and now her bones are in a vault at the
Mungo National Park headquarters. The Aborigines care for her
and treat Lady Mungo as a symbol of their heritage, coming out
of the ground to guide them.
It happens that the Lady is not only guiding the Aborigines but
also all of us. The daters have taken another look at her and now,
using three different methods, have come up with a revised figure
of about 62,000 years! Why the exclamation point? This puts the
Mungo Lady in Australia some 30,000 years ahead of the first
fossils of the Cro-Magnon in Europe. The Cro-Magnon, named
after the location in France where the fossils were first found,
were the first of the Homo species to exhibit modern behavior in
Europe. The Mungo Lady therefore was an affront to the
common assumption that humans spread to Europe before
spreading eastward to Asia and then South Pacific. I would be
remiss if I didn''t mention that the 62,000-year date is still
controversial in spite of the three different dating techniques.
Although everything points to Homo sapiens arising out of
Africa, there wasn''t much evidence of modern behavior there
before the evidence found in Europe, or now in Australia.
However, this changed in quite a spectacular manner when the
diggers, or perhaps I should call them spelunkers, investigated
Blombos Cave at the southern tip of South Africa. In Blombos
Cave they found one of the accepted indications of modern
human behavior, the presence of bone tools such as awls and
possibly spearheads. The daters believe this site to be over
70,000 years old. Now we''re getting there! Another telling
finding was the presence in the cave of fish bones, including some
from a sizeable fish called a black musselcracker, still in the area
today. The hypothesis is that the inhabitants of Blombos Cave
had figured out how to attract and spear the fish and that this in
turn implies an ability to share information through language. If
all this stands the scrutiny of the scientific community, "Out of
Africa" is certainly more than the title of a good book or movie,
it''s people like us!
Meanwhile, the diggers and daters and DNAers continue their
work. Someday, the three groups might come together in
harmony and there won''t be any controversy. But then that
wouldn''t be any fun, would it? Is it perverse of me to think of all
this as being like a hockey game? The only shots you see on TV
on the nightly sports segments, are the scoring of the goals by the
Gretzkys of the game. But most of the action is by the diggers at
the two ends of the rink, down and dirty, trying to get that puck
out of danger and back up the ice for the scorers to get a chance.
Allen F. Bortrum
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