History Forum / General / Ancient History / July 2009

The following page lists the mutation rates for various short tandem
repeats (STRs) in the Y chromosome. These mutations are used to track
the movement of populations over time.

http://freepages.genealogy.rootsweb.ancestry.com/~geneticgenealogy/ratestuff.htm

(It turns out, from very recent studies, that the very slow-mutating
STRs vary in their mutation rate over time, so this table is not 100%
correct, but it's good enough for the timescales we're concerned with.
Those changes are only important to those studying humanity back tens
or hundreds of thousands of years.)

The way it works is simple enough. There is an equal (roughly)
probability of an STR lengthening or shortening, so the standard modal
type (the mode of the frequency of the various STRs across the whole
of a population) will - for a statistically large enough population -
be identical to the founder's STR's. (There will only ever be one
founder for a given SMT, as the Y chromosome is inherited along the
male line only.)

A given STR is known as a genetic marker and the number of times it
occurs is the value of that marker.

Because values can both increase and decrease, it's not just a simple
matter of looking at a handful of markers to see where someone fits
in. Even the broadest classifications usually require about 12
markers, and generally nobody trusts a result under 37 markers, and
most serious investigators go into the 67+ range.

However, STRs aren't the only thing people check these days. It's long-
since evolved into hunts for single-nucleotide polymorphisms (SNPs).

http://en.wikipedia.org/wiki/Single-nucleotide_polymorphism

Again, these are inherited along the male line, and as with STRs, they
change very slowly. If a person has an identified grouping of STRs and
SNPs, there is no serious question of them having a direct common
ancestor along the paternal line with every single person with those
same STRs and SNPs. By looking at the mutation rates of the various
STRs, and by examining when specific SNPs appear, it is possible to
build a very sparse but entirely sound family tree of haplogroups. An
example for the R haplogroups can be found here:

http://www.isogg.org/tree/ISOGG_HapgrpR09.html

You can then calculate the time to the most recent common ancestor of
any two people with the same haplogroup, by looking at the precise
differences in the more rapidly-changing markers and the probability
of those changes happening. What you end up with is a probability
curve that obviously can't extend beyond the age of that given
haplogroup. This curve tells you the probability that this most recent
common ancestor appeared more recently than a given amount of time
back. You can further refine the curve by taking into account what you
know about your family tree and therefore what you can exclude as
possibilities.

A similar technique exists for mitochondrial DNA, which is only
inherited on the female line, but the haplogroups for that tend to be
much more ancient. This makes it great for validating where ancient
populations as a whole were at a given time, but it's less useful for
genealogical work at this time.

(Although there has been some good work done since on non-European
populations, a good introduction is Syke's book "Seven Daughters of
Eve", named for the seven haplogroups he identified as stemming from
Mitochondrial Eve, the earliest woman we can uniquely identify through
mtDNA.)

Now, with respect to the people of the near East, the page of interest
is this one:

http://www.isogg.org/tree/ISOGG_HapgrpJ09.html

Now, remember boys and girls, displacement from the modal type means
you are no longer of the modal type, so nobody who belongs to ANY
haplogroup for which there has been a mutation on ANY marker out of
those used for the Cohen Modal Type =OR= carries an SNP belonging
specifically to a haplogroup whose modal type is NOT of the Cohen
Modal Type is Cohen Modal Type.

As the CMT occurs in both J1 and J2 (but not J), there is a precursor
haplogroup currently unidentified that is common to J1 and J2 and
descended from J that this modal type appears in. The majority of
descendent haplogroups will have mutated off the modal type. Since J1
and J2 spawn off it, and there are no other J lines, there's clearly a
genetic bottleneck at the time the CMT appears, at least within the J
haplogroup, and may extend back indefinitely into J but no further
back than that. There is nothing significant in any other haplogroup
at the time, so no planetary disaster need be considered, but clearly
there's a regional problem and it only affects these people, not other
populations in the region at the time.

I will leave it to others to bicker and squabble over how to interpret
the data, though doubtless those most inclined to bicker and squabble
will neither reference the data nor make use of it. Doubtless, too,
we'll see repeated arguments over what "modal" means, who knew what
when, and who defined what how, but that sort of trash is best left to
the experts on trash. Those interested in the science can reach their
own conclusions, WHETHER OR NOT THOSE AGREE WITH ANYONE ELSE'S VIEW.
There is no requirement to adhere to a specific viewpoint in science,
merely an obligation to use the best data available and a gentlemen's
agreement to respect when that data contradicts a cherished belief.

How does this impact history, which is written, not bred? Well, it's
quite fascinating. It turns out that technology and culture flow quite
independently of populations. Since culture includes traditions and
socially-maintained belief systems, and technology leaves the
artefacts, this can seriously screw with written histories and
archaeology in unpredictable ways. As a result, genetic archaeology
(studying the genetic markers of the DNA found in human bones and hair
from archaeological sites) is proving important in unscrambling the
omelette.