More Bioarchaeology: DNA for Relatedness and Migration
Introduction to Archaeology F 2002 / Owen: More bioarchaeology p. 1
Introduction to Archaeology: Class 15
More bioarchaeology: DNA for relatedness and migration
Copyright Bruce Owen 2002
Ancient DNA studies
DNA is sometimes preserved in soft tissues, hair, or well-preserved bone
but this is so mostly in relatively recent material or material from especially good preservation conditions
artificial or natural mummies
bone that was not exposed to the elements and was buried in a fairly dry or frozen context
The DNA is usually broken into short segments; the less well preserved, the shorter the pieces
longer pieces are more likely to include complete nucleotide base sequences that can be used for matching or contrasting with other samples
very small amounts can be copied or "multiplied" into usable quantities using PCR
Polymerase Chain Reaction: a test-tube method that essentially imitates the natural process of replication of DNA
but all the DNA in the test tube will get multiplied, including any from microscopic bits of skin, finger oils, saliva droplets, etc. of the lab technician, the excavator, etc.
so control of contamination is crucial
ideally, all the people ever associated with the sample provide DNA samples
and the "ancient" DNA is first checked against these to make sure that none of it comes from the modern people involved
DNA studies often use mtDNA
found in mitochondria, not nuclei of cells
there are many mitochondria per cell, vs. only one copy of each chromosome in the nucleus
so there is a lot more mtDNA to start with
this improves the odds of getting usable mtDNA from an ancient sample
inherited only from the mother
no recombination due to sexual reproduction
so offspring's DNA is theoretically identical to its mother's
this allows for reconstructing or confirming family relationships, as in Thomas's example of the bodies of Czar Nicholas II and his family
so the only changes are from the occasional error in natural DNA replication
these build up slowly over generations
so people who share a female ancestor in their maternal descent line (mother-grandmother-greatmother-etc.) have
identical mtDNA if no errors have occurred in either line since that common ancestor
almost, but not quite identical mtDNA if one or more errors have occurred since the common ancestor
the more differences in the two people's mtDNA, the more time has passed since they had a common maternal ancestor
by comparing these differences among many individuals in different populations
we can reconstruct the order of splitting of biological populations
useful for reconstructing population movements like migrations, colonization, etc.
examples
reconstructing the broad patterns of population of the globe from an apparent origin of modern Homo sapiens in Africa
mtDNA from Africans varies more than any other population around the globe
and other populations are more similar to each other than they are to African ones
this suggests that all human populations are descended from African ones
that is, humans first appeared in Africa
then different African populations that were relatively isolated from each other began to accumulate differences in their mtDNA
later, some members of these populations left Africa and colonized other parts of the world
these started off with only a subset of the variants that were already present in Africa, so the rest of the world's populations started off more homogeneous than the ones in Africa
and since these spreading populations in turn split into relatively isolated populations as they filled up the world, they are more closely related to each other than to many African populations, and have had less time to accumulate differences in their mtDNA
"African Eve" hypothesis
the claim that one woman is ancestral to all living humans on their maternal side
not that she is the only female ancestor, but just that if you look far enough back, there must be some woman who is part of every single living person's family tree (on the maternal line)
this is a statistical fluke, interesting but not really very important…
the "molecular clock"
assumes that changes in mtDNA occur at a relatively constant rate
say one error (changed nucleotide base) per so many replications or generations
so the total number of "letters" in the DNA code that differ between two individuals should be proportional to the amount of time since they had a common female ancestor
this allows us to say not only that certain populations are more or less closely related, but also to estimate how many years ago the populations split up
incidentally, the assumption of a relatively constant rate of change of mtDNA is reasonable on two grounds
first, it is supported empirically
the degree of difference has been measured between many populations and even different species
and a relatively constant rate does fit the branching pattern
second, it is reasonable from an evolutionary standpoint
most of the mtDNA does not actually do anything; it is "junk" DNA that is never involved in protein synthesis
so natural selection does not act on it
most of the mtDNA it is free to accumulate changes without any influence from the outside world
Examples in Thomas concerning the population of the New World
solid archaeological evidence of people here at Monte Verde (Chile) and Meadowcroft Rock Shelter (Pennsylvania) by 12,500 BP (10,500 BC)
likely but debated evidence back to 14,000 BP
before that, there is possible evidence but nothing certain
mtDNA to the rescue?
assuming that most New World populations descended from a single population that crossed the Bering strait
then the amount of difference between New World populations should indicate how long it has been since that founding population split up and spread in different directions across the New World
(actually, there were apparently one or two much later migrations, but these are thought to be known and their descendents are excluded from this study)
result: that founding population split up around 22,000 to 29,000 BP
of course, there are lots of methodological issues about how this number was calculated, but we can treat it as probably very roughly in the ballpark
so, where is the early archaeological evidence?
just to scarce to have been found yet?
maybe more than one group crossed the Bering strait
in that case, the time of split could be correct, but it might refer to a population splitting event that happened in northern Asia, well before anyone crossed to the New World
testing the idea of Tiwanaku colonies in the coastal valleys of southern Peru
the research problem
Tiwanaku was a major urban and ceremonial center in the altiplano around lake Titicaca
Around 700 AD, there started to be sites in the Osmore valley, far from the altiplano, that have almost exclusively Tiwanaku-style artifacts
The question is: do these sites mean that Tiwanaku people actually moved to the Osmore, or did local people start using Tiwanaku-style goods due to trade, religious proselytizing, military conquest, etc.?
that is, bioarchaeology is needed to distinguish between patterns of movements of people, and patterns of spreading of ideas and cultural traits
archaeologists are constantly accused of thinking that "pots are people"
bioarchaeology should help to separate the two
initially done using visible traits of crania
small holes (foramina) and other features that are thought to be genetically controlled
the supposed colonist's crania proved to be much more like those from one area of the Tiwanaku heartland than like the local people in the coastal valley
this suggests that the supposed colonists really were people who immigrated from the Tiwanaku area, not just local people who started using Tiwanaku-style goods
now data is being collected to do the same sort of thing using ancient mtDNA
considered more definitive than cranial traits
but this project is still underway; no answers yet…