berangere: (anthropo fun)
[personal profile] berangere posting in [community profile] archaeology
This post belongs to the Frequently-Or-Not-So-Frequently-Asked-Questions in Archaeology project.

prompt by [personal profile] yvi

I would like to know what modern scientific methods (especially to do with molecular Biology, but feel free to mention others) are used in archeology and for what.


  I will deal now with molecular biology, and will list the others sciences I can think of in another post (that will probably be dating-methods heavy). First, I'd like to make clear that I had to look for the exact definition of 「Molecular Biology」 in wikipedia. I felt that the limits of this science are quite blurry so I am not sure everything that will be dealt with in this article really relates to molecular biology. Let's say I tried to focus on anything related to genes, cells or proteins. Therefore, it is essentially linked to human remains, that are not found in every archaeological excavation. I suppose the techniques can be applied to animal remains either, according to the problematic of the excavation or the study.
  I suppose [personal profile] yvi  is quite familiar with molecular biology, hence the question, but I tried to include really basic explanations for the article to be understandable to people with no scientific background. Note that I am not a biologist and that my explanations may not be particularly good.


    1- population genetics

  Most of the studies of population genetics in an archaeological context have a link with migration of past human groups. But it is interesting to note that those studies are generally made with present-day people's ADN. As the techniques evolve, it has been more common recently to see studies based on fossil ADN either, but this has still to be developed.
  First note there is two type of ADN : mitochondrial ADN that is inside the mitochondrias in the cell, and nuclear ADN that is in the nucleus of the cell (the 23 pairs of chromosomes you see everywhere are nuclear ADN). When a foetus is created, the ovocite of the female parent is used as a base for creating the new cell. In this ovocite, there is mitochondrias from the female parent, but no mitochondrias from the male parent.

  The studies are generally of two types, that may be combined :

- the ones on the mitochondrial ADN (ADNmt) that is inherited from the mother, and the mother only. From this ADNmt you can reconstruct maternal lineage only : no information about paternal lineage can be retrieved.

- the ones on the nuclear ADN, that are generally made on the Y chromosome, hence the reconstruction of paternal lineage only.

  The choice of mitochondrial ADN and Y chromosome is explained by the fact the mutations that occur in them are *real* mutations.
In a cell, there are 23 pairs of chromosome. In those pairs, one is inherited from the mother, the other from the father. During the cell division, the chromosome of each pair come so close to each other that they can exchange parts of them : this is a recombination, it counts as mutation, but does not really alter the genes since the gene is still present, just not on the same chromosome as before.
Then, there are real mutations, that are not exchanges with another chromosome, but real modifications of the ADN.
ADNmt do not pair with anything, so, only the latter mutation can occur.
In the nucleus, the Y chromosome being unique (the other part of the pair being a X chromosome), the recombinations are not possible either.

  Studies are conducted on the similarity rates of different populations to determine which ones are the closest. Globally : you take a gene or a segment of ADN that is known to have different versions and you look at the rates of each version in each population (population 1 have 80% of people with the A version, 5% of people with the B version, 6% of people with the C... and so on). The results are generally displayed in phylogenetic trees.

  It is important to note that ADN studies only deal with the mutations and genes that survived till now, since it uses present-day ADN. It means there is a huge drift in the data involved. Those studies also often consider that mutation rate is stable over time to propose a date when two populations got separated, but we know that this rate is not that stable.

  As far as I'm concerned, I dealt with population genetics linked to the origin of modern Japanese. The studies are numerous and concern present-day ADN as well as fossil ADN but it seems none agrees with the others, so I must say I am perplex about the use of this science to solve this type of archaeological problematic.


    2- family lineage in collective burials or cemetery

  Collective burials are places where the bones of different persons are put, in a successive way (when in a contemporaneous way, it is a multiple burial). This type of burial was really fashionable at the end of the Neolithic in France and we have dolmens filled with the bones of hundreds of individuals.
  Those studies are conducted on fossil ADN, and really often concern ADMmt (maybe for reasons of better conservation ?). In some burials, it has been proved that in the apparent capharnaum of bones, family regroupments were observed.
  The same type of studies can be conducted at the scale of a cemetery, to see if people of the same family were buried in the same corner of the cemetery.


    3- sexing a skeleton

  As it has been discussed in two previous articles (1 ; 2) by [personal profile] marshtide  and [personal profile] ossamenta , ADN analysis can be used to determine the sex of a skeleton. I must say that even if I read about this test being performed in a few articles, I never myself encountered a case when it's been used (neither have any of my archaeologist friends ever used this technique)


    4- lipid dosage

  OK, I clearly think this is not molecular biology. It has more to do with chemistry. Anyway, this is something I encounter frequently since I study a part of the world where the acid soil do not allow the conservation of bones. To know if a pit was a burial, Japanese archaeologists sometimes dose the quantity of lipids contained in the pit earth. A high concentration of lipid indicates the presence of a corpse (I am not sure how they determine it is human though).


    5- diets
 
  This also is more chemistry than molecular biology... or so do I think.
  It is possible, analyzing the composition of the bones (expecially carbon and nitrogen atoms) to determine the type of diet the person had when living. I mean : primarily relying on meat, or fish, or vegetable. It is even possible to determine if it was freshwater or saltwater fishes, and wild ot domesticated vegetables.
  I have in mind a study (still in Japan, sorry), where this type of analysis was combined with spatial study in the cemetery, grave goods distribution, as well as with sex repartition, to determine there was two type of populations in one village : those who ate primarily meat and those who ate primarily fish. A fact that was probably linked to the specialization of the persons in hunting or fishing.


  Those are the only application of molecular biology I can think of. There may be others that I just forgot. I really hope I got the definition of molecular biology correct.
  Most of those techniques are quite expensive, therefore not systematically used in a domain like archaeology where the lack of funds is so common it has become a feature we are trained to deal with from the beginning of our formation.

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