So last night I think I fell asleep before finishing the point I wanted to make: these population events lie on a continuum, with total replacement on one end and complete assimilation on the other.
The degree of leaning towards one side or the other can be thought of as more or less probable depending on a number of factors, namely:

Number of Population Difference.
We can easily imagine one (numerous) group having a much larger influence than the other as they come into local proximity due to their population advantage. This can give them power towards replacement, if that path is taken. On the extreme ends we have an individual moving into a population (assimilation or replacement both possible), and a group moving into a region with a sole inhabitant. (either total replacement or assimilation, depending)

larger population groups have more power than smaller ones, a power differential which can lead towards replacement http://www.elrst.com/wp-content/uploads/2008/05/refugees.jpg

Genetic Difference
With two contending groups the degree of genetic similarity will also affect the outcome. Differing species will have a very difficult time absorbing into each other as they are incapable of reproducing together. on the other end, two groups with nearly identical phenotypes (not necessarily genotypes but one does contribute to the other) may not have any conflict and readily absorb.

Very well known graph of neanderthal genetic differences - were they close enough genetically to assimilate? http://www.thefastertimes.com/bones/files/2009/07/genetic-differences-graph.jpg

Cultural Difference
Groups with similar cultures will tend towards assimilation whereas conflict can arise from differing cultures. These differences are readily seen in the archaeological record as differences in burial practices, ornamentation, or toolmaking techniques, though a sharper change is required to differentiate from simple evolution of technology/culture.

Misinterpreted signals can lead to conflict as different cultures interact http://www.1000ventures.com/design_elements/selfmade/ccd_example_ok-gesture.gif

 

One last note on this focus towards area-displacement; Archaeological methods can give us a continuous record in one location but time resolution across sites lies in the order of tens of years at best, so even though it can be argued that these population units may travel from area to area, and it's possible one group displaced another geographically without affecting them otherwise, we can define the events as a changing of access to locational resources, which helpfully constricts our analysis to singular locations.

I'm tired but I'll just point something out before I go to sleep.

In the same event, replacement and absorption can both be present. If we look at historical population turnovers such as the invasion of the new world by Europeans and the change in Aleutian islanders from the article I mentioned last post, we see that, since the two populations are of the same species, at least some degree of hybridization occurs. The colonization of the Americas mostly replaced the indigenous population, as native American population numbers from before until today will attest, but they were not eradicated and still form a part of the population.

As for prehistoric cases, such as Homo Erectus in asia, it seems the natives were exterminated, as there are no further living H. Erectus today, and they haven't contributed to the modern DNA pool. Since H. Sapiens and H. Erectus are different species and have noticeably differrent morphology, it isn't a far leap to imagine the two groups regarding each other as sub-human (or sub-erectus in our case) and causing a complete replacement to occur. How can hybridization even happen if they are different species?

I don't think a complete replacement event can occur anymore, at least on a planetary scale, due to modern technology making the effective distance between any two points on the earth to a human much much smaller. Whereas before ~2kya a human living in Africa, for example, would never have been able to find himself among an Alaskan village in his lifetime. Today, there is continual mixing of population from any point on the globe to another, so I think that differentiation of humanity into separate species is highly unlikely. The isolation required for specialization is just not there. If humanity colonizes planets light years away perhaps enough isolation will be present for the different lineages to diverge and eventually become separate species. Until then, barring a drastic technological reduction, complete replacement seems impossible today because the two populations involved will be of the same species and have a degree of mixing. (even in something barbaric like nuclear extermination, there will have been some of the victim population who were traveling, even some living in the aggressor's area)

time to sleep~

first post!

I haven't written a blog before, so this should be interesting. Let's hope this one isn't as self-absorbed as the blog concept would imply (:

I'll start with the class. Anthro 128 is an anthropology class regarding human origins and prehistory, but it is saturated with archaeology. Indeed, pre-history -by definition- leaves us with no first-hand accounts or witnesses aside from physical evidence. That's not to say other disciplines don't provide other pieces of raw clay which we can weave into an elegant tapestry describing human history.

Recently, genetics have come into play to investigate the divergence of modern human location-groups from a common ancestor, and the contributions of particular remains to the current species (given that the DNA is salvageable) This study  is prototypical of such investigations of archaeological DNA extraction, a cursory glance (search ancient DNA)will find many such studies, here's some examples. Incidentally, that last link also describes some problems with this method. DNA from researchers can contaminate the results, because tiny samples are vastly multiplied, which also multiplies error. Structural mechanisms which favor the decay of certain aspects of DNA can also bias what remains of ancient DNA.


The other genetic approach is to analyze the degree of similarity between modern human groups to estimate how 'distant' they are from in each other, if we assume mutations occur at a fairly common rate. This has the advantage of using very available DNA from living humans, as opposed to rare and chance-based ancient DNA extraction, as well as the advantage of DNA not selected by the decay profile of the specimen.

If we look at an event which was long enough ago to not be patently obvious, but which happened recent enough to provide a large amount of evidence, we can use both approaches. An example of this is a study which looks at Aleutian populations ~1-2kya and thus has much surviving DNA evidence. In this manner, or even without genetics, (for instance observing how modern-day hunter gatherers live in their environment) we can extrapolate backwards to understand what we find.

source: xkcd.com

The problem with extrapolation, of course, is the old cliché of "past performance is not an indicator of future returns", or, in this case, "past performance is not an indicator of even further past performance which was so far ago we're not sure what really happened." Regardless, some information is usually better than none at all, and the models created are often tested and either confirmed or refined by newer research.