Cultural Transmission, Copying Errors, and the Generation of Variation in Material Culture and the Archaeological Record. Eerkens and Lipo 2005.

In this paper, Eerkens and Lipo make a model to understand the means by which variation in material is produced. It is important to realize that here the authors are discussing variation among the same type of artifact, for example variability in thickness of the same type of ceramic pot or projectile point. They look specifically at copying errors as a source of variation, and how methods of cultural transmission either reduce, increase or maintain this variation. They have thus constructed simple models to quantify expected distribution of variation through time under different conditions. The authors claim their arguments can be extended to other aspects of human culture, although they never go into the meaning of this statement.

The authors discuss what produces variation. Although previous work has been done on the subject, it has focused on variation of types and not variation within types.

They begin by exploring cultural transmission, noting its difference with biological transmission, and thus the difficulties in tracking its complex processes.

According to the authors, variation can occur at different points in the creation of material culture in the archaeological record. There are: (1) transmission of instructions, (2) execution of instructions, (3) result of using different raw materials (Eerkens and Lipo 2005, page 319).
The authors propose a schematic for the generation of variation of material culture, which I have replicated here: (sorry about the bad reproduction, I had no choice but to up it up as an image)

The authors argue that the left and right column (stochastic and biased errors, respectively) create different kinds of variation patterns. Stochastic errors are compared to random mutations in genetic transmission, and therefore do not have a predetermined direction. This supposedly leads to smaller scale variation. On the other hand, biased errors, which are intentional, are greater and can be directional. This is because cognitive processes can “sort variation” (page 320). Thus when a hunter-gather wants to create a new form (ei: invent something new), it is not produced by random errors or from a blank slate, but with intentional modifications of a prior known (inherited) form that is believed to be an improvement.

However the authors are mostly concerned with the first column and variation as a result of stochastic copying error. They describe the Weber Fraction, a well known concept, which says due to the human threshold of perception; humans produce 3% error when copying something without being able to directly measure it. Thus we can expect a variation of 3% across a type of artifacts stemming from imperceptible human error. As the errors accumulate, they become perceptible over time, resulting in drift.

In their simulations, unbiased copying errors cause variation to be transmitted and increase over time, although the rate of increase slows down over time. They simulate biased transmission, notably conformist and prestige transmission. Conformist transmission means “individuals conform to the average value […] of the entire previous generation” (page 323). In conformist simulations, variance equilibrium is always eventually reached, although the time it takes to reach that equilibrium depends on the strength of conformity. Overall, this reduces the effect of drift. In prestigious bias, individuals are more likely to try to copy from prestigious flintknappers, which are assumed to produce variants closest to the mean. Thus, prestigious bias should reduce variance. However, prestigious individuals may not necessarily be producing traits that are closest to the means, and individuals may have to choose between multiple prestigious flintknappers, therefore drift is possible. Overall, biased transmission reduces variability caused by the accumulation of copying errors throughout time.

The authors then apply this model to two case studies. The first involves the basal width and thickness of Owens Valley projectile points. Since variation of basal width reduces over time, it suggests non-copying errors may be factors, in other words a biased transmission process may be operating. Thickness however, becomes increasingly variable throughout time, suggesting stochastic copying errors. The second case study involves Illinois Woodland Pot Sherds. Thickness remained constant over time, probably because thickness is important to the function of a vessel and thus non-functioning thicknesses would be weeded out. It would seem that a particular thickness was a trait that was selected for, thus drift was not caused by stochastic copying errors.

The authors admit in their conclusions that there are surely many other factors at play, including changes in settlement patterns, social organization, individual memory, concept of ideal form, etc. However copying error seems to be universal, and thus this paper provide us with a null hypothesis with which we can test data to determine the source of variance over time in the archaeological record.

I like that the authors created a null hypothesis. They build a model based on a universal human trait, and therefore can theoretically be used in any case study and is then very useful to archaeology.

I find it interesting that the trend of random error accumulating over time is the same as that in biological evolution with a rate of increase slowing down over time. The cause of this in biology is history; although here is always variation in traits of organisms, the extent of that variation depends on the organisms’ genetic history (Bell 2008). If this the same in cultural information, even though there are errors, a previous artifact from a long time ago is still remembered, even though it is not the one being copied? I think there are many implications of this that can be more thoroughly investigated.

Clearly, creating a theory of cultural evolution based on Darwinian evolution has many challenges, but as the authors have demonstrated, with more research in this area it is possible to build models appropriate for archaeology.

References:
Bell, Graham. (2008). Selection: The Mechanism of Evolution, 2nd edition. Oxford University Press.
Eerkens, JW; Lipo, CP. (2005). Cultural transmission, copying errors, and the generation of variation in material culture and the archaeological record. Journal of Anthropological Archaeology 24 316–334.