Network of ancient Thai bronze Buddha images

This blog post continues the theme from the previous post (Trees and networks of written manuscripts), in which I noted that anthropological data are very likely to involve horizontal flows of phylogenetic information as well as vertical ones. My own analyses of anthropological datasets that are available online seem to confirm this suggestion. The simplest way to illustrate this point is to take a dataset and analyze it using a network method. If the network method produces a tree-like diagram then we can safely conclude that vertical descent has had a larger influence on the transmission of the cultural information than has horizontal transfer.

The dataset that I will use here is provided by Marwick (2012). It involves photographic images of 42 cast metal Buddha statues from the Alexander B. Griswold collection of the sacred sculpture of Thailand (Walters Art Gallery, Baltimore, USA). The statues cover seven widely recognized chronological Thai culture-historical groups.

The morphological features of the statues' heads were coded as 17 binary characters, representing the face of the Buddha image; and these data are included in Marwick (2012). Statues CN65 and CN66 had identical codings for the features used.

Originally, Marwick (2012) analyzed these data by first summarizing the characters for each of the seven culture-historical groups. The phylogenetic analysis was then performed with these seven groups as the taxa. The exhaustive-search parsimony analysis produced three maximum-parsimony trees, and the bootstrap consensus tree was not well-supported, as shown in the figure.

This result suggests that the data may not be particularly tree-like. To assess this, I have performed a network analysis using the hamming distance and a NeighborNet graph, as shown in the next figure. The seven culture-historical groups have been colour-coded as follows (in chronological order):

Dvaravati Khmer Thirteenth_Century Sukhothai Early_Ayutthaya Lan_Na Late_Ayutthaya

light green dark green dull blue bright blue purple-brown pink red

Click to enlarge.

Clearly, the network is not very tree-like, and so we can infer that there has been a considerable influence of horizontal flow of phylogenetic information, as well as the vertical flow through time. There are, however, distinct temporal patterns in the network, which we can infer are probably phylogenetic patterns.

The samples from the earliest three periods (Dvaravati, Khmer, Thirteenth_Century) are at the right-hand end of the network, while the samples from the next period (Sukhothai) are at the bottom-left. This implies that a large stylistic change occurred between the Thirteenth_Century and the Sukhothai periods. Furthermore, the Khmer period style is rather distinct from that of the immediately preceding period (Dvaravati) and the immediately following one (Thirteenth_Century), which are themselves not distinct. That is, there was no stylistic change between the first two periods, but there was a small change to the next period, and then a large change to the following period.

The samples from the latest two periods (Lan_Na, Late_Ayutthaya) are collected mainly in two locations, at the bottom of the graph and at the top-left. This indicates that, although there are two distinct styles, they do not correlate with the two culture-historical periods. So, the pattern here is not a strictly phylogenetic one, and we need to look for some other explanation

The samples from the Early_Ayutthaya period are scattered throughout the top and left of the network, suggesting that this is an intermediate style between that of the immediately previous Sukhothai period and the earliest three periods, rather than being an innovative style leading to the succeeding Lan_Na period.

Importantly, these interpretations of the phylogenetic patterns do not accord with those from the tree-building analysis, where the possible patterns of horizontal flow of information are not made explicit.

Reference

Marwick B (2012) A cladistic evaluation of ancient Thai bronze Buddha images: six tests for a phylogenetic signal in the Griswold Collection. In: Bonatz D, Reinecke A, Tjoa-Bonatz ML (editors) Connecting Empires. National University of Singapore Press, pp. 159-176.