In order better to conceptualize aspects of shared ancestry and subsequent divergence amongst the populations within a major ethnolinguistic grouping it is appropriate to introduce here the concept of the “phylogenetic unit”. This concept has been applied recently to one branch of the Austronesians, namely the Polynesians, by Kirch and Green (1987). It can also be applied carefully to the Austronesians as a whole, albeit on a much larger scale in both time and space.^[2] Basically, the idea of phylogenetic relationship revolves around derivation from a common source, in cultural terms identifiable through shared patterns of language and society, in biological terms identifiable through shared configurations of the gene pool. Phylogenetic units, whether defined culturally or biologically, are subject to divergence or radiation of their internal elements through the operation of processes such as population fission with subsequent geographical separation, founder or bottleneck effects, selective adaptations to differing or changing environments, and the effects of contact with external societies.

Identifying Austronesian societies as members of an Austronesian phylogenetic unit in the broad sense does not imply that they belong, past and present, to a kind of sealed species-like entity with sharp and unbreachable boundaries. They do not, any more than do speakers of any other family of languages. We see the undeniable significance in many times and places of interaction between Austronesians and various non-Austronesian populations, not only in language but also in biology and other aspects of society. The reality of the past 5000 years of Austronesian prehistory and history reflects both “bifurcative” and “rhizotic” (or reticulate) processes of cultural change in the terminology favoured by Moore (1994).

It should be clear, therefore, that in order to approach questions of Austronesian history and common ancestry in the broad sense we need to keep separate, for heuristic purposes, biology, language and culture, even though many aspects of culture are inextricably linked to language. Languages, populations and cultures evolve, diversify and mix through different, albeit conceptually overlapping, mechanisms. When considering something on the whole Austronesian scale it would be naïve to assume that linguistic, cultural and biological entities and their boundaries must correlate precisely, or will ever have done so in any absolute way, although relatively high degrees of coordination and correlation, despite variations, are an essential part of the concept of a phylogenetic unit.

One of the main implications of this book is therefore that Austronesian peoples and societies are all linked by branching but not sealed lines of common ancestry spanning the past 6000 years or so. But anyone inquisitive about the nature of the human species might wish to ask why such a unit should exist. In other words, why did such a phenomenal process of colonization occur, ultimately extending over half way around the world, and what major stimuli or constraints came to bear upon its regional expressions? These are important questions which will be considered from different viewpoints spanning various time-depths in some of the chapters which follow. There may be no simple answers, but the questions are worthy of articulation and asking.