Yesterday Russell Gray visited Penn and gave a talk based on his much-discussed Nature article with Quentin Atkinson, "Language-tree divergence times support the Anatolian theory of Indo-European origin." (Nature, 426, 435-439). In the audience were Don Ringe and Tandy Warnow, whose reactions I cited in an earlier post, and a collection of linguists, biologists and computer scientists that included Bill Poser and me.
Gray's presentation explained a lot more about their methods than the (necessarily brief) Nature article did. Much of the additional material can be found in this draft chapter (though I understand that a newer version will soon be available).
There was a lively and sometimes heated discussion, both during and after the talk. I have other tasks today, so a detailed account will have to wait for later, but I'll give a few general impressions now. I'm sure that Bill will have some comments as well.
First, everything that I learned reinforced my earlier belief that this is serious and interesting work. Its methods and conclusions remain controversial but they are worthy of very close attention. This is also not a one-shot deal -- Gray is continuing experiments on the Indo-European issues, and has new work on Austronesian in progress.
Second, Gray and Atkinson draw different (and in fact roughly opposite) conclusions from Warnow and Ringe about the reliability of various phylogenetic inferences. As I noted earlier, Warnow and Ringe argue that we can often get good information about tree topology, but (in the present state of knowledge) can't expect any reliable information about times. In contrast, Gray and Atkinson argue that even when tree topology is very uncertain (and even if the history is substantially untreelike as well), it may still be possible to get fairly tight time estimates.
I'm not sure who is right about this. This is partly because I still don't know enough about the details of the models involved. But as far as I can tell from yesterday's discussions, even the folks who know a lot more are really in a similar state. It comes down to an argument about which simplifying assumptions to make, and what effects these assumptions will have on the conclusions that result. I'll go over some of this argument in more detail when time permits.
In thinking about the general problem, an analogy with physics may be helpful. If we assume that the sun, planets and other heavenly bodies are point masses in calculating their orbital dynamics, our model is obviously false to fact. But does this simplification invalidate our conclusions? Well, it might or might not, depending on what calculations we do and what conclusions we want to draw. Any model of orbital dynamics will be simplified -- and therefore false -- to one extent or another. The question is whether this matters with respect to some specific quantitative or qualitative prediction. Giving a correct answer to that question requires a mixture of detailed mathematical reasoning, relevant empirical testing and luck.
One of Russell Gray's slides made this point by quoting the well-known scientific proverb that "A model is a lie that leads us to the truth". I believe that this was originally adapted (by whom?) from a remark made by Picasso:
"We all know that art is not truth. Art is a lie that makes us realize truth, at least the truth that is given us to understand. The artist must know the manner whereby to convince others of the truthfulness of his lies." (The Arts, Picasso Speaks, 1923)
Yesterday Russell Gray made considerable headway in convincing me of the validity of his approach. His talk, and the discussion around it, clarified for me the nature of the simplifying assumptions that he's making, and the (empirical and logical) questions to be addressed in determining whether those simplifications invalidate his conclusions about the dating of Indo-European. He also convinced me that he's continuing a serious program of efforts to test the effects of his assumptions, and that he's serious about understanding and addressing objections. In other words, he's doing science.
Posted by Mark Liberman at April 28, 2004 09:31 AM