A paper by Gell-Mann & Ruhlen in PNAS this week conducts a phylogenetic analysis of word order in languages and concludes that SOV is the most likely ancestor language word order. The main conclusions from the analysis are:
(i) The word order in the ancestral language was SOV.
(ii) Except for cases of diffusion, the direction of syntactic change, when it occurs, has been for the most part SOV > SVO and, beyond that, SVO > VSO/VOS with a subsequent reversion to SVO occurring occasionally. Reversion to SOV occurs only through diffusion.
(iii) Diffusion, although important, is not the dominant process in the evolution of word order.
(iv) The two extremely rare word orders (OVS and OSV) derive directly from SOV.