of the seven language families under examination. Five separate runs produced the same (unrooted) tree, with three sets of language families: an eastern grouping of Altaic, Inuit–Yupik, and Chukchi–Kamchatkan; a central and southern Asia grouping of Kartvelian and Dravidian; and a northern and western European grouping of Indo-European and Uralic. Two rootings were considered, using established age estimates for Proto-Indo-European and Proto-Chukchi–Kamchatkan as calibration. The first roots the tree to the midpoint of the branch leading to proto-Dravidian and yields an estimated origin for Eurasiatic of 14450 ± 1750 years ago. The second roots to tree to the proto-Kartvelian branch and yields 15610 ± 2290 years ago. Internal nodes have less certainty, but exceed chance expectations, and do not affect the top-level age estimate. The authors conclude "All inferred ages must be treated with caution but our estimates are consistent with proposals linking the near concomitant spread of the language families that comprise this group to the retreat of glaciers in Eurasia at the end of the last ice age ~15,000 years ago." Many academics specializing in historical linguistics via the comparative method are skeptical of the conclusions of the paper, and critical of its assumptions and methodology. Writing on University of Pennsylvania blog Language Log, Sarah Thomason questions the accuracy of the LWED data on which the paper was based. She notes that LWED lists multiple possible proto-word reconstructions for most words, increasing the possibility of chance matches. Pagel et al.. anticipated this criticism and state that since infrequently used words generally have more proposed reconstructions, such errors should "produce a bias in the opposite direction" of what the statistics actual shows (i.e. that infrequently used words should have larger cognate groups if chance alone was the source). Thomason also argues that since the LWED is contributed to primarily by believers in Nostratic, a proposed superfamily even broader than Eurasiatic, the data is likely to be biased towards proto-words that can be judged cognate. Pagel et al.. admit they "cannot rule out this bias" but say they think it is unlikely bias has systematically impacted their results. They argue certain word types generally believed to be long lived (e.g. numbers) do not appear on their 23 word list, while other words of relatively low importance in modern society, but important to ancient people do appear on the list (e.g. bark and ashes), thus casting doubt on bias being the