*Am J Phys Anthropol*DOI: 10.1002/ajpa.22237

**Exploring prehistory in the North American southwest with mitochondrial DNA diversity exhibited by Yumans and Athapaskans**

Cara Monroe et al.

A recent study of mitochondrial DNA variation in Native American populations from the American Southwest detected signatures of a population expansion of subhaplogroup B2a, dated to 2,105 years before present (99.5% confidence interval, 1,273–3,773 YBP), following the introduction and intensification of maize agriculture in the region. Only one Yuman group and no Athapaskan speakers were analyzed in previous studies. Here we report mtDNA haplogroup and hypervariable region (HVR I, and II) sequence data from 263 extant Yuman speakers, representing the major branches of the Yuman language family, in addition to the Western Apache (Athapaskan) to further investigate the demographic context and geographic extent of this expansion.

**Data presented indicate that the expansion of B2a is only slightly older [2,410 YBP (99.5% CI: 1,458–4,320 YBP)] than previously estimated and not significantly.**Despite large confidence intervals there are implications for the origin and expansion of the Yuman language family. Cultural transformations due to the inundation and draining of Lake Cahuilla may explain in part the frequencies of this lineage among the Kumeyaay and other Yuman and Takic groups in Southern California. This may have been the result of group fissions and fusions followed by migration and interaction that included expanded trade networks and intermarriage among Yuman speakers.

**In addition, a series of in-situ genetic bottlenecks is proposed to have occurred among the Western Apache leading to increasing homogeneity within haplogroup A, culminating in an admixture event with the Yavapai.**

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It is worth noting that the three sigma 99.5% confidence intervals used are wider than the customary two sigma 95% and one sigma 68% confidence intervals use in most contexts.

ReplyDelete2410 YBP (+667/-317 YBP) would be a more conventional way of stating the new result, as would 2410 YBP (95% CI: 1,776-3,744 YBP).

Also, given the fact that there are two independent data sets, one really ought to go on to make a combined estimate of the mean value weighted relative to the margin of error in each data set (the first result has slightly smaller margins of error so it would be weighted 53% relative to the recent result), for a combined mean of 2249 YBP. Then one should calculate an approximate combined margin of error by taking in each direction (forward v. backward in time), by taking the square root of the sum of the squared one sigma margins error in that direction divided by the number of errors averaged (which is -211 and +423) (this ignores some small residual terms and makes some model dependent assumptions that aren't precisely right in an asymmetrical error distribution case like this one, but are very close to the precisely calculated value).

Expressed in the most common way in this kind of study, the combined estimate, which is the number that really matters, should be (stating in a conventional way):

2,249 YBP (95% CI: 1,827-3,095 YBP).

Of course, this CI only measures uncertainty for a given assumed mtDNA mutation rate. But, as the recent ancient mtDNA H study whose abstract appears at the AAPA 2013 post on this blog suggests, the actual mutation rate measured in similar conditions was 45% higher than the conventional value.

This empircally calibrated mtDNA mutation rate would give a date of 1,552 YBP (95% CI: 1,261-2,136 YBP) (the errors are actually calculated as a percentage of the mean and then translated into dates so they scale proportionately to the mean).

The combined estimate would put the expansion of mtDNA subhaplogroup B2a in two ethnically distinct Southwestern American populations (the Yuman who were centered around Arizona more or less) and the Apache (whose center of gravity would have been closer to eastern Colorado and migrated to the region around that time) in a time period that overlaps with the time period from 700 CE to 1130 CE during which the American Southwest "saw a rapid increase in population due to consistent and regular rainfall patterns. Studies of skeletal remains show that this growth was due to increased fertility rather than decreased mortality. However, this tenfold increase in population over the course of a few generations could not be achieved by increased birthrate alone; likely it also involved migrations of peoples from surrounding areas." In particular, the Na-Dene migration from Alaska and the Pacfic Northwest to the American Southwest is believed to have happened at around this time based on archaeological and linguistic evidence.