A very useful new paper on the origin and spread of mitochondrial (mtDNA) haplogroup U7 has just appeared at Scientific Reports.
It re-iterates some key points that I've made about this haplogroup; that it's a South Caspian-specific lineage and conspicuous by its absence from all Yamnaya samples sequenced to date. In fact, along with other South Caspian-specific lineages, such as U1, U3a, HV2 and HV0, it's missing from all Early Bronze Age steppe samples sequenced to date (see here).
This is surely a major problem for those positing that ancient populations from the South Caspian, in other words what is now mostly Iran, made a significant contribution to the formation of Early Bronze Age steppe pastoralist groups, including Yamnaya.
However, I'd say the paper's conclusion that U7 probably spread into Europe before the Early Bronze Age is a bit iffy. Based on the available ancient European mtDNA, it looks to me as if it mostly spread into Europe after the Early Bronze Age. So why are there European-specific U7 lineages, such as U7a19, seemingly with coalescent times dating to the Neolithic in Europe? Well, perhaps because after these lineages moved to Europe, they went extinct in the Near East? From the paper, emphasis is mine:
Abstract: Human mitochondrial DNA haplogroup U is among the initial maternal founders in Southwest Asia and Europe and one that best indicates matrilineal genetic continuity between late Pleistocene hunter-gatherer groups and present-day populations of Europe. While most haplogroup U subclades are older than 30 thousand years, the comparatively recent coalescence time of the extant variation of haplogroup U7 (~16–19 thousand years ago) suggests that its current distribution is the consequence of more recent dispersal events, despite its wide geographical range across Europe, the Near East and South Asia. Here we report 267 new U7 mitogenomes that – analysed alongside 100 published ones – enable us to discern at least two distinct temporal phases of dispersal, both of which most likely emanated from the Near East. The earlier one began prior to the Holocene (~11.5 thousand years ago) towards South Asia, while the later dispersal took place more recently towards Mediterranean Europe during the Neolithic (~8 thousand years ago). These findings imply that the carriers of haplogroup U7 spread to South Asia and Europe before the suggested Bronze Age expansion of Indo-European languages from the Pontic-Caspian Steppe region.
Compared to other subclades of hg U, both the phylogenetic structure and the ancestral origin of hg U7 are rather obscure. This haplogroup is characterized by generally low population frequencies and limited sequence diversity, despite a geographic distribution ranging from Europe to India [14,16,25,27,30,31,32,33]. Recently, it has been detected in skeletal remains from Southwest Iran [my note: that was U7a] dated ~six thousand years ago (kya)  as well as in remains from the Tarim Basin in Northwest China (3.5–4.0 kya) .
Another major episode of gene flow affecting the European gene pool appears to have occurred during the Late Neolithic and Early Bronze Age, from a source in the Pontic-Caspian Steppe region north of the Caucasus [3,54,66,72]. It has been suggested that this migration resulted in a further substantial shift in the genetic profile of Europeans and was a major vehicle for the movement of Indo-European languages to Europe [3,72], and likely also to South Asia54. Interestingly, the autosomal genetic component in Europeans considered to derive from the Steppe is almost fixed in two pre-Neolithic ancient genomes from the South Caucasus. This component is distributed eastwards towards South Asia as well54, where it mimics the distribution of U7 (Pearson’s r = 0.65, p = 0.01). Our time estimates for the expansion and differentiation of hg U7 in the Near East, Central Asia, South Asia, and Europe, however, predate these putative late Neolithic-early Bronze Age migrations and thereby rule them out as a major vehicle for the spread of U7 to Europe and South Asia. In this respect, it is also noteworthy that Yamnaya herders of the Steppe so far analysed (n = 43) show no traces of U7 [3,55,72,73] – and U7 is rarely found in this region today (Fig. 2).
The expansion time of hg U7 in the Near East, Central Asia and South Asia is more consistent with autosomal multi-locus estimates for the genetic separation of these regions during the Terminal Pleistocene74, suggesting a common demographic process, whose origin was unclear previously. Here, we show that the frequency and distribution of U7b lineages indicate an origin of this clade in the Near East, whilst for U7a these statistics cannot differentiate between South Asia and the Near East (including the Caucasus) as a possible homeland.
Sahakyan et al., Origin and spread of human mitochondrial DNA haplogroup U7, Scientific Reports 7, Article number: 46044 (2017), doi:10.1038/srep46044
Mitogenomes reveal post-Neolithic gene flow from the Near East to Tuscany
Big deal of 2016: the territory of present-day Iran cannot be the Indo-European homeland