units. In Haplogroup BT and chimpanzee chromosomes, this marker consists of 9 T nucleobase units. This pattern suggested that the 9T stretch of Haplogroup BT was the ancestral version and that Haplogroup A was formed by the deletion of one nucleobase. Haplogroups A1b and A1a were considered subclades of Haplogroup A as they both possessed the M91 with 8Ts. But according to Cruciani et al. 2011, the region surrounding the M91 marker is a mutational hotspot prone to recurrent mutations. It is therefore possible that the 8T stretch of Haplogroup A may be the ancestral state of M91 and the 9T of Haplogroup BT may be the derived state that arose by an insertion of 1T. This would explain why subclades A1b and A1a-T, the deepest branches of Haplogroup A, both possess the same version of M91 with 8Ts. Furthermore, Cruciani et al. 2011 determined that the P97 marker, which is also used to identify Haplogroup A, possessed the ancestral state in Haplogroup A but the derived state in Haplogroup BT. Likely geographic origin As current estimates on TMRCA converge with estimates for the age of anatomically modern humans and well predate the Out of Africa migration, geographical origin hypotheses continue to be limited to the African continent. According to Cruciani et al. 2011, the most basal lineages have been detected in West, Northwest and Central Africa, suggesting plausibility for the Y-MRCA living in the general region of "Central-Northwest Africa". Scozzari et al. (2012) agreed with a plausible placement in "the north-western quadrant of the African continent" for the emergence of the A1b haplogroup. The 2013 report of haplogroup A00 found among the Mbo people of western present-day Cameroon is also compatible with this picture. The revision of Y-chromosomal phylogeny since 2011 has affected estimates for the likely geographical origin of Y-MRCA as well as estimates on time depth. By the same reasoning, future discovery of presently-unknown archaic haplogroups in living people would again lead to such revisions. In particular, the possible presence of between 1% and 4% Neanderthal-derived DNA in Eurasian genomes implies that the (unlikely) event of a discovery of a single living Eurasian male exhibiting a Neanderthal patrilineal line would immediately push back T-MRCA ("time to MRCA") to at least twice its current estimate. However, the discovery of a Neanderthal Y-chromosome by Mendez et al. suggests the extinction of Neanderthal patrilineages, as the lineage inferred from the