Haplogroup I is a human mitochondrial DNA (mtDNA) haplogroup. It is believed to have originated about 21,000 years ago, during the Last Glacial Maximum (LGM) period in West Asia ((Olivieri 2013); Terreros 2011; Fernandes 2012). The haplogroup is unusual in that it is now widely distributed geographically, but is common in only a few small areas of East Africa, West Asia and Europe. It is especially common among the El Molo and Rendille peoples of Kenya, various regions of Iran, the Lemko people of Slovakia, Poland and Ukraine, the island of Krk in Croatia, the department of Finistère in France and some parts of Scotland and Ireland.

Haplogroup I is a descendant (subclade) of haplogroup N1a1b and sibling of haplogroup N1a1b1 (Olivieri 2013). It is believed to have arisen somewhere in West Asia between 17,263 and 24,451 years before present (BP) (Behar 2012b), with coalescence age of 20.1 thousand years ago (Olivieri 2013). It has been suggested that its origin may be in Iran or more generally the Near East (Terreros 2011). It has diverged to at least seven distinct clades i.e. branches I1–I7, dated between 16–6.8 thousand years (Olivieri 2013). The hypothesis about its Near Eastern origin is based on the fact that all haplogroup I clades, especially those from Late Glacial period (I1, I4, I5, and I6), include mitogenomes from the Near East (Olivieri 2013). The age estimates and dispersal of some subclades (I1, I2’3, I5) are similar to those of major subclades of the mtDNA haplogroups J and T, indicating possible dispersal of the I haplogroup into Europe during the Late Glacial period (c. 18–12 kya) and postglacial period (c. 10–11 kya), several millennia before the European Neolithic period. Some subclades (I1a1, I2, I1c1, I3) show signs of the Neolithic diffusion of agriculture and pastoralism within Europe (Olivieri 2013).

It is noteworthy that, with the exception of its northern neighbor Azerbaijan, Iran is the only population in which haplogroup I exhibits polymorphic levels. Also, a contour plot based on the regional phylogeographic distribution of the I haplogroup exhibits frequency clines consistent with an Iranian cradle ... Moreover, when compared with other populations in the region, those from the Levant (Iraq, Syria and Palestine) and the Arabian Peninsula (Oman and UAE) exhibit significantly lower proportions of I individuals ... this haplogroup has been detected in European groups (Krk, a tiny island off the coast of Croatia (11.3%), and Lemko, an isolate from the Carpathian Highlands (11.3%)) at comparable frequencies to those observed in the North Iranian population. However, the higher frequencies of the haplogroup within Europe are found in geographical isolates and are likely the result of founder effects and/or drift ... it is plausible that the high levels of haplogroup I present in Iran may be the result of a localized enrichment through the action of genetic drift or may signal geographical proximity to the location of origin.

— Terreros 2011

A similar view puts more emphasis on the Persian Gulf region of the Near East (Fernandes 2012).

Haplogroup I ... dates to ~25 ka ago and is overall most frequent in Europe ..., but the facts that it has a frequency peak in the Gulf region and that its highest diversity values are in the Gulf, Anatolia, and southeast Europe suggest that its origin is most likely in the Near East and/or Arabia ...

— Fernandes 2012

Haplogroup I is found at moderate to low frequencies in East Africa, Europe, West Asia and South Asia (Fernandes 2012). In addition to the confirmed seven clades, the rare basal/paraphyletic clade I* has been observed in three individuals; two from Somalia and one from Iran (Olivieri 2013).