The Population Genetics of Iranian Zoroastrians: Evidence of Genetic Isolation and Bottleneck Effects

Zoroastrianism is one of the earliest religions, originating in ancient Iran. There is a debate about its place of origin, but it is generally believed to have originated in West Azerbaijan or Baluchistan (north-west and south-east of modern Iran, respectively) and has influenced many subsequent religions over the centuries (including northern Buddhism, Judaism, Christianity, and Islam). From the 6th to the 7th century AD, Zoroastrianism was the state religion of Iran, and most Iranians practised it. Zoroastrianism existed in Iran during the Median era (approximately 678-550 BC) and the Achaemenid Empire (550-330 BC); however, Alexander’s invasion of Persia in 331 BC and the subsequent establishment of the Seleucid Dynasty (312-63 BC) greatly reduced its influence and number of followers. Many priests, teachers and lawyers who orally transmitted the ancient teachings were executed, and many Zoroastrian holy temples (Fire Temples) were destroyed. The Parthians (247-224 CE) ruled Persia after the Seleucids, but little is known of the status of Zoroastrianism during their time. When the Sassanid Dynasty came into power in 224 CE, they aggressively promoted Zoroastrianism and rebuilt fire temples to promote their religion. During the Sasanian Empire (224-651 CE), Zoroastrianism became the official state religion and was deeply intertwined with imperial power and centralised authority. The Sasanians standardised the faith, compiled the religious text, the Avesta, and established a strong, hereditary clerical system, resulting in a highly ritualised, influential, and often oppressive religious institution. Following the Arab invasion of Persia in the 7th century, which is known as a period of oppression, misery and massacre, Zoroastrianism experienced a long, gradual decline from the official Persian state religion to a persecuted minority faith. Zoroastrians faced systemic discrimination, heavy taxes (jizya), and destruction of fire temples, leading many to convert or migrate to remote and inaccessible areas of Iran or India (so-called Indian Parsis) to escape persecution. Recent genetic studies show that the Zoroastrians in Iran and India possess unique genetic profiles, characterised by a genetic bottleneck due to isolation, high levels of endogamy (marriage within the community), and distinct ancestry that sets them apart from other Persian ethnic groups. In this research, we have explored and compared the genetic diversity of Zoroastrians with that of other Iranian ethnic groups and populations.