There has been much debate and speculation about the origin of Bengalis. Questions have been going around as to whether they were Mongoloid, Dravidian, Caucasian, and so on and so forth. Some concrete facts need to be considered before we establish this theory.
Bengalis, both from West Bengal, India and from Bangladesh speak “Bangla” – which belongs to the Indo-Aryan group of the Indo-Iranian branch of the Indo-European language family. The Bengalis are of diverse origin, having emerged from the confluence of various communities that entered the region over centuries. Bengalis are mainly Mongoloid tribes (the first to enter India) who got mixed with Caucasians (all other Caucasians including South Indians) from Magadha. However, some 1000 years ago, a South Indian Chola King, Rajendra Chola conquered Bengal and the Chola army passed on their genes to the local Bengalis. This gives one the impression and similarity between Bengalis and South Indians.
There is a lot of difference and diversity in phenotype among Bengalis. Firstly, Bengal was inhabited by Australoid tribes who were subjugated by Austronesian tribes, who in turn were further subjugated by Sinitic tribes. Then entered the Aryans, who named the region Bengal, which derives from BONGA + AL. BONGA is the Santhal word for Sun God and AL means boundary.
In the 1200s, when Bengal was overrun by Turkish raiders, Bengal was mostly inhabited by Hindus and Buddhists. Many Tibetan tribes settled in the Bengal region who were fiercer than their Aryan counterparts. Thus, core Bengalis are primarily Ancestral South Indian admixed with Indo-Aryans from Northern India and Mongoloids from East Asia. The Turkish Central Asians merely enhanced the existing Aryan and Mongoloid genetic components.