<sigh>
Which part do I single out?
The same Brahmi-derived script is used for several languages, and was a common alternative to the Nagari version that is now used for Hindi, as far back as the eleventh century at least, although this script or a predecessor has been used for the Kamarupa Inscriptions as far back as the 7th century. It was used for Bengali, Assamese and, in later years, for variations found in Manipur. The script was in no way connected organically with the language Bengali, until as late as the 18th century, and the centralisation which happened as a consequence of British research into the language. Iswarchandra Vidyasagar's name is associated with this adoption of the script for Bengali as a standard, but it is possible that the general direction was set by the Fort William College and the first researches into Bengali by scholars from that college, assisted by Sanskrit scholars.
The Sanskritisation of Bengali and the purging of Persian words that took place then is a separate matter; this misunderstanding of the British, guided by Sanskrit-oriented pundits, led to the reaction known as Muhammedan Bengali, or Mussulman Bengali, which existed earlier but not in particularly widespread a form.
That is for the script. For the language, what we need to know is that all these eastern languages were derivatives of 'eastern' Prakrit, or Magadhi Prakrit, which was the Prakrit that was taught to newcomers to the language. In contrast, Gujarati, Rajasthani, Marathi, Punjabi and other western languages of the Indo-Aryan family were derived from 'western' Prakrit, or Suraseni. Assamese drew apart and achieved linguistic autonomy as far back as the 7th century.
Thirdly, the impression conveyed that until recent years, Bengali itself was one large, happy family is an egregious myth. Magadhi Prakrit itself split into perhaps four parts, of which one was the predecessor of modern-day Assamese, right at the outset, perhaps as much as a millennium ago, but probably around the time of the Bhakti movement in BengalThree of these splits coalesced into modern-day Bengali. The fourth was used as a foundation for modern-day Assamese.
A request: please leave your language chauvinism parked somewhere before getting on to this forum. Some of these interventions are crass. They are humiliating for other Bengalis to read.
This note is from memory, and dates are approximate.
It was coined for you, it fits you, wear it.
) .. Dont you think you contradict yourself? How did you loose your original language. We dont understand Bodo. Can we? NO. Can we understand Mizo? NO. Can we understand Nagas? No. But we understand Assamese.. Why? Because its Bengali.. 
