A thread on discovery of Proto Indo-European (PIE) Language, the ancestral Language of modern Indo-European languages that are spoken by 3.2 billion people in several continents, and the established relationship between Indo-European languages.

Some of the popular Indo-European languages are Greek, Latin, Sanskrit, Hindi, English, French, Persian, Bengali, Spanish etc.

The Biblical explanation of different languages set forth in Genesis, in the story of Tower of Babel, was the only source to understand the origin of different languages, before the advent of linguistics as a field of study.

The story is as follows:
Men tried to built a tower to heaven, but the God was displeased at men's act of challenging Him which made Him to destroy the tower, dispersed the population and gave them different languages so they wouldn't communicate again to challenge the God.

Beyond this, there was an understanding that Romance languages like Spanish, Italian, Portuguese and French evolved from a common language, Latin. The prolonged Roman rule over the areas where these modern languages are spoken was seen as the probable reason.

It was also obvious that Romance languages were similar to Germanic languages of Central and Northern Europe, and Slavic Languages of Eastern Europe. Borrowing of words due to geographical proximity was attributed to this, and it was true.

But, the idea that all of these languages evolved from a common language didn't come into play. This was discovered not in Europe, but in India in 18th century. Much of modern day subcontinent was ruled by the English East India Company.

It is the British involvement in India that paved the way for the discovery of PIE, the probable source of Indo-European languages.

Warren Hastings who had the apprection for native Indian culture became the first Governor General of Bengal Presidency in 1774. He also believed that there should be very limited involved of the company in the traditional institutions like judiciary.

He devised a plan by which English judges would supervise over Indian courts and apply Hindu laws in Hindu areas, and Muslim laws in Muslim regions. Most of the Hindu legal traditions were written in Sanskrit, but English judges didn't have proficiency in it.

Sanskrit was not spoken but used in administration and religion extensively. Latin was similar to the role of Sanskrit in middle ages in Europe.

Enter Sir William Jones, philologist and jurist, who published the translation of Persian manuscript to French in 1770, at the age of 22. In 1771, he published the Grammar of Persian language in English, and mastered Orientalism at Oxford. He was proficient in 28 Languages.

He was appointed as the judge of Supreme Court of Fort Williams, Calcutta, in 1783. It was Hastings who was able to convince British monarchy for this appointment as monarchy was hostile towards Jones for his support for American War of Independence.

Jones founded the Asiatic Society in Calcutta in 1784, and was given the responsibility of translating Hindu laws in Sanskrit to English. He began to study Sanskrit for this and discovered the similarities between Sanskrit and European languages.

He was bewildered to find the similarities between Sanskrit and other European languages, as there could be no borrowing due to geographical remoteness.

His discovery led to the foundation of Linguistics as the branch of study to determine whether a language is an Indo-European or not.

In a lecture at Asiatic Society, he announced that Sanskrit derived from the same source from which Greek and Latin derived. He established a connection not just between Sanskrit and Greek or Latin, but also between Gothic and Germanic languages.

Here's an excerpt from his lecture:

He arrived at the similarities based on phonological shifts in Languages. It was later theorised by Grimm with reference to Germanic languages.
Example of phonological shift:

Another example: a compound word for God meaning sky-father
In Sanskrit, it was Dyáus pitr.
In Greek, it was Zeus Patér.
In Latin, it was Dies Pater which became Jupiter after an another sound shift.

The "Patér" part was dropped in Greek later and became Zeus.

Phonological shift is due to several reasons. But, the important discovery here is that all these words originated from a same root word and are cognates. Grimm and Verner tried to formulate this sound shift with respect to Germanic languages.