You don't have to have spent long learning Latin and Greek before you realise that the languages are related somehow. It doesn't take a genius to work out that fero, fers, fert is related to phero, phereis, pherei ("bear, carry").
For a long time, no-one really understood what the relationship was. The breakthrough came in the 1500s. Jesuit missionaries in India discovered Sanskrit, an ancient scholarly language which plays a similar role in Hinduism to that played by Latin in Catholicism. It became obvious that Sansrkit was related to the other two great ancient languages. Other such discoveries followed, and by the late 18th century Indo-European studies was up and running as a discipline. The premise of the new discipline was that the languages of Europe and northern India are nearly all related - "sprung from some common source", as the English scholar Sir William Jones famously put it in 1786. The "common source" has become known as Proto-Indo-European, or PIE.
To illustrate how it works, let's take the word "father":
Latin - paterAnd, for contrast, the same word in some unrelated languages:
Ancient Greek - pater
Sanskrit - pita
Old Persian - pita
Old High German - fater
Albanian - atë
Irish - athair
Japanese - chichiThere is no question that the words in the first group are related while the words in the second group aren't. The same trick can be repeated for a large number of other words.
Navajo - azhé'é
Arabic - ab
Between the attested languages known to scholars and the lost language of PIE there seem to have been intermediate daughter-languages of PIE. English, German and Norwegian are thought to descend from PIE via a "Proto-Germanic" language, Irish and Welsh via "Proto-Celtic", and so on - in much the same way as French, Italian and Spanish can be shown to descend via Latin.
For a long time, it was thought that there was a major division between Western and Eastern Indo-European languages. This was known as the centum/satem split, from the words for "hundred" in Latin and Sanskrit. However, the remains of an extinct IE language known as Tocharian were discovered in China in the early 20th century. In spite of its easterly location, this proved to be a centum language (the word was känt or kante, depending on the dialect). This is the sort of discovery that comparative linguists find terribly exciting. Almost as exciting was the decipherment, at around the same time, of the ancient middle-eastern language known as Hittite - it turned out not only that this was a member of the Indo-European family, but that it preserved some particularly ancient linguistic features from PIE.
The early reconstructions of PIE made it look very like Sanskrit. Modern reconstructions use technical phonetic notation which makes it look closer to Klingon:
h₂ówis, (H)jésmin h₂wlh₂néh₂ ne éh₁est, dedork’e (h₁)ék’wons, tóm, wóg’ʰom gʷérh₂um wég’ʰontm, tóm, bʰórom még’oh₂m, tóm, dʰg’ʰémonm h₂oHk’ú bʰérontm. h₂ówis (h₁)ék’wobʰos ewewkʷe(t): k’ḗrd h₂gʰnutoj moj widntéj dʰg’ʰmónm (h₁)ék’wons h₂ég’ontm. (h₁)ék’wōs ewewkʷ: k’ludʰí, h₂ówi! k’ḗrd h₂gʰnutoj widntbʰós: dʰg’ʰémō(n), pótis, h₂wlnéh₂m h₂ówjom kʷnewti sébʰoj gʷʰérmom wéstrom; h₂éwibʰoskʷe h₂wlh₂néh₂ né h₁esti. Tód k’ek’luwṓs h₂ówis h₂ég’rom ebʰuge(t). [Source]This is not as obscure as it looks, however. The first word, h₂ówis, is the word for "sheep", and is connected with the Latin ovis and the English ovine. Similarly, the term (h₁)ék’wons means "horse", and is related to equus and equine.
Don't forget, PIE is only a reconstruction, put together on a best-guess basis. Many aspects of the language are still unclear. We can't tell how far the Indo-European people themselves would have recognised PIE in the form currently put forward by scholars. The doyens of Indo-European studies, J.P.Mallory and D.Q.Adams, have written in the definitive Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World:
A time traveller, armed with this book and seeking to make him- or herself understood would probably engender frequent moments of puzzlement, not a little laughter, but occasional incidences of lucidity.Where and when was PIE spoken? No-one really knows, except that it must have been before c.2000 BC, which is when the first historical texts of IE languages start to appear. The most plausible guesses for the time period are somewhere in or around the 3000s BC. In any event, PIE was not a monolithic entity. It would have been spoken for a long time, undergone changes and had different dialects.
The location of the Indo-European "homeland" has been a source of controversy ever since the discovery of PIE itself. For a long time, scholars thought that it was possible to pin down the location by reconstructing the words for geographical features, flora and fauna, and then seeing which areas in Europe and western Asia possessed the same. But this is a lot more difficult than it sounds. Even if we succeed in reconstructing the words correctly, this method is very slippery. Fifty years ago, it might have been plausible to argue in the following way. There are reconstructible PIE words for "salmon", "beech tree" and "eel". The beech tree, Fagus silvatica, is native to central and eastern Europe; and the salmon and eel are found in the Baltic Sea but not in or around the Black Sea. So PIE must have originated in eastern Europe near the Baltic coast - mustn't it?
Not necessarily. There are several problems with this method. Most obviously, we can't be sure exactly where in the Eurasian landmass particular flora and fauna were to be found several thousand years ago. We also can't know precisely what the Indo-Europeans meant by the reconstructed terms. By "salmon", did they mean the Atlantic salmon (Baltic Sea) or the salmon trout (Black and Caspian Seas)? By "beech", are we sure that they meant Fagus silvatica and not another species with a different geographical distribution? There are other problems with such arguments too, which need not detain us. There is, however, one intriguing clue of this kind which may be genuinely significant. There is no reconstructible PIE word for "sea".
It is much easier to say where the IE homeland wasn't than where it was. We can be fairly confident that it wasn't in western Europe, or in India itself. But that still leaves an enormous expanse of territory, from eastern Europe to southern Russian via Turkey and Iran. The theory that the homeland was located among the steppes of southern Russia is as likely as any other.
Even if we can't reliably locate things like beech trees and salmon on the map, reconstructing items of vocabulary can give us interesting insights into the Indo-Europeans' social world. If we can reconstruct a term in PIE, the concept which it refers to must necessarily have been known to the speakers of PIE. We can discover quite a bit about the Indo-Europeans and their society using this method. They were not especially primitive people - we're not talking about cavemen here. They were farmers who lived in settled villages. They wore woollen clothes and used wheeled vehicles and ceramic vessels. The horse was culturally important to them. They worshipped a pantheon of gods headed by a Sky Father (Zeu Pater in Greek, Iu-piter in Latin). They had a creation myth in which the world was made out of the body of a primaeval giant. The PIE vocabulary has a lot of reconstructible words that may refer to fighting, and this has been used to suggest that they were a warlike people. Their poets seem to have sung of the fame of heroes, just as Homer and the Indian epic poets did in later times.
This post would not be complete without a reference to the political use and abuse of the Indo-European theory. During the 19th and 20th centuries, IE studies became bound up with current ideas of racial supremacy, and our IE ancestors were sometimes seen as being a kind of master race - the original "Aryans". There was a crossover between IE research and the political far right, in the form both of Nazi sympathisers like Jan de Vries and of archconservative nationalists like Georges Dumézil. Some neo-Nazis have continued to promote an old theory that the Indo-Europeans came from the North Pole - the purest, whitest place of all. There is sometimes a fine line between reconstructing ancient myths and creating new ones of our own.