In 1796, William Jones, chief justice of British India, indicated the similarities between Sanskrit, Greek and Latin, and Persian. This led to the widely accepted theory that there existed a mother tongue designated proto-Indo-European, located ca6500 BCE in a possible homeland north of the Caucasus. The denomination Indo-European derives from the hypothesis that modern European languages might well have derived from a language closer to Sanskrit that to Greek or Latin. It is confusing because in fact European languages evolved independently of Indic languages. The originary Indo-Europeans separated into three branches: the Indo-Aryan, the Indo-Iranian, and the Indo-European proper.
The Indo-European branch, which begat most of the languages of Europe and two minor extinct languages in the Tarim Basin, probably separated first, perhaps ca4000, mostly in the direction of the west but also, a smaller group, towards the east. The main groups of the Indo-European branch (in terms of survival, speakers, and influence) are: Greek, Slavic, Baltic, Germanic, and Italic. Other surviving groups are Armenian, Celtic, and Illyrian (modern Albanian). Indo-European groups and individual languages that have not survived are: Thracian, Phrygian, Anatolian (Hittite and Luwian languages, more properly Nesite), Venetic, and Messapic.
From linguistic analysis and from historical hindsight, a possible sequence in which the Indo-European migrations towards the west might have started is as follows (the names are of course ex post facto): Thracians and Nesites or future Hittites (before 3000); Greeks and Illyrians (before 2300); Italics (ca1900); Slavs (ca1750); Balts (ca1500); Armenians, Celts, and Germans (before 1200). (These datings are more for the purpose of sequential tabbing than for indicating accurate times, which anyway no one has. What they indicate is basically the order in which these different peoples might perhaps have begun to emerge in pre-history as the antecessors of historical peoples and languages. There are no correspondences between these speculations and the historical attestations of these peoples.)
Because the Celts were occupying western Europe when the Romans subjugated them (1st century BCE), it is assumed that they had been there for many centuries before. This incidentally does not mean that there weren't other peoples in Europe before the proto-Celts. And it doesn't mean either that Celts and Germans, as Julius Caesar apparently believed, were separate ethnic groups. The Basques, who speak a language with no other linguistic relatives, are thought to have inhabited their Pyreneean homeland and far into adjacent lands since "time immemorial". Iberian and Tartessian are known to have been spoken in Spain before the arrival of the Celts. And Ligurian, though Celticized, was not an originally Indo-European language.
As opposed to generalized techniques, such as the use of certain tools or pottery-making with basic, primitive decorative variations, advanced cultures are characterized by a set of associated features, such as architecture (albeit primitive), metal-working capability, ceramic-types, and mode of burial. Archaeological research has found evidence for advanced cultures (Urn-Field and Hallstatt) in western Europe as far back as ca1200 BCE or before, so it is natural to associate them with the ancestors of the Celts. In Italy archaeology has found remains of a characteristic culture ca1100. It is known that Italy was occupied by a non-Indo-European-speaking people called Etruscans, who developed a civilization of their own when the Italics (the future Romans among others) were still in a backward tribal stage. This means either of two: (1) that the Italics arrived after 1100 or (2) that they were there before or arrived simultaneously and co-existed with the ancestors of the Etruscans. As there is no evidence of destructive discontinuities in the archaeological record for Italy, the second alternative seem the likelier. The Italics spoke the ancestors of the Latin-derived Romance languages of Italy, France, Portugal, Spain, and Romania.
It is believed that the Nesites conquered the native Hatti of Anatolia ca2000. By one of the many ironies in the relations between peoples, the Nesites became known as Hittites. Very little is known of the Hatti. The first written records of the Hittites date from ca1600. If the Nesites entered the Balkans ca3000, then the proto-Greeks, who for various reasons can be assumed to have founded Mycenaean Civilization (dated to ca2000), could have been following them ca2500. As far as history goes, all that can be said of the Illyrians is that they settled north of the Greeks, perhaps with Thracians and Phrygians.
In this schematized chronological sequence, the Germans moved into Scandinavia and northern Germany (pivotal date 1200), where they had the Celts to the west and south and the Balts and Slavs to the east. The Armenians migrated into the Balkans, where they lingered (pivotal date between 1200 annd 1000), then followed the Phrygians into Anatolia. After ca1500, the Balts were occupying the Baltic coastline and its hinterland. The Slavs occupied the lands from the Elbe to central Russia. In the steppes south of the Slavs were Indo-Iranians. During the 2nd century BCE, the Germans were on the move displacing the Celts from Germany proper and expanding along the Danube towards the Black sea. The Slavs were pressured to the east on the Elbe frontier and they in turn constrained the Balts. The only records we have for these movements and displacements are what the Romans found out after they occupied the Rhine frontier and during the course of the history of the Roman Empire and its relations with its neighbors along the Danubian limes. Beyond that there are shreds of archaeological finds and what paleolinguistics can reconstruct about the relative geographical location of Balts and Slavs, such as evidence from the names of rivers.
At the height of the Roman Empire (2nd century CE) descendants of Indo-Europeans were occupying the lands from central Russia to the shores of the Atlantic Ocean in Portugal. In much of the empire Latin was superimposed on a variety of other languages, especially those belonging to Afro-Asiatics, a family in which Egyptian and Semitic were principal branches.
The smaller branch of the Indo-Europeans that migrated to the east eventually ended up in the Tarim Basin (southern Xinjiang). In 1900, a large cache of manuscripts was spirited out of Dunhuang, a Chinese city on the eastern edge of the Tarim Basin. They were being kept in Buddhist caves in the area. These texts, which contain Buddhist, Manichean, and Nestorian literature, were written from the 6th to the 8th centuries in two different Indo-European languages related to the languages of the Indo-Europeans who migrated to Europe. They are called Turfan and Kuchan and there are considerable dialectical differences between them.
When did these Indo-Europeans arrive in the Tarim? There is archaeological evidence for the presence of the ancient Chinese in the Tarim Basin until ca2000 BCE. Therefore, the eastern Indo-Europeans probably arrived there after that time. Assuming these Indo-Europeans separated from the proto-Indo-Europeans at the same time as the western Indo-Europeans (ca4000), then they must have been wandering in the steppes and moving gradually through Transoxiana towards the Tarim ahead of the Indo-Aryans and the Indo-Iranians. There is evidence of an extinct higher culture that existed in the 3rd millennium in the area between Transoxiana and Iran. It has been dubbed BMAC (Bactria-Margiana Archaeological Complex). This civilization was abandoned towards ca1500. The eastern Indo-Europeans might or might not have had to do with it. The eastern Indo-Europeans might have entered the Tarim Basin at the time the Indo-Aryans were moving into the Indus River Valley in Pakistan (ca2000) and the Indo-Iranians were in the steppes. In any event, the Tarim Basin, which the Chinese started dominating during the 1st century BCE, by the 6th century CE was being infiltrated by Uigur Turks, who finally gained ascendancy in the region leading to the final extinction of the Indo-European Tarim Basin civilization.
An interesting question which has not received much attention from either linguists or archaeologists, is what got the Indo-Europeans on the move in the first place. Population density is one obvious answer. A fascinating possibility lies in the demonstration in recent years that the formation of the Black sea was the result of catastrophic flooding with the breaching by the sea of Marmara of a natural barrier in the Bosporus ca5600 BCE. Submerged farming settlements have been found offshore at levels where one would have expected them to be if the shoreline of the Black sea was much further out than it is today. This flooding, which has been compared to a thousand Niagaras set off at once, would have had immediate repercussions on the peoples occupying the lands of the hypothesized Indo-European homeland. Its is even conceivable that the flooding of the Black Sea might have been the source of the Great Deluge legends common to Mesopotamia and the Levant. One significant Indo-European datum is that in the legends associated with the Indic law-giver Manu there is mention of a flood similar to the deluge.