[Here's a more classical version of the Indo-European migration map without the Caspian hypothesis (2008)]

Indo-European Origins and Geography

"Recent concerns for integrating
factors of time (date of languages) and space
(location of the speakers when they entered history)
call for a re-evaluation of all models."
[Carol F. Justus, Indo-European Document Center]

A Theory of Symmetric Expansion
(This first part of the article may still be valid)

A new alternative theory is being created to reconcile historical data (Where do we first see the Indo-Europeans in historical record?) with linguistic and geographic evidence (Where should the Indo-Europeans have traveled before we first see them in historical record?). The theory attempts to integrate these two different aspects of Indo-European studies into one visual representation.

It is based on two assumptions:

1) Any kind of demographic migration proceeds in symmetric, circular waves.
Therefore, each ethnic group or language should have a focus for an outward wave expansion, which has a form of a primary, circular wave on a smooth surface. After this primary wave runs into an obstacle, secondary circular waves are formed with new radial migrations spreading in all directions from that point. (This is roughly an equivalent of the Huygens-Fesnel principle in wave optics where all wave processes are described in this way).

2) All radial lines denoting these wave migrations should be superimposed on local, (paleo)climatic, and (paleo)geographic data to produce meaningful results. In other words, peoples travel only where the (paleo)climate and (paleo)geography allow. They cannot migrate just anywhere across mountains, deserts, or countries occupied by other ethnic groups. Therefore, out of a total number of possible radial routes, only a small number of routes are taken in real life.

Moreover, these actual migration routes may often coincide with the high population density areas of today, since people could have first travel approximately along the same lines they settle down and live for centuries or thousands of years afterward (this is true if we assume for simplicity that there were no drastic climatic or geographic changes over the past 5000 years).

These two key ideas are corroborated by data from historical record, e.g.
(1) the expansion of Ancient Rome (from the city of Rome outward, -- the Mediterranean Empire)
(2) the expansion of Greek culture (from the Aegean Sea outward along all sea routes to Sicily, Spain and the Caucasus)
(3) the growth of the Chinese civilization (from a small region near the Huang He outwards) (fig. below)
(4) the growth of Persian Empire (from Media outward) (fig. below)
(5) the expansion of Egyptian civiliztion (from the Upper Nile to Nubia in the south and Canaan in the north)

[The inverted night illumination maps were used because they supposedly reflect the actual population densities both at present and during the antiquity.]

The first figure (based on Earthlights, NASA) shows the expansion of the Chinese civilization from the Han Dynasty level (c. 1000-1500 BC) to the late Zhou level (by 350 BC) and to modern times (1700 AD). Even though the Chinese civilization may look assymetrical at first sight, a closer examination reveals slow symmetrical expansion from the region near the lower Huang He, which proceeded using only geographically feasible routes and areas.

The second figure demonstrates that the location of Media approximately coincides with the center of the Persian Empire at its maximum, so by tracing the borders at any period we can possibly predict the location of its craddle near the Caspian Sea.

Similar maps may be created for other instances, e.g. tracking the location of the homeland of the Roman Empire to northern Italy, etc.

In some cases (such as North America – the conquest of the Wild West, and Russia -- the conquest of Siberia, the expansion of Austronesian and Bantu languages) the expansion seems asymmetrical and lop-sided, but this can be explained using of the second postulate: the absense of free land at one side forbids the spread in the direction in question.

Temporal considerations for the wave motion

The present wave theory (as applied to the Indo-European studies) suggests that the Indo-Europeans might have travel at a predictable speed in various directions. This is further supported by the fact that the time of arrival of the Indo-Europeans to the utmost eastern and western points is nearly the same, which probably implies a gradual expansion from a common center at roughly the same rate.

For example, the Lusitanians arrived in Portugal by c. 500 BC, which is about at the same time as the Sinhalese arrived in Sri Lanka (543-482 BC, the Prince Vijaya legend). The arrival of Aryans in Mohenjo-Daro around 1800-1700 BC is matched by the migration period of the Hellenic tribes to northern Greece about 1600-1800 BC, especially if one takes into consideration the differences in distance required to migrate around the Black Sea, as well as other factors that may affect the rate of migration.

In other words, the symmetric expansion is natural, whereas the asymmetric one should somehow be explained and accounted for. In many older theories of the Indo-European Urheimat, the focus was commonly dislocated to one side, often being placed near the author's own homeland. Such are some of the Northern Europe hypotheses or the Out-of-India theory. It's important to note that this kind of nationalistic subjectivism must be excluded from further research, and this can be done using the current symmetry approach.

The Caspian Sea Hypothesis (see "Notes" below)

As far as the Proto-Indo-European origins are concerned, the present representaion shows that the initial migration focus of the IE expansion must be located in the Caspian Sea region – most likely, in the area of present day Dagestan, Azerbaijan or the Province of Gilan in northern Iran. This conclusion comes from backtracking all routes symmetrically to one central point in space and time. The three large concentric circles on the large map show how this was done. (Comment: the Caspian Sea hypothesis could be oversimplified, as the Eurasian continent has no symmetric shape.)

Also note that the Caucasus mountains would serve as a natural barrier for the separation of PIE dialects. The PIEs that remained to the north of Azerbaijan – in Dagestan (Russia) – could have preserved the ancient consonant system and remained Centum, while the group that lived along the Caspian coast to the south of Azerbaijan – in Gilan (Iran) – acquired the palatalization charactaristcs and became Satem.
(Of course, if the distinction between Centum and Satem dates back to the period of the intial split, and is not, at least partly, a later development).

The backtracking in time indicates that the Indo-European nation might have still existed as a single unity until 3000 BC (The dating is very uncertain, but the present version of this theory is not sufficiently elaborated for temporal applications)

Why the expansion?

We suggest that the reasons for the expansion were mainly technological. The IEs could have had a better technology, and this is why they were able to spread their language over such and enormous territory.

Note that the conquest of the Americas, Australia and Siberia by the Spanish, French, English or Russian invaders during the 17-19th centuries is nothing but a continuation of the same ancient Indo-European expansion (!), which this time was brought about by the invention of modern ships and firearms. By analogy, the technological reasons for migration could also have been significant in the past.

Thus, we suppose that the most plausible explanation (aside from the theories of sudden climatic changes) could be linked to the invention of the horse-drawn wheeled cart or horse-drawn chariot, an event that constituted a tremendous technological advancement to the whole of the PIE proto-civilization allowing ancient tribes to gradually migrate in all directions searching for new farming territory. (Not necessarily correct, but this technology principle might still be valid)

Indeed, we know that the horse was probably domesticated between the Dniepr and the Don around 4200-3500 BC (The Sredny Stog Culture), while the wheel seems to be invented in Sumer c. 3300-3100 BC. As you can easily see, the imaginary expansion circle (according to the present theory of wave expansion) for these two technological inventions seem to overlap in the area of the Caspian Sea at just about the same time as the mass migration of IEs supposedly began there, that is, circa 2900 BC.

It seems to be a proven fact that the Indo-Europeans already used horses at this period of time, so if they discovered the wheel manufactured by Sumarians (who used only donkeys and oxen at the time – e.g. earliest depictions c. 2600 BC, Ur), they could quickly adapt the new invention to horses, thus creating the wheeled horse-drawn cart – a very effective means of transportation of the time. (The other possibility being that the wheel was their own invention). In this way, they were able to spread all over Eurasia. In other words, the horse came from the north, the wheel came from the south, and the only narrow passage where they could have possibly met is the west coast of the Caspian Sea (!).

The wheel is indeed known to the Kura-Araxes archeological culture in the South Caucasus (3200-2500 BC). The earliest known spoke-wheeled chariots date from c. 2000-1800 BC (Sintashta-Petrovka findings southeast of Magnitogorsk, see map) which seems to coincide with the advances of Proto-Tocharians near the Urals in 1900-1800 BC.

Interdisciplinary data that support this view

1) Lexical evidence
Geography and plant &animal life data seem to corroborate the Caspian Sea hypothesis. The classic requirements for the names of "salmon" and "beech" are fulfilled. Indeed, salmon live in the Caspian, and beech forests abound in the area. Interestingly, the preservation of many other lexemes denoting natural life and resources can be explained just as well.

Proto-Indo-European language: birch, beech, pine, oak, hornbeam, cedar, aspen/poplar, willow, cherry, apple, maple, alder, hazel, nut/chestnut/walnut, elm, linden, ash, yew; horse, sheep, pig, ox, cow, sow; wolf, bear, wild boar, deer, fox, lynx, lion, antelope, wild ox (bison), squirrel, beaver, otter, mouse, tortoise, snake, salmon; bird, eagle, crow, thrush, grouse, crane, goose (swan); bronze, copper

Dagestan: oak, pine, hornbeam, birch, beech, maple, poplar, black alder, willow, cultivation of cherries, apples, apricots, pears, melons, wheat, rice; European bison (aurochs), mountain goat, roe deer, chamois, bear, deer, leopard; wild turkey, partridge, jackdaw, eagles; coal, iron ore

Azerbaijan: beech, oak, and pine forests; cultivation of wheat, persimmon, wine grapes; Alpine meadows in the mountains used for pasture; bear, roe deer, mountain goat, wild boar, lynx, leopards, European bison (wisent), fox, porcupine, bat, rabbit, weasel, wildcat, badger, gazelles, jackals, hyenas, dormouse, rat, squirrel, snakes (viper);
extensive pastures for sheep, cattle, and goats;
prey birds, pheasent, flamingo, waders, pelican, partridge, turtledove, grouse, ringdove, vulture, owl, partridge, ortolan, spoonbill, kingfisher, pigeon, heron, geese, cranes, ducks, eagle
iron ore, copper, zinc, lead, salt

Gilan and the Elburz region: beech, oak, and hornbeam (ironwood) forests, conifers (cedar), wild fruit trees (almond, pear, pomegranate, walnut), silk tree, walnut, pistachio, plane, alder, yew, acacia; cultivation of tea, olives; wolf, fox, bear, mountain goats, red mountain sheep, roe deer, rabits, gerbils, Caspian tiger, hyena, leopard, deer, gazelles, lynx, snakes, lizards; waterfowl; coal, iron ore, lead, zinc

Caspian Sea: sturgeon (and its varieties), salmon, pike, perch, carp, herring, mullet, sprat, tortoises, porpoise, Capian seal, pelicans, swans, flamingos, herons, egrets, sandpipers, partridges, various migratory birds

The few remaining words (hazel, elm, linden, ash, beaver, otter) should be added to the common list just as well, because linden, ash, and elm should grow in the Caucasus and could be lost in the lists above due to incomplete information; beavers and otters were hunted and exterminated in modern times, while their original habitat was larger covering most of Europe and Asia. In other words, this looks like almost one-to-one correspondance.

Also, please note a remarkable preservation of the word "salt" throughout most IE languages – L sal, Gr. hals, Sans. salila, Latv sals, Rus. sol', Germ. salt. (The salt is found in abundance around the Caspian due to sea level variations and intense evaporation)

2) Archaeological evidence
The territory of Caucasian Albania has been inhabited for more than a million of years. Copper needles and other artifacts appear in Azerbaijan and eastern Georgia by 4000 BC. In 3300-2200BC, Dagestan is the location of the Kura-Araxes culture that extended over all of the Lesser Caucasus to Armenia. It was characterized by farming and live-stock raising economy, and the use of wheeled vehicles.
Unlike the IE culture, the Kura-Araxes was also characterized by the cults of hearth, sun, aurochs, depictions of helixes and other apparently purely Caucasian features, therefore it should be rather considered as Nakh-Dagestanian. Still, the IEs may be somewhere near, for example, they may be located to the south or even below the sea level. In some theories, the Kura-Araxes is identified with the homeland of Proto-Anatolian speakers, an idea that seems to be supported by the current symmetric expansion theory.

A very short list of archealogical findings has been added to demonstrate that artifacts typically associated with IEs (rich burial sites, catacombs, wheel, horse, early bronze and copper tools) tend to show up in the target region as part of the Kura-Araxes culture.

1) Derbent (Dagestan), Bronze Age agricultural communitities, c. 3000 BC

2) Velikent (near Derbent, Dagestan), site discovered in the 19th century, burial catacombs (no mounds), numerous ceramic vessels, models of wheels, bronze axes, knives, hews; jewelry, early layers c. 3000-2500 BC, most layers c. 2500-2000 BC.

3) Kultupe (Nakhichevan, on the border of Armenia and Azerbaijan), a figurine of horse and horseman, c. 3000 BC

4) Baba-Dervish (Azerbaijan), dugout huts, bone and stone objects, furnaces, wheat and barley cultivation, weaving, pottery, figurines of wheels, aurochs, clay models of woman, c. 3000-2000 BC

3) Paleoclimatic evidence: the Caspian Sea level influence
The Caspian Sea is an inland body of water located in a deep depression and separated from the ocean at least since the end of the Ice Age or earlier. It is characterized by regular regressions and transgressions due to natural climatic changes, so its level and shoreline vary constantly with the time. As the graph shows, in 4200 BC the level began to fall. It reached its historical minimum of -36 m (-115 ft) (c. 3000 BC, and then it slowly began to rise again until it reached the present day mark of -29 m (-93 ft) by 2700 BC. Coincidentally, the split of IE dialects matches the strongest demonstrated decline in the last 15 000 years (!). Therefore, we arrive at the idea of the Caspian Sea Bridge that seems to explain how the PIEs could have spread to India in the 3rd millenium BC.

[A. G. Kasymov, The Caspian Sea, 1987]

The probable scenario:
1) 4000-3300 BC: the PIEs migrate to Dagestan from the Pontic steppes riding on horseback along the western, narrow, plane Caspian shore formed by the dried up sea bed. 2) 3300-3000 BC: Bronze Age agricultural settlements are formed near Derbent (Dagestan). The wheel is invented in Sumer (or independetly by the PIEs) and reaches the area of Azerbaijan largely improving the PIE technology. 3) 3000-2500 BC: Using new technology the IEs begin to spread to the north, south and along the Kura estuary as the sea level begins to rise again. By the time a new regression starts in 2500 BC, the PIEs must have already advanced far enough from their original habitat.

The map of the Indo-European expansion
from the viewpoint of the Caspian hypothesis

Although the map below might contain some errors, it shows how the Indo-European migrations could have proceeded in principle (So far, I have not found any similar detailed representation elsewhere). (As of 2007: most dates, except the well-established ones (black color) are probably too recent)
[See a more standard version of the Indo-European migrations map without the Caspian hypothesis (2008)]

Arrows were carefully placed around natural obstacles and along suitable trails which supposedly coincide with the areas of dense population of today. A number of other factors (such as the presence of foreign tribes or ancient kingdoms, winter temperatures, migration along shortest and quickest routes) were also taken into consideration. When fully elaborated, this type of map based on the ideas of circular expansion is supposed to predict the IE migration routes with considerable precision.

orange region: the expansion focus (the late Caspian Urheimat)
yellow: probable early migrations from 3000 to 2000 BC
white: migrations from 2000 to 1000 BC
light blue: late migrations since 1000 BC
red: areas occupied by foreign nations
black dots: ancient cities
symbols of wheel: regions where the wheel was first found or invented
symbols of horse: probable regions of horse domestication c. 4000-3000 BC
numbers: where and when the Indo-Europeans are almost definitely first seen in historical record

Geographical Evidence: Why the Caspian Bridge?

(This reasoning could in fact apply to the Indo-Iranian migration, not necessarily just Proto-Indo-European)

How did the IEs find their way to India from Pontic steppes? Why the western coast of the Caspian Sea, not the eastern one, for instance?

What looks like a tiny bit of terrain on a map, might in fact be an enormous, impenetrable territory. Curiously, Asia and Europe seem to be separated by many obstacles along thousands of miles with suitable passages only at several locations. Consequently, it's vital to take a close look at local geography and paleoclimate before tracing any migration routes.

1. The Ustyurt Plateau between the Caspian and Aral Sea. Arrid, no-man's-land for hundreds of miles with scarce saline water sources. (Note: in 2000 BC, precipitation could have been greater by 20-50%)

2. The ridge of the Ustyurt Plateau (150-300 m high) is in fact an ancient sea shoreline. Hardly any roads or humans, though half-wild horses manage to survive in the area.

3. The Karakum ("The Black Sand") Desert is just to the south of the Ustyurt Plateau. So after you cross the Ustyurt, you have to cross or go around these sand dunes before you get to Iran or Afghanistan. Note: The Amu Darya River used to flow along the northern edge of the Karakum into the Caspian Sea until c. 500 BC (its dried up bed is still known today as the Uzboy). Obviously, the Uzboy delta was covered by oases, and the climate in the area was generally a little milder than it is today. The question is whether the climate of all of the eastern coast was mild enough to allow for settlements of agricultural and cattle raising communities for hundreds of years. More research on local paleoclimatic conditions is needed.

4. In Abkhazia, the Caucasus Mountains are too close to the Black Sea – one can even see snow ridges from a boat at some locations. In some places, the engineers had to build hanging bridges over the shoreline. It's unlikely that this could be an easy trail for migrations on horseback, by cart, or chariot in 3000 BC, especially considering the possibility of intermittent attacks from local inhabitants (the Proto-Abkhazians?).

5. The trail across the northern Caucasus goes along mountain rivers, through narrow, winding gorges and over deep canyons. The Scythians were able to penetrate the Daryal Gate from the north c. 800 AD which is evidenced in its ancient name of "The Alan Gate" and the fact that their descendents (Ossetians) still live in this region. Natural hardships as well as potential conflicts with the Proto-Nakh and ancient Kartvelians raise too many questions about the validity of this trail as the main route for the Indo-European migration.

6. Finally, this is Derbent (Dagestan), the oldest and southermost city in the Russian Federation inhabited by agricultural communities since 3000BC(!!). The urban settlements date from 700 BC. Derbent is situated in the most narrow strip of land between the Caspian and Caucasus, yet still wide enough to accomodate a city with fairly large population. Known to ancient Greeks, occupied along the timeline by whoever followed this route from south to north and back (Caucasian Albanians, Scythians, Romans, Persians, Huns, Khazars, Arabs, Turks, Tatars, Mongols, Russians), it is this city that has always been the Gate to Europe for Asians and the Gate to Asia for Europeans, second only to Constantinople in this role. The ancient fortress was built by the Sassanids c. 550 AD. The 30-mile long walls used to extend down all the way to the sea protecting Persia from the Huns in the north.

Therefore, Gilan, Azerbaijan and Dagestan (the territory of Caucasian Albania) is the most probable area to search for the natural trail to India for the PIE tribes.

7. The upper Kura River valley in eastern Georgia and Azerbaijan has arable land suitable for agriculture. The climate is mild, the average temprature in January is about 1oC (34oF). Anyone moving from the Caspian Sea to Armenia or Anatolia has to cross this region.

8. A village in Dagestan where oak, pine, hornbeam, beech and birch still grow in dense mountain forests. A probable homeland to Centum languages in 3000 BC according to the theory.

9. The Caspian Sea coast in Gilan (northern Iran). Even if this were not the homeland to the Satem languages, the Indo-Europeans could have passed through these hills by 2700 BC.

10. Mount Ararat in Turkey. It's possible that the Anatolians had lived around these hills for centuries in close contact with the Hurrians and Urartians.

11. A village in Kurdistan, where both the early Mitanni (c. 1700 BC) and early Iranians (the Medes) (c. 1000 BC) seem to have started their expansion. Looks like nothing changed here in all these millenia.

12. Horses in eastern Kazakhstan – today like 4000 years ago when this territory was first occupied by Proto-Tocharians (c. 1500 BC). If the Indo-Aryan tribes would have come as far as this region to find their trail to India, the IE languages would probably be scattered over much larger territory in western China, and their early presence would not be so evident in western Iran, Kurdistan, Armenia or Anatolia.

Difficulties with the Anatolian group
(Nothing new, the Anatolian group, and possibly even Armenian (!) poorly fit into the IE family)
We know that Hittite is characterized by changes that set this language aside from other groups. Most likely, the changes in Proto-Anantolian were due to the strong influence of Proto-Hurrian and Proto-Urartian adstratum. The Proto-Anatolians must have been highlanders who inhabited the territory of modern Armenia in 3000-2000 BC slowly pushing their way to the west. Therefore, the Anatolian branch can in fact be an offspring of the two language famililes (?)– the Caucasian and IE. This seems to be a natural phenomenon, since Armenian has also undergone considerable changes as compared to Indo-Iranian or Balto-Slavic.
It is also possible that Phrygian (known since 700 BC) is just a continuation of Proto-Armenian moving along the same route of the Hittites into Anatolia and then the Balkan Penisula or vice versa, because it is not even clear if they moved westwards or eastwards. Illyrian (known since 600 BC) may be of the same stock, too (?). Note: Illyrian and Armenian are normally viewed as Satem, and Phrygian is also seen more closely related to the Satem branch and the eastern languages. Moreover, both Herodotus and Eudoxus of Rhodes related the Armenians to the Phrygians.

Agreement with former theories
There's no need to reject any other theories, especially the Kurgan hypothesis by Gimbutas (1956). We could assume that the southern Ukrainian territory before 3000 BC was just an earlier habitat for the Proto-Indo-European tribes – an area where they might have lived before they moved to the Caspian coast.

Many of the Gamkrelidze and Ivanov ideas (1986), who viewed the PIEs as southern highlanders and placed the original homeland in the region of present day Armenia, are also supported.

On the other hand, this approach rules out theories that place the Urheimat too far to the west, yet, it is possible that such theories refer to earlier stages of IE expansion.

Note that the traditional Kurgan hypothesis is far from perfect.

[The Great Soviet Encyclopedia, 1970-77]

The main problem is that the reconstructed late PIE flora and fauna is simply NOT of the steppe type. Neither oak, nor hornbeam, nor pine, nor beech normally grow in the Ponto-Caspian steppes, in fact, they grow anywhere except this region. And if the steppes had been covered by light woods in the past, the existing conditions might not have been suitable for domestication of horses. The late PIE wild life must have been of a mixed type consisting of many varieties of trees and animals, with sufficient meadows for pasturing horses, sheep and cattle, and this kind of environment is typically found in mountainous and coastal regions.

The Urheimat Continuity Approach
(This part could still be correct in general terms)

Instead of searching for one specific original epicentric spot, it is now possible to attempt to reconstruct a continual track of events and show how the PIE tribes could have been moving around Eurasia since the Neolithic. It turns out that many theories proclaimed by different authors do not necessarily contradict to each other — rather they work together to produce one single timeline.

Possible PIE timeline:
7000 BC Anatolia, farming
6000 BC settling down around the western coast of the Black Sea
5000 BC elevation of the Black Sea level
4500 BC pushed to northern Pontic steppes
4200 BC domestication of the horse in the Sredny Stog culture
4000 BC emergence of the Yamna culture
3600 BC early influence on the early Maykop culture
3500 BC arrival in Dagestan and Azerbaijan
3400 BC separation of Proto-Anatolian (Proto-Hittite)
3400 BC early expansion of PIEs into Europe (Globular Amphora, Baden, Corded Ware) begins
3300 BC the Yamna at its peak
3200 BC interaction of Proto-Anatolian with the Kura-Araxen culture
3100 BC satemization of the southern branch
3000 BC first use of wheeled carts by IEs
2900 BC elevation of the Caspian sea level
2900 BC expansion of classic IE dialects begins
2800 BC expansion of the Yamna culture beyond the steppes begins

The ancient and modern history can, in fact, also be seen as part of IE migrations
500 BC arrival in Portugal and Ceylon
200 BC decline of the Greek civilization
400 AD decline of the Roman civilization
1100 AD crusades of Europeans to the Middle East
1500 AD expansion of the Romance group to America
1600 AD expansion of Slavs to Siberia after the implemenation of firearms
1700 AD early Germanic settlements in America, Africa, Australia
1800 AD conquest of the wild west in America
1900 AD The German "Drang nach Osten" and the two world wars

Arguments for the "Caspian Bridge"

1. Physical arguments. It is finally possible to explain space and time distribution of IE dialects and languages and draw detailed maps using physical ideas of symmetric expansion around natural obstacles
3. Lexical arguments. The flora and fauna of the target area coincide with the reconstructed IE stock by 95%.
4. Technological arguments. It is possible to give a good technological reason for the immense Indo-European expansion – the invention of a horse-drawn wheeled cart.
5. Paleoclimatic arguments. The reason for moving in or out of the original area may be linked to variations in the Caspian sea level. (The Caspian Sea Bridge to Asia)
6. Archeological arguments. Bronze and copper age equestrian cultures seem to be spread in the target area at the right time. Although, data are too scarce so far.
7. Indirect metallurgical argument. Early metallurgy is connected with mountainous regions.
8. Indirect climatic argument. Since most early agricultural civilization tend to arise along parallel 35, the PIE Urheimat should be relocated further to the south.
10. Indirect heuristic argument. Partial integration with other theories is achieved.

References

Encyclopedia Britannica, 2002
Encarta Encyclopedia, 98-2005
Great Soviet Encyclopedia, 1977
Lingvisticheskii Enciclopedicheskii Slovar, 1990
A. G. Kasymov, The Caspian Sea, 1987
V.A. Safronov, Indoevropeyskie Prarodiny, 1989
T.V. Gamkrelidze, V.V. Ivanov, The Indo-European Language and Indo-Europeans, 1984
Various web sources

NOTE (as of 03.2007):

After much argument, I believe the present version of this theory should be taken with a large grain of salt. There are many discrepancies as to the dating (3000 BC is too late), and the oversimplified positioning of all of the PIE ethnic groups in one single point. Besides, the asymmetry of the Eurasian continent indicates that positioning the PIE Urheimat near the Caspian Sea is rather contrived, so the more generally accepted placement in southern Ukraine looks more plausible. But the wave method itself as described in the initial paragraphs could still be valuable if sufficiently corrected and elaborated. The idea of the Caspian Bridge route could also apply to the spread of the Indo-Iranians, or possibly other ethnic groups moving to Asia.

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Indo-European Origins and Geography (c) 2004-2007
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