The Aryan invasion theory is a historical theory first put forth by the German Indologist Friedrich Max Müller and others in the mid nineteenth century in order to provide a historical explanation for the existence of Indo-European languages in India. According to the most common version of the Aryan Invasion Theory (AIT), the Aryans originated in South Russia (Ukraine), from where they invaded or migrated to Iran, India, Central Asia and Europe.
The theory itself has a complex history — initial acceptance, subsequent modifications, and currently new challenges in terms of counter theories. No single conclusive theory now prevails. Rather, combinations of theories are generally accepted.
The best-known form of the theory was developed by European historians in the late nineteenth century. As expressed, for example, by Charles Morris in his 1888 book "The Aryan Race", this theory holds that a Caucasian race of nomadic warriors known as the Aryans, originating in the Caucasus mountains in South-Eastern Europe, invaded Northern India and Iran, somewhere between 1800 and 1500 BC. The invaders entered the Indian sub-continent from the mountain passes of the Hindu Kush, possibly on horseback, bringing with them the domesticated horse. The theory further proposes that this race displaced or assimilated the indigenous pre-Aryan peoples and that the bulk of these indigenous people moved to the southern reaches of the subcontinent or became the lower castes of post-Vedic society. The Aryans would have brought with them their own Vedic religion, which was codified in the Vedas around 1500 to 1200 BC. Upon arrival in India, the Aryans abandoned their nomadic lifestyle and mingled with the native peoples remaining in the north of India. The victory of the Aryans over the native civilization was quick and complete, resulting in the dominance of Aryan culture and language over the northern part of the subcontinent and considerable influence on parts of the south.
The theory was first proposed on linguistic grounds, following the discovery that Sanskrit was related to the principal languages of Europe (the Indo-European language group). It was assumed that Northern India, in which languages derived from Sanskrit were spoken, must have been occupied by migrants speaking Indo-European languages. The dominant languages in Southern India, known as "Dravidian", were assumed to have been spoken by autochthonous pre-Aryan peoples, who had been displaced southward. Hence the Aryans were said to have supplanted the Dravidians in the north of the subcontinent.
Initially Max Müller assumed that the migrants would have been farmers, but later writers envisioned an invasion by nomadic warriors. It was proposed, on the basis of passages in the Rig-Veda and assumptions about surviving racial hierarchies (see Dasa), that these invaders were light-skinned people who had subdued darker aboriginal people and then mixed with them. The theory fit some existing ideas that justified contemporary European colonization. Initially, the aboriginal 'Dravidian' occupants of India were assumed to have been primitive, and the achievements of ancient India were credited to the descendants of the Aryan invaders. In the 1920s, however, the Indus Valley Civilization was discovered. It was obviously advanced for its time, with planned cities, a standardized system of weights and bricks, etc, and it was understood that if the Aryans had invaded, then, regardless of their later achievements, they had in fact overthrown or at least supplanted a civilization more advanced than their own.
The association of Aryans with a physical "race" also has been slowly dropped. Max Müller clarified late in his career that by Aryan, he only meant a group of languages and not a race. Romila Thapar also maintains that Aryan never meant race in the Rig Veda and that the Proto-Indo European speaking people were already "a mixed bunch" and not a purely Caucasian race.
Questioning the theory
Accepted generally when it was first propounded, this theory has since been questioned on two fundamental grounds: firstly, whether the Aryans came through bloody invasions or through peaceful migration, and secondly, whether the Aryans came from outside the Indian subcontinent at all.
The issues raised by these lines of questioning are discussed in the subsequent paragraphs.
Theory of migration rather than invasion
The first responses to the Aryan invasion theory accept the basic premise that the Aryans came from outside India but speculate and differ on the nature of their ingress. The proponents of this camp are of the opinion that there is very little archaeological evidence for an invasion. For the invasion theory to be viable, the Aryans would have had to discover mountain passes among the treacherous Hindu-Kush mountains, most of which are snow free only three months a year. The Aryan invaders, being a nomadic people would be far smaller in number to the Indus Valley civilization, which was spread over an area greater than 1.8 million km², with an estimated population greater than the combined populations of all the other river civilizations at that time except ancient China. They would then have to quickly and completely rout an advanced civilization living in fortified cities over a large geographic area and impose their culture, language, cosmology and religion on the local population without leaving any physical traces of themselves.
In addition, there are practically no archaeological signs of an invasion, such as carbonized layers in the Indus Valley city sites. Nor are there oral or written legends of an invasion. It seems much more likely that Aryan migrants found mountain passes and entered the sub-continent during the snow free months and settled within or close to the Indus Valley civilization. Multiple waves of migration are possible, causing a mingling of the immigrant and local populations. There may have been significant exchange and assimilation of culture and language on both sides. The immigrants may have travelled back and forth to their original lands taking language and culture to other Indo-European peoples, especially Ancient Persia. Human skeletal remains excavated from sites of the Indus Valley civilization show a mixed ethnic composition similar to the present, showing support for migration rather than an invasion. Thus the idea of "invasion by barbarian Aryan hordes" has been replaced by "immigration and acculturation by a small group of linguistically Indo-European people".
Currently, there is a general acceptance of this theory of migration or gradual incursion.
Theory that Vedic Aryan culture originated in India
In recent times a different viewpoint has been proposed: that no such Aryan migration or invasion occurred; that the Indus Valley civilization was the civilization described in the Vedas; and that the Aryans originated in India. Some advocates of this position propose that the Proto-Indo-European language actually originated in India, from which its earliest speakers spread westwards. Others believe that the Indo-European languages originated outside India, but that they spread into India before the development of the Indus Valley Civilisation. On this view, the Indo-Aryan sub-branch of the IE languages evolved within India, along with the beliefs that became Vedic culture.
Based on recent discoveries of what are interpreted as Vedic elements in the Harappa and Mohenjodaro sites, as well as newly excavated cities in Gujarat and off the coastlines of Eastern and Western India, the counter-theory proposes that the great Vedic Saraswati River is the dry river bed that has been identified in North-Western India and that the 'Aryan race' is nothing more than those Indian tribes considered 'noble' for adherence to Vedic principles, not for their racial characteristics or lineage. This theory of the Aryan culture being indigenous sometimes proposes Vedic Indian culture coming into being as early as 5000 BC, and slowly developing till around the time of the dissolution of the Harappa and Mohenjodaro cultures, whose disappearance is linked to the drying of the Saraswati River. In other versions it may have developed within the conventional time-frame, but from long established inhabitants of the area. Researchers remain divided on this topic with the majority adhering to the established account.
Evidence relating to the theory
Over two thousand Indus Valley sites have been identified, but only five percent of them have been excavated. The ongoing investigation of the Aryan question involves:
- archaeology of a large area and a long period of time;
- archaeogenetic evidence from the existing population;
- linguistics involving Indo-European branches, Vedic Sanskrit and Dravidian;
- hermeneutics involving Indian and other scripture (Vedas, Puranas and the Avesta);
- geography of the areas involved.
It is hard to be an expert in all the above disciplines over such a large area and over such a long time period, so observations or claims made by any person may show accuracy and thoroughness in one area but faulty analysis or oversight in another.
The opponents of continuity primarily focus on showing that the Rig-Vedic culture is pastoral, external to the Indian sub-continent and that a chronological gap exists between the Indus Valley and the Rig Vedic cultures.
Also, strong similarity of ideas between early Indian culture and other ancient cultures make scholars suspect that the cultures might have been originated somewhere in central asia and then migrated in different directions. For instance, the attack Homer describes in Iliad against City of Troy , where soldiers hide in the body of a horse, has a strikingly similar counter-instance in sanskrit where playright Bhasa writes about capturing an enemy king by luring him to an elephant hunt where enemy soldiers are hiding inside the stomach of a wooden elephant.
Proponents of continuity focus on stressing that the Rig-Vedic culture is native to the sub-continent, urban in nature, makes constant references to bodies of water (Central Asian nomads would not have been exposed to seas) and a chronological peer of the Harappan culture, and that perhaps they are the same culture.
The individual arguments may focus on linguistics, use of metals, domestication of horses or differences in described geography, but the basic focus is to identify the Rig-Vedic culture with or against the Indus Valley civilization.
Only five percent of the known Indus Valley sites have been excavated, so one can expect a constant stream of archaeological evidence to be unearthed in the future. Unlike hermeneutic evidence, there are very few issues with archaeological evidence, primarily due to the reliability of Carbon-14 and Thermo-luminescence dating.
The discovery of the Harappa and Mohenjo-daro sites changed the theory from an invasion of implicitly advanced Aryan people on an aboriginal population to an invasion of nomadic barbarians on an advanced urban civilization. Recent DNA evidence showing a change in the ethnic makeup of the people in the subcontinent once between 6000BC and 4500 BC and then again between 800 and 200BC caused Romila Thapar to state that the Aryans were already a mixed bunch when they arrived in India.
Among the archeological signs claimed to support the theory of an invasion are the many unburied corpses found in the top levels of Mohenjo-daro. Some interpret these as victims of a conquest of the city, while others suggest that they were victims of an epidemic, left unburied as a result of the breakdown of city sanitation.
An important piece of archaeological evidence mentioned in support of the invasion theory was the absence of horses in the Indus Valley civilization, while the Vedas make frequent mention of the horse. (Though the earliest domestication of the horse is widely agreed to have occurred in the grasslands of Central Asia, the first use of horses in South Asia is a topic of great dispute.) The horse specialist Sandor Bökönyi (1997) believed that excavated teeth from the Harappan site Surkotada could "in all probability considered remnants of true horses", but others like Meadow (1997) disagree, because remains of the Equus caballus horse are difficult to distinguish from other horse species like Equus asinus or Equus hemionus. However, terra-cotta figurines claimed to represent horses, and faunal remains were excavated from the sites at Lothal, Surkotda and Kalibangan. The identity of these objects is, however, disputed.
Similar weight has been placed on differences in the types of metals used in either civilization; the importance of the bull to the Indus Valley civilization as evidenced by imagery in seals and pottery, in contrast to the Vedic cow-worship; the importance of the tiger in the Indus Valley civilization and its absence in the Vedic texts; the absence of the six spoked Aryan wheel and the heavy consumption of fish by the Indus Valley dwellers in contrast to the virtual absence of fish in the Vedas.
Proponents of a continuous civilisation point out that the bull is mentioned numerous times in the vedas (next only to the horse), for example verses comparing Soma to the bull [Rig Veda 1:32, 9:92] and Exploits of Indra [Rig Veda 1:33, 7:24, 10:86]. The sacred place of the cow is not Vedic; it originated in later Hinduism during the time of Krishna the cowherd. There are no verses in the Vedas that speak about the need to refrain from cow-slaughter. Verses mentioning fish do exist in the Rig Veda (7:18, 10:68) and the tiger is mentioned in the Yajur Veda (4:4, 5:3, 6:2, 7:7). Terra-cotta figurines excavated are claimed to show chariots with spokes painted (at KaliBangan) or shown in relief (at Banawali).
Recently, the excavation of Dholavira in the Gujarath province of India is claimed by the same camp to show a city that is consistent with Vedic principles of city planning: arameshthina, madhyamesthina and avameshtina or upper, middle and lower cities .
Brian E. Hemphill and Alexander F. Christensen's study (1994) of the migration of genetic traits does not support a movement of Aryan speakers into the Indus Valley around 1500 BC. According to Hemphill's study, "Gene flow from Bactria occurs much later, and does not impact Indus Valley gene pools until the dawn of the Christian era."
Kenneth Kennedy (1984), who examinaed 300 skeletons from the Indus Valley Civilization , concludes that the ancient Harappans “are not markedly different in their skeletal biology from the present-day inhabitants of Northwestern India and Pakistan”(p.102).
Kennedy in a later study (1995) remarks that if an Aryan invasion had taken place, obvious discontinuities in the skeletal record should be found. The two discontinuities that Kennedy finds in the prehistoric skeletal record do not corresopond with a period around 1500 BC. The first of these discontinuities occurred between 6000-4500 BCE (a separation of the Neolithic and Chalcolithic inhabitants of Mehrgarh), and the second occurred between 800- 200 BCE. He concludes that "if Vedic Aryans were a biological entity represented by the skeletons from Timargarha, then their biological features of cranial and dental anatomy were not distinct to a marked degree from what we encountered in the ancient Harappans.” Comparing the Harappan and Gandhara (eastern Afghanistan) cultures, Kennedy remarks that: “Our multivariate approach does not define the biological identity of an ancient Aryan population, but it does indicate that the Indus Valley and Gandhara peoples shared a number of craniometric, odontometric and discrete traits that point to a high degree of biological affinity.”
- Hemphill & Christensen: “The Oxus Civilization as a Link between East and West: A Non-Metric Analysis of Bronze Age Bactrain Biological Affinities”, paper read at the South Asia Conference, 3-5 November 1994, Madison, Wisconsin; p. 13.
- Kennedy, Kenneth 1984. “A Reassessment of the Theories of Racial Origins of the People of the Indus Valley Civilization from Recent Anthropological Data.” In Studies in the Archaeology and Palaeoanthropology of South Asia (99-107). --- 1995. “Have Aryans been identified in the prehistoric skeletal record from South Asia?”, in George Erdosy, ed.: The Indo-Aryans of Ancient South Asia, p.49.
Genetics and Archaeogenetics
The recent advances in Archaeogenetics have some interesting results for the Aryan invasion theory but are still in the early stages. Genetic study shows that Indian population as a whole has little similarity to other areas of supposed Indo-European settlement, indicating there was no mass settlement. Indian maternal DNA is generally similar right across the country indicating that the mass of population has been in place there for a long period. 
More recent results (Kivsild et al. 2003b, see also Cordeaux et al. 2003) show that the combined results from mtDNA, Y-chromosome and autosomal genes indicate that "Indian tribal and caste populations derive largely from the same genetic heritage of Pleistocene southern and western Asians and have received limited gene flow from external regions since the Holocene."
The Haplogroup R1a has been previously linked with the ancient Kurgans and/or Indo-Europeans of southern Russia/Ukraine, who supposedly migrated to Europe, central Asia and India between 3000-1000 BC. (Passarino et al. 2001; Quintana-Murci et al. 2001; Wells et al. 2001). However, the high frequency of R1a found in Punjab and in the South Indian Chenchu tribe, together with a highter R1a-associated short tandem repeat diversity in India and Iran compared with Europe and central Asia, indicates that R1 and R1a differentiation may have originated in South or West Asia.(Kivisild 2003b) The defining M17 mutation has also been found in several south indian tribes (Kivisild 2003b, Ramana et al. 2001, Wells et al. 2001). Stephen Oppenheimer, who reports upon the results of the Human Genome Diversity Project in his book "The Real Eve: Modern Man's Journey out of Africa, (p.152)" comments these findings with the conclusion that: "For me and for Toomas Kivisild, South Asia is logically the ultimate origin of M17 and his ancestors; (...),thus undermining any theory of M17 as a marker of a `male Aryan Invasion of India'." Oppenheimer further believes, that it is highly suggestive that India is the birthplace of the eurasian mtDNA haplogroups, the Eurasian eves. According to Oppenheimer, nearly all human maternal lineages in Europe descended from only four mtDNA lines that originated in South Asia 50'000-10'000 years ago.
The neolithic spread of farmers to Europe from Levant/Middle East has also been linked to 12f2 (haplogroup 9) and the markers M35 (haplogroup 21) and M201. But while M35 is present in Europe, Anatolia, South Caucasus and Iran, Indians generally do not have the Alu insertion in their Y chromosomes. The lack of YAP+chromosomes in India suggests that M35 appeared in the Middle East only after a migration from Iran to India had taken place, but earlier than the later migration of near- and middle eastern farmers to Europe. (Kivisild 2003a)
Since virtually all central asian haplogroups of M seem to belong to the Mongolian, and not the Indian type of haplogroup M, this indicates that no large-scale migration from the present Turkish-speaking populations of Central Asia to India could have occurred (Kivisild 2000)
According to a study by Bamshad et al. (2001), higher caste Telugus have a higher frequency of haplogroup 3 than lower castes, Haplogroup 3 is also characteristic of eastern Europeans. However, further studies have revealed that a high frequency of haplogroup 3 occurs in about half of the male population of northwestern India and is also frequent in western Bengal. These results, together with the fact that haplogroup 3 is much less frequent in Iran and Anatolia than it is in India, indicates that haplogroup 3 among high caste Telugus must not necessarly have originated from eastern Europeans. The high diversity of haplogroup 3 and 9 in India suggests that these haplogroups may have originated in India. (Kivisild 2003a)
Genetic evidence on the origins of Indian caste populations (Bamshad et al. 2001.)
However, a 2001 examination of male Y-DNA by Indian and American scientists indicated that higher castes are genetically closer to West Eurasians than are individuals from lower castes, whose genetic profiles are similar to other Asians. These results indicates that at some point male West Eurasians provided a significant genetic input into the higher castes, a result which supports the notion that the caste system was an attempt by these predominantly male arrivals to keep themselves separate from the native population. 
The genetic studies by Michael J Bamshad and his team (2001) from University of Utah and Dr. Spencer Wells (2003) from Harvard University, give strong backing to the Aryan invasion theory.
In the study by M.J Bamshad and his team  they wrote, "Our results demonstrate that for biparentally inherited autosomal markers, genetic distances between upper, middle, and lower castes are significantly correlated with rank; upper castes are more similar to Europeans than to Asians; and upper castes are significantly more similar to Europeans than are lower castes."
The genetic study involves the analysis of genetic material known as the Mitochondrial DNA which is only passed maternally and so it is used to study female inheritance. The male-determining Y chromosome, is passed along paternally and is therefore used to study male inheritance. The evidence implies that few millennia ago group of males with (Eastern) European affinities invaded the Indian subcontinent from the Northwest of the sub-continent.
The researchers went on to state that the genetic variations among the upper castes and lower castes is the evidence to the origin of the caste system. The people who were either migrating or invading the sub-continent had descendants in the male population largely in the higher castes than in the lower castes. The researchers state that these invading or migrating people might have instituted the caste system.
In the abstract to their paper the researchers stated, "In the most recent of these waves, Indo-European -speaking people from West Eurasia entered India from the Northwest and diffused throughout the subcontinent. They purportedly admixed with or displaced indigenous Dravidic-speaking populations. Subsequently they may have established the Hindu caste system and placed themselves primarily in castes of higher rank."
The study also revealed another classic anthropological observation, that of women being significantly more mobile in terms of caste and hierarchical class than men, who are almost not socially mobile at all in terms of caste and hierarchical class. Genetic evidence reveals that over millennia men have married women from lower castes but women have rarely married men from lower castes. Thus the researchers imply that caste and class to a large extent is perpetuated by women and has also thereby contributed to the minimal mixing of Aryan blood with the natives.
The latest genetic research paper (2004) on Indo-European origins, support the conclusions of the (2001) study, backing the Aryan invasion theory.
(However see Kivisild  and ,for a different view and also Renfrew ).
However, most of the pro-invasionist papers imply that R1a1 is the genetic marker that is representative of an invasion, due to its high frequency in Euro-asia. But an equally likely genetic marker is haplogroup L. This haplogroup is present in Greek, Turkish, Lebonese, Iranians, Central Asian and Indian populations (and Europe, see Kivisild). This marker is found in locations where written sources record the presence of Indo-European languages and people: i.e. Greek, Hittite, Hyksos, Mitanni, Iranians and Indians. Its peak frequency is found in Indo-Iranian populations. Another possible marker is J2. Also the 'Western Euroasian' components that are found in Indian mtdna show a distribution closer to that found in the Southern Caucasus and Middle East, than in Eastern Europe. There is also the question of why one should assume only one Y haplogroup is representative of the Aryan gene pool. R1a1, R1b, J2, L and H - all of which are present in India and Central and West Asia - are all possibilities.
- Bamshad M., Kivisild T., et al; (2001) Genetic evidence on the origins of Indian caste populations, Virus Research 75(2): 95–106, Jun. , ---A review of Michael Bamshad's work is given in the New Scientist (2001) : 'Written in blood'; New Scientist vol 170 issue 2291 — 19 May 2001, page 17.
- Basu et al. (2003) Ethnic India: A genomic view, with special reference to peopling and structure, Genome Research, 13, 2277–2290.
- Cann, R. (2001) Genetic clues to dispersal in human populations: Retracing the past from the present, Science, 291, 1742–1748.
- Richard Cordaux; Robert Aunger; Gillian Bentley; Ivane Nasidze; S.M. Sirajuddin; and Mark Stoneking; (2004) Independent Origins of Indian Caste and Tribal Paternal Lineages; Current Biology, Vol. 14, p. 231–235, February 3
- Kivisild, Toomas et al. 1999a. "Deep common ancestry of Indian and western-Eurasian mitochondrial DNA lineages"  ---1999b. "The Place of the Indian mtDNA Variants in the Global Network of Maternal Lineages and the Peopling of the Old World"  ---2000a. "An Indian Ancestry: a Key for Understanding Human Diversity in Europe and Beyond"  ---2000b. "The origins of southern and western Eurasian populations: an mtDNA study" ---2003a. "The Genetics of Language and Farming Spread in India"  ---2003b. "The Genetic Heritage of the Earliest Settlers Persists Both in Indian Tribal and Caste Populations" , 
- Mait Metspalu et al. 2004. Most of the extant mtDNA boundaries in South and Southwest Asia were likely shaped during the initial settlement of Eurasia by anatomically modern humans  and 
- Oppenheimer, Stephen; (2003) "The Real Eve: Modern Man's Journey ouf of Africa" , 
- Spencer Wells; (2003) 'The Journey of Man: A Genetic Odyssey', Princeton University Press, January.
- Y-Chromosomal DNA Variation in Pakistan 
- Jats (Genetics Section) 
The linguistic facts of the situation are little disputed; however, their historical interpretation is contentious. Most linguists interpret them as implying an Aryan migration into India; the linguistic arguments provide no data that would determine whether this migration was peaceful or invasive, and different linguists have argued for either or for a combination of both on extra-linguistic grounds. However, some historians contest this interpretation, and conversely advocate an Indian origin for the speakers of Proto-Indo-European.
The languages of north India mostly belong to a single family, the Indo-Aryan subgroup (related to Sanskrit) of the Indo-European family of languages. The languages of south India belong to a different linguistic family, the Dravidian languages, including Tamil, a very distinct language in its own right (with literature and tradition from at least 300 BC, disjoint from the Vedic), which has not been proven to be linked with any other language family. While Dravidian languages are primarily confined to the south of India, there is a striking exception: the Brahui, which is spoken in the Indus Valley area, the linguistic equivalent of a relict population , indicating that Dravidian languages were formerly much more widespread and were supplanted by the incoming Indo-European languages, such as Sanskrit. The Elamite language, an extinct language of southeastern Iran, has also often been linked to Dravidian (in a proposed Elamo-Dravidian or Zagrosian family); if this turns out to be true, it would even more strongly imply a more northerly former distribution of the Dravidian languages.
Linguists have several rules of thumb they use to gauge the place of origin of a family. One is that the area of highest linguistic diversity of a language family is usually fairly close to the area of its origin; thus, for example, while most speakers of Germanic languages live in the United States, the highest diversity of Germanic languages is found in northern Europe. By this criterion, India seems to be an exceedingly unlikely candidate for the origin of the Indo-European languages — it has only one Indo-European subfamily, Indo-Aryan, not counting recent introductions of European languages — and eastern Europe appears much more promising; conversely, the highest diversity in Dravidian is found among its northern branches.
However, the early formation of political states also affects the distribution of languages.The Punjab was in historical times settled by Iranians ,Greeks and Huns yet Indo-Aryan languages
dominate,probably due to dominance of latter Indian empires and states.Hence in regions where Persian and India empires dominated many languages died out.This process can be seen in the elimination of Saka and Tocharian languages through the influence of Persians ,Buddhism and Turks.
Another linguistic rule of thumb is that the earliest members of the family to diverge are usually found near the place of origin; the earliest member of Indo-European to diverge appears to have been the Anatolian languages, as Hittite grammar's many peculiarities (including an animate/inanimate gender system which appears to predate the three-gender system reconstructed for the rest of Indo-European) show. The second major divide is often considered to be the centum versus satem divide, a sound shift affecting palatals. Both types are found in Europe, but only satem languages appear to be found in India (with the possible exception of Bangani ; see below.) For reasons such as these, most linguists believe Indo-European to have originated somewhere around the Black Sea: a favorite candidate is the Kurgan culture.
The presence of retroflex consonants (including L) in Vedic Sanskrit is generally taken by linguists to indicate the influence of a non-Indo-European speaking substratum population, since these sounds are found throughout Dravidian and Munda and are reconstructed for proto-Dravidian and proto-Munda, but are not reconstructable for proto-Indo-European — nor even proto-Indo-Iranian — and are extremely rare among other Indo-European languages (having phonetically emerged in Swedish and Norwegian only in recent centuries, as a result of combinations with r.) This argument is strengthened by the presence of words with Dravidian and Munda etymologies in Sanskrit, argued to be borrowings from a previous Dravidian and Munda population, or substratum; some of these etymologies have been challenged, though most have not.
While, to many, all of this may clearly suggest an Indo-European migration into India, critics of the Aryan invasion theory note that this does not automatically imply a migration at 1500 BC from the North-West. Any migration could have occurred much earlier and may not have resulted in any conflict; see Colin Renfrew. They also argue that the "substratum" influences from Dravidian and Munda could equally well be adstratum influences through mutual contact without conquest.
The presence of words describing a temperate climate in Proto-Indo-European — such as a root for "snow" — has also been taken as evidence against the theory of Indian origin for Indo-European; however, this argument is weak, since the Himalayan foothills have a temperate climate.
The argument from the centum/satem divide has been challenged: in the 1980s, Claus-Peter Zoller announced the discovery of apparent traces of a centum language in the Bangani language of the western Himalayas. However, George van Driem and Suhnu Sharma later went there to do further fieldwork , and claim that it is in fact a satem language, and that Zoller's data were flawed. Zoller does not accept this , and claims that their data was flawed. The question is unlikely to be resolved without further fieldwork.
Indo-Europeanists note that the names of several temperate-climate flora and fauna — for instance the salmon and the beech tree — seem to be reconstructible for proto-Indo-European; critics note that the meaning of these words varies from branch to branch, and consider the exact referent of the terms to be as yet unestablished. Proponents of the claim that Indo-European originated in India note that Sanskrit names of purely Indian animals have IE etymologies: mayUra for peacock; vyAghra for tiger; mahiSa for buffalo; pRshatI for spotted deer; iBha and hasthi for elephant. Critics note that these names appear to be derived rather than basic words — for instance, hasthi comes from the Sanskrit word for "hand" — and that they cannot be reconstructed for proto-Indo-European (unsurprisingly, since one would expect such words to have been lost by people traveling to regions without peacocks and elephants.)
A major hurdle with the hermeneutics of the Vedic age is the complexity of the scripture and the Vedic language itself. At the least, a passing knowledge of Vedic Sanskrit is required and scholars who rely solely on translations inherit mistranslations and any prejudices that may be present in the translator's commentaries. Fortunately, the Rig Veda is easy to understand with some knowledge of classical Sanskrit.
A major argument offered against identifying the Indus Valley civilization with a continuous, indigenous Vedic civilization is that the society described in the Vedas is primarily a pastoral one, whereas the Indus Valley civilization was heavily urbanized, and that few of the elements of such an urban civilization (e.g., temple structures, sewage systems) are described in the Vedas. However, proponents of continuity note that the Rig Veda does contain some phrases referring to elements of an urban civilization: city's lord [Rig Veda 1:173], shrine [Rig Veda 9:113], ship with a hundred oars [Rig Veda 1:116] and iron forts [10:101]. Frequent references to the ocean and large tracts of water are also suggested as indicating the idea of continuity, since the most obvious route for IE-speakers to have entered India by would have been through the sea-less inland areas of Afghanistan; although the steppes of Russia (often proposed as an origin for Indo-European) border on two seas, and Central Asia contains two seas, proponents of continuity argue that the people would have forgotten such ideas on their route. They also note that a primarily pastoral society does not exclude the existence of urbanisation, especially since the Vedic books appear to have been composed over a long period of gradual change, rather than being a snapshot of society at one particular moment.
Proponents of continuity state that evidence in the Vedas points to a considerably earlier dating of the text. As an example, they argue that the positions of stars described in the Vedas occurred in 3500 to 4000 BC and point out that there is no account in the text of an invasion, of a great migration, or of an ancestral homeland in Central Asia.
There is, as well, considerable description of a river Saraswati. Recent geological evidence (taken from satellite photographs) has uncovered the existence of a dry riverbed — the Hakra River — going through the Punjab area in the Indian subcontinent.
A few historians believe this river is the Saraswati described in the Vedas. Many of the archaeological Indus Valley sites lie along the remains of this riverbed, suggesting that the Indus Valley civilization may have flourished between these two rivers. Around 1900 BC, however, the Hakra river appears to have dried up (due to earthquakes and the shifting of the path of the tributary Yamuna river, which turned from feeding the Hakra to feeding the Ganges), causing the decline of the Indus Valley civilization.
Opponents of continuity argue that the identification of the Saraswati with the Hakra would lead to inconsistencies, and that the Saraswati is very probably a particular river in Afghanistan, which is known to have had a similar name. They also point to the linguistic and religious similarities between the Vedas and early Iranian sacred literature such as the Avesta, as well as the earlier Mitannian kings of Syria. The languages and the names of gods are very similar and both involve the ritual drinking of Soma. Proponents of continuity retort that it could have been Indian people that moved from India to Iran and interacted with, or founded, the Zoroastrians.
The issue might be settled definitively by the deciphering of the many seals found at Indus Valley sites, which are written with an unknown script. If the language of these seals turned out to be Dravidian or Munda (or any other non-IE language group), this would confirm the theory that an indigenous culture was supplanted by an outside one. (However note the adoption of Aramaic as the official language of the historically IE speaking Persian empire without any such invasion/migration). If it were Indo-Aryan it would support the alternative claim. What the script says would also be of great significance, shedding new light on the Indus Valley culture and possibly on ancient movements within the Indian subcontinent. However, the Indus Valley script remains undeciphered; several decipherments have been proposed — the best-known being Parpola's which interprets it as Dravidian, although some others interpret it as an early form of Sanskrit — but none has been widely accepted among scholars, and the sparseness of the corpus makes it difficult to test such claims. Some even suggest that it may not have been a form of writing after all.
Influence of politics
Like much of history, this question is immensely politically charged with ulterior motives being ascribed to proponents of both camps.
Supporters of the migration theory are faced with several accusations. The major one is that the British Raj and European Indologists from the 19th century to the present day forwarded an Aryan Invasion (and now Migration) in support of Euro-centric notions of white supremacy. Assertions that the highly advanced proto-Hindu Vedic culture could not have had its roots in India are seen as attempts to bolster European ideas of dominance. Many opponents of the Aryan-Vedic continuity in India, like Romila Thapar, are Marxist.
The proponents of a continuous, ancient, and sophisticated Vedic civilization are seen by some as Hindu nationalists who wish to dispense with the foreign origins of the Aryan for the sake of national pride. Another motivation may arise from the desire to eradicate the problem associated with the Indian caste system; the hypothesis that it may originally have been a means of social engineering by the Aryans to establish and maintain a superior position compared to the Dravidians in Indian society may be a source of discomfort.
Socialist accounts of history proliferate in post-independence Indian universities, and Hindutva has emerged as a significant force in Indian politics. Until legitimate and widely corroborated archeological evidence for either side of the argument emerges, ulterior motive rather than genuine scholarship will be seen as underpinning their respective theories.
Influence in theology
Certain modern theories of the origins of Hinduism and Buddhism as they are known today are based around the Aryan invasion theory. In particular there are theories that Soma and Amrita were the Fly Agaric mushroom (Amanita muscaria), which were used by tribes in the Russian steppes; that they were essential to the Aryan religion but did not grow in India; and that this absence led to the development of "spiritual" versions of the substances and a more organized religious system. Differences between 'Aryan' (north Indian) and 'Dravidian' (south Indian) religious practices have also been explained by reference to the theory.
Some Hindu thinkers have reacted against the theory on spiritual rather than historical grounds, claiming it to be 'materialistic'. Sri Aurobindo denies the Aryan invasion theory in his works. He writes that the ancient Hindu scriptures, like, in his opinion, the Bible, cannot be interpreted in the materialistic sense, and there was no such a thing as an "Aryan invasion" in the physical sense.
See also: Indo-Aryan languages, Indo-Iranian languages, Proto-Indo-European, Indo-European languages, Dravidian languages, Sanskrit, Indus script, Urheimat, Hinduism, Rig Veda, Vedic Sarasvati River, Indus Valley Civilization, Caste, History of India, Vedic civilization, Aryan, Aryan Race, Dasa
- Bryant, Edwin: The Quest for the Origins of Vedic Culture. 2001. Oxford University Press.ISBN 0195137779
- Elst, Koenraad Update on the Aryan Invasion Debate. 1999. ISBN 8186471774 , 
- Frawley, David The Myth of the Aryan Invasion of India, 1995. New Delhi: Voice of India
- Sethna, K.D. 1992. The Problem of Aryan Origins. New Delhi: Aditya Prakashan.
- Update on the Aryan Invasion Theory Koenraad Elst's online book.
- The Myth of the Aryan Invasion
- Indus Valley Rig-Vedic Connection Archaeologist B.B.Lal presents visual evidence.
- DMOZ listing
- From A Tribute to Hindusim - compilation
- The Aryan question revisited by Romila Thapar
- Cache of Seal Impressions Discovered in Western India
- Central Asia 2000-1000BC