The Aryan Invasion is a theory first put forward by British Indologist Frederick Max Müller and others in the late 19th century. The theory states that a race of light-skinned nomadic warriors known as Aryans (although this name is etymologically incorrect, see Aryan) originating in the Caucasus mountains invaded India and Iran somewhere between 1800 and 1500 B.C.E, displacing the indigenous Dravidian people and their Indus Valley Culture. The Aryans brought with them their own Vedic religion, which were codified in the Vedas around 1000 B.C.E. Upon arrival in India the Aryans abandoned their nomadic lifestyle and intermixed with the Dravidians in the north of India.
The theory was built off scant archaeological evidence, and many of the dates were based on calculations made from Müller's belief that the world was created in the 4th millenium B.C.E. However, the theory was buttressed by the undisputable linguistic split between North and South Indian languages, with Sanskrit from the North belonging to the Indo-European languages.
At the moment most historians accept the theory, although the idea of a large-scale invasion that was alive around 1900 has made place for the idea of a much more modest invasion, where the Aryans either merged in with the existing population or formed its upper layer. However, a recent generation of archaeologists and historians, mostly from India, has challenged the hypothesis and is convinced that the Vedas point to a presence of the peoples who originally wrote them in India long before the presumed date of the Aryan invasion (ca. 1700 BCE).
Arguments of Opponents
Opponents of the theory state that evidence in the Vedas points to a considerably earlier dating of the text (for example, the positions of stars described occurred in 3500 to 4000 B.C.E.), and there is no account in the text of an invasion, of a great migration, or of an ancestral homeland in Central Asia. There is, however, 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. Some historians believe this river, which at some points may have been five miles wide, and may have been as vast as the Amazon River in its heyday, is the Saraswati described in the Vedas. Many of the archaeological Indus Valley sites lie along the remains of this riverbed, indeed far more than lie along the Indus (which the Vedas name the Sindhu river), suggesting that the Indus Valley civilization may have flourished between the two rivers. Around 1900 B.C.E., however, the 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 Saraswati to feeding the Ganges), causing the decline of the Indus Valley civilization.
In addition to the above archaeological evidence, much of Vedic culture is inconsistent with a nomadic lifestyle, like the use of metallurgy. This suggests that, contrary to the Aryan invasion theory, the Indus Valley civilization was in fact the Vedic civilization and both Aryan and Dravidian were indigenous to north and south India respectively, and that no conflict between Aryan and Dravidian or any Aryan invasion ever occurred.
Arguments of Proponents
On the other hand, certain contradictions between what is known of the Vedic civilization and the Indus Valley civilization have to be taken into account. There is little or no evidence of the use of horses in the Indus civilization, while the Vedas make frequent mention of the horse (a subject of great dispute). Similar weight has been placed on the type of metals used in either civilization, the interpretation of the Vedic people as pastoral in contrast to the urban Indus Valley dwellers, the worship of the bull in contrast to the Vedic cow-worship, the importance of the tiger in the Indus civilization and its absence in the Vedic texts, the heavy consumption of fish by the Indus Valley dwellers whereas fishing goes virtually unmentioned in the Vedas, etc.
Proponents of the theory of Aryan invasion argue that the identification of Saraswati with the Hakra would lead to inconsistencies, and that the Saraswati must be identified with some other river, perhaps in Afghanistan. They also point to the similarities between the Vedas and early Iranian literature such as the Avesta.
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.
Politics
Like much of history, this question is immensely politically charged. Followers of Hindutva very much wish to dispense with the Aryan invasion theory in favor of a continuous, ancient, and sophisticated Vedic civilization. In contrast there are many South Indians who have adopted Dravidian as a matter of ethnic pride. Hindu rejection may also be based on suggestions that the Indian caste system would originally have been a religious means for the Aryans to establish and maintain a superior position in Indian society. On the other hand, the original formulation of the theory probably had a racist background, bringing the Indian civilization back to a Caucasian source.