Neolithic tools found in the Kathmandu Valley indicate that people were living in the Himalayan region in the distant past, although their culture and artifacts are only slowly being explored.

Written references to this region appear only by the first millennium BCE.

During this period, political or social groupings in Nepal become known in north India.

The Mahabharata and other legendary Indian histories mention the Kiratas, who, as the Kirat people, still inhabit eastern Nepal in 1991.

Some legendary sources from the Kathmandu Valley also describe the Kiratas as early rulers there, taking over from earlier Gopals or Abhiras, both of whom may have been cowherding tribes.

These sources agree that an original population, probably of Tibeto-Burman ethnicity, lived in Nepal twenty-five hundred years ago, inhabiting small settlements with a relatively low degree of political centralization.

Monumental changes occur when groups of tribes calling themselves the Arya migrate into northwest India between 2000 BCE and 1500 BCE.

By the first millennium BCE, their culture has spread throughout northern India.

Their many small kingdoms are constantly at war amid the dynamic religious and cultural environment of early Hinduism.

A series of migrations by Indo-European-speaking semi-nomads, known as Aryans, takes place during the second millennium BCE.

These preliterate pastoralists speak an early form of Sanskrit, which has close philological similarities to other Indo-European languages, such as Avestan in Iran and ancient Greek and Latin.

The term Aryan means pure, and implies the invaders' conscious attempts at retaining their tribal identity and roots while maintaining a social distance from earlier inhabitants.

Although archaeology has not yielded proof of the identity of the Aryans, the evolution and spread of their culture across the Indo-Gangetic Plain is generally undisputed.

Modern knowledge of the early stages of this process rests on a body of sacred texts: the four Vedas (collections of hymns, prayers, and liturgy), the Brahmanas and the Upanishads (commentaries on Vedic rituals and philosophical treatises), and the Puranas (traditional mythic-historical works).

The sanctity accorded to these texts and the manner of their preservation over several millennia—by an unbroken oral tradition—make them part of the living Hindu tradition.

These sacred texts offer guidance in piecing together Aryan beliefs and activities.

The sanctity accorded to these texts and the manner of their preservation over several millennia—by an unbroken oral tradition—make them part of the living Hindu tradition.

Permanent Aryan settlements and agriculture lead to trade and other occupational differentiation.

As lands along the Ganga (or Ganges) are cleared, the river becomes a trade route, the numerous settlements on its banks acting as markets.

Trade is restricted initially to local areas, and barter is an essential component of trade, cattle being the unit of value in large-scale transactions, which further limit the geographical reach of the trader.

Custom is law, and kings and chief priests are the arbiters, perhaps advised by certain elders of the community.

An Aryan raja, or king, is primarily a military leader, who takes a share from the booty after successful cattle raids or battles.

Although the rajas have managed to assert their authority, they scrupulously avoid conflicts with priests as a group, whose knowledge and austere religious life surpasses others in the community, and the rajas compromise their own interests with those of the priests.

The route taken by peoples who migrated to India until the entry of the Europeans by sea in the late fifteenth century, and with the exception of the Arab conquests of Muhammad bin Qasim in the early eighth century, has been through the mountain passes, most notably the Khyber Pass, in northwestern Pakistan.

Unrecorded migrations may have taken place earlier, but it is certain that migrations increase in the second millennium BCE.

The records of these people—who speak an Indo-European language—are literary, not archaeological, and are preserved in the Vedas, collections of orally transmitted hymns.

In the greatest of these, the "Rig Veda," the Aryan speakers appear as a tribally organized, pastoral, and pantheistic people.

The later Vedas and other Sanskritic sources, such as the Puranas (literally, "old writings"—an encyclopedic collection of Hindu legends, myths, and genealogy), indicate an eastward movement from the Indus Valley into the Ganges Valley (called Ganga in Asia) and southward at least as far as the Vindhya Range, in central India.

A social and political system evolves in which the Aryans dominate, but various indigenous peoples and ideas are accommodated and absorbed.

The caste system that will remain characteristic of Hinduism also evolves.

One theory is that the three highest castes—Brahmins, Kshatriyas, and Vaishyas—were composed of Aryans, while a lower caste—the Sudras—comes from the indigenous peoples.

The Aryans are a pantheistic people, following their tribal chieftain or raja, engaging in wars with each other or with other alien ethnic groups, and slowly becoming settled agriculturalists with consolidated territories and differentiated occupations.

Their skills in using horse-drawn chariots and their knowledge of astronomy and mathematics give them a military and technological advantage that lead others to accept their social customs and religious beliefs.

By around 1000 BCE, Aryan culture has spread over most of India north of the Vindhya Range and in the process assimilated much from other cultures that preceded it.

The Aryans have brought with them a new language, a new pantheon of anthropomorphic gods, a patrilineal and patriarchal family system, and a new social order, built on the religious and philosophical rationales of varnashramadharma.

Although precise translation into English is difficult, the concept varnashramadharma, the bedrock of Indian traditional social organization, is built on three fundamental notions: varna (originally, "color," but later taken to mean social class), ashrama (stages of life such as youth, family life, detachment from the material world, and renunciation), and dharma (duty, righteousness, or sacred cosmic law).

The underlying belief is that present happiness and future salvation are contingent upon one's ethical or moral conduct; therefore, both society and individuals are expected to pursue a diverse but righteous path deemed appropriate for everyone based on one's birth, age, and station in life.

The original three-tiered society—Brahman (priest), Kshatriya (warrior), and Vaishya (commoner)—eventually expand into four in order to absorb the subjugated people—Shudra (servant)—or even five, when the outcaste peoples are considered.

The extended and patriarchal family is the basic unit of Aryan society.

A cluster of related families constitutes a village, while several villages form a tribal unit.

Child marriage, as practiced in later eras, is uncommon, but the partners' involvement in the selection of a mate and dowry and bride- price are customary.

The birth of a son is welcome because he can later tend the herds, bring honor in battle, offer sacrifices to the gods, and inherit property and pass on the family name.

Monogamy is widely accepted although polygamy is not unknown, and even polyandry is mentioned in later writings.

Ritual suicide of widows is expected at a husband's death, and this might have been the beginning of the practice known as sati in later centuries, when the widow actually burns herself on her husband's funeral pyre.

Historians believe that Bengal, the area comprising present-day Bangladesh and the Indian state of West Bengal, was settled in about 1000 BCE by Dravidian-speaking peoples who were later known as the Bang.

Their homeland bears various titles that reflect earlier tribal names, such as Vanga, Banga, Bangala, Bangal, and Bengal.

The area known as Bangladesh will, for most of its history, be a political backwater—an observer rather than a participant in the great political and military events of the Indian subcontinent.