The Incas were originally one of several insignificant tribes that lived in the Cuzco valley in the central highlands of the Andes. Their myths relate how their first ruler, Manco Capac, brought the tribe to that valley either from the shores of Lake Titicaca or, in another version, from the 'windows' or caves at Paccari-Tambo, a place some eighteen miles south-east of Cuzco. Like Quetzalcoatl for the Toltecs and Aztecs, the mythical figure of Manco Capac was revered as the bringer of civilization to the world; he was also worshipped for being a direct descendant of the Sun God. a family connection which gave Inca rulers a quasi-divine status.
In the late fourteenth century the Incas subjugated the other tribes in the Cuzco valley, and their imperial career began in earnest in 1438, when the great conqueror. Yupanqui Inca Pachacuti. 'the Transformer', ascended the throne Between 1438 and 1463 he extended Inca rule to the region of Lake Titicaca, and thence north-west. Other conquests by both Pachacuti and his son Topa Inca brought northern territories as far as Quito under Inca control. Pachacuti it was who laid the foundations of the well-organized Inca state centred upon Cuzco, a holy city which possessed the imposing Temple of the Sun representing the very source of Inca power. In 1471 Pachacuti was succeeded by Topa Inca, who, having already subdued the great Chimu kingdom in the north, ventured south into present-day Chile, establishing the limit of Inca power at the River Maule in the territory of the Araucanian Indians. From 1493 the next supreme Inca. Huayna Capac, became involved in an extended campaign against rebellious ethnic kingdoms on the northern frontier zones, especially around Quito. Upon Huayna Capac's death some time between 1525 and 1528, his natural son Atahuallpa seized these frontier territories with the help of several important generals and launched a coup d'etat against the legitimate successor, his half-brother Huascar. It was during the ensuing civil war that Francisco Pizarro happened to arrive in the kingdoms of the Sun.
In less than a hundred years the Incas had built the most formidable empire in the Western Hemisphere. Like that of the Aztecs, their dominion was characterized essentially by the levying of tribute from scores of subject kingdoms and tribes. But the Incas went much further than the Aztecs in developing a centralized bureaucratic state at the service of a supreme ruling class. In this the physical peculiarities of the Andean region were directly influential.

The geography of the area covered by the Inca empire is marked by great contrasts of climate and terrain. Ascending from the rainless deserts of the coast to the snow-capped peaks of the Andes, one passes through sharply varying ecological environments. On the coast, agrinculture is possible only in the vicinity of rivers or on land under irrigation; fishing has therefore always been important. In the highlands, altitude determines the kind of crops that can be produced; for instance, maize will grow well up to 11,000 feet while at higher levels tubers and grams can be cultivated. In the cold, windswept puna - steppe-like grasslands just below the snow-line - no agriculture is possible, though pasture is available for the llamas, vicunas and other ruminants that provide meat and wool. Each level forms an 'ecological tier' yielding a particular range of produce, and yet there is not enough fertile land on any one tier to sustain a large population.
Over the centuries Andean societies developed a way of overcoming this problem by sending out settlers to cultivate crops at different altitudes in order to complement the produce of their native territories. Andean societies were not therefore territorially integrated units, but took rather the form of 'vertical archipelagos* comprising the ancestral homeland - which provided the core of tribal identity - and outlying agrarian settlements on a number of ecological tiers specializing in various types of produce for distribution and exchange among the dispersed branches of the tribe. Geography thus produced a unique economic structure, which, in turn, determined social values and practices. Where fertile land, being scarce, needed to be so carefully husbanded, it is little wonder that its distribution had to be closely regulated by the community and that a spirit of co-operation should be so highly prized among members of the tribe. As a result, the two ruling principles of Andean tribal society were redistribution and reciprocity.

The basic social unit was the ayllu, an extended kinship group or clan similar to the calpulli in Middle America. Each ayllu possessed land which it allocated to heads of families, who could cultivate it for themselves but were not allowed to sell it to others. It was common practice within the ayllu for an individual member to work a neighbour's fields in return for similar assistance; he would also render tribute by taking turn to labour for a time in the fields of the ayllu headman and the tribal chieftains. Given the intrinsic difficulties of the terrain, the ayllus had to join together to perform certain collective tasks, such as the building of terraces to enlarge the area of cultivation and the construction of systems of irrigation. Because of the threat of crop failure in such a fickle climate, a number of public warehouses were used to store grain and other crops for distribution in case of famine. The produce from the different ecological tiers had to be distributed to all the ayllus in the tribe. Thus the geography of the Andes dictated a high degree of collective action and central regulation within the tribal communities.
The Incas elaborated upon these traditional practices of communal regulation and reciprocal services in order to build an imperial state. Inca imperialism did not eliminate local identities; rather, it added a higher stratum of authority to the pre-existing tribal hierarchies. Traditional ethnic chieftains, under the supervision of Inca governors and Inca garrisons, were responsible for the collection of tribute from their people for the Inca aristocracy. Tribute was received from subject peoples largely in the form of labour. The local traditions of collective work were taken over by the imperial state and transformed into the mita, a system of forced labour by which the colonized Supreme Being, Viracocha, creator of the universe, from were ultimately derived. The doctrine of Viracocha's pre-eminence, however, did not displace the cult of the Sun God. Inca cosmogony, like that of the Aztecs and the Mayas, divided the history of the universe into 'suns', each age having been brought to an end by a cataclysm. The Fifth Sun had been inaugurated by the Inca Manco Capac, and it was his descendants who were charged with ensuring the continuing survival of the world through sacrifice and expiation. Although not as prodigal of human life as the Aztecs, the Incas sacrificed youths, girls and children - all of whom had to be physically perfect - on occasions when the usual sacrifice of llamas was deemed insufficient to save the world from calamity.

The religious establishment was large and influential. A hierarchy of priests, headed by immediate relatives of the Inca himself, served the many temples and huacas found throughout the empire: at the Temple of the Sun in Cuzco some 4,000 people were engaged in the ministry of the state religion. Religious communities of 'chosen women', who were recruited as young girls from all parts of the empire, performed a variety of duties: some might be selected for sacrifice to the gods, others as concubines for the Inca and his favorites; the rest would be employed in weaving precious vicuna wool into garments for the royal family, or in preparing food and libations for the frequent ceremonies that were held by priests and nobles. This very close identification of religion with government afforded enormous power to the Inca state. In the remains of great buildings in Cuzco, in the massive fortress of Sacsahuaman or in the ruined city of Macchu Picchu, all constructed with huge boulders cut to shape and fitted exactly into place, there survive impressive monuments to its extraordinary capacity to mobilize human labour. Perhaps extreme regimentation by the state was necessary to compensate for technical deficiencies in an otherwise sophisticate civilization: without beasts of burden or knowledge of the wheel, the Incas depended crucially on manpower. But their true success lay in the skill with which they built up a polity that transcended by far the limits of the tribe. The business of government was turned into a dynastic monopoly based on privileged knowledge, not just as regards the arcana of religion but also at a more mundane level: the absence of a system of writing restricted important information to a closed oligarchy, who had access to records kept on knotted cords known as quipus. Such privileged knowledge increased the possibilities of political control over the passive multitude of commoners whose cultures remained entirely oral. So long as belief in the divine origin of the Inca dynasty and in its right to extensive privileges could be upheld, the edifice of state would remain in place. In 1531, however, Francisco Pizarro and his company of infidels found their way into the Inca empire, Tahuantin-suyu, causing that great pyramid of state to collapse when they violated the sacred pinnacle of its authority.
In the Lima museum, there are hundreds of skulls which have undergone trepanning and the insertion of gold and silver plates by Inca surgeons.