These children [to be sacrificed to the mountain and other gods] would be collected from all over the land and would be carried in litters together … They should be very well dressed, paired up female and male.

Juan de Betanzos, 1551

Human nature would not allow them to kill their own children … if they did not expect some reward for what they were doing or if they did not believe that they were sending their children to a better place.

Bernabé Cobo, 1653

The Incas were an ethnic group of superlatives: although never numbering more than 100,000 individuals, they nevertheless created the largest native empire in the New World, 2,500 miles long, from what is now southern Colombia to central Chile and across some of the world's most mountainous and difficult terrain.

The Incas' breathtaking mastery of their natural environment was acutely brought home to me this weekend as I climbed 2,000ft up a cloud forest trail in south-eastern Peru to Machu Picchu, a royal retreat built for an Inca emperor that clings to a mountain spur 8,000ft up in the Andes. As I wandered about the cloud-wreathed city of gurgling fountains, sacrificial altars, celestial observatories and exquisitely fashioned buildings of white granite blocks – some of which weigh more than 50 tons – I couldn't help but reflect on the sheer genius the Incas obviously possessed.

Although their empire existed for a scant 100 years before being cut short in 1533 by the arrival of the Spaniards, the Incas managed to create 26,000 miles of roads, ruled an empire of 10 million people and imposed their language and culture from one end of the Andes to the other. In a very real sense, the Incas were the "Romans" of the New World and, like the Romans, they were excellent administrators and empire builders. Like the Romans, however, they borrowed many aspects of their culture – from metallurgy and warfare and architecture to agriculture and animal husbandry and astronomy – from other, previous cultures.

They also appropriated, transformed and incorporated elements of many other South American religions, including animal and human sacrifices. Last week the photograph of a 15-year-old Inca girl appeared in the press, a beautiful and unblemished teenager who was sacrificed more than 500 years ago on top of a 22,000ft volcano in northern Argentina. Her mummified body was found by archaeologists in 1999 and is now on display for the first time in a museum in Argentina. Drugged with coca leaves and plied with alcohol, the girl was left to freeze to death high in the Andes, a seemingly senseless death to modern readers. The revelation of the girl's untimely death raises an obvious question: why did the Incas, despite being one of the most powerful, sophisticated and accomplished cultures in the New World, feel the need to sacrifice their children on mountain tops?

The answer can be found in a strange melange of Inca religious beliefs, natural catastrophes, and the sheer difficulty of trying to survive amid the frozen heights of one of the most volatile mountain chains in the world.

Credit: Observer graphics

The Incas arose in western South America, one of only six areas in the world where state-level societies arose (the others are Mesoamerica, China, Mesopotamia, the Indus valley and Egypt). The Incas were only the latest of a multitude of civilisations that had arisen in western South America and had borrowed from earlier cultures such as the Chimú, Moche, Nazca and Tiwanaku. The Incas began their sudden surge to power in the early 15th century, led by an emperor called Pachacutec, "overturner of worlds".

Through threat, negotiation, or bloody conquest, Pachacutec and his successors began to subjugate nearby provinces, determining the number of taxpaying peasants and installing local Inca governors and administrators before their armies moved on. If co-operative, local elites were allowed to retain their privileged positions and were rewarded for their collaboration. If uncooperative, they were exterminated, with their supporters.

Like other agriculturally based empires, Inca rule was built on reciprocity between the Inca elite and peasants, who were expected to pay taxes in the form of goods and labour; in return, the state was expected to provide the empire's citizens with security, laws and administration and also with emergency relief in times of famine or natural catastrophe.

The Incas constructed huge storehouses filled with foods and goods. If one area of the empire suffered drought or some other form of calamity, the Incas withdrew food and supplies from the storehouses and replaced them when local production increased again. If another area was attacked by marauding tribes, Inca armies soon arrived to repel the attackers and restore order. Through their labour tax, a succession of Inca rulers built new cities, constructed networks of roads, marshalled vast armies, erected and filled storehouses, and enlarged their empire.

Although the Incas created a finely tuned imperial state with engineers who could convert rugged rainforest mountains into well-ordered stone cities such as Machu Picchu, even they came up short when it came to the natural calamities that repeatedly struck western South America. The Inca empire straddled the Andes, a mountain chain formed by the continuing collision of a giant tectonic plate called the Nazca plate that slowly smashed into the South American plate, whose western edge also forms the western edge of South America. The Incas thus built their empire within the Pacific's "ring of fire" where volcanoes periodically erupt. Because of the colliding plates, violent earthquakes are common, destroying cities and towns. In addition, the empire was beset by the climatic havoc wreaked by El Niños every seven years, resulting in savage floods that disrupted food supplies.

In response to such natural phenomena, the Incas resorted to religion. In the Inca world, lightning, earthquakes, volcanic eruptions, rain, weather and fertility were controlled by a panoply of gods. To survive in an unpredictable world, the Incas sought to form reciprocal relationships with their gods, just as they formed reciprocal relationships with one another, or with other tribes.

The Incas' primary god was the sun god, or Inti, which made agriculture possible. The Inca ruler himself was considered to be the son of the sun god, thus Inca emperors were worshipped and considered divine, inhabiting the apex of a vast theocratic state. To create and maintain relationships with their gods, the Incas gave them a variety of offerings. These ranged from simple prayers, food, coca leaves and woven cloth to animals, blood and, in the ultimate sacrifice, human beings. In especially uncertain times, such as when an emperor died, or when volcanoes erupted or severe earthquakes or famine struck, priests sacrificed captured warriors or specially raised, perfectly formed children to the gods. The Incas believed in an afterlife and that the children they sacrificed would inhabit a better, and more abundantly provided for, world.

Although the Spanish invaders did their best to exterminate Inca religion, the Incas were not the first culture to resort to human sacrifice in times of great stress or need. The Celts of Ireland and Britain frequently made human sacrifices to their gods. Mongols, Scythians, early Egyptians and various Mesoamerican groups all made human sacrifices, for one reason or another. Closer to home, the Greek author Homer wrote of how Iphigenia was set to be sacrificed by her father Agamemnon to ensure success in the Trojan war (he ultimately sacrificed a deer instead). And in the Hebrew Bible, when God asked Abraham to sacrifice his son, an angel stopped Abraham at the last moment. Abraham sacrificed a ram instead, but only after "learning to fear God".

The Incas did their best to fathom what, at the time, was unfathomable – the violent, unpredictable catastrophes of nature which, in some cases, had ended cultures that preceded them. To their credit, the Incas did their best to ensure the survival of their people and empire by paying close attention to nature and doing their best to use every means at their disposal, including human sacrifice, to gain control over it.

The irony is that, more than 500 years after the Spaniards put an end to perhaps the most spectacular empire in the New World, not only are Andean and worldwide glaciers shrinking at unprecedented rates as the Earth heats up, altering and damaging native ecosystems, but the new cultures that have replaced the Incas seem apathetic, at best, in making any kind of sacrifice in order to gain control over a potentially self-created environmental disaster.

Kim MacQuarrie is a multiple Emmy-winning documentary film-maker, writer and anthropologist who lived for five years in Peru. His most recent book is The Last Days of the Incas, the story of the conquest of the Inca empire