In 1519, when the Spanish Conquistadores crossed the ring of mountains that surrounded the Aztec capital Tenochtitlán, and beheld the astonishing cities which formed the core of the Aztec empire, they could scarcely believe their eyes. What the Conquistadores discovered was that (despite being themselves capable of a very calculating brutality) the Mexica, as the Aztecs were also known, “presided over a city of pyramids and sacred temples that reeked with the blood of human sacrifice”.


“The dismal drum sounded again,” wrote Bernal Diaz, an aide to Hernán Cortés, the leader of the Conquistadores. The drum, he wrote, was “accompanied by conches, horns, and trumpet-like instruments. It was a terrifying sound, and we saw [the captives] being dragged up to the steps to be sacrificed. When they had hauled them up to a small platform in front of the shrine where they kept their accursed idols we saw them put plumes on the heads of many of them; and then they made them dance with a sort of fan… Then after they had danced the [priests] laid them on their backs on some narrow stones of sacrifice and, cutting open their chests, drew out their palpitating hearts which they offered to the idols before them. Then they kicked the bodies down the steps, and the Indian butchers who were waiting below cut off their arms and legs and flayed their faces.”

Diaz claimed to have been in 119 battles, so he may have been given to exaggeration, but he insisted his account of the Mexica campaign was ‘A True History of the Conquest of New Spain’.

European explorers dubbed the Americas the ‘New World’, as opposed to the Old World – Africa, Asia, and Europe – that had been known since antiquity. When the Spanish first arrived on mainland America, there were two civilisations that were prominent in the New World: the Aztecs in what is now Mexico, and the Incas in Peru. Both were flourishing, boasting elaborate capital cities, organised religion and associated art works. Both were rigidly divided into social classes, and had successful methods of food production. The Aztecs were reached first, in 1519, 13 years before the Incas.

We find the events described by Diaz abhorrent today. David Carrasco, professor of the history of religions at Princeton University, concedes that Aztec practices have troubled modern readers for centuries and that “the scholarly community has been remarkably hesitant to explore the evidence and nature of large-scale ritual killing in Aztec Mexico”. (That is now beginning to change: I have counted 29 titles published in the last 31 years devoted to human sacrifice, cannibalism or other forms of ritual violence.)

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It was an awareness of these startling differences between the hemispheres, and a desire to make sense of the context, that sparked the idea for my new book The Great Divide, which describes a period of history, 15,000 BC to AD 1500, when there were two populations in the world, each unaware of the other. This came about because in around 15,000 BC, during the Ice Age, the world’s oceans were lower than they are today and early humans were able to walk from Siberia across the Bering Strait, and enter the Americas. At the end of the Ice Age, the Strait refilled with water, and people in the Americas were cut off from those elsewhere – until the Americas were ‘discovered’ just before 1500.

There are three fundamental differences between Eurasia and the Americas that together account for the fact that civilisation in these two hemispheres developed along very different lines and, in the case of the New World, explain its bloody practices. These differences are: first, geography, climate and weather; second, the anomalous distribution around the world of domesticable mammals; and third, the likewise anomalous distribution of psychotropic plants – in particular hallucinogens.

Geography first. South America was a separate continent from North America until about two million years ago. Thanks to the La Brea tar pits in what is now downtown Los Angeles, which have preserved their remains, we know what animals flourished when early people arrived in the New World, about 15,000 BC: sabre-toothed cats, lions, tapirs and bobcats. This is a “breathtaking proportion of carnivores”, according to one account, whereas most of the domesticated mammals as we know them originally evolved in the great steppes of Eurasia.

Moreover, the Old World – Eurasia anyway – is an east-west continent, as the German 19th-century philosopher Hegel and most recently Jared Diamond, professor of geography at the University of California, have pointed out, whereas the Americas are north-south in orientation. This means climate is less varied in the Old World – hours of daylight, temperature and rainfall have a much narrower range.

As a result, plants and animals (people included) find it much easier to move around than in the Americas.

Fertility worship

In the Old World the dominant weather phenomenon is the Asian monsoon which, recent research has shown, has been decreasing in strength for the past 8,000 years. This drying has ensured that fertility worship is the most common form of religion across Eurasia. In the New World, in contrast, there have been two dominant phenomena. The first is El Niño – sudden, periodic changes in the southern Pacific Ocean, producing devastating storms and winds (which have been increasing in frequency over the past 6,000 years, from a couple of times per century to every few years now). The second is the fact that the tectonic plates of the world are so configured that earthquakes and volcanoes were concentrated where civilisations existed in ancient times in central America.

These factors, along with hurricanes and tornadoes, which occur mainly in the tropics, means that religion in the New World developed into what scholars have called ‘tectonic faith’ – worship to avert natural disasters and catastrophes.

It is a striking anomaly that whereas the Old World has around a dozen species of large mammal that have been domesticated (cows, horses, sheep, goats, and so on), the New World has only three (llama, vicuña, alpaca). Moreover, the New World mammals are relatively small and so never offered themselves as powerful sources of energy, as did cattle, horses and camels in the Old World. As a result, four things that we take for granted in the Old World – ploughing, riding, driving and milking – never existed in the new.

Domesticated animals had a much greater effect on the development of Old World history than is generally credited. In particular, the great steppes of Eurasia became the home of pastoral nomads, who invented riding, metallurgy – and perhaps milking. In addition it was the pastoral, nomadic Israelites who, because their lifestyle encompassed so many forms of terrain, conceived the idea of one God, who had ‘dominion’ over all landscapes. Monotheism, which has so shaped life in the Old World, never developed in the new. Pastoral nomadism was simply not possible there.

In the Americas life was dominated by wild animals, in particular the jaguar, the king of the rainforest (all ancient civilisations in Meso- and South America worshipped the jaguar); the bison on the Great Plains (where American ‘Indians’ established a hunting relationship that lasted until historical times); and the super-abundant salmon, which enabled stable coastline chiefdoms to endure in the Pacific north-west, again for millennia.

Plant life also varied dramatically. The Old World was dominated by grasses – cereals which spread rapidly with the aid of domesticated animals pulling ploughs and carts. In the Americas, tubers and roots were dominant in many areas, keeping people tied to the same land in perpetuity – because they were harvested throughout the year, not all at once, and therefore did not denude the land of its fertility.

In the New World there was one important grass – maize. But being tropical, not temperate, as wheat and barley were, it naturally had a high sugar content (due to strong sunshine) and was used at first mainly for its sugar, to make beer, whose psychoactive properties were regarded as sacred. In these latter circumstances, surpluses, to be traded, were much harder to build up. Civilisations are built on surpluses.

Of just over 100 psychoactive plants known worldwide, between 80 and 90 per cent are exclusive to the New World. Given that early people entered the New World from the Old via the Bering Strait during the Ice Age, this becomes very relevant. This is so because the earliest religious manifestation known to archaeology is shamanism, a practice that began with the ancient reindeer-hunting tribes of Siberia. In these early religions, shamans ‘negotiated’ with the animals their ‘permission’ to kill them for food. They did this by entering trance, achieved via psychoactive plants.

Simple shamanism

All of the main New World civilisations (Olmec, Maya, Chavin, Aztec, Inca in South and Meso America, and the pueblo chiefdoms in north America – the Anasazi, the Hopi and Mogollon cultures) practised shamanism and used psychoactive plants.

But, what began as a simple shamanism in villages turned violent when complex, urban civilisations developed. Now, more theatre was needed to keep the populations compliant and this is where bloodletting and sacrifice evolved. The leaders let blood, often in huge amounts, and research has shown this can stimulate the excretion of endorphins in the brain, chemicals which produce effects akin to trance.

This practice developed because, during the natural catastrophes that were more common in the New World, people would be killed or injured or they disappeared altogether – and so the idea gained ground that the gods needed humans and blood to survive.

The Mayan kings could not accede without letting massive amounts of blood, the pain of which would send them into trance. Bloodletting was accomplished by passing spikes or ropes through the tongue or penis, the blood falling on to special strips of paper, which were burned, sending the blood to the heavens. After that the blood supply for the gods was maintained by the sacrifice of captives.

This culminated in the Aztec religion, where people needed to be sacrificed every day to nourish the sun, which was born every day and died every night and which, owing to the eruption of volcanic ash, periodically did not appear at all, filling people with dread. The Aztecs fought wars in which the object was not to kill the enemy, but capture for sacrifice later.

The crucial difference, accounting for the greater organised violence in New World religions, arose because, when you worship fertility, the cycles of nature ensure that, sooner or later, worship works. In the New World, worship takes the form of propitiation – petitioning the gods to make something not happen. But the nature of climatological events, as in the New World, mean there is no rhythm and, more often than not, worship doesn’t work. Therefore, you have to redouble your efforts.

This is why, as David Carrasco found, Aztec sacrifice grew steadily more terrible. In contrast, human sacrifice died out in the Old World, to be replaced by animal sacrifice (domesticable mammals were readily available). This itself died out around the time of Christ, because monotheistic, abstract Gods, without human qualities, no longer required blood to sustain them.

This was a profound ideological difference between the two hemispheres because, as many scholars have noted, the Christian monotheistic God, whose nature could be discovered by study, helped encourage investigative knowledge, leading to science and progress. This explains, ultimately, why it was Columbus who sailed west to discover the New World rather than Moctezuma sailing east to discover Africa and/or Eurasia.

Timeline: Old World

c150,000 years ago Modern humans develop in Africa

c125,000 years ago Humans migrate out of Africa to populate the world

c15,000 BC Humans cross the land bridge between Siberia and Alaska to populate the

c10,500 BC Cereals first cultivated

c10,000–8,000 BC Transition from hunter-gathering to sedentism (year-round settlements)

c9500 BC Worship of the Bull and Great Goddess, a step beyond shamanism

c8000 BC Cattle domesticated

c4000–3000 BC Plough invented; milking; traction complex; ‘the secondary products revolution’

c3500 BC First cities emerge in Mesopotamia

c3000–2000 BC Metallurgical revolution, leading to the Bronze Age (peaking at 1,400 BC)

c2500 BC Writing in the order of speech (at Lagash)

c1800 BC Chariot developed

c1500 BC Pastoral nomadism emerges

c1200 BC Gods change gender, from female to male

c900–200 BC Axial Age, when the main forms of religion, as we know them, evolve: Buddhism, Upanishadic Hinduism, Confucianism, Taoism, Rationalism in Greece, Judaism

c33 AD onwards Christianity, an amalgam of Greek philosophy and Judaism, produces the idea that God may be known by investigative knowledge, eventually giving rise to modern business and science

Timeline: New World

c9000 BC First evidence of bison hunting

c8440 BC First evidence of Sophora use as hallucinogen

c6000 BC Sea levels stabilise; vast shoals of salmon off the coast of NW America

c4000 BC Maize used for beer

c3500 BC Teosinte (wild form of maize) first cultivated

c3055 BC Early city of Aspero in Peru

c2250 BC Image of staff god – earliest religious figure in the New World

c850 BC Chavin culture. Lanzon, lance-like God. Temple statues of people turning into jaguars

c1–1250 AD Many cultures flourish in New World: Nazca, Tiwanaku, Moche, Maya, Zapotec, Mixtec

c200 AD Maize in North America

c600–1000 AD Ball courts where ceremonies and ball games took place proliferate. Rubber balls were then unique to the New World

c750 AD Rise of Tula and the Toltecs

c1050 AD Cahokia culture, based on maize. The only New World culture to follow an Old World trajectory at all closely

c1426 AD Rise of the Aztecs

c1436 AD Rise of the Incas

Peter Watson is a research associate at the McDonald Institute for Archaeological Research at the University of Cambridge. His book, The Great Divide, was published in January by Weidenfeld & Nicolson


This article was first published in the February 2012 issue of BBC History Magazine