Denisovans were discovered only in 2010. It's possible that other human species could still be recovered from the oblivion of prehistory. They all had large brains, walked upright, and were able to make tools. They were all human. So Sapiens were only one type of human species among many and were no shoo-in to become masters of the universe. Why then did Neanderthals, Denisovans and other human species disappear?

"One possibility is that drove them to extinction," Harari wrote. "Imagine a Sapiens band reaching a Balkan valley where Neanderthals had lived for thousands of years. The newcomers began to hunt the deer and gather the nuts and berries that were the Neanderthals' traditional staples ... The less resourceful Neanderthals found it increasingly difficult to feed themselves and slowly died out."

There is an even more brutal possibility. Again Harari: "Another possibility is that competition for resources flared up into violence and genocide. Tolerance is not a Sapiens trademark. In modern times, a small difference in skin colour, dialect or religion has been enough to prompt one group of Sapiens to set about exterminating another group.

"Would ancient Sapiens have been more tolerant towards an entirely different human species? It may well be that when Sapiens encountered Neanderthals, the result was the first and most significant ethnic-cleansing campaign in history."

Sapiens may have a horrible, long-buried secret that accounts for our heart of darkness. There is another, gentler possibility. In 2010, a four-year study found that up to four per cent of the uniquely human DNA of the populations in the Middle East and Europe were Neanderthal. In another study, up to six per cent of the uniquely human DNA of modern Melanesians and Aboriginal Australians were Denisovan. Given the small exchanges of intra-species genes, it appears there was occasional intermixing of a Neanderthal or Denisovan Juliet with a Sapien Romeo. But the Capulet house of Neanderthals or Denisovans could still have been wiped out by the Sapien Montagues.

If our genocidal prehistory is correct, our phenomenal capacity for violence - the most sophisticated form being state-organised warfare - should not surprise anyone. Until recently, the study of history has been the study of the history of states; and virtually all early states - from Mesopotamia and Egypt to India and China - were formed to wage war. States may make war, but war makes states.

A good study of this is another book from last year, by British-American historian Ian Morris. For Morris, we owe practically everything we treasure to past wars. In the past 10,000 years, warfare compelled Sapiens to form larger societies and civilisations to enjoy greater security and prosperity.

Wars have made our lives 20 times safer than our prehistoric ancestors or contemporary tribes with no state-like organisations. But the West made a quantum leap by perfecting weapons of mass destruction through the industrial, scientific and technological revolutions. In 500 years, Westerners have made it possible not only to wipe out all the species including ourselves on earth but the planet itself. From the Thirty Years War to the Napoleonic and two world wars, Europeans invented total and universal warfare. Faced with the possibility of extinction, the West has pulled back from the brink with counter-movements called the humanitarian and environmental revolutions. Like a religion, the universality of human and democratic rights as the birthright of Sapiens' humanity is a nice little story that many people believe in as a matter of faith.

Having invented the capability for universal destruction, the West has also, often inadvertently, spread its technology, science, medicine, and ideologies, around the world. It is perhaps making amends of sorts by advocating, as an ideal if not always in practice, that governments should be instruments of peace and rights protection.

Alex Lo edits the science page