Giant leaps for mankind

In the space of a few dozen millennia – the blink of an eye in terms of the long aeons of our planet's history – our own species, Homo sapiens, emerged from the melting-pot of primate evolution to become the definitive version of the human race.

This guide attempts to piece together some of the ways in which this happened: from the earliest waves of migration from Africa through assorted natural cataclysms and world-changing fluctuations in climate. By the end, we find the earliest hunter-gatherer societies giving way to the first settled civilisations, as early experiments in agriculture open up alternatives to traditional nomadic lifestyles – while humanity is already beginning to have a significant impact on the planet's ecology.

The coming of man: how a single species of humans – sapiens – became the last to survive on earth

Any idea how close we are to midnight on our 24-hour clock of Earth history? A few minutes left to go, perhaps? After all, we still have all human "recorded" history to tackle, starting from when the first large-scale human civilisations emerged in the Middle East and continuing through to today, with approaching 7 billion of us living on the planet.

How about three seconds? That's it. Imagine how short sitting doing nothing for just three seconds would seem compared with doing nothing for the length of a full day. Anyone attempting that rather daft experiment would get a good impression of how much Earth history actually took place before the first Homo sapiens could be heard calling across the hot, dusty African plains.

We now know that it was from Africa that Homo sapiens, or modern humans, came. We did not descend from Neanderthals. The two species lived at the same time for thousands of years, and while there was probably a little interbreeding, recent genetic evidence suggests that there was not much. Red hair, freckles and pale skin are features that may have been handed down from the Neanderthals.

Instead we must look back to Africa, to the descendants of Homo erectus, Turkana Boy's people ( see Part 4). Bones from 21 sites across Africa have been found, stretching back almost 500,000 years, and they paint a picture that shows how small evolutionary steps along the Homo line led to who we are today.

The oldest Homo sapiens fossils yet found came from southern Ethiopia. In 1967 two human skulls were discovered buried deep in mud at the bottom of the Omo River. They have recently been re-dated, and are now thought to originate from about 195,000 years ago. These skulls, called "Omo I" and "Omo II", certainly look as if they belong to the immediate predecessors of modern humans. They are slightly larger than modern human skulls, but otherwise strikingly similar. They have been classified as a subspecies of Homo sapiens called Homo sapiens idaltu. They have flat faces and prominent cheekbones, but not the protruding brow ridge of the earlier Homo erectus and the Neanderthals.

Genetic research supports the idea that all humans alive today emerged from a single evolutionary line in Africa from about this time. But one strange feature remains unsolved. There is surprisingly little genetic variation between people alive today – much less than in most species of mammals. Even our closest cousins, the chimpanzees, show genetic differences 10 times greater across the spectrum of their species than we do.

Such a small variation can only mean one thing: at some point our species, Homo sapiens, must have shrunk to a very few individuals – perhaps between 1,000 and 10,000 people – all of whom shared a very similar genetic code.

This idea has set experts off on another hunt, to find some "event" that fits in with the idea of Homo sapiens suffering an almost fatal collapse in population early on in its history. One candidate for such an event is a massive Category 8 volcano – a super-eruption – that occurred about 75,000 years ago at Toba, on the Indonesian island of Sumatra. A giant hot-spot of molten lava is thought to have burst through the Earth's crust, releasing energy 3,000 times greater than that of the huge eruption of Mount St Helena in Washington state, USA, in 1980. One site in India today still has ash deposits from the eruption of Toba that are 6m thick. Such an enormous explosion would have created a blanket of dust in the atmosphere, blocking out the sunlight for months, even years, triggering a sudden drop in global temperatures and possibly even starting an Ice Age all by itself.

Maybe there are other reasons. A virus, perhaps, that wiped out large parts of the human population? At the moment, no one knows. But from a very small stock of African ancestors, Homo sapiens walked its way around the world, copying the migrations of its ancestors, eventually to supplant all other species of humans. The last of these, the Neanderthals, held off from extinction until about 24,000 years ago, which is the date of their very last known traces, in Gorham's Cave, Gibraltar.

Did we kill them? Did we eat them? Or did they die because of climate change or through lack of food during an especially nasty Ice Age snap? It seems that changes in the climate about 30,000 years ago were especially severe in central Asia and northern Europe, which is where the Neanderthals lived.