The new batch - 150,000 years ago

With over 6 billion people living in the world today, human beings are a phenomenally successful animal. But our species, Homo sapiens, once came close to outright extinction.

Clues from genetics, archaeology and geology suggest our ancestors were nearly wiped out by one or more environmental catastrophes in the Late Pleistocene period. At one point, the numbers of modern humans living in the world may have dwindled to as few as 10,000 people.

Flirting with extinction

By a strange twist of fate, the harsh conditions that caused this near extinction may also have allowed the cultural explosion that gave rise to human behaviour as we know it today.

Professor David Goldstein, a molecular biologist at University College in London, has uncovered evidence of a very ancient population bottleneck. A bottleneck is an event that reduces the genetic difference, or diversity, in a population of animals.

One way this can occur is through a catastrophe that wipes out a large proportion of a population. If we compare the genes of modern people from all over the world, they are remarkably similar, suggesting that the ancestors of all living people expanded from a small population that survived a bottleneck. The ancient bottleneck proposed by Professor Goldstein must have occurred in Africa, where modern humans evolved.

"Our data suggests there was a bottleneck that was not that recent," says Goldstein. The genetic data puts the likely date for this event at just before 100,000 years ago.

Water shortage

It's not known what caused this bottleneck. But a plausible candidate is emerging. By measuring the ratios of different oxygen isotopes in ice cores, scientists can reconstruct climatic changes over time. Oxygen isotope data suggests that between 190,000 and 130,000 years ago - a period known as 'oxygen isotope stage 06' - Africa was drained of moisture and became a parched wasteland, with little to sustain populations of modern humans.

"I'm not in a position to say what caused the bottleneck, but it certainly could be a something like that (drought). That scale of climatic change could be responsible for what we see in the genetic data," says Goldstein.

There may also have been other bottlenecks that contributed to the small amount of genetic diversity we see in modern humans. Professor Stanley Ambrose of the University of Illinois at Urbana-Champaign believes that the eruption of the volcano Toba in Sumatra roughly 70,000 years ago was responsible for a volcanic winter that caused an instant ice age.

Big freeze

The large amount of sulphur thrown up into the atmosphere by the eruption reflected sunlight away causing temperatures around the world to plummet. Temperatures in Africa may have fallen by as much as 9°C, creating a freeze that lasted 1,400 years.

"It was a long time, it was unrelentingly cold," says Ambrose. But it didn't just get cold, a temperature change of this magnitude would almost certainly have caused another terrible drought. "Lakes dried up, the earth turned to sand. Every year of drought was geometrically worse than the year before," adds Ambrose.

Daring to share

Ambrose believes it is no coincidence that around this time, modern humans in Africa were undergoing drastic changes in the ways they organised their societies.

The harsh climatic conditions that accompanied the volcanic winter may have placed pressure on humans to cooperate with each other. Small foraging groups became larger societies. Ambrose calls this the 'troop-to-tribe transition'.

Social whirl

This transition seems to have involved systems of gift exchange between distant peoples. Beads made out of ostrich eggshell seem to have been important items in this system of gift-giving, as they are today amongst South African !Kung San hunter-gatherers. The earliest examples of these beads have been dated to 40,000 years old. These beads were exchanged over distances of 200 kilometres in order to secure future favours when times became tough. In this way, humans increased the likelihood of survival or 'spread the risk of survival'.

Another important innovation after 70,000 years ago is the invention of a stone tool technology called 'microliths' in Africa. Microliths are small flakes and blades that characterise the Later Stone Age in Africa. These tools are very diverse, because each was specialised for a task. Ambrose describes previous stone technologies as jack-of-all-trades (master of none) toolkits, whereas microliths reflect modern humans using the right tool for the job.

On the move

Ambrose thinks the appearance of this new, specialised toolkit represents an enhanced ability to plan and anticipate tasks. Microliths were also given as gifts in gift-exchange systems. By adopting a strategy of spreading the risk of survival, hominids were able to disperse out of Africa and dominate the new habitats they encountered.

"People with reliable resources do not co-operate, they revert to tight-knit, territorial societies," says Ambrose. "If you look at modern humans, they were able to go out there and exploit unpredictable resources."

Homo sapiens reached the Near East by at least 90,000 years ago, Australia by at least 50,000 years ago, Europe by 40,000 years ago and the Americas by 12,000 years ago. But the story of human evolution does not end here: we are still evolving in subtle ways that we cannot detect. Perhaps our descendants in years to come will be able to describe how we fit into their human family tree.