Prehistoric World

150,000

Fully modern Homo sapiens ('wise man') became established in Africa after descending from Homo heidelbergensis and going through a transitional phase with Homo sapiens idaltu. Homo sapiens had a characteristic look: their faces were (and of course still are) small and tucked under a high, domed braincase. They had small eyebrow ridges and their lower jaw ended in a prominent chin. On average, their bodies were less muscular than those of earlier hominids. The appearance of modern humans coincided with the appearance of highly crafted tools, efficient food-gathering strategies and a complex social organisation.

100,000

In search of new food supplies, Homo sapiens began to cross from Africa into the Middle East. By this Late Pleistocene period, its numbers appear to have been dramatically reduced by a lack of food stocks, perhaps to as low as 2,000 individuals (according to recent genetic research) in the main groups, which meant that for a while Homo sapiens was perilously close to extinction. Other, smaller groups seem to have remained in Africa.

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 UCL in London, has uncovered evidence to back up the idea 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.

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.

95,000

A tiny species of human evolved in Indonesia at the same time as Homo sapiens was first migrating out of Africa.

Homo floresiensis was a one-metre- (3ft) tall species which lived on Flores Island (near Java) from between 95,000 to at least 12,000 years ago. It had long arms and a skull the size of a large grapefruit and shared its habitat with a golden retriever-sized rat, giant tortoises and huge lizards - including Komodo dragons - and a pony-sized dwarf elephant called Stegodon which floresiensis probably hunted.

Floresiensis probably evolved from Homo erectus, whose remains have been discovered on the Indonesian island of Java. Homo erectus may have arrived on Flores about one million years ago, evolving its tiny physique in the isolation provided by the island in response to the local scarcity of resources (later influxes of Homo sapiens appear to have evolved in the same way).

Homo sapiens reached the area by 50,000 years ago, but the last evidence of this hominid at Liang Bua dates to just before 12,000 years ago, when a volcanic eruption snuffed out much of Flores' unique wildlife, although there are hints that floresiensis could have lived on much later than this.

90-70,000

Some groups of Homo sapiens followed the coastline east from northeastern Africa, heading into Persia and South East Asia.

Other Homo sapiens groups settled along the Fertile Crescent and in the south eastern corner of Anatolia. They lived alongside tribes of Homo neanderthalis, and there was a considerable overlap in their competition for resources which lasted for at least 30,000 years (as proved by the discovery of Kebara, a Neanderthal fossil found in Israel and dated to 60,000 years ago) - this was nearly five times longer than when sapiens later reached Europe. The considerably smaller numbers of sapiens at this time, as they moved into a land dominated by neanderthalis probably goes a long way to explaining this.

However, one significant drawback for Neanderthals was the fact that their physique forced them to maintain a high calorific intake. They were forced to hunt for food containing twice as much energy as Homo sapiens. Once they found themselves in direct competition with increasing numbers of Homo sapiens, their hunt successes would have been harmed, perhaps significantly. This would certainly have had a detrimental effect on their existence.

70,000

There may 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 at this time was responsible for a volcanic winter that caused an instant ice age.

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.

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'.

50,000

Homo sapiens first reached Australia after spreading slowly through South Asia and Sunda-land. Lowered sea levels had created a landbridge which meant that the South Asian islands were joined together above sea level as far south as Java.

But the peopling of Australia was a long-term process involving numerous subsequent movements of people out from Asia. Two routes were possible: one from Southern China through the Philippines and into New Guinea, the second from South East Asia through the islands of Indonesia and into Northern Australia.

Although the frequent lowering of sea levels during Ice Ages caused the Sunda shelf to become fully exposed, and the same thing happened to the Sahul shelf - comprising Australia, New Guinea and Tasmania -  the two were never linked. Both of the colonisation routes therefore required a sea journey of at least 60 kilometres of open sea, so sea-going craft would have been essential. What these groups of Homo sapiens used for this so far remains unknown.

Settlement in Sahul-land expanded rapidly, with the southern and eastern areas becoming fully occupied within 25,000 years. During this time the environment changed quite dramatically, becoming increasingly dry and reaching a peak of aridity 18,000 to 16,000 years ago, followed by a slight improvement in conditions. The same dramatic changes brought about the extinction of the giant marsupials at the end of this period.

After 40,000 years of Homo sapiens settlement on on Sahul-land,  rising sea levels cut off New Guinea and Tasmania, and covered the rich coastal sites. This was simultaneous with the appearance of another variety of human type in Australia, fully modern but with a heavy robust skull. These individuals may be a later wave of immigrants, but it is also possible that they are the result of local breeding effects on a small community.

Homo erectus was still in existence in Java, but probably not for long.

Recent studies into the complicated stratigraphy of the Java Homo erectus sites have revealed some surprising information. Researchers have dated the deposits thought to contain erectus fossils near the Solo River in Java to only 50,000 years ago. This would mean that at least one population of Homo erectus in Java was a contemporary of Homo sapiens.

The two species had some level of direct contact, as is proven by the distribution of lice. Lice have stuck with hominids and their primate ancestors for at least 20 million years, and their lineages are remarkably close to those of the hominids. The distribution of lice around the world proves that there was either some interbreeding on a limited scale between erectus and sapiens, or sapiens hunted erectus for food and the lice were  transmitted during the process of preparing the meat. This very individual strain of lice (found only in the Americas) was then carried by sapiens as they headed northwards towards Beringia. The first wave of humans arrived there around 48,000 years ago.

Homo erectus may have held on in Java for another 20,000 years before it died out totally.

45,000

Something happened that transformed the world of Homo neanderthalis, especially in Europe. There was a sudden change in the weather. The climate in Europe began to deteriorate significantly, getting colder and drier. Although this had happened before, during other gaps between Ice Ages, this was different. The changes were more rapid, and very unsettled, with no particular pattern emerging. And they were simply getting worse and worse.

Neanderthals were built to survive the cold, but the speed of this climate change was different to anything they had experienced before. These changes were not happening at a scale of thousands of years, sometimes they were at a scale of tens of years or hundreds of years, within the space of a few Neanderthal generations.

They faced a crisis of survival. The forests in which they lived were dying out because of the weather. And in this new, more open landscape, they would have found it increasingly difficult to hunt. Neanderthal hunting methods - on the edge of woodland where their prey wouldn't see them coming - just wouldn't work so well on open step land environments because the size and weight of their heavy spears made them almost useless for throwing any distance.

40,000

Homo sapiens' 'troop-to-tribe' 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 areas of 200km 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'. Gift-giving was a key activity of early modern humans, as it is today.

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.

At the same time, Homo sapiens first emerged into Europe and began to compete with the indigenous Homo neanderthalis population for food and resources. Neanderthals were already facing problems from abrupt climate change, so when the first Homo sapiens arrived in northern Europe with their new technology - a much lighter spear that could be thrown - Neanderthals were hit again with the blow of being out-hunted.

In fact, it could have been Neanderthal's loss of dominance in Europe that opened the door for Homo sapiens to begin moving west from Anatolia and competing directly with them. Homo sapiens was established across all of habitable Europe within 5,000 years.

24,000

What characterised all Neanderthals was their extraordinarily short lower limbs, built for power but not so good for speed or long distance running. They also had a pelvis that itself was extraordinarily broad. This meant that the pelvis was not going to be as bio-mechanically efficient in long distance locomotion as that of Homo sapiens.

It seems that the very features that made Neanderthal perfectly adapted to the rigours of the Ice Age had also locked him into an evolutionary dead end. Homo sapiens may not have been adapted to the cold, but they were tailor made for the open plains. They were better able to exploit the open spaces, the step land habitats that were expanding in Ice Age Europe. And as the forest retreated, the Neanderthals retreated along with them.

Unable to survive in the open Neanderthals could only have clung on in ever decreasing woodland refuges. And as their habitat collapsed around them their population fell.

The effect seems to be that the populations that were once closely connected across Europe began to get fragmented and scattered. There came a point at which these populations were no longer viable and the Neanderthals became extinct, with their last communities on the western edges of Europe, in modern Portugal, disappearing.

12,000

A new wave of Homo sapiens crossed the frozen Baring Straits - Beringia - and entered the Americas. This species was now the most successful ever, having colonised and thrived on all six habitable continents and out-competed all related Homo species.

All images copyright © BBC or affiliates unless otherwise stated. No breach of copyright is intended or inferred. Text copyright © P L Kessler, adapted from numerous sources and notes, most notably the BBC tv series, Walking with Cavemen. An original feature for the History Files.