The heat was unbearable. The land was barren. Early homo sapiens dragged his weary feet across the dusty African desert. It was around this time, ninety thousand years ago, that our ancestors experienced a change in fortune.

Leaving behind their crude stone tools, which had been in use for millions of years, these early humans underwent a huge transformation with a massive boost in brain power. In the space of a few tens of thousands of years, give or take, they developed new and better weapons and tools, as well as an appreciation for art and jewellery, such as shell bead necklaces. This period has since been labelled “the origins of modern human behaviour”, and, anthropologically speaking, we have never looked back.

A diverse team of scientists at UCL have put the transition from stone age brute to modern cultural human down to increases in population density. The team used a computer model to simulate social learning in subpopulations, where people can move between groups. They found that, with a higher number of subpopulations, not only do individuals learn better, but populations are better able to hold onto new ideas. Dr Mark Thomas, co-author of the study, says “it’s all very well having a good idea, but not if it doesn’t hold. Over time, a more dense population will hold onto ideas.”

The computer data was tested against estimates of population density during the origins of modern human behaviour. By tracing humans back, via DNA, to a common female ancestor, it is possible to tell when population was constant and when it expanded. This, combined with estimates of land size, can be used to work out population density. “Our model correlated with evidence in Africa and the Middle East,” claimed an excited Thomas. However, it is difficult to study some parts of the world, such as South East Asia and India, because most of what was habitable land is now underwater.

On top of a potential underwater world of ancient artefacts, there are a host of other mysteries surrounding this period. All evidence of modern human behaviour disappears in Africa for around thirty thousand years, before returning. A similar dramatic transition in behaviour wasn’t seen in Europe for another forty-five thousand years, but when it did come, it stayed. “Some people say it’s not the same, that the European thing was special,” says Thomas. “I actually think that’s entirely unfair.” One theory is that some sort of mutation took place in the brain. The main advocate of this idea, the so-called “brainy mutation”, is Richard Klein, of Stanford University.

Klein’s argument is that the early humans in Africa weren’t so modern, after all. Klein attacks the idea that modern human behaviour came “Out of Africa” (as the theory has come to be known), accusing other academics of using ambiguous data to fit into their preconceived ideas. “[The ‘Out of Africa’ model] does not explain why social relations changed when they did or why they changed at all.” Based on this, Klein argues, “it becomes at least as plausible to tie the basic behavioural shift at fifty thousand years ago [i.e. the European one] to a fortuitous mutation that promoted the fully modern brain.” Dismissing the ninety thousand year old evidence in Africa, Klein believes that “the fully modern capacity for culture may have appeared only about fifty thousand years ago”, in other words, in the Europeans.

This idea that truly modern brains arose in our expansion to Europe has, for many in the field, unsavoury consequences to say the least. As humans had already dispersed throughout the planet by this time, the theory would suggest that some people in the world are more cognitively able, or brainy, than others. “As well as being a socially irresponsible thing to say, there is just no evidence for it,” says Thomas. “We absolutely expected right from the off that Richard Klein would be rabidly anti [our model] because it flies in the face of his crazy idea about this recent mutation.”

On the contrary, Thomas and his UCL colleagues reckon that the first humans, two hundred thousand years ago, were mentally capable of modern behaviour. “It’s like somebody flicked a switch two hundred thousand years ago, but the light didn’t come on for another hundred thousand years,” Thomas muses. One idea is that human braininess evolved for another reason altogether. Speculating, Thomas suggests a modern relevance, “to track and coordinate social relationships, work out who we trust and gossip about who we think is dodgy.” Perhaps the ancient caveman trekking through sub-Saharan Africa isn’t so far removed from the modern-day Tweeter after all.