We’ve seen that there is little physical difference between a woman or man 100,000 years ago, and you or I today. We can see almost certainly that language is older than the onset of the full human package. Our brains are not significantly different from when we were just dabbling with art, and indeed they don’t seem fundamentally different from those artists who were not us, but our cousins the Neanderthals. The symptoms of modernity have been with us for tens of thousands of years longer than its arrival. Evidence is found scattered in Europe and Indonesia by 40,000 years ago. There are examples of modernity in Africa and Australia within a few millennia after it’s seen in Europe as well. These make a genetic basis for the change unlikely, as they are spread over the world with no interaction, no gene flow, between these peoples. If we are to assume that all the humans that spread around the world had emerged from Africa, and were genetically similar, then it’s unlikely that they independently will have enjoyed the same DNA mutations that triggered the arrival of a complex mind. If the Palaeolithic people of the world were biologically similar, the question is this: why did it take so long to become modern when we were physically ready for thousands of years?
There are many pieces of this puzzle that remain elusive. These are areas of research that are beginning to blossom, such as theory of mind, and the nature of consciousness. They are questions that have languished in fascinating philosophical realms for decades and centuries, and are beginning to be examined with the more precise scientific tools of the twenty-first century. We inch towards a better understanding of those areas as they become entwined with neuroscience.
There’s one idea that I think is crucial, that has been emerging in the last few years, but is not yet discussed widely, though I hope it will be soon. It is that population size and structure changed, and with those changes, modernity followed. The full package came about because of how we organised our society.
The first clue to this theory is that populations seem to grow larger at the time of the onset of modernity, in multiple locations. We see it in Africa 40,000 years ago, and at a separate time in Australia, more like 20,000 years ago. These expansions may be in line with the local environment, simply that as the climate changed, life became easier. They might also be a manifestation of our huge migrations. No other creature has moved permanently in such a short period of time – within 20,000 years of leaving Africa, we had settled in Australia.
We also see the opposite effect: a loss of cultural sophistication in societies whose populations do not grow, migrate or are cut off from a bigger populace. For example, Tasmania became an island around 10,000 years ago, as the last Ice Age thawed and the seas rose, and was separated from mainland Australia by what Europeans named the Bass Straits. The indigenous people of Tasmania managed to maintain a tool kit of only twenty-four pieces in that isolation, and lost the skills to make dozens of others over thousands of years in the Neolithic. Indigenous Australians on the mainland developed more than 120 new tools during the same period, including multi-toothed bone harpoons.
In the Tasmanian archaeological record, we see the gradual disappearance of fine bone tools, the loss of the ability to make cold-weather clothing, and perhaps most significantly, the degradation of fishing technology. Hooks and spears for catching cartilaginous fish vanish from archaeology, as does evidence of fish bones (although they did continue to forage and eat crustaceans and sessile molluscs). When Europeans arrived in the seventeenth century, the indigenous people expressed both surprise and disgust at the colonisers’ skill at catching and eating large fish, yet 5,000 years earlier it was a key and thriving part of their diet and culture.
Scientists who are interested in the full package have developed models to try to understand how the cultural transmission of skills is affected by the size and structure of a population.1 In this way, they can test how and why we see the hallmarks of modern behaviour come and go, and eventually stay in the archaeological record. These are effectively equations that model how an idea or skill is passed around in a community. They plug in hypothetical numbers for the size and density of a population, and a skill level for an imagined expert task – maybe knapping an arrowhead, or tooting a flute – and then they run simulations that work out how that skill level can be transferred between people. Mathematical models of this sort are pretty technical, but what they are effectively doing is saying: ‘Here are people with a very particular set of skills, which can be taught to others. How does the size of a population affect the efficiency of teaching?’
The answer appears to be ‘enormously’. Larger populations enable the transfer of complex cultural skills with far greater efficiency than smaller ones. The maintenance of skill levels is heavily dependent on population size (which is also affected by migration). According to the models, small populations, especially isolated ones, will lose skills through an inefficient transmission. When populations grow, they accumulate culture more readily. Only we do this. Though there is a smattering of other examples of cultural transmission in other animals, we do it all the time.
I don’t think that demographics is necessarily an obvious link to how we became who we are, which might be why it has been relatively neglected. But when we look at what humans are, it makes perfect sense. We are social, meaning that we depend upon the interactions with others for our own well-being. We are cultural transmitters, meaning that we pass on a wealth of knowledge that is not encoded in our DNA. We do this horizontally, not just vertically, meaning that we teach to people who are not our children, but are our peers, and may not even be genetically close kin. And we are highly skilled and creative, but that expertise is not distributed evenly throughout our populace – some people have skills that others don’t, and when we need to find out how to do something, we ask an expert.
There’s a second reason why this might not be as popular an idea as I think it ought to be. For many years in the infancy of evolutionary biology, scientists fiercely debated a question absolutely fundamental to Darwin’s greatest thought, natural selection: what is being selected?
Of the potential answers available, they scale all the way from the gene, to the individual, to the family, to the larger group, to the species. We put all that to bed in the middle of the twentieth century with unequivocal evidence that the answer is the gene. A gene encodes a phenotype – that is, the physical manifestation of a piece of DNA – and differences in those physical manifestations in a population are visible to nature as a means of selecting what works better. The gene that encodes that phenotype is what is transmitted from generation to generation, the unit of inheritance. A gene for processing goat’s milk after weaning was selected in humans over a gene that did not permit digestion of a nutritious drink. Individuals are merely carriers of genes, which drive the necessity of procreating simply so that the existence of the gene is perpetuated.
This gene-centric view of evolution was discovered and developed by some of the titans of twentieth-century biology – Bill Hamilton, George Gaylord Simpson, Bob Trivers, and others – and was immortalised in one of the great works of popular science, Richard Dawkins’ The Selfish Gene. It is correct, and now textbook. What this new model suggests is that there is selection for cultural transmission of things that are adaptive and therefore of great benefit to us, pivoting not on genes but on a population. We biologists are rightly groomed away from ideas about group selection because they are not correct – the data does not fit the idea that evolution acts on groups. But cultural transmission is not encoded in DNA, and in some ways, is exempt from the precise mechanisms that happen in the formation of egg and sperm that engineers genetic difference in a population, and therefore is subject to Darwinian evolution.
Put together like that, it seems obvious that the demographic structure of a society is going to be essential in maximising the way information and skills are transmitted within a group. Any group of people relies on an internal organisation to be effective. It seems from these models, that our modernity – the full package of being the humans that we are today – is dependent on us being able to accumulate culture, to pass it on, and to pass it on in a society that grew to optimise the overall success of its members.
This is a territory that is actively being researched right now. It is the model that I think is right, for what that is worth, though much more work is needed. A tiny proportion of the ground has been dug to uncover our pasts. A fraction of the genes of our ancestors have been sampled. As ever in science, the answers are never complete, and we mould and carve ideas, throw them away if the data doesn’t fit, or build them up if it does. The idea that demography was an essential pivot in the ascent of us is an idea that is young.
The truly amazing thing is that Darwin was thinking along the very same lines, one and a half centuries ago. He writes in The Descent of Man:
As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all members of the same nation, though personally unknown to him. This point being once reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races.
1 Notably spearheaded by Mark Thomas and colleagues at University College London, and Joseph Henrichs at Harvard University, among others.