A particular genetic variant associated with fat metabolism, that’s curiously prevalent in modern Mexican populations, appears to have come from Neanderthals originally. Perhaps it conferred some sort of advantage in the past, linked to a particular diet, but interacting with a different sort of food intake today it increases the risk of developing diabetes. Other genetic variants that have entered our genomes from these ‘lost tribes’ are associated with differences in skin and hair colour. Seven out of ten modern Europeans possess a certain gene of Neanderthal origin that’s associated with freckles. With other genes inherited from archaic populations, it’s less clear what the functional significance is in modern genomes. On the other hand, it’s obvious that a lot of archaic DNA has been weeded out – most likely because it was linked to a reduction in fertility.
Interbreeding with lost tribes meant that our ancestors tapped into a rich reservoir of genetic variation – potentially picking up useful adaptations to local environments, including the pathogens that lived in them. This is an important and relatively new insight into the mechanism of evolutionary change: the introduction and spread of a genetic variant might start with a new mutation, or with an old mutation in that population suddenly proving useful – but it can also arrive from another, closely related population, through interbreeding. From apples to humans, we all bear the evidence for our hybrid origins in our genomes.
But it’s not just closely related human species we’ve interbred with that have left their mark on us today. We’ve found firm allies in other species – including plants as well as animals – and we’ve met nine of them in this book. By teaming up with these other species, domesticating them – or providing them with an opportunity to ‘domesticate themselves’ – the course of human history has been profoundly affected, in ways that can be difficult to comprehend. The influence of the Neolithic has rippled down through the centuries and millennia.
The Neolithic Revolution
It’s only with hindsight, and with that deep and wide perspective that geography, archaeology, history and genetics provides us with, that we can appreciate this grand narrative. There’s such a gulf between mapping out events and processes over thousands of years, on a continental scale, and the personal experiences, the everyday lives, of our ancestors. But on the other hand, it feels as though we’re getting closer to a convergence: charred grains, polished stone sickles, traces of milk on fragments of pottery, DNA from ancient wolf bones and echoes of the word for ‘apple’ in a truly ancient language – each provide us with astonishing glimpses of detail.
Just as we’ve elaborated the stories of the origin of species, adding more facets and complexity as new evidence comes to light, the story of the Neolithic has become immeasurably more complicated over time. Rather than a linear, predictable march of progress, driven by human intent, developments – new alliances and the new technologies that accompany them – have emerged in a much more haphazard way. The Neolithic – a switch from hunting and gathering nomadism to farming sedentism – was inevitable as human populations grew. But the particular trajectory varied from place to place, and external factors were hugely influential. As the Ice Age loosened its grip on the world, agriculture developed independently in separate areas. Each time it emerged in fits and starts – then the idea, the technology and the newly domesticated species rippled out from their origins, with the power to feed an expanding human population.
The emergence of agriculture in western and eastern Asia near-simultaneously, around 11,000 years ago, must be more than coincidence: global climate change was affecting people – and grasses – separated by thousands of miles. A global increase in atmospheric carbon dioxide levels after 15,000 years ago would have boosted plant production – fields of wild cereals were there for the picking. Then there was the climatic downturn during the Younger Dryas of 12,900 to 11,700 years ago. Hunters would have started to come back empty-handed more often. Easily harvestable fruits and berries would have been thin on the ground. Foragers would have fallen back on their fallback resources – including the difficult-to-collect but energy-rich seeds of grasses: oats, barley, rye and wheat in the west; broomcorn and foxtail millet and rice in the east. Technologies to make harvesting more efficient and to grind the hard seeds into flour, like the Natufian sickles and stone mortars, came long before domestication and farming itself. By the time the climate started improving, this dependence on cereals had developed into proto-farming.
Those early centres of domestication were hugely influential. The broadly Mesopotamian ‘cradle of agriculture’ provided the founder crops of the western Eurasian Neolithic. From the fertile land between and around the Rivers Euphrates and Tigris came the first domesticated peas, lentils, bitter vetch, chickpeas, flax, barley, emmer wheat and einkorn wheat. From the land around the Yellow and the Yangtze Rivers came millets, rice and soybean. But there were plenty of other locations around the world where domestication took off. At the end of the Younger Dryas, people from the southern half of Africa migrated north to colonise the green, fertile Sahara. They were hunter-gatherers, subsisting on fruits, tubers and cereals as well as the animals they hunted. They’d been using grinding stones since 12,000 years ago, and cultivation of indigenous sorghum and pearl millet may have started there soon after. But Saharan agriculture was wiped out around 5,500 years ago when a southwards shift in the monsoon transformed the once-fertile landscape into a desert. Sugar cane was domesticated in New Guinea some 9,000 years ago, and teosinte was domesticated to become maize, in Mesoamerica, around the same time.
The more we look, it seems the more centres of domestication we find. The Fertile Crescent is fascinating – but it has tended to draw our gaze away from other, equally important wellsprings of the Neolithic. Vavilov identified seven centres of domestication. Jared Diamond posited nine or ten around the world. More recent studies suggest there were as many as twenty-four. Domestication of species has happened many times, in many different places. Quite a few of the environments where domestication took place were – as Vavilov pointed out – mountainous. These are environments where diversity tends to be rich – physical conditions vary with altitude. But for any potential domesticate, the fit with human nature, as well as the timing, had to be right. Species that reacted positively to human intervention, at the same time as humans were open to changing their way of life – that was the winning combination which led to the formation of these crucial allegiances. And conscious decision-making rarely played any role.
The term ‘artificial selection’ perhaps implies an agency, a consciousness, that is not always at work. Although our modern selective-breeding programmes represent carefully planned interventions and incredibly thoughtful selection, it wasn’t always so – especially at the dawn of domestication. The wheat that grew up around the threshing floors was not deliberately sown – but it paved the way for the first fields. The separation between natural and artificial selection is, perhaps itself, artificial. Humans are not the only species to affect the evolution of other species. Our very existence depends on interdependency. We might be able to understand what it is that we think we’ve done, peering into genomes, but bees have influenced the evolution of flowers just as surely as we’ve influenced the evolution of dogs, horses, cattle, rice, wheat and apples. The bees may not know it, and reflect on it as we do, but they have still driven change. What we’ve called artificial selection, ever since Darwin used the phrase to help build his argument, is no more than human-mediated natural selection.
Domestication may have started, in many cases, as a thoughtless process: species coming into contact, knocking up against each other, growing closer until their evolutionary histories became intertwined. We’re so used to thinking of ourselves as the masters, and other species as our willing servants, even our slaves. But the ways in which we entered into these contracts with plants and animals were various and subtle, evolving organically into a state of symbiosis and co-evolution. There was rarely any thoughtful intent be
hind the initial construction of this partnership. Anthropologists and archaeologists have described three main pathways to domestication of animals – and it was never an ‘event’, rather a long, drawn-out evolutionary process. One pathway involves animals choosing humans, borrowing resources from us. As they moved in closer, they began to co-evolve with us, becoming tame long before any sort of human-directed selection – like the creation of dog breeds in the last few centuries – could even begin to happen. Dogs and chickens both became our allies in this way. The second route is the prey pathway. Even here, there would have been no initial intention to domesticate animals – only to manage them as a resource. This would have been the route for medium and large herbivores such as sheep, goats and cattle – first hunted as prey, then managed as game, and finally herded as livestock. The final pathway is the most intentional – where humans set out to capture and domesticate animals, right from the start. Usually these animals were seen as useful for something other than just meat – and horses, tamed as our steeds, are a prime example.
Even when conscious intent did start to play a role, as farmers and breeders began to select particular traits – by weeding out the ones they didn’t want – the aim still wasn’t particularly long-sighted. Darwin himself recognised this. He wrote that, whereas ‘eminent breeders try by methodical selection, with a distinct object in view’, others would have been focused on just the next generation, with ‘no wish or expectation of permanently altering the breed’. Nevertheless, those choices would, over decades and centuries, lead to ‘unconscious modification’ of a variety or cultivar. Darwin thought that even ‘savages’ and ‘barbarians’ (to the modern reader, he’s extremely un-PC at times) could modify their animals, by even less conscious selection – simply by saving their favoured beasts from being eaten during famines.
The final blow to our mastery of nature comes when we consider the relatively small number of species that we’ve been able to successfully recruit as our allies. Many, as the nature writer Michael Pollan so succinctly put it, have ‘elected to sit it out’. For a species to become a successful ally, it had to possess certain qualities which would – when the occasion presented itself – act as predispositions to becoming human domesticates. Without the curiosity of the wolf, the submissive nature of the mare, the potential for grasses to develop a non-shattering rachis, the plumpness of the central Asian wild apples – we probably wouldn’t have dogs, horses, wheat and cultivated apples.
Nevertheless, our domestication of other species has had far-reaching – global – consequences. The concept of interdependency with other species – that lies at the heart of the Neolithic – became an idea, a part of human culture, which would prove so successful that it was destined to spread around the world. Having struck up particular relationships with certain plants and animals, our ancestors could move those species – modifying the local environment to suit them, as they went. It was an extremely successful strategy, even if its origins had come about through pure serendipity.
Today, hunting and gathering is a lifestyle practised by a diminishingly small number of people. Among others, there are still a few, tiny populations of hunter-gatherers in Africa, including the Bushmen in Namibia and the Hadza people of Tanzania. They live in relatively inhospitable places, in semi-deserts – landscapes where farmers can’t farm. They’ve resisted the Neolithic Revolution right up until now, but their way of life is under threat, and will probably disappear this century.
Co-evolution and the course of history
Human history would have played out very differently if the other species we interacted with had been different – missing altogether, impossible to catch or to domesticate, for example. We sometimes approach history and prehistory as though we humans are so much the lords of our own destiny that external forces have little or no role to play. But the story of any species can never be told in isolation. Every species exists in an ecosystem – we are all interlinked and interdependent. And serendipity and contingency are woven into all the interactions that have played out in the course of our intertwined histories.
The alliances that we’ve formed with other species over millennia have changed the course of human history in ways that the earliest farmers, the first hunters with their dogs, and the first horse-riders couldn’t possibly have dreamt of. Cultivated cereals have provided the energy and protein for the expansion of human populations – far beyond the potential that gathered wild foods could support.
The wheats from the centres of domestication in the Middle East provided the fuel for a population boom that spilled over into migrations, with farmers spreading across Europe during the Neolithic. Domesticated sheep, goats and cattle provided that crucial means of storing protein and energy – as ‘walking larders’. Forming alliances with plants lately used as fallback foods, and with once-hunted animals, humans began to buffer themselves a little from the immediate effects of climatic disturbances. With a more secure source of energy and protein, and a more settled way of life, families could grow larger. It sounds like such an unmitigated success story, but the slightly counter-intuitive reality of the Neolithic Revolution was that it chained people to lives of hard labour and took its toll on the health of individual women, men and children.
An archaeological site in central Anatolia – spanning just over a millennium, between 9,100 and 8,000 years ago – provides us with an astonishing snapshot of those living through the transition. The early farming community of Catalhöyük inhabited a dense settlement of mud-brick houses, packed tightly together. At first, just a few families lived there, then the village grew in size dramatically. The farmers grew mainly wheat, but also barley, peas and lentils, and they kept sheep, goats and a few cattle, as well as hunting aurochsen, wild pigs, deer and birds, and gathering wild plants. Their fields were located several miles south of the settlement, and they also had to range widely to hunt and herd their animals. The skeletal remains of more than 600 people have been discovered at Catalhöyük, and these bones tell tales. There’s an extraordinary number of juveniles, including the bones of many newborn babies. On the face of it, it looks like infant and child mortality was particularly high, but in fact this pattern is likely to represent an unusually large number of babies being born in the first place. Teasing apart the numbers by date, the birth rate appears to have risen with the transition from foraging to early farming, and again, with a move to more intensive farming. The number of houses in the village was growing accordingly. Analysis of nitrogen isotopes in the bones of infants indicates that weaning was starting when infants were relatively young, around eighteen months of age. Early weaning in such populations is linked to a shorter space between births – a population boom in the making.
But it wasn’t all rosy. Catalhöyük presents a picture of increased physiological stress and health problems, compared with earlier foraging communities. A diet focused on cereals provides plenty of energy but not necessarily all the essential building blocks of protein, or vitamins, that the body needs. Although other sites have turned up evidence of reduced growth rates, this doesn’t seem to have been the case at Catalhöyük. Nevertheless, there’s ample evidence there of low-level physiological stress, including bone infections – and a high rate of tooth decay that is probably linked to a starch-rich diet.
Today, industrialised farming means that the hard graft of agriculture has, to a large extent, become shouldered by machinery rather than humans. But we’re all chained to systems of food production where cereals – the fallback foods of our hunter-gatherer ancestors – have become staples, just as they were at Catalhöyük. With globalised food supplies, we can access other sources of important vitamins (and now we can even insert those into cereals, with gene editing), but our teeth are still suffering from the effects of the Neolithic Revolution. One of the most maligned villains is the sugary derivative of maize: high-fructose corn syrup. It seems to be a food that encapsulates the best and worst of the Neolithic legacy – a fantastic source of e
nergy, certainly, but also an insidious threat to health that we’re only just starting to recognise. Maize itself has played a huge part in human history. It fuelled the Inca and Aztec civilisations, and went global after Columbus (and possibly Cabot) reached the New World. Today, we produce more of it by weight than any other grain. It fuels us – but we grow four times as much as we humans eat to feed livestock, and almost as much again to create biofuels.
The impact of domesticated species on our own journey through history is perhaps easiest to grasp if we imagine what would have happened without them. This approach is analogous to the way that geneticists set out to find out about the function of a particular gene – by creating a knockout. We can’t test our alternative histories in the same way, but our thought experiments can still give us some idea of how different the world could have been without these various species.
Without our domesticated cereals, where would we be today? The Neolithic would have unfurled in an unfamiliar manner. Pastoralism alone would surely not have supported the population expansions that saw the spread of people, livestock and crops from the Middle East, right across Europe. Would early civilisations – the Sumerian in the Middle East, the Yellow and Yangtze River civilisations in the Far East, and the Mayan in Mesoamerica – have taken off? Perhaps not in the same way, but the horse-riding nomads of the Eurasian Steppe remind us that civilisation can evolve on the move. In a world without cereals, would we all still be nomads, living in yurts rather than houses? Or would starchy tubers like potatoes have filled the void? As we consider the absence of each domesticate, it becomes harder and harder to imagine a world without the species we’re so familiar with, and so dependent on.
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