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First Steps

Climbing the winding path towards the Mount Carmel Caves in modern-day Israel, it is possible to envision the majestic environment that would have surrounded this site in prehistoric times. The Mediterranean climate would have been pleasant across the seasons, with moderate temperature fluctuations. The creek snaking through the mountains in the adjacent verdant valley would have been a source of potable water. The forests beside the mountain range would have been suitable for hunting deer, gazelles, rhinoceroses and boar, and out in the wild, in the open areas abutting the narrow coastal plain and the Samarian mountains, there would have grown prehistoric species of cereals and fruit trees. The warm climate, ecological diversity and raw materials surrounding the Mount Carmel Caves would have made them ideal homes for numerous bands of hunter-gatherers over the millennia. Indeed, remains unearthed in these ancient caves, now a UNESCO World Heritage Site of human evolution, attest to a sequence of prehistoric settlements over hundreds of thousands of years, as well as tantalising potential encounters between Homo sapiens and the Neanderthals.

Archaeological findings from this and other sites across the globe indicate that archaic and early modern humans slowly but steadily acquired new skills, mastered the use of fire, developed increasingly sophisticated blades, handaxes, and flint and limestone tools, and created artworks. A key driver of these cultural and technological advancements, which came to define humankind and set us apart from other species, was the evolution of the human brain.

Genesis

The human brain is extraordinary: large, compressed and more complex than the brain of any other species. It has tripled in size over the last six million years, with most of this transformation occurring 200,000-800,000 years ago, largely before the emergence of Homo sapiens.

Why have the capabilities of the human brain expanded so significantly over the course of the history of the human species? At first glance, the answer might appear self-evident: having an advanced brain has clearly allowed us to achieve levels of security and prosperity that no other species on Earth has managed to attain. Yet the reality is considerably more intricate. If a brain resembling the human one is indeed so unambiguously beneficial for survival, why has no other species developed a similar brain over billions of years of evolution?

Consider this distinction for a moment. Eyes, for example, developed independently along several evolutionary tracks. They evolved among vertebrates (amphibians, birds, fish, mammals and reptiles), cephalopods (including cuttlefish, octopuses and squid), as well as in a simpler form - ocelli - in invertebrates such as bees, spiders, jellyfish and sea stars. The distant ancestor of all these species, which lived more than 500 million years ago, seems to have had only basic light receptors, capable of distinguishing light from dark.Nevertheless, since accurate vision has provided a distinct survival advantage in different environments, complex eyes evolved independently in some of these different groups, uniquely adapted in each case to the individual species' habitat.

This phenomenon, whereby similar traits evolved independently in different species rather than emerging from an existing trait in a common ancestor, is known as convergent evolution. There are numerous other examples, such as the development of wings among insects, birds and bats, and the comparable body shape that evolved in fish (shark) and marine mammals (dolphins) to suit life underwater. Evidently, various species have acquired similar beneficial traits by independent means - but not brains capable of crafting literary, philosophical and artistic masterpieces, or inventing the plough, the wheel, the compass, the printing press, the steam engine, the telegraph, the aeroplane and the internet. Such a brain has only evolved once - in humans. Why is such a powerful brain so rare in nature, despite its apparent advantages?

The resolution of this puzzle partly lies in the brain's two major drawbacks. First, our brain exhausts enormous amounts of energy. It consists of only 2 per cent of the body's weight while consuming 20 per cent of its energy. Second, its large size makes it difficult for a baby's head to pass through the birth canal. Consequently, the human brain is more compressed or 'folded' than other species' brains, and human babies are born with 'half-baked' brains that need years of fine-tuning to reach maturity. Human infants are therefore helpless: while the young of many other species can walk by themselves shortly after being born, and are rapidly able to acquire their own food, humans need a couple of years before they can walk by themselves in a stable fashion, and many more before they can reach material self-sufficiency.

Given these drawbacks, what led to the development of the human brain in the first place? Researchers have argued that several forces may have contributed jointly to this process. The ecological hypothesis suggests that the human brain evolved as a result of the exposure of our species to environmental challenges. As the climate fluctuated and nearby animal populations adapted accordingly, prehistoric humans with more advanced brains would have been better able to identify new food sources, devise hunting and gathering strategies, and develop cooking and storage technologies that allowed them to survive and thrive in the shifting ecological conditions of their local habitat.

The social hypothesis, in contrast, maintains that the growing need to cooperate, compete and trade within complex social structures gave a more sophisticated brain, with its better ability to understand the motives of others and anticipate their reactions, an evolutionary advantage.Likewise, being able to persuade, manipulate, flatter, recount and amuse - all of which would benefit one's social standing, as well as conferring advantages in themselves - spurred the development of the brain and the capacity for speech and discourse.

The cultural hypothesis, meanwhile, highlights the ability of the human brain to assimilate and store information, allowing it to be passed from one generation to the next. According to this viewpoint, one of the unique advantages of the human brain is its capacity to learn efficiently from the experiences of others, facilitating the acquisition of habits and preferences that boost survival in diverse settings without relying on the far slower process of biological adaptation.In other words, human babies may be physically helpless but their brains are equipped with unique learning capacities, including the ability to grasp and retain the behavioural norms - the culture - that enabled their ancestors to survive and will help their descendants to prosper.

One mechanism that may have further contributed to the development of the brain is sexual selection. It could be that humans developed a preference for mates with more advanced brains, even in the absence of overt evolutionary advantages of the brain itself. Perhaps these intricate brains attested to invisible qualities that were important for protecting and raising children, and potential mates were able to infer these qualities from perceptible attributes such as wisdom, articulation, quick thinking or a sense of humour.

The evolution of the human brain was the main impetus for the unique advancement of humanity, not least because it helped bring about technological progress - ever more sophisticated ways to turn the natural materials and resources around us to our advantage. These advancements, in turn, shaped future evolutionary processes, enabling human beings to adapt more successfully to their shifting environments and to further advance and utilise new technologies -  an iterative and intensifying mechanism that has led to ever greater technological strides being made.

In particular, it is thought that developments in the mastery of fire, which allowed early humans to begin cooking their food, spurred additional growth of the brain by reducing the energy required to chew and digest, thus making calories more accessible and freeing space in the cranium previously occupied by jaw bones and muscles. This reinforcing cycle may have fostered further innovation in cooking technologies, which may have led to further growth of the brain.

Yet our brain is not the only organ that sets us apart from other mammals. The human hand is another. In conjunction with our brains, our hands too evolved partly in response to technology, specifically the benefits of creating and utilising hunting tools, needles and cooking gear. In particular, when the human species mastered the technology to carve stones and make wooden spears, the survival prospects of those who could use them forcefully and accurately improved. Better hunters could support their families more reliably, and therefore raise more children to adulthood. The intergenerational transmission of these skills increased the share of proficient hunters in the population, and the advantages of further innovations, such as sturdier spears and, later, stronger bows and sharper arrows, contributed to the evolutionary advantage of these hunting skills.

Positive feedback loops of a similar nature have emerged throughout our history: environmental changes and technological innovations enabled population growth and triggered the adaptation of humans to their changing habitat and their new tools; in turn, these adaptations enhanced our ability to manipulate the environment and to create new technologies. As will become apparent, this cycle is central to understanding the journey of humanity and to resolving the Mystery of Growth.

Exodus from the Cradle of Humankind

For hundreds of thousands of years, the human species roamed in small bands of hunter-gatherers in Africa, developing complex technological, social and cognitive capabilities along the way. As prehistoric humans became ever better hunters and gatherers, their population in the fertile regions of Africa increased significantly, ultimately reducing the living space and natural resources available to each of them. Thus, once climatic conditions permitted, humans started branching out to other continents in search of additional fertile grounds.

Homo erectus, arguably the first hunter-gatherer species of human, spread out to Eurasia nearly two million years ago. To date, the oldest fossils of early Homo sapiens to have been discovered outside of Africa are 210,000 years old (uncovered in Greece), and 177,000-194,000 years old (found on Mount Carmel in northern Israel). Yet it appears that the descendants of these first modern humans to leave Africa became extinct or retreated into Africa due to adverse climatic conditions during the glacial period.

It was in Africa, then, about 150,000 years ago that the most recent (matrilineal) ancestor of all living humans, Mitochondrial Eve, emerged. Although there were of course numerous women in Africa at the time, their lineages ultimately became extinct. All humans on Planet Earth today are descended from this one African woman.

The widely accepted 'Out of Africa' hypothesis suggests that the current population of anatomically modern humans across the globe descends predominantly from a more significant migration of Homo sapiens from Africa as early as 60,000-90,000 years ago. Humanity flocked to Asia via two routes: the northern via the Nile Delta and the Sinai Peninsula to the eastern Mediterranean region known as the Levant, and the southern one via the Bab-el-Mandeb Strait at the mouth of the Red Sea into the Arabian Peninsula (Fig. 3). The first modern humans reached South East Asia more than 70,000 years ago, Australia 47,000-65,000 years ago, and Europe nearly 45,000 years ago. They settled Beringia approximately 25,000 years ago, crossing the land bridge over the Bering Strait during several periods of the Pleistocene Ice Age, and entered deeper into the Americas 14,000-23,000 years ago.

These waves of migrations out of Africa contributed to the size and the diversity of the human population across Planet Earth. As prehistoric humans settled new ecological niches, they enjoyed access to new grounds for hunting and gathering and started to multiply more rapidly. Meanwhile, their adaptation to diverse new environments led to greater human and technological diversity, fostering the spread and cross-pollination of innovations, and leading to further population growth.

Ultimately, however, population growth led to the same scarcity of fertile land and resources that had spurred the migration from Africa in the first place. Despite their new tools and techniques, humans' living standards gradually reverted towards the subsistence level. The inability to sustain the growing population, as well as climatic changes, eventually induced humanity to explore an alternative mode of subsistence - agriculture.

Early Settlement

Nearly 12,000 years ago, as the climate gradually warmed in the aftermath of the latest glacial period, Homo sapiens experienced a dramatic transformation. Across the world, people gradually swapped their nomadic wandering for sedentary lifestyles, and began to make great strides in art, science, writing and technology.

Evidence from the Natufian culture (13,000-9500 BCE), which flourished in the Levant, suggests that in some places the transition to permanent dwellings predated the onset of agriculture. Despite being predominantly hunter-gatherers, the Natufians lived in stable residences, made typically of a drystone foundation with brushwood superstructure. Each settlement contained up to a few hundred people, who ventured out for hunting expeditions and for gathering native wild crops. But for the majority of the world's human population at the time, it was the transition to agriculture that was the primary inducement to sedentism.

The Agricultural Revolution, also known as the Neolithic Revolution, first emerged in the Fertile Crescent - a lush region along the Tigris and Euphrates rivers, down the eastern Mediterranean coast, and around Egypt's Nile Delta - which was abundant in a wide variety of domesticable species of plants and animals. Agriculture emerged independently, about 10,000 years ago, in South East Asia, and from these distinct locations it spread swiftly across the Eurasian land mass. The rapid diffusion of agricultural practices within this vast region was enabled by the east-west orientation of these continents and the feasibility of the dispersal of plants, animals and technologies along similar lines of latitude without encountering major natural obstacles.

In contrast, as argued by the American geographer and historian Jared Diamond in his Pulitzer Prize-winning book Guns, Germs and Steel, sub-Saharan Africa and the Americas, which contained far fewer domesticable species of plants and animals, experienced the transition to agriculture significantly later. Despite an early onset of agriculture in Meso-America and in some regions of Africa, the diffusion of agricultural practices was slower within these areas because the north-south orientation of these continents created major differences in climate and soil between regions. Moreover, the Sahara and the largely impassable tropical rainforests in Central America served as natural barriers to this diffusion process.

Nonetheless, after hundreds of thousands of years of painfully slow technological and social change, this process - the transition from hunter-gatherer tribes to agricultural societies, and from nomadic lifestyle to sedentary living - spread within a few thousand years to most of humanity. During the Neolithic Revolution, humans domesticated a wide range of wild plants and animals around the world. Wheat, barley, peas, chickpeas, olives, figs and date palms, as well as sheep, goats, pigs and pigeons, were first domesticated in the Fertile Crescent. Grapes and pomegranates in the nearby Transcaucasian region. Rice, buffalo and silkworms were domesticated in China, and ducks in South East Asia. Sesame, aubergines and zebus in the Indian subcontinent. Sorghum, yams, coffee and donkeys in Africa. Sugar cane and bananas in New Guinea, and maize, beans, squash and potatoes, as well as turkeys, llamas and alpacas, in the Americas.