The Big Chill
When I was growing up in a city called Lubbock, in the so-called Panhandle region of Texas, we used to relate geographic distance in the form of time. The distance between Lubbock and Brownfield, a nearby town, was often given as ‘around forty-five minutes’, rather than 50 miles. This stems from the fact that everyone taking this journey would be driving a car, and most drivers would settle on a speed of around 60 mph – giving us a rough-and-ready conversion between time and distance.
For most of human history, distance has been expressed in a similar way. The earliest humans would have described distances in terms of the time taken to walk there. I am writing this in a house in East Anglia, near the market town of Sudbury, but if I were describing it to a Palaeolithic ancestor I might mention that it is around three days’ walk from London. Similarly, our ancestors living tens of thousands of years ago would have envisioned their territories in terms of the time and effort required to traverse them. Luca Cavalli-Sforza and archaeologist Albert Ammerman have calculated that agricultural populations expanding into new territory disperse at a rate of approximately 1 km per year. Hunter-gatherers, being more mobile, can move at several times this rate. Of course, this is actual expansionary movement – the total distance walked in any year would be much more than this. But a few kilometres per year is a good estimate of the average rate at which modern-day hunter-gatherers, living in much the same way as our Upper Palaeolithic ancestors, migrate through new territory.
Based on this rate of movement, the trip from north-eastern Africa to the Bering Strait, on the opposite side of the Eurasian landmass, would have taken several thousand years. Today it is theoretically possible to make this trip in a single aeroplane flight – taking off in Djibouti (just across the Gulf of Aden from the Arabian peninsula) and landing in Provideniya, Russia, a short hop from Alaska. But around 50,000 years ago, when our ancestors began their voyage across the continent, it would have been unimaginable to make such a massive leap in one go. Rather, the journey across Eurasia would have happened imperceptibly, measured on a different time scale – one of intergenerational distances. This ‘deeper’ clock would have ticked away as individual bands gradually migrated into new territory, following animals, searching for water or plants, or perhaps stone for making tools. Some movement may even have been instigated by conflicts with other human groups. It was probably a combination of all of these reasons, as well as others we can’t envision today. Whatever forces led to what palaeoanthropologist Chris Stringer has called the ‘African Exodus’, the journey must not be seen as a conscious effort to traverse the continent, but rather as a gradual expansion in range driven largely by seemingly insignificant local decisions. It is not unlike the act of squeezing toothpaste through a tube, where climate is both the stick and the carrot of the scenario. Difficulty at home forces the migration, but climatic change may lead to the appearance of new resources in distant regions. The human population is gradually forced through the geographic ‘tube’ by the combination of these forces, pushing and pulling over thousands of years until humans have dispersed far from their original homeland.
While this is a fair description of what motivated the earliest humans to move across Eurasia, we are interested in using the genetic data to infer the details of how it might have happened. Genetics has answered the question of who (Africans) and when (50,000 years ago), and we have some theories as to why (environmental change), but we now must ask how our ancestors of 50,000 years ago made the leap into Eurasia – and what route they would have followed. For this, we need to go back to our study of palaeoclimatology and ask what north-eastern Africa would have looked like fifty millennia ago.
The world was getting colder around 70,000 years ago, as the last ice age accelerated into a deep-freeze. This may have been the catalyst for the Great Leap Forward, favouring intelligence and complex social structures as the climate deteriorated and life became more difficult. The forests were shrinking, replaced in eastern Africa by savannah and steppe grasslands, with their wealth of large ungulates. It was on these grasslands that humans tracked and hunted, developing ever more complex tool-making and social skills. Life was incredibly active, with all effort focused on killing and gathering enough food to survive. The bell-shaped mtDNA mismatch distribution suggests that they were quite successful at this, with an expanding population even as the world turned colder and nastier.
No doubt it was the competition and difficulty of living inland, with dwindling access to both water resources and easy prey, which led some populations to live on the coast. These would have been the ancestors of the Australians, who almost certainly began to migrate out of Africa along the southern coastal route as soon as the conditions created an easy exit to the Eurasian landmass. This would have been easiest between Djibouti and present-day Yemen, a straight shot out of the Rift Valley to the endless beaches of southern Asia.
The lifestyle of these coastal people would have been relatively sedentary, tied as they were to gathering from the sea. Their days almost certainly would have been dictated by the exposure and submersion of the intertidal zone, with its rich beds of molluscs and crustaceans. Although they would probably have hunted as well, they would have been guaranteed a better return on their labours if they stayed near the coast. As we saw in the previous chapter, the genetic and archaeological data bear this out, suggesting that they did not stray very far inland at this early stage. The interior was left to the more active hunters, who would have had to move great distances to obtain the resources they needed to survive – animals, plants and water. They are the ones who made the leap into the unknown beyond the coast, into the wilds of interior Eurasia.
One of the apparent conundrums of biology is that the more temperate parts of the world actually contain the largest animals. In ecology there is an observation known as Bergmann’s rule, which states that body size increases with latitude. While this isn’t strictly true for every species, it is a good generalization. The woolly mammoths, largest land mammals of the past few hundred thousand years, lived in the tundra regions of far northern Eurasia and America. In the sea, there is actually more biological material in the colder parts of the planet than there is in the warmer. In spite of the incredible diversity found on a coral reef, the total mass of organisms is significantly less than that found in more polar regions. The polar oceans, for instance, contain the world’s densest concentrations of plankton. These tiny plants and animals support the largest animals on earth, the filter-feeding baleen whales which, over time, have become almost completely dependent on this unusual food source, the remnants of their terrestrial lives tens of millions of years ago being nearly invisible today.
Similarly, the tropical rainforest contains a huge number of species, but the size – and density – of any particular species is quite low. Furthermore, because all of the nutrients are tied up in organisms, the soil actually contains very little in the way of minerals and organic matter. In reality, overgrown bushes do not typically clog the ground in a mature tropical forest, despite the Hollywood cliché of machete-wielding explorers. The tragedy of deforestation is that it is all too easy, in the space of a few years, to reduce a teeming ecosystem to a lifeless desert. The tropical environment is poised precariously on the edge of fecundity and death, extremely susceptible to relatively trivial disturbances.
The temperate parts of the planet, on the other hand, are endowed with rather more resilience. While the species diversity is a small fraction of that seen in the rainforest, the organisms living there are better able to withstand drastic upheaval. This is primarily due to the vicissitudes of life in the temperate zone. Tropical climatic stability has nurtured the evolution of species over tens of millions of years in virtually unchanged conditions (save for variations in geographic range). On the other hand, vast tracts of the Eurasian landmass have been periodically covered in ice or reduced to deserts during the same time period. This long-term cycle actually mirrors the annual variation in we
ather that produces the temperate zone’s seasons: the dry heat of a Mongolian summer yields to icy winter storms in the space of a few months. Because of the enormous environmental variation seen there, animals living in the temperate zone have had to rely on two crucial adaptations to keep themselves alive: investment and migration.
In the same way that you or I might choose to forgo the instant gratification of spending every penny we earn in a non-stop shopping binge, with an eye to using the money saved to see us through difficult times or old age, so animals that are used to encountering difficulty set aside some of their resources during times of plenty. It is not a conscious decision, but rather an evolved instinctual behaviour – an adaptation to life on a meteorological seesaw. Every spring and summer, for instance, the arctic tundra explodes into an orgy of growth and reproduction. Plants flower, pushing shoots above the permafrost for the first time in nearly ten months. Mosquitoes engulf everything that moves in a buzzing, bloodsucking cloud and the mammals that live in the Arctic – such as reindeer and walrus – give birth to their young. During this benign period, when temperatures can soar to nearly 100°C above their winter lows, you would be forgiven for thinking that the far north is one of the most productive places on earth – a teeming mass of life, hell-bent on one last bang before winter sets in again and everything dies. However, there is a method in the madness of the creatures living in the far north. It is during this time that every species in the Arctic is preparing for the end of the party, which will come like clockwork in early September when the temperature drops below freezing once again. No tropical mammal would ever evolve the behaviour of building up fat reserves to prepare for times of famine, but most temperate species do this as a matter of course. During the Arctic summer, reindeer add as much as a third to their body weight, storing resources for the long, dark winter. This allows them to survive the period of dearth that comprises 70 per cent of the year. It also makes them tempting targets for carnivores.
Humans, as they adapted to life on the plains of east Africa, would have become more and more adept at hunting the large mammal species that lived there. These include several species of antelope, an animal that has been called the ‘takeaway pizza’ of the Upper Palaeolithic. One of the behavioural changes that may have taken place around the time of the onset of the Upper Palaeolithic is the specialization of human populations on particular prey species, with presumed adaptations in hunting methods and weapons. The techniques you use to take down a gazelle, for instance, are quite different from those you might use to kill a mammoth or a rhinoceros. Specialization would have allowed efficient use of the animal resources in a region – but it would also have led to more movement, as the herds were depleted in one region and it became necessary to move on to find others in distant places. Seasonal hunting also appears to have made its appearance around this time, with evidence that early human populations followed herds of grazing animals – particularly antelope – from summer pastures in the hills surrounding the Mediterranean and the Red Sea, down to the warmer coastal regions in the winter. It was this gradual ebb and flow of animals, over hundreds or thousands of years, that may have brought fully modern humans and their Upper Palaeolithic toolkits into the Middle East around 45,000 years ago.
Modern humans had been present in the Levant (the eastern region of the Mediterranean) since at least 110,000 years ago, but the population was never extensive and was limited to a few sites. During this early phase of the last ice age, the eastern Mediterranean was effectively an extension of northern Africa, with similar climatic conditions and animals. The cave sites of Qafzeh and Skuhl in present-day Israel contained typically Ethiopian animals during the time when modern humans occupied them. Then, during the period between 80,000 and 50,000 years ago, modern humans abruptly disappear from these sites. In some cases they were replaced by Neanderthals, with their thick skulls and robust skeletons. This gives us a clue as to what was happening in the Levant during this time.
The climate was getting much colder after 80,000 years ago, and temperatures around the eastern Mediterranean were plummeting. It is likely that the average global temperature during this time dropped by around 10°C, with massive knock-on effects on plant and animal distributions. The early modern humans who had migrated out of Africa via Egypt and the Levant during the wetter and warmer times found that they were unable to rely on animals they had hunted for thousands of years. They may have died off, or perhaps they simply migrated back to Africa, but they do not seem to have made any further headway in their conquest of the Eurasian interior. These early modern humans are probably best viewed as a tentative stab at the world beyond Africa – one that simply did not make it any further.
Then, around 45,000 years ago, modern humans appear again in the Levant. This time, though, there was a critical difference. While the humans of 40,000 years earlier had used tools very similar to those of their Neanderthal contemporaries, the latest invaders carried with them the ‘killer app’. These people were the recent inheritors of the Great Leap Forward, with its advanced technology and complex culture in tow. Their Upper Palaeolithic tools and cooperative hunting behaviour – as evidenced by seasonal migrations and prey specialization – gave them an edge that the earlier moderns had lacked. Once they entered the scene, the path was open to the rest of the continent.
The route they followed in their blitz across Eurasia is revealed by the genetic patterns, so for the next part of the journey we’ll need to leave aside stones and bones and return to our DNA excavation.
6
The Main Line
Now here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!
Lewis Carroll, Through the Looking Glass
As I mentioned at the beginning of the last chapter, my lineage of Y-chromosome markers coalesces back to a DNA polymorphism known as M168, the ancestor of everyone living outside of Africa. M168 unites me with the Australian coastal migrants, tracing us both back to Africa around 50,000 years ago. This places all non-Africans in that continent immediately after the earliest archaeological evidence for the Great Leap Forward, and suggests a causal relationship between this ancient cultural revolution and the migration of modern humans out of Africa. The people who stayed in Africa, as well as the ones who left, would have been fully modern in every respect – technologically, culturally and artistically. The mitochondrial DNA results suggest that a massive expansion in human populations began around this time, consistent with the range expansion we see in the archaeological record. The Y and mtDNA data hint at two routes, one of which would have followed the coast to Australia around 50–60,000 years ago. What about the other, which accounts for the majority of people in the world today?
Before we begin to trace the order of the additional markers on my lineage, and their significance to our story, we need to clarify what the order actually signifies. There are two issues to be considered here, and both involve timing. The first is what we might call relative dating. To understand this, it is worth revisiting our hypothetical kitchen. Like the maternal and paternal soup recipes, the genetic recipes we have all inherited contain a combination of ingredients, or markers, that distinguish them from everyone else’s soup. In order to establish the order in which the ingredients were modified, we need to compare many different recipes before we begin to see patterns. So let’s do a bit of genetic cooking.
Imagine having an international potluck supper, where everyone invited is asked to bring a soup that is specific to his or her own country. In our kitchen we have several dozen bowls of soup sitting on the table. Each has a slightly different recipe, but they all come from the same source. How do we know this? Because each recipe uses as its basic ingredient impala – a species of antelope that occurs naturally only in Africa. It is extremely difficult to obtain impala meat in many parts of the world, but it is the cornerstone of all the soup recipes and it must be included.
In this recipe game, we will make use of Ockham’s insight into historical change to infer the order in which the ingredients were added. If we assume that the addition of ingredients occurs at a regular rate, and there is no loss or substitution of ingredients once they are added, then the most common ingredients have usually been added the earliest. This is because this order minimizes the total number of changes required to explain the soup recipes. For example, if we were to sample five soups from the ones sitting on the table, we might find the following recipes:
The Journey of Man: A Genetic Odyssey Page 12