Let’s look now at how this most recent ice age, and the consequent drop in global sea levels, also provided a crucial opportunity for us to spread around the world. We are all children of Africa, but we didn’t remain in our cradle.
EXODUS
Roughly 60,000 years ago, our ancestors began to disperse out of Africa. It’s difficult to know exactly which routes we took around the planet, or the precise timing of when we first reached new areas, because the fossil record is very patchy and it is often hard to tell from the archaeological evidence exactly which branch of hominins they were left by. So most of our understanding of humanity’s expansion comes from studying the genetics of indigenous populations living around the world today. By analysing the DNA, and being able to estimate the rate at which mutations accumulate in the genetic code, we can work out how long ago different populations diverged from each other. Mapping this genetic variation around the globe allows us to work out when humans first arrived in different regions, and so enables us to follow the ancient migration paths.
Two main kinds of DNA have been most useful in this detective work. Inside each of our cells are tiny structures called mitochondria, which run the biochemical reactions for providing energy. These mitochondria are the powerhouses of the cell, and they contain their own little loop of DNA. When you were conceived, you inherited the mitochondria from your mother’s egg cell, but none from your father’s sperm: the mitochondrial DNA passes continuously down the maternal line, from mother to daughter. Analysing the genetics of mitochondrial DNA, and calculating the time it took for different populations to split from each other, allows us to backtrack to where they converge – that particular woman in the deep past who happened to be the ancestral mother of all people alive today. This most recent matrilineal common ancestor has been dubbed Mitochondrial Eve, and she lived in Africa around 150,000 years ago. If instead we look at the DNA held on the Y chromosome that is only passed from father to son we can backtrack to the most recent male ancestor, nicknamed Y-chromosome Adam. Dating the root of this genetic tree is more uncertain, but the male common ancestor is believed to have lived between 200,000 and 150,000 years ago.
This doesn’t mean that there was only one woman and one man alive at the time, or indeed that the female and male most recent common ancestors ever met each other – they lived at different times and in different places. In fact, it would be a staggering coincidence if the female mitochondrial line happened to date back to the same time as the male Y-chromosome line. (The biblical nicknames are misleading in this sense.) The only significance of Mitochondrial Eve (and analogously Y-chromosome Adam) is that she just so happened to give birth to daughters who themselves had daughters, and so on down the line to all people alive today; by chance the other lineages in the family tree died out or had no female children.
The most surprising result to come out of these global genetic studies is that the human species is exceedingly uniform. Despite superficial regional differences in hair and skin colour, or skull shape, the genetic diversity among the 7.5 billion humans living in the world today is astonishingly low.30 In fact, there’s more genetic diversity between two groups of chimpanzees living on opposite banks of a river in Central Africa than there is between humans on opposite sides of the world.31 Human genetic diversity is greatest within Africa, however, so even if we had never discovered any fossilised bones or early archaeological evidence and had only the DNA of modern humans to go by, it would still be clear that we all originated in Africa and spread from this birthplace. Moreover, the genetic studies suggest that humanity around the world today descends from a single exodus event out of Africa, rather than multiple waves of migration, and probably from no more than a few thousand original migrants.32
Modern humans, Homo sapiens, first entered the Arabian Peninsula during a regional climatic shift to wetter conditions and a greening of the area,33 either by walking north across the Sinai peninsula, or by taking a more southerly route by raft across the Bab-el-Mandeb strait.34 As our ancestors began to spread into Eurasia we encountered other species of hominins that had already left Africa much earlier. Modern humans underwent a small degree of interbreeding in the Middle East with Neanderthals so that we picked up a trace of their DNA and then carried it with us as we populated the rest of the world35 – it makes up around 2 per cent of the genetic code of non-Africans today.36 The fact that modern East Asians appear to have more Neanderthal DNA than Europeans would suggest that humans mixed with Neanderthals on at least one other occasion as we migrated eastwards through Eurasia.
More interbreeding seems to have occurred with another mysterious, extinct hominin species known as the Denisovans when we moved through Central Asia. We know of the Denisovans only from a few teeth and the fragments of a finger and a toe bone that were discovered in a cave in the Altai mountains on the border between Siberia and Mongolia, and the DNA analysis reveals that they were probably a sister species of the Neanderthals.37 Between 4 and 6 per cent of the DNA of modern people in Melanesia and Oceania derives from the Denisovans, and they also made a small contribution to the genetic code of Native American populations.38 It’s incredible to think that an entire human species, living alongside our own just a few tens of thousands of years ago, is known to us only by a some bone fragments and the trace of DNA they left imprinted in our genome. An even earlier hominin species, Homo erectus, had left Africa almost 2 million years ago and reached as far as China and Indonesia, but had already fallen extinct by the time humanity spread across Asia.39 No indigenous peoples remaining in Africa carry DNA from either the Neanderthals or Denisovans.
As the first human migrants reached each new area their population grew and their descendants continued the dispersal. The region now covered by Iraq and Iran acted as a major dispersal hub, with migration streams heading up into Europe, across the rest of Asia, and into Australia and the Americas.40 It seems likely that humans first headed east, following the southern margin of Eurasia to India and South East Asia;41 an early offshoot from this path took humans into Europe around 45,000 years ago.42 The eastward migration split into two routes either side of the Himalayas, like a river flowing around a rock, with one path heading north across Siberia and eventually into the Americas, and a second taking a southerly route across South East Asia towards Australia. The spread through southern Asia seems to have been relatively quick, possibly due to the similarity in climate with our ancestral home in sub-Saharan Africa, and we reached South East Asia and China around 50,000–45,000 years ago.43
From the Indochinese Peninsula, we crossed into New Guinea and Australia about 40,000 years ago.44 With global ocean levels over 100 metres lower than today owing to ice-age conditions, the shallow seas around Indonesia were exposed as dry land. The Indonesian archipelago became part of an extension of South East Asia known as Sundaland, and Australia, New Guinea and Tasmania were all united as a single landmass called Sahul. These two lands faced each other across a narrow sea studded with chains of islands, which helped our migration into this south-eastern corner of the world.45
The slow wave of dispersal eventually reached the north-eastern tip of Eurasia, and it is here that the ice-age conditions proved most crucial for human migration: they gave us our route of entry into the Americas.
Today, the coasts of Russia and the US are separated by the 80-kilometre wide Bering Strait, with the two Diomede Islands sitting right in the middle of that sea channel.fn2 During the last ice age, with the dropping sea levels, the land of Siberia and Alaska would have extended to reach towards each other, like the outstretched fingers of Adam and God painted by Michelangelo on the ceiling of the Sistine Chapel, until eventually they touched and the two vast continents of Eurasia and the Americas were joined. This land corridor would have widened until, at the glacial maximum around 25,000 years ago, it stretched up to a thousand kilometres north to south.
Although clear of ice sheets, the Bering land bridge would still have been a decidedly harsh environment: co
ld and dry, with dunes of silt that had been eroded by the glaciers and blown in the wind. The land bridge was little more than an Arctic wasteland, but dotted with enough hardy vegetation to support animals – woolly mammoths, ground sloths and steppe bison, as well as the sabre-toothed tigers that preyed on them.
Humans made it across this land bridge into America sometime after 20,000 years ago.46 But other animals had already made the crossing in the opposite direction into Eurasia during an earlier ice age, some of which would become crucial to civilisations through history. Both the camel and horse had evolved in North America and crossed into Eurasia along the Bering land bridge, subsequently dying out in their birthplace. (We’ll return to the significance of this in Chapter 7.)
After walking across the land bridge into Alaska, humanity worked its way down through the Americas as the ice sheets receded. At the time, two huge expanses of ice – the Cordilleran and Laurentide ice sheets – covered most of Canada and large areas of the northern USA. At its maximum extent, the Laurentide ice sheet was larger than the entire Antarctic ice cover today, with an immense dome up to 4 kilometres thick over Hudson Bay.47 To get round these ice sheets to head south, the migrants may have travelled down the western coastline, or perhaps passed between the two along an ice-free corridor.48 But once safely past the ice sheets in North America, humanity rapidly spread across the continent as the Ice Age subsided. They crossed the Panama Isthmus into South America around 12,500 years ago, and had reached the very southern tip of the continent within another millennium. Humanity had encompassed the globe.
The Ice Age world with the migration paths of H. sapiens and the approximate ranges of Neanderthals and Denisovians.
Thus the Ice Age and its lowering of the global sea levels enabled the peopling of the Americas. As they had moved through Europe and Asia our ancestors had made contact with Neanderthals and Denisovans, but here in America they encountered no previous peoples. After they crossed the Bering land bridge into the new world, humanity was walking where no hominin species had ever trodden before.49
Then, around 11,000 years ago, as the world warmed again after the last glacial maximum and sea levels rose, the Bering land bridge once again disappeared beneath the waves. The connection between Alaska and Siberia was severed, and the Eastern and Western hemispheres were cut off from each other. Lasting contact was not made again between the peoples of the Old World and the New for another 16,000 years, until Columbus set foot on the Caribbean islands in 1492. Genetically similar, but living in different landscapes with access to different plants and animals, these two isolated populations of humanity formed civilisations independently from each other but remarkably similar in their domestication of crops and livestock and the development of agriculture.fn3
I may have given the impression that our spread was a rapid, or even directed, migration to all corners of the globe – as if our ancestors mindfully turned their backs on their original homelands in Africa and boldly strode towards the horizon, perhaps furrowing their brow in an expression of fortitude, systematically filling in all the nooks and crannies in the outline of the continents. But these movements are more accurately described as dispersals, with groups of hunter-gatherers ranging widely across the landscape with very low population densities, slowly moving with the seasons and over the years with changing local climates, roving to avoid the cold and drought and to seek warmer, moister, more favourable conditions for finding food.53 Over the generations we drifted further and further afield. Humanity’s spread from the Arabian Peninsula along the southern Eurasian coast to China, for example, occurred at an average rate of less than half a kilometre a year.
Ultimately, however, humanity inherited the Earth. Our cousin hominin species – the Neanderthals and Denisovans – slid into extinction. As we discussed in the last chapter, it seems likely that they were simply outcompeted by humans rather than hunted and killed, or else succumbed to the harsh conditions as the Ice Age reached its peak. The last Neanderthals disappeared between 40,000 and 24,000 years ago, and we became the sole surviving human species on Earth. Within 50,000 years of leaving Africa we had colonised every continent apart from Antarctica to become the most widespread animal species on the planet. Our mastery of fire and skill in making clothes and wielding tools allowed us, a bunch of savannah apes, to inhabit every climate zone from the tropics to the tundra. We moved out of the environment that made us, and learned to create our own artificial habitats of huts and farms, villages and cities.fn4
That this global expansion took place during the punishingly cold climate of the last ice age is perhaps surprising, but it is in fact these very icehouse conditions that enabled us to accomplish this. The growth of the northern ice sheets drew so much water out of the oceans that the dropping sea levels exposed great areas of the continental shelves. It is the Ice Age that allowed us to simply walk across dry land to Indonesia, cross the narrow sea into Australia, and crucially make our way along the Bering land bridge into the Americas. Lower sea levels also meant that there was a much greater area of land to live on – an additional 25 million square kilometres, roughly the equivalent of present-day North America.55
But alongside providing the conditions that enabled humanity to spread across the globe, past ice ages have had other far-reaching implications for the moulding of the landscape we inhabit and the course of our history.
RAMIFICATIONS
You may know that the ‘crinkly edges’ of Norway made up of innumerable U-shaped fjords were carved by advancing glaciers during the ice ages, as were the lochs of Scotland. And even if glaciation was much less prominent in the Southern Hemisphere, if you look at a map of Chile you’ll see identical fjord features along the Pacific coastline at the tip of South America. During the ice ages, the Patagonian Ice Sheet expanded down from the Andes, covering a full third of Chile at its greatest extent, and glacially gouged out these valleys. They were subsequently drowned by the rising sea levels, becoming an incredibly intricate jumble of rugged little islands, headlands and interlinked channels – almost as if the coastline itself had been shattered by the frost.
When the Portuguese explorer Ferdinand Magellan found a route around the tip of South America in 1520 on the first circumnavigation of the world, he did so along a passage created by these flooded, glacially carved valleys. The narrowest points at the Atlantic entrance to the Magellan Strait were formed by ‘terminal moraines’ – debris pushed ahead by the glaciers acting like bulldozers and then, as they began to retreat again with the end of the Ice Age, dumped at their base.56 The 600-kilometre-long Magellan Strait was a crucial sailing link between the Earth’s two greatest oceans for almost four centuries before the construction of the Panama Canal in 1914. Although it is narrow and difficult to navigate, with unpredictable currents, it is still shorter and (as an inland channel) far more sheltered from storms than the tempestuous passage between the southernmost cape and Antarctica found by Sir Francis Drake in 1578.
Glaciation also had profound implications for the reshaping of North America’s geography and the subsequent history of the United States. Here the extensive ice sheet diverted the course of the mighty Missouri and Ohio rivers, and when the glaciation thawed these rivers continued to flow along what had been the edge of the ice sheet. Today, they meet the Mississippi in a huge Ψ shape and offer easy east-west transport right across the interior of the continent. The Missouri in particular reaches over 2,000 kilometres west to the Rocky Mountains. It was this river, previously diverted by the Ice Age, that carried the explorers Lewis and Clark most of the way towards the Pacific coast in 1804, and enabled the establishment of an American presence across the huge tract of Louisiana and the Northwest Territories. Other rivers too were diverted by glaciation, such as the Teays and St Lawrence; without these river transportation routes around the Appalachian Mountains, the original Thirteen Colonies might have remained confined to the Atlantic seaboard.
The Great Lakes of North America too are features left
behind by the Ice Age, their deep basins gouged out by the advancing Laurentide ice sheet and then filled with its meltwater as it retreated again around 12,000 years ago.57 Once they were linked by canals, these extensive waterways became hugely important for inland transport from the Atlantic coast before the construction of long-distance railroads, and saw New York, Buffalo, Cleveland, Detroit and Chicago develop into major commercial centres.58
Moraines, rubble ridges 40–50 metres high, can be seen stretching right across the north of the US. Long Island in New York is formed of two long moraine ridges dumped at the head of the Laurentide ice sheet, as is Cape Cod, further up the coast in Massachusetts.59 Moreover, Boston, Chicago and New York are built on thick deposits left behind by the melting of this ice sheet.60 We mine these moraines and glacial stream deposits around the world for sand and gravel for the aggregate in concrete, road-surfacing, and base material for foundations or railway tracks. In addition, the frigid frontier of the North American ice sheets drove fierce winds, which picked up fine particles of silt, sand and clay that had been ground out of the bedrock, and deposited them further south to create the fabulously fertile loess farmland of the Midwest.61
It’s on the other side of the pond, however, that we find perhaps the clearest example of the influence of the ice ages on history.
ISLAND NATION
Half a million years ago, Britain was not an island. It was still part of continental Europe, physically connected to France – like conjoined twins – by an isthmus running between Dover and Calais. This land bridge was a continuation of the hump-shaped geological structure known as the Weald–Artois anticline that stretches from south-east England to north-east France, formed of layers of rock buckled upwards in the same tectonic upheaval that created the Alps when Africa slammed into Eurasia.
Origins Page 5