The Incredible Human Journey

by Alice Roberts

  The Ice Age

  This story of ancient human migrations across the world is set almost entirely within the later stages of what geologists call the Pleistocene period, or the Ice Age. In its entirety, the Pleistocene lasted from 1.8 million years ago to 12,000 years before the present. Although our species appears only in the late Pleistocene, by the end of that period modern humans had made their way into every continent (except Antarctica). In some chapters we will also dip our toes into the Holocene, the period that followed the Pleistocene or Ice Age, and in which we’re still living today.

  As we look deep into the past, over vast stretches of time, the apparent stability of geography and climate that we perceive as individuals melts away and we see instead a picture of changing climate, with sea levels and whole ecosystems in flux. The population expansions and migrations of our ancient ancestors were governed by climate change and its effect on the ancient environment. Reconstructing past climates, or palaeoclimates, is an exciting field of science that draws on ancient clues that have been ‘frozen in time’ as well as our understanding of the relationship between the earth and the sun.

  The earth’s orbit is not a perfect circle, and so there are times (spanning thousands of years) when the earth is nearer the sun, and warmer, and other times when it is further away, and consequently colder. These cycles last around 100,000 years. As well as this, the tilt of the earth’s axis varies, on a 41,000-year cycle, affecting the degree of difference between the seasons. The earth also wobbles a little around its axis, on a 23,000-year cycle. There are times when the factors affecting tilt and orbit work together to create exceptional chilliness – a glacial period. At other times, the factors come together to produce a very warm period, called an interglacial. This theory was developed by the Serbian mathematician Milutin Milankovitch in the early twentieth century.5, 6

  During the 1960s and 1970s, researchers were able to pin down ice ages with increasing levels of accuracy using deep-sea cores, samples drilled from the seabed. Those cores contain the shells of tiny marine animals, called foraminifera, and the carbonate in their shells contains different isotopes of oxygen. The two isotopes of relevance here are 16O, the lighter, ‘normal’ kind, and 18O, a heavier version. Both are present in the ocean, but water that evaporates from the oceans contains more of the lighter kind. This means that water precipitating from the atmosphere – as rain, hail, snow or sleet – also contains more of the lighter 16O than the seas. And it’s that water, falling on to land or ice caps, which becomes frozen into large ice sheets during an ice age. That means there’s more of the heavier 18O left behind in the seas, and more of it gets incorporated into those tiny shells, during an ice age.7 So marine cores, which can be dated using uranium series dating and by looking at the way the earth’s magnetic pole has switched in the past, hold an amazing record of past climate and ice ages.

  Formations in limestone caves – in stalagmites, stalactites or flowstone, or in the useful, all-embracing jargon, ‘speleothem’ (from the Greek for cave deposit) – also contain a record of past climate, depending on the proportions of oxygen isotopes that are present in the water that forms them. At any one time, the ratio of heavy and light oxygen isotopes in that water depends on global temperatures – and how much water is locked up as ice, as well as on local air temperatures and the amount of rainfall. While deep-sea cores are useful for looking at global climate, speleothem is very useful for investigating how climate has varied in a specific locality. Another indicator of past climates is pollen: soil samples containing pollen can be analysed to show the range of plants that were living in a particular area.

  The Pleistocene was a period marked by repeated glaciations and ending with the last ice age. As ice sheets grew and then shrank back, sea levels would fall and rise. With differences of up to 60 million km3 in the amount of water locked up as ice, sea levels fluctuated by up to 140m.7 The oxygen isotopes trapped in deep-sea cores and speleothem can be used to draw up a series of alternating cold and warm stages called ‘oxygen isotope stages’, often abbreviated to OIS. Just looking at the last 200,000 years, there have been three major cold periods (corresponding with oxygen isotope stages or OIS 2, 4 and 6) interspersed with four warmer periods (OIS 1, 3, 5 and 7). But the Pleistocene really was one long, cold ice age. Interglacials account for less than 10 per cent of the time.7

  At the moment we’re enjoying a nice, warm interglacial, oxygen isotope stage 1. The last full-glacial period, OIS 2, lasted from 13,000 to 24,000 years ago. The peak of this most recent cold phase, around 18,000 to 19,000 years ago, is known as the ‘Last Glacial Maximum’ (or ‘LGM’). OIS 3, from 24,000 to 59,000 years ago, was a bit warmer and more temperate, though still much colder than the present, and is called an ‘interstadial’. OIS 4, from 59,000 to 74,000 years ago, was another full-glacial period, though nowhere near as cold as OIS 2.4, 8 OIS 5, the last (sometimes called the ‘Eemian’ or ‘Ipswichian’) interglacial, was a warm, balmy period, lasting from about 130,000 to 74,000 years ago. Before that, there was another glacial period, OIS 6, starting at about 190,000 years after the preceding interglacial, OIS 7.

  This level of detail might seem a bit excessive, but our ancient ancestors were very much at the mercy of the climate (as we still are today). For instance, there was a population expansion during the wet warmth of OIS 5, and a crash, or ‘bottleneck’, during the cold dryness of OIS 4. And sea levels fluctuated according to how much water was locked up in ice: during cold, dry periods, sea levels were significantly lower – by as much as 100m – than during warm, wet periods. Between 13,000 and 74,000 years ago (i.e. during OIS 2–4), the world was a drier, colder place than it is today. Although the map of the world was generally very similar, there was more land exposed; many of today’s islands would have been joined to the mainland, and in places where the coast slopes gently the shoreline would have been much further out than it is today. This is of particular significance to archaeologists looking for traces of our ancestors along those ancient coasts – which are now submerged.

  Stone Age Cultures

  Archaeologists classify periods differently from geologists, depending on what humans were doing at the time. During the Stone Age, humans (including Homo sapiens and their ancestors) were making stone tools. This is before metals – copper, tin, iron – were discovered and used. In fact, in the scheme of things, metal-working is a very recent invention.

  The Stone Age is traditionally divided up into the Palaeolithic (old stone age – roughly corresponding with the Pleistocene period), Mesolithic (middle stone age) and Neolithic (new stone age). These stages happened at different times in different places, so it can become quite confusing. The categories are also based on European prehistory, where much early archaeological work was carried out. But in terms of global archaeology, western Europe is a bit of a backwater, even a cul-de-sac9 and so the terminology that has grown up there is sometimes rather unhelpful when we’re trying to understand what was happening in the rest of the world. However, the categories at least provide us with a vocabulary and some kind of framework to help us think about the deep past.

  Table showing the relationship between geological periods, oxygen isotope stages and what humans were getting up to at the time.

  Each stage is characterised by different styles and ways of making stone tools, but also by differences in the broader lifestyles of people. Put very simply (really too simply, as we shall see later), the Palaeolithic lifestyle was that of a nomadic hunter-gatherer, the Mesolithic saw a trend towards settling down, and the Neolithic saw the beginning of settled villages, cities, agriculture, pottery and organised religion.

  Throughout the Palaeolithic, and in the Mesolithic, too, our ancestors were nomadic. They left barely a trace of their passing – no buildings, and very little in the way of possessions – and those material possessions that they did have were often made of what we now think of as biodegradable materials, so they have long since disappeared. When we find stone tools, we
are often looking at something that was just a part of a more complex piece of equipment. Sometimes there are hints from polished areas of the stone tool suggesting how it might have been tied to something. Very rarely are the conditions right for organic materials – like pieces of wood or animal hide – to be preserved. When you consider the scarcity of the remains, it’s quite amazing that we can find the occasional trace and, from this, reconstruct part of our collective (pre)history.

  During the Palaeolithic period, there are changes in the types of stone tools people were making, and the period is divided up into Lower, Middle and Upper Palaeolithic (or, in Africa, the Early, Middle and Later Stone Age). Stone tools start to appear in the ground, in what is grandly termed the ‘archaeological record’, around 2.5 million years ago, made by early members of our own genus, Homo. These are crude, pebble tools, and the toolkit or stone tool ‘technology’ is called Oldowan after the sites excavated by Mary Leakey in the Olduvai Gorge. These basic tools continued to be made for hundreds of thousands of years. Our early ancestors were not great innovators! But we have to grant them some skill. In the wild, chimpanzees make tools out of easily modified materials like sticks or grass stems, and use stones to crack nuts; chimpanzees in captivity can be taught to make stone tools, but their products are still not as good as those Oldowan tools.10

  The next stone toolkit to come along is called the Acheulean. And this toolkit is not found only in Africa. In fact, it is named after the site of St Acheul in France, where a characteristic ‘hand axe’ was discovered in the nineteenth century. Acheulean tools are found in Africa from about 1.7 million years ago, but it’s not until 600,000 years ago that they are found in Europe. The tool from St Acheul is actually quite late: it dates to between 300,000 and 400,000 years ago. By 250,000 years ago, this technology had disappeared. Slightly strangely, this hand axe technology never reached East Asia. The fossil record suggests that people – probably Homo erectus – first made their way out of Africa around a million years ago, so it’s unlikely that the East Asian pebble-tool-makers were direct descendants of the Oldowan people in Africa; they are more likely to have been, culturally, ‘Acheuleans’ who gave up making hand axes as they moved east.10

  Hand axes are pointed, teardrop-shaped tools, flaked on both sides. It seems that nobody knows much about how these tools were used: were they designed for use in the hand, or hafted on to a shaft? Many archaeologists prefer simply to call them ‘bifaces’ (a general term for tools flaked on two sides). Acheulean bifaces are much more refined (though still big, chunky things) than Oldowan tools. Some of the bifaces are quite beautifully symmetrical, and many archaeologists have suggested that their form is governed by aesthetics as well as function. It’s a tempting but ultimately conjectural idea, as there is no other evidence for any art at this time. And, once again, there seems to be extreme conservatism in tool-making throughout this period: there was very little invention. Over the huge time span of the Acheulean – from 1.7 million to 250,000 years ago – that culture hardly changed.10

  But then a new sort of culture appeared. South of the equator in Africa, it’s called the Middle Stone Age (MSA). Similar tools in North Africa, Europe and western Asia are called Middle Palaeolithic, or Mousterian, the latter a name that comes from the Neanderthal site of Le Moustier in south-west France. These labels carry with them a great deal of historical baggage, and the distinction between Africa and Eurasia is not particularly helpful. What we can say is that these tools seem to have been made by the archaic species Homo heidelbergensis, as well as by its (probable) daughter species: Homo sapiens and Neanderthals.

  The MSA/Middle Palaeolithic differs from the Acheulean in that bifaces disappear from the toolkits. And tools are often made from stones that have first been shaped into a tool blank, or ‘prepared core’ – although, actually, the distinction isn’t that easy as this technique was also used in the Acheulean. From studies of the wear on MSA/Middle Palaeolithic tools, it seems that the people were regularly mounting, or ‘hafting’, stone points on to shafts (although, as I mentioned, it is possible though not proven that so-called Acheulean bifaces were hafted). The new generation of tools, and ways of making them, were much more varied than the preceding stone tool technologies. There were other developments during this stage: people started to collect reddish, iron-rich rocks, perhaps for use as pigment; the first hearths appear; they had control of fire; and they started to bury their dead. From the composition of their bones, it also seems that people started to eat more meat in this period. Although people had hunted before, judging from artefacts such as the 400,000-year-old Schöningen spears from Germany, archaeologists believe that it is in the MSA/Middle Palaeolithic that hunting – not just scavenging – became routine.10

  About 40,000 years ago, there was another change: to what is called the Later Stone Age (LSA) in Africa, and the Upper Palaeolithic in Eurasia. A huge and varied range of stone tools emerged, and people were also regularly making things out of bone. They were also using ‘true’ projectile weapons – spear-throwers with darts and bow and arrow (as opposed to just hand-cast spears)11 – and they were building shelters, fishing and burying their dead with a degree of ritual that had not really been seen before. They also created magnificent art – particularly in Europe. Although this was probably not the first art (as there’s much earlier evidence of pigment use in Africa), the painted caves of Spain and France are quite exceptional. From the fossils that have been discovered alongside archaeological finds, it is generally believed that the Later Stone Age and Upper Palaeolithic were made by just one species: Homo sapiens, modern humans. Us. Some palaeoanthropologists believe that the appearance of this new phase marks the relatively sudden beginning of truly ‘modern’ human behaviour10 but others think that it is possible to see traces of fully modern behaviour much earlier, even before 100,000 years ago. They also suggest that this behaviour developed gradually, mirroring the physical, biological transition to a modern form.9, 12

  The continuing debate goes to show that it’s actually very difficult trying to tease out how and when this behavioural transition, to something we can truly recognise as modern human, happens. As far as stone tools are concerned, it’s hard to find a clear signature of the earliest modern human tools. To begin with, the earliest modern humans were manufacturing exactly the same type of toolkits as their parent and sister species, heidelbergensis and Neanderthals; they all made bog-standard, Middle Stone Age tools. But there is a distinct MSA toolkit, from the northern Sahara, that has been attributed to modern humans. Similar to other MSA toolkits in many ways, the Aterian includes stemmed or ‘tanged points’ (perhaps spear- or arrowheads). At a site in Morocco, further evidence of ‘modern behaviour’ has been discovered, alongside Aterian tools, in the form of shell beads.13 Even so, before the LSA and the Upper Palaeolithic, it is difficult to discern the presence of modern humans based on stone tools alone. So fossilised skeletons become like holy grails for those seeking evidence of the earliest modern humans.

  Dating Fossils and Archaeology

  It is important to understand something about the techniques archaeologists can now draw on to date their discoveries. Dating is at the very centre of some of the biggest controversies and knottiest problems in palaeoanthropology.

  Relative dating often involves judging the age of something by its position in the ground. So, for instance, you might judge something to be Iron Age if it lay underneath a Roman mosaic but on top of a Bronze Age burial. A more scientific approach, sometimes referred to as ‘absolute dating’, involves some means of measuring the age of the object itself, or at least the layer that it is buried in. Absolute dating techniques relevant to the period we’re considering include radiometric and luminescence dating.

  Radiometric techniques work by measuring the levels of different radioactive isotopes in materials. Radioactive isotopes decay over time, from one form to another. If the proportions of those forms can be measured, and the rate of decay is known, then the
date of the object can be calculated.

  The best-known radiometric dating technique is radiocarbon dating. The radioactively unstable C14 isotope decays to stable C12 over time. As C14 is present in the atmosphere, plants trap it when they photosynthesise, and animals eating plants also obtain it. This means that a living plant or animal contains a proportion of C14 to C12 that matches the ratio in the atmosphere. But when the plant or animal dies, it stops taking in any more C14; the C14 it already contains gradually decays to C12. So, by knowing the rate of decay, and then by measuring the proportions of the carbon isotopes in an organic object, whether that’s a piece of wood, some charcoal or a bone, you can work out when that organic thing was last alive.

  The precision of radiocarbon dating has recently improved with the use of accelerator mass spectrometry (AMS), which has also pushed the useful limit of radiocarbon dating back to 45,000 years ago. Accuracy has also improved with pre-treatment of samples to remove contamination from modern carbon, and with calibration, to take account of the fact that the amount of C14 in the atmosphere has changed over time (dates given in this book are calibrated, calendar years, rather than ‘radiocarbon years’). Radiocarbon dates published before these advances – before 2004 – need to be treated with caution. Generally speaking, when archaeological materials have been redated using the new, improved techniques, the dates turn out to between 2000 and 7000 years older than the previous estimates. An added advantage of AMS radiocarbon dating is that it requires only a minute sample from a precious archaeological object. AMS radiocarbon dating is the best way of dating organic things – as long as they are less than 45,000 years old.14 Beyond that, and if we’re interested in early modern humans and their forays out of Africa, going back more than 50,000 years ago, we have to look to other methods.