The Incredible Human Journey Read Free

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 16 O, the lighter, ‘normal’
kind, and 18 O, 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 16 O 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 18 O 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