The Incredible Human Journey (3 page)

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 behaviour
¹⁰
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.

Two other radiometric dating techniques that can be used to date rocks are uranium series and potassium-argon dating. Uranium
series dating uses radioactive isotopes of uranium and thorium, which decay to stable lead isotopes. It depends on soluble
isotopes being precipitated and then changing to insoluble forms, so it can be applied to speleothem and coral. Potassium-argon
(and argon-argon) dating is used to date volcanic rocks. Argon can escape from molten rock, but is trapped in solidified
lava. So if archaeological finds or fossils are found between layers of speleothem (in limestone caves) or between layers
of volcanic tuff from ancient volcanic eruptions, these techniques can provide a date, or at least a date range, for the discoveries.

A relatively new technique that is proving incredibly helpful in Palaeolithic archaeology is luminescence dating. It is used
to pin a date on the last time that grains of quartz or feldspar were exposed to either heat or light. It can be used to date
the layers of sediment that an object is buried in, or sometimes even to date an object itself if it was heated – for instance,
a piece of pottery or a hearth stone. Luminescence dating is a very powerful tool, useful for pinning an age on objects that
are just a few years old, all the way to things that have existed for a few million years.
¹⁵

The way that luminescence dating works is, I think, quite mind-blowing. When grains made of natural quartz crystals (i.e.
grains of sand) are exposed to ionising radiation – from naturally occurring radioactive elements like uranium as well as
cosmic rays – electrons get trapped in tiny flaws inside their crystal structure. Light or heat makes the crystal release
its electrons. But once a quartz grain is buried, it starts to accumulate electrons again … until somebody comes along and
digs it up. Samples for luminescence dating have to be kept completely in the dark when they are collected.

Back in the lab, the quartz grains are sorted out from the sample, under very dim, red light. Then they are exposed either
to heat (in thermoluminescence, or TL, dating) or light (in optically stimulated luminescence, or OSL, dating). Then the
crystals within them release their trapped electrons – making them glow. By measuring this luminescence, while knowing levels
of natural radiation at the place where the quartz grains were buried (from other sediment samples and measures of cosmic
radiation), the length of time that the crystal was buried can be estimated.
¹⁵

Another method that measures levels of trapped electrons, again resulting from bombardment with radiation within sediments,
is electron spin resonance (ESR). This technique works well for ageing tooth enamel, which is also a crystalline material
– so it is very useful for dating hominin fossils.
¹⁶

Genetic Studies

Quite recently, another branch of science has begun to provide important clues about our ancestry, about how we are all related
to each other and even about the way the world was colonised. This time, however, the evidence is not buried in the ground
but inside
us
, because the DNA contained in each cell of each of our bodies holds a record of our ancestry. Getting samples of DNA is surprisingly simple – and painless. DNA can be collected from volunteers using just a cheek brush
or saliva swab. These samples contain cells, and inside those cells is the precious DNA.

While everyone’s DNA is mostly identical, there are some differences. There have to be, otherwise we’d all look exactly the same, clones of each other. Some genes govern our appearance, while
others control the machinery of life. There are differences in those genes, too. You can’t tell just by looking at someone,
but they might have a different blood group from you, a slightly different enzyme for breaking something down inside their
cells. The differences in these active genes and the protein products they make are constrained by natural selection. If a
mutation happens in an important gene, it could make the protein product of that gene work better, worse, or perhaps have
no effect at all.