The Incredible Human Journey

To Jonathan Musgrave and Kate Robson-Brown,
wise friends and mentors

Introduction

We are very familiar with the idea that humans are everywhere; that wherever you go in the world you will probably find people
there already. We are an unusual species in that we have a near-global distribution. And although people around the world may look quite
different from each other, and speak different languages, they can nevertheless recognise each other as distant cousins.

But where and when did our species first appear? What are the essential characteristics of our species? And how did people end up being everywhere? These are rephrasings of fundamental questions. Who are we? What does it mean to be human? Where do we come from? For thousands of years, such questions have been explored through philosophy and religion, but the answers now seem to lie firmly within the grasp of an empirical approach to the world and our place within it. By peering deep into our past and dragging clues out into the light, science can now provide us with some of the answers to the questions that people have always asked.

They are questions that have always captivated me. As a medical doctor and anatomist (I lecture in anatomy on the medical
course at Bristol University), I am fascinated by the structure and function of the human body, and the similarities and differences between us and other animals. We are certainly apes; our anatomy is incredibly similar to that of our nearest relations,
chimpanzees. I could put a chimpanzee arm bone, or humerus, in an exam for medical students and they wouldn’t even notice
that it wasn’t human.

But there are obviously things that mark us out – not as special creations, but as a species of African ape that has, quite
serendipitously, evolved in ways that enabled our ancestors to survive, thrive and expand across the whole world. There are
aspects of anatomy that are entirely unique to us; unlike our arms, our spines, pelvis and legs are
very
different from those of our chimp cousins, and no one would mistake a human skull for that of another African ape. It’s a
very distinctive shape, not least because we have such enormous brains for the size of our bodies. And we use our big brains
in ways that no other species appears to.

Unlike our closest ape cousins, we make tools and manipulate our environments to an extent that no other animal does. Although
our species evolved in tropical Africa, this ability to control the interface between us and our surroundings means that we
are not limited to a particular environment. We can reach and survive in places that should seem quite alien to an African
ape. We have very little in the way of fur, but we can create coverings for our bodies that help to keep us cool in very hot
climates and warm in freezing temperatures. We make shelters and use fire for warmth and protection. Through planning and
ingenuity, we create things that can carry us across rivers and even oceans. We communicate, not just through complicated
spoken languages but through objects and symbols that allow us to create complex societies and pass on information from generation
to generation, down the ages. When did these particular attributes appear? This is a key question for anyone seeking to define our species – and to track the presence of our ancestors through the traces of their behaviour.

The amazing thing is – it
is possible
to find those traces, those faint echoes of our ancestors from thousands and thousands of years ago. Sometimes it could be an ancient hearth, perhaps a stone tool, that shows us where and how our forebears lived. Occasionally
we find human remains – preserved bones or fossils that have somehow avoided the processes of rot and decay and fragmentation
to be found by distant descendants grubbing around in caves and holes in the ground, in search of the ancestors.

I’ve always been intrigued by this search, by the history that can be reconstructed from the few clues that have been left
behind. And at this point in time, we are very lucky to have evidence emerging from several different fields of science, coming
together to provide us with a compelling story, with a better understanding of our real past than any humans have ever had
before. From the study of bones, stones and the genes within our living bodies comes the evidence of our ancestors, of who
we are, of where we came from – and of how we ended up all over the world.

When the BBC offered me the opportunity to follow in the footsteps of the ancients, to delve into the past, to meet people,
see artefacts and fossils for myself and visit the places that seem most sacred to those searching for real meanings, I couldn’t
wait to get started. I took a year off from teaching anatomy and looking at medieval bones in the lab, and set off on a worldwide
journey in search of our ancestors.

The Human Family Tree

My journey would take me all around the world, starting in Africa and then following in the footsteps of our ancestors into
Asia and around the Indian coastline, all the way to Australia, north into Europe and Siberia, and, eventually, to the last
continents to be peopled: the Americas.

Modern humans are just the latest in a long line of two-legged apes, technically known as hominins. We’ve grown used to thinking
of ourselves as rather special, and a quick glance at the human family tree shows us that we’re in a rather unusual position
at the moment, being (as far as we know) the only hominin species alive on the planet. Going back into prehistory, the family
tree is quite bushy, and there were often several species knocking around at the same time. By 30,000 years ago, it seems
there were only two twigs left on the hominin family tree: modern humans and our close cousins, the Neanderthals. Today, only
we remain.

The ancestral home of hominins is Africa, although some species, including our own, have made it out into other continents
at various times. Whether we actually met up with our ‘cousins’ on our ancient wanderings is something I’ll be investigating in this book. Certainly,
it seems that there was some overlap in Europe, and that for a good few thousand years modern humans and Neanderthals were
sharing the continent.

It may sound strange but it’s actually quite difficult to know exactly how
many
different species of ancient hominins there were. It’s something that prompts a huge amount of debate. The world of palaeontology
– the branch of science that peers into the past to examine extinct and fossil species – is inhabited by ‘lumpers’ and ‘splitters’.
Lumpers use very wide definitions of species to group lots of fossils together under one species name. Splitters, as the
term suggests, divide them up into lots of different species. But which group is right? It’s hard to know, and this is one
of the debates that enlivens this science. Both lumpers and splitters are looking at the same evidence – but making different
interpretations.

Trying to decide if two populations really are different enough from each other to be labelled as separate species is more
difficult than it seems. Some species can even interbreed and produce fertile, hybrid offspring. But, basically, species are populations that are diagnosably different from each other, in terms of their genes or their morphology
(the way their bodies are constructed) – or both.

With long-dead fossil animals, all palaeontologists have to go on is the skeleton, and sometimes only fragments of bones.
So the problem of species definition becomes even more difficult. Looking at the skeletons of living animals, they can get
an idea of the range of morphological variation in a species (because, even within a species, animals come in a variety of
shapes and sizes). They can also measure the level of morphological difference between species. This gives them a benchmark
for how similar skeletons are within a species, and how different they need to be to be classed as separate species. Then
the palaeontologist can apply that standard to sort fossil animals into species. It’s a knotty problem, and it’s not really
that surprising that different palaeontologists, each of whom may have spent a lifetime studying the fossils, can reach different
conclusions.

Indeed, some palaeontologists shy away from talking about ancient ‘species’ at all. The eminent physical anthropologist William
White Howells suggested we call these groupings ‘palaeodemes’ (‘ancient populations’) instead. But we can recognise evolving
lineages in the fossil record, and giving distinct populations a genus (e.g.
Homo
) and species (e.g.
sapiens
) name provides a useful handle and helps when we’re trying to reconstruct family trees.
¹

Within palaeoanthropology, the discipline that looks at fossil hominins in particular, taxonomies range from extreme lumping,
with some researchers calling all hominins from the last million years
Homo
sapiens
, to extreme splitting, with other scientists finding room for eight or more species. Chris Stringer, palaeoanthropologist
at the Natural History Museum in London, has recognised four species during and since the Pleistocene (in the last 1.8 million
years):
Homo erectus
,
Homo
heidelbergensis
(the putative common ancestor of modern humans and Neanderthals),
Homo sapiens
and
Homo neanderthalensis
,
²
although the recent discovery of the tiny ‘Hobbit’ skeletons in Indonesia requires us to make room for
Homo floresiensis
as well.

Throughout this book I use the word ‘human’ in an inclusive but nevertheless precise sense to mean any species in the genus
Homo
, whereas ‘modern human’ refers to our own species,
Homo sapiens
. In the same way, ‘Neanderthals’ are
Homo neanderthalensis
.

Each one of these human species made it out of Africa, to Eurasia.
Homo erectus
got all the way to Java and China by about one million years ago. About 800,000 years ago, another lineage formed and expanded:
Homo heidelbergensis
fossils have been found in Africa and Europe. The European branch of this population went on to give rise to Neanderthals,
about 300,000 years ago. Modern humans sprang from the African population, some around 200,000 years ago – and
their
descendants spread across the globe.

This is a version of events that is now accepted by most palaeoanthropologists, and is supported by the weight of fossil
evidence and genetic studies. It is known in the jargon as the ‘recent African origin’ or ‘Out of Africa’ model. But although
this is now the majority view, it is not the only theory about how modern humans evolved and ended up everywhere. Some palaeoanthropologists
still argue that archaic species like
Homo erectus
and
heidelbergensis
, having spread from Africa into Asia and Europe, then ‘grew up’ into modern humans in all of these continents. At the end
of the twentieth century, a great debate raged over which version, the recent African origin or the ‘Regional Continuity’
(also called ‘Multiregional Evolution’), was a more accurate representation of events. Since then, the evidence – genetic,
fossil and climatological – has stacked up quite impressively in favour of a recent African origin,
³
,
⁴
but there is still a minority of scholars who argue for multiregionalism.
There are also some palaeoanthropologists, who, while accepting a recent African origin, suggest that modern humans may have
interbred with other archaic species as they spread into other continents, for instance mixing with the Neanderthals in Europe.
²

Reconstructing the Past

Generally speaking, someone who studies ancient human ancestry is a called a
palaeoanthropologist
. Palaeoanthropology is a sphere of enquiry that essentially started off with fossil-hunting, but which today draws on many
other disciplines (so that, for example, people who now end up contributing to palaeoanthropology may come from fields as
disparate as genetics and climate science).

When Charles Darwin wrote
The Descent of Man
in 1871, not a single early human fossil had been discovered, but he nonetheless tentatively suggested that Africa might just
be the homeland of the human species:

In each great region of the world the living animals are closely related to the extinct species of the same region. It is,
therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as
these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African
continent than elsewhere.

Then fossils started to appear. For a long time, the study of fossils formed the basis of palaeoanthropology, supplemented
with comparisons with the anatomy of living humans and our closest relatives, the African apes: chimpanzees and gorillas.
Scientists in this specific field might call themselves
physical
or
biological anthropologists
. Much of their work is focused on bones; after all, that’s normally all that is preserved in fossils.