The Accidental Species: Misunderstandings of Human Evolution (17 page)

The problem is that the type specimen of
Homo habilis
—that is, the fossil used to name the species—is a jawbone with teeth, and the type specimen of
Homo rudolfensis
is a skull, complete in most respects except that it lacks a jaw and teeth. This means that there is no way to compare the two species directly, so any decisions about the status of these species has to be made in a roundabout way, by comparing them with other fossils that might very well have their own problems of interpretation.
⁴²
The confusion deepens with the possibility that any and all early
Homo
species might really be australopiths, and not
Homo
at all. The murk is thickened by a general fogginess about what features make a hominin
Homo
—but mostly by the general lack of fossil evidence. Even after almost a century of sustained effort, the fossil record of hominins is too slender for us to say anything definite about the origins and general characteristics of the earliest members of our own genus,
Homo
.

Some scientists, notably Bernard Wood of George Washington University in Washington, DC, have suggested that these early species of
Homo
look sufficiently archaic—and too much like australopiths—that to include them in the genus
Homo
makes defining our own genus even more difficult than it is already.
⁴³
Wood prefers to cast these hominins as australopiths, similar in many ways to
Australopithecus africanus
—a species that has also been hard to define, given that its name was originally coined to refer to a baby, rather than an adult in which the full expression of a species’ distinctive traits might be seen. The situation has been complicated further with the detailed description of two partial skeletons of an australopith from Malapa, a cave near Sterkfontein.
⁴⁴
This hominin, named
Australopithecus sediba
, had a small brain, but features of its skeleton are reminiscent of early
Homo
.

What of the tools? Evidence for tools now goes back at least 2.5 million years—for tool use, perhaps as long as 3.39 million years, if the scratches seen on animal bones excavated from a site in Ethiopia were deliberately made by hominins.
⁴⁵
This makes toolmaking far more ancient than the genus
Homo
, even if
Homo habilis
is admitted to the club.
It is possible, indeed likely, that australopiths made tools. If
Homo floresiensis
is the descendant of an australopith, rather than
Homo erectus
, then the case is made, given that tools have been found associated with Flo.

The error is to construct an argument that is both circular and spurious. If we assume that only members of the genus
Homo
can make tools, then anything associated with stone tools must be in the genus
Homo
. This ignores the possibility that the tools might have been made by Zinj or indeed any other hominin, including species as yet undiscovered. Now that we have good reason to think that australopiths indeed made tools, the necessary restriction to the genus
Homo
becomes nonsense. The alternative is to admit australopiths to
Homo
, which would then make
Homo
even harder to define than it is already.

Worse, though, is the conceit that toolmaking necessarily accompanies a bigger brain, such that when a brain becomes big enough, facilities such as technology become possible. As I show later in this book, many animals with brains much smaller even than that of
Homo floresiensis
make and use tools. Conversely, organisms as simple as bacteria can make structures at least as elaborate as stone tools, but nobody would accuse such creatures of having any brains at all. Leakey, like the Piltdown committee before him, was in danger of making unwarranted assumptions about the progressive evolution of hominins where no such assumptions were justified.

After Leakey, paleoanthropology split into two streams, one going further forward in time, the other, backward.

Forward first. With
Homo erectus
in Asia and Neanderthals in Europe, but
Australopithecus
and very early
Homo
in Africa, the consensus view emerged that
Homo
migrated out of Africa with, or soon after, the evolution of
Homo erectus
, around 1.8 million years ago. Further hominins found in Eurasia appeared to confirm this view.

Recent discoveries include several spectacular specimens of hominin skulls and skeletons in caves in the Sierra de Atapuerca in northern Spain.
⁴⁶
These finds, while remarkable, are not alone. Remains of hominins have been found across Eurasia from Britain to China. Some appear to belong to
Homo erectus
, whereas others are much harder to place, and are conventionally lumped into a kind of dustbin called
Homo heidelbergensis
, named after a mandible discovered in Germany in 1907, and conventionally regarded as a generalized Eurasian form whence descended the Neanderthals, and possibly also
Homo sapiens
, if enigmatic
finds from Africa such as
Homo rhodesiensis
(a distinctive skull found in 1921 in what is now Zambia) belongs to this increasingly inclusive transitional form. Another species,
Homo antecessor
, comes from deposits at Atapuerca thought to be older than those yielding the bones of supposed
H. heidelbergensis
.
⁴⁷

This conventional view has run into problems. The first is the nature of
Homo erectus
itself. Did this species really originate in Africa? Finds assignable to this species have indeed turned up in Africa, notably a near-complete skeleton of a youth discovered at Nariokotome on the shores of Lake Turkana in Kenya in 1984, by the legendary fossil hunter Kamoya Kimeu, one of Richard Leakey’s “hominid gang.”
⁴⁸
The youth clearly belonged to
Homo
, based not just on the features of the skull, but on the skeleton, which had the cylindrical ribcage and long legs seen in
Homo
, rather than the more conical ribcage and shorter legs typical of
Australopithecus
. Some researchers, though, found sufficient differences between the Nariokotome skeleton and some other African finds and later
Homo erectus
to create a new species,
Homo ergaster
, to encompass early
erectus
-like hominins from Africa.
⁴⁹
Very early examples of hand axes, a style of stone tool very much associated with
Homo erectus
, have also been found in Ethiopia and Kenya
⁵⁰
—with, of course, the usual health warnings about linking tools and their makers.

So, what’s the problem? Here’s the deal:
Homo erectus
evolves in Africa around 1.8 million years ago; evolves advanced hand axes rather than the simpler pebble tools of
Homo habilis
(and maybe
Australopithecus
); colonizes Rest of World. To me, this narrative seems rather too biblical to be credible.

The first crack in the story was the discovery of a remarkable collection of hominin skulls and bones from rocks beneath a medieval monastery at Dmanisi in the Republic of Georgia, associated with primitive tools, and dated to between 1.85 and 1.78 million years ago.
⁵¹
The Dmanisi hominins are arguably the oldest known hominin fossils outside Africa—but if they are the descendants of the first bold exiles from Africa, they seem to have taken a step backward. They are sufficiently primitive to have drawn comparisons with
Homo ergaster
rather than
Homo erectus
but have also, lately, acquired their own species name,
Homo georgicus
.
⁵²

It’s very tempting to view the Dmanisi hominins as primitive members of
Homo erectus
(or something closely related to it) caught in the act of migrating out of Africa. Such temptations should be resisted. To
be sure, there are bits and pieces of
Homo erectus
, and of stone tools indicative of their passing, recovered from all over the Old World that could quite plausibly be strung together to make a story of migration from Africa, but that would be to ignore the gaps in time and space that must be bridged. Rather than
Homo erectus
having evolved from an earlier form of
Homo
in Africa and moved into Eurasia, it is perfectly possible for
Homo erectus
to have evolved in Asia from some even earlier form and migrated back into Africa, replacing
Homo habilis
. The Dmanisi hominins might therefore be creatures caught in the act of coming home, not venturing forth. This scenario might seem a little contrived, especially as no fossils of hominins older than 1.7 million years are currently known from outside Africa. But the dates for many early hominins in Europe are constantly being pushed backward toward the 2-million-year mark. And the existence of
Homo floresiensis
, which looks arguably more like
Australopithecus
than a dwarfed
Homo erectus
, suggests that hominins left Africa much, much earlier than had been thought possible.

The recent discovery of early hand axes together with pebble tools from the western shore of Lake Turkana in Kenya further blurs the picture.
⁵³
It suggests that the first hominins to have left Africa might have fled without their distinctive hand axes. It also raises the possibility that the first exiles were more primitive than
Homo erectus
—and the even more remarkable possibility that
Homo erectus
didn’t evolve in Africa at all, but having evolved in Asia—perhaps from even earlier African roots—went back to Africa again.
⁵⁴
Everyone knows that the Israelites crossed the Red Sea, but nobody said anything about them coming back.

Rolling the tape forward, from about 1.8 million to 200,000 years ago, we see in the fossil record the first signs of behavior that seems distinctively human, as opposed to just hominin. The earliest known remains attributable to
Homo sapiens
are almost 200,000 years old and come from Ethiopia.
⁵⁵
At about the same time, cave sites in South Africa show signs of new things, such as shells pierced to make ornaments, the use of a natural pigment called ocher in decoration, and the extensive exploitation of seafood.
⁵⁶
When
Homo sapiens
evolved, the first thing it did was head for the beach.

The story goes that once modern humans evolved in Africa, they spread throughout the world, displacing any other hominins they might have come across, most notably Neanderthals in Europe. This
“out-of-Africa” tale was vigorously countered by another view, called “multiregional continuity,” that
Homo sapiens
evolved several times, independently, from various forms scattered throughout the world, including Neanderthals. At this point I shall point you to
figures 5
and
6
in
chapter 1
and ask you whether you think, on the basis of the fossil evidence, either view stands up.

The out-of-Africa idea received a huge boost in 1987 with a study on human evolution that broke new ground by not being based on sparse, fragmentary fossil evidence, but on comparisons between people alive today. Our inheritance is encoded in the genetic material, DNA, almost all of which is found in the nucleus of each cell. But cells also contain other bodies, called mitochondria, which have DNA. It so happens that this mitochondrial DNA (or mtDNA) is passed strictly down the female line. Writing in
Nature
, the late Allan C. Wilson and his colleagues described how they analyzed the mtDNA from 147 people of diverse origins and used the pattern of similarities and differences between the samples to sketch a kind of evolutionary genealogy of humanity.
⁵⁷
The results showed that the greatest diversity of mtDNA was to be found in Africa, and that mtDNA from everywhere else seemed to have been an offshoot of an ancient African form. This idea gave credence to the view that modern humans evolved in Africa and spread throughout the rest of the world. Calculations of the rate at which mtDNA would acquire new variations suggested that humans left Africa roughly 200,000 years ago. Given that mtDNA is passed down exclusively from mothers to daughters, the authors wrote that “[a]ll these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa.” The significance was not lost on the author of an accompanying commentary in
Nature
entitled “Out of the Garden of Eden,” which described the Wilson paper as reporting that “Eve was alive, well, and living in Africa around 200,000 years ago.”
⁵⁸
With Genesis in your PR department, you can hardly go wrong. It’s perhaps fortunate that subsequent work has largely borne out the idea that modern humans originated in Africa.

The picture has, perhaps inevitably, become more complicated. More recent studies on nuclear DNA, including DNA recovered—remarkably—from Neanderthals, shows that modern humans didn’t completely replace earlier species of hominin. If you are of European descent, then around 4 percent of your genes came from Neanderthals.
⁵⁹
If you’re from New Guinea, you might boast an even more remarkable
heritage, for some of your genes come from the still-obscure Denisovans.
⁶⁰
In his book
The Origin of Our Species
, Chris Stringer suggests that some archaic hominins might have survived even in Africa until a few tens of thousands of years ago. This idea is supported by genetic work showing traces of interbreeding with ancient hominins as yet unknown in some modern African populations.
⁶¹
Everywhere you look, we all bear some genetic traces of hominins past.

In any case, it must be remembered that “mitochondrial Eve” was not the only woman around at the time, only the one whose mitochondrial DNA appears to have survived until the present day. It is, perhaps, no more than luck that it was she, rather than any other female, living earlier or later, who turned out to have been the ancestor of all the mtDNA found (so far) in modern humans.
⁶²