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How We Came To Be The Only Humans On Earth
By Chris Stringer
Henry Holt and Company Copyright © 2012 Chris Stringer
All rights reserved.
The Big Questions
It is barely 150 years since Charles Darwin and Alfred Russel Wallace presented their ideas on evolution to the world. A year later, in 1859, Darwin was to publish one of the most famous of all books, On the Origin of Species. Then, the first fossil human finds were only beginning to be recognized, and paleontology and archaeology were still in their infancy. Now, there is a rich and ever-growing record from Africa, Asia, and Europe, and I have been privileged to work in one of the most exciting eras for discoveries about our origins. There have been highly significant fossil finds, of course, but there have also been remarkable scientific breakthroughs in the amount of information we can extract from those finds. In this first chapter I will outline the evidence that has been used to reconstruct where our species originated, and the very different views that have developed, including my own. There are in fact two origins for modern human features that we need to consider. Here, I will talk about our species in terms of the physical features we humans share today, for example, a slender skeleton compared to our more robust predecessors, a higher and rounder braincase, smaller brow ridges, and a prominent chin. But there are also the characteristics that distinguish different geographic populations today — the regional or "racial" characteristics, such as the more projecting nose of many Europeans, or the flatter face of most Orientals. I will discuss their quite different origins later in the book.
In The Descent of Man (1871), Darwin suggested that Africa was the most likely evolutionary homeland for humans because it was the continent where our closest relatives, the African apes, could be found today. However, it was to be many years before the fossil evidence that was ultimately to prove him right began to be discovered. Before then, Europe with the Neanderthals, "Heidelberg Man," and the spurious "Piltdown Man," and Asia with "Java Man," were the foci of scientific attention concerning human ancestry. But the 1921 discovery of the Broken Hill skull in what is now Zambia, and the 1924 discovery of the Taung skull (from South Africa), started the process that gave Africa its paramount importance in the story of human evolution, even if that process still had many years to run. By the 1970s a succession of fossils had established that Africa not only was the place of origin for the human line (that is, the continent in which the last common ancestor of humans and chimpanzees lived) but was probably also where the genus Homo (humans) had originated. But where did our own species, Homo sapiens (modern humans), originate? This was still unclear in the 1970s and remained so until quite recently.
When Charles Darwin wrote in the Origin of Species, "light would be thrown on the origin of man and his history," he was reluctant to say any more on the subject, as he admitted twelve years later in the introduction to The Descent of Man: "During many years I collected notes on the origin or descent of man, without any intention of publishing on the subject, but rather with the determination not to publish, as I thought that I should thus only add to the prejudices against my views." But in the intervening years he had been fortified by a growing number of influential supporters and thus felt ready — finally — to tackle the controversial topic of human origins. He then went on to say: "The sole object of this work is to consider, firstly, whether man, like every other species, is descended from some pre-existing form; secondly, the manner of his development; and thirdly, the value of the differences between the so-called races of man." However, Darwin acknowledged that there were still many doubters, something that unfortunately remains as true today as it was then: "It has often and confidently been asserted, that man's origin can never be known: but ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science."
Darwin then proceeded to pay tribute to a number of other scientists for their work on human origins, particularly the German biologist Ernst Haeckel, and this is especially interesting as Haeckel differed from him and Thomas Huxley ("Darwin's bulldog") over a critical question about our origins, a question that continues to be debated even today. In The Descent of Man Darwin wrote: "We are naturally led to enquire, where was the birthplace of man at that stage of descent when our progenitors diverged from the catarrhine stock [the catarrhines group includes apes and monkeys]? ... In each great region of the world the living mammals 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." However, he then proceeded to caution, "But it is useless to speculate on this subject ... as there has been ample time for migration on the largest scale."
Not only did Darwin have to deal with a dearth of fossil evidence in 1871, including a complete absence of any humanlike fossils from Africa, but there was also no knowledge of the concept of continental drift (the idea that landmasses migrated in the past, splitting and realigning as they moved across the Earth's surface). This process is now known to underlie many of the present distributions of plants and animals (for example, the unique assemblages of species found in places like Australia and New Zealand). Previously, to explain puzzling links between species in different regions, now-sunken continents were often postulated. For example, lemurs are rather primitive primates that today are found only on the island of Madagascar, some three hundred miles off the coast of Africa, but ancient lemurlike fossils had been found in the Indian subcontinent, and such similarities led the British zoologist Philip Sclater to hypothesize in 1864 that there was once a large continent, which he named Lemuria, stretching across much of what is now the Indian Ocean.
Using the concept of this lost continent, Haeckel argued for a different ancestral homeland for humans: "There are a number of circumstances which suggest that the primeval home of man was a continent now sunk below the surface of the Indian Ocean, which extended along the south of Asia ... towards the east; towards the west as far as Madagascar and the southeastern shores of Africa. By assuming this Lemuria to have been man's primeval home, we greatly facilitate the explanation of the geographical distribution of the human species by migration." Moreover, Haeckel differed from Darwin and Huxley in favoring the gibbon and orangutan of southeast Asia as better ape models for human ancestry than the gorilla and chimpanzee of Africa. And whereas Darwin followed the geologist Charles Lyell in arguing that the fossil record of human evolution was still unknown because the right regions had not yet been searched (in particular Africa), Haeckel preferred the explanation that most of the critical evidence was now sunk beneath the Indian Ocean.
During Darwin's lifetime, the Neanderthals were already known from their fossil remains as ancient inhabitants of Europe. While some scientists pushed them into the position of "missing links," reconstructing them with bent knees and grasping big toes, others like Huxley recognized them as big-brained, upright, and unmistakably human. Darwin never lived to see the first discovery of a really primitive human fossil, announced by a Dutch doctor, Eugène Dubois, in 1891. Dubois had been inspired by Haeckel's writings to get an army posting to what was then the Dutch East Indies (now Indonesia), to search for ancient remains. Haeckel had created the name "Pithecanthropus alalus" ("Ape Man without Speech") for a hypothetical link between apes and humans that he believed had once lived in Lemuria. Dubois was blessed with luck in his excavations on the island of Java and soon found a fossilized and apelike skullcap and a human-looking thighbone. He named these "Pithecanthropus" (in honor of Haeckel) "erectus" (because the femur indicated this creature walked upright, as we do). We now know this species as Homo erectus, a wide-ranging and long-lived species of early human. But because this first find of the species was made on the Indonesian island of Java, it tended to reinforce Haeckel's and Dubois's notions of a Lemurian/southern Asian origin for humans, rather than an African one.
In naming "Pithecanthropus erectus," Dubois was following the system laid down over a century earlier by that greatest of all classifiers, the Swedish naturalist Carl Linnaeus. The Chinese sage Confucius said that it was "a wise man" who specified the names of things, and by happy coincidence this was the name, in Latin, that Linnaeus chose for the human species: Homo sapiens. Before Linnaeus there were many different ways of naming and grouping plants and animals, often based at random on particular features that they showed — color, say, how they moved around, or what they ate. But Linnaeus believed in grouping living things by the bodily features they shared, and at the heart of his system were the two names applied to every natural kind, or species: its group or genus name capitalized, and its particular species name. Thus Homo ("Man") and sapiens ("wise"). The system is a bit like a surname (the genus name Homo) and a first name (differentiating the different children with a particular surname, in our case sapiens). In the most-cited tenth edition of his book Systema Naturae (1758) he also named four geographic subspecies: "europaeus," "afer," "asiaticus," and "americanus," introducing some dubious anecdotal behavioral distinctions in line with then current European notions about the superiority of the European subspecies. For example, while "europaeus" was, of course, governed by laws, "americanus" was governed by customs, "asiaticus" by opinions, and the African subspecies "afer" by impulse.
In the early 1900s, evidence continued to accumulate in favor of a non-African origin for humans, and the focus returned to Europe. Further Neanderthal remains were found in Croatia and France, and a more ancient and primitive fossil jawbone was unearthed in the Mauer sandpit near Heidelberg in Germany in 1907. As enough material began to accumulate, scientists started to build evolutionary trees from the fossil evidence. These tended to fall into two main categories: ones where the fossils were arranged in a linear sequence leading from the most primitive form (for example, Java Man or Heidelberg Man) to modern humans, with few or no side branches (like a ladder); and others (like a bush) where there was a line leading to modern humans, and the other fossils with their primitive features were placed in an array of side branches leading only to extinction.
The combination of Darwin's and Wallace's publications on the transmutation of species and a proliferating Pleistocene fossil human record led to the expectation that there must have been many more ancient species of humans (the Pleistocene is a recent geologic epoch, poorly dated during Darwin's time, but now believed to stretch from about 12,000 to 2.5 million years ago). William King had named the first fossil-based species Homo neanderthalensis in 1864, from the skeleton discovered in the Neander Valley in 1856. Within fifty years, the new European finds were being assigned to dozens of new human species in an unfortunate tumult of typology, where trivial differences were elevated to assume real biological significance. Thus, the completely modern-looking remains that had been found in the sites of Cro-Magnon, Grimaldi, Chancelade, and Oberkassel became the human species "spelaeus," "grimaldii," "priscus," and "mediterraneus," respectively, while the remains from Spy, Le Moustier, and La Chapelle-aux-Saints became "spyensis," "transprimigenius," and "chapellensis," despite their resemblance to the remains already designated H. neanderthalensis from the Neander Valley. This trend for what we can call extreme "splitting" continued up to about 1950, when the pendulum swung back to the opposing tendency to "lump" fossils together in just a few species.
Suggestions that Europe may have hosted even more primitive human relatives started to emerge from a gravel pit at Piltdown in southern England in 1912, giving rise to yet another species called "Eoanthropus dawsoni" ("Dawn Man of Dawson" — Charles Dawson being the principal discoverer). Parts of a thick but large-brained skull, coupled with a distinctly apelike jaw, turned up there with ancient animal fossils and primitive stone tools, suggesting an age as great as that of Java Man. Africa had nothing to compare with these burgeoning finds, but that finally began to change in the 1920s. However, circumstances were such that these first finds still failed to switch the focus of human origins to Africa.
The Broken Hill (Kabwe) skull, discovered in 1921, was the first important human fossil from Africa, but it was a puzzling find. Although it was assigned to the new species "Homo rhodesiensis" by Sir Arthur Smith Woodward of the British Museum, the Czech-American anthropologist Ale Hrdlicka dubbed it "a comet of man's prehistory" because of the difficulty in deciphering its age and affinities. The skull was found in cave deposits that were being quarried away during metal ore mining, in what is now Zambia (then the British colony of Northern Rhodesia). It's one of the most beautifully preserved of all human fossils, but it displays a strange mixture of primitive and advanced features, and its face is dominated by an enormous brow ridge glowering over the eye sockets. And because it was found during quarrying, which eventually destroyed the whole Broken Hill mine, its age and significance remain uncertain even today (but see the final chapter for the latest developments).
Three years later an even more primitive find was made in a limestone quarry at Taung, South Africa — a skull that looked like that of a young ape. It was studied by a newly established professor of anatomy in Johannesburg, named Raymond Dart, and in 1925 he published a paper in the scientific journal Nature, making some remarkable claims about the fossil. He argued that it showed a combination of ape and human features, but that its teeth, brain shape, and probable posture were humanlike. Dart named it Australopithecus africanus ("Southern Ape of Africa"), and he declared that it was closely related to us, and even a potential human ancestor. Dart's claims were treated with great skepticism by the scientific establishment, particularly in England. This was partly because of judgments about Dart's youth and relative inexperience, and partly because the fossil was that of a child (young apes may look more "human" than adult apes). Others thought that the finds from Java, Heidelberg, and Piltdown provided much more plausible ancestors than Australopithecus africanus. And finally, the location and estimated age of Taung also counted against it.
No one (not even Darwin and Huxley) had considered southern Africa as a location for early human evolution, and as the Taung skull was guessed to be only about 500,000 years old, it was thought too recent to be that of a genuine human ancestor. Instead, it was considered to represent a peculiar kind of ape, paralleling humans in some ways. We now know, of course, that the australopithecines represented a long and important phase of human evolution that lasted for over 2 million years, and which is recognized at sites stretching from Chad in the Sahara to many more in eastern and southern Africa. And we have also known since their exposure in 1953 that the misleading Piltdown remains were fraudulent and had nothing to do with our ancient ancestry.
Excerpted from Lone Survivors by Chris Stringer. Copyright © 2012 Chris Stringer. Excerpted by permission of Henry Holt and Company.
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