Darwin deliberately deferred his treatment of human evolution to another book, The Descent of Man. Perhaps it is not surprising that the two volumes of that later work devote more space to the topic of its subtitle, Selection in Relation to Sex (investigated largely in birds), than to human evolution. Not surprising because, at the time of Darwin’s writing, there were no fossils at all linking us to our closest relatives among the apes. Darwin had only living apes to look at, and he used them well, arguing correctly (and almost alone) that our closest living relatives were all African (gorillas and chimpanzees – bonobos were not recognized as separate from chimpanzees in those days, but they are African too), and therefore predicting that, if ancestral human fossils were ever to be found, Africa was the place to search. Darwin regretted the paucity of fossils, but he maintained a robustly bullish attitude to it. Citing Lyell, his mentor and the great geologist of the time, he pointed out that ‘in all the vertebrate classes the discovery of fossil remains has been an extremely slow and fortuitous process’ and added, ‘Nor should it be forgotten that those regions which are the most likely to afford remains connecting man with some extinct ape-like creature, have not as yet been searched by geologists.’ He meant Africa, and the quest was not helped by the fact that his immediate successors largely ignored his advice and searched Asia instead.
It was indeed in Asia that the ‘missing links’ first began to become less missing. But those first fossils to be discovered were relatively recent, less than a million years old, dating from a time when hominids were pretty close to modern humans and had migrated out of Africa and reached the Far East. They were called ‘Java Man’ and ‘Peking Man’ after their discovery sites.* Java Man was discovered by the Dutch anthropologist Eugene Dubois in 1891. He named it Pithecanthropus erectus, signifying his belief that he had realized his life’s ambition and found ‘the missing link’. Disagreement came from two opposite sources, which rather proved his point: some said his fossil was purely human, others that it was a giant gibbon. Later in his rather embittered and cantankerous life, Dubois resented the suggestion that the more recently discovered Peking fossils were similar to his Java Man. Fiercely possessive about, not to say protective of, his fossil, Dubois believed that only Java Man was the true missing link. To emphasize the distinction from the various Peking Man fossils, he described them as far closer to modern man, and his own Java Man of Trinil as intermediate between man and ape.
Pithecanthropus [Java Man] was not a man, but a gigantic genus allied to the gibbons, however superior to the gibbons on account of its exceedingly large brain volume and distinguished at the same time by its faculty of assuming an erect attitude and gait. It had the double cephalization [ratio of brain size to body size] of the anthropoid apes in general and half that of man . . .
It was the surprising volume of the brain – which is very much too large for an anthropoid ape, and which is small compared with the average, though not smaller than the smallest human brain – that led to the now almost general view that the ‘Ape Man’ of Trinil, Java was really a primitive Man. Morphologically, however, the calvaria [skullcap] closely resembles that of anthropoid apes, especially the gibbon . . .
It can’t have improved Dubois’ temper that others took him to be saying that Pithecanthropus was just a giant gibbon, not intermediate between them and humans at all, and he was at pains to reassert his earlier stand: ‘I still believe, now more firmly than ever, that the Pithecanthropus of Trinil is the real “missing link”.’
Creationists have from time to time used as a political weapon the allegation that Dubois backed off from his claim that Pithecanthropus was an intermediate ape-man. The creationist organization Answers in Genesis has, however, added it to their list of discredited arguments which they now say should not be used. It is to their credit that they maintain such a list at all. As I said, both the Java and Peking specimens of Pithecanthropus have now been shown to be quite young, less than a million years old. They are now classified along with us in the genus Homo, retaining Dubois’ specific name erectus: Homo erectus.
Dubois chose the wrong part of the world for his single-minded quest for the ‘missing link’. It was natural for a Dutchman to head first for the Dutch East Indies, but a man of his dedication should have followed Darwin’s advice and gone on to Africa: for Africa is where our ancestors evolved, as we shall see. So what were these Homo erectus specimens doing out of Africa? The phrase ‘out of Africa’ has been borrowed from Karen Blixen* to refer to the great exodus of our ancestors from Africa. But there were two exoduses and it is important not to confuse them. Relatively recently, maybe less than 100,000 years ago, roving bands of Homo sapiens looking pretty much like us left Africa and diversified into all the races that we see around the world today: Inuit, native Americans, native Australians, Chinese, and so on. It is to this recent exodus that the phrase ‘out of Africa’ is normally applied. But there was an earlier exodus from Africa, and these erectus pioneers left fossils in Asia and Europe, including the Java and Peking specimens. The oldest fossil known outside Africa was found in the central Asian country of Georgia and dubbed ‘Georgian Man’: a diminutive creature whose (rather well-preserved) skull is dated, by modern methods, to about 1.8 million years ago. It has been called Homo georgicus (by some taxonomists, although others don’t recognize it as a separate species) to indicate that it seems rather more primitive than the rest of the early refugees from Africa, who are all classified as Homo erectus. Some stone tools slightly older than Georgian Man have just been discovered in Malaysia, sparking a new search for fossil bones in that peninsula. But in any case, all these early Asian fossils are pretty close to modern humans and all are nowadays classified in the genus Homo; for our earlier antecedents we must go to Africa. First, though, let’s pause to ask what we should expect of a ‘missing link’.
Homo georgicus
Chimpanzee
Suppose, for the sake of argument, we take seriously the original confused meaning of the term ‘missing link’, and seek an intermediate between chimpanzees (see right) and ourselves. We are not descended from chimpanzees, but it is a fair bet that the common ancestor that we share with them was more like a chimp than like us. In particular, it didn’t have a huge brain like ours, it probably didn’t walk upright as we do, it probably was a lot hairier than we are, and it surely didn’t have such advanced human features as language. So, even though we must remain adamant, in the face of common misunderstanding, that we are not descended from chimpanzees, there’s still no harm in asking what an intermediate between something like a chimpanzee and us would look like.
Well, hair and language don’t fossilize well, but we can get good clues about brain size from the skull, and good clues about gait from the whole skeleton (including the skull, for the foramen magnum, the hole for the spinal cord, points downwards in bipeds, more backwards in quadrupeds). Possible candidates for missing links might have any of the following attributes:
1 Intermediate brain size and intermediate gait: perhaps a sort of stooping shamble rather than the proudly erect bearing favoured by sergeant majors and deportment mistresses.
2 Chimpanzee-sized brain with human upright gait.
3 Large, more human-like brain, walking on all fours like a chimp.
So, bearing these possibilities in mind, let’s examine some of the many African fossils that are now available to us, but unfortunately were not available to Darwin.
I’M STILL MISCHIEVOUSLY HOPING . . .
Molecular evidence (which I shall come on to in Chapter 10) shows that the common ancestor we share with chimpanzees lived about six million years ago or a bit earlier, so let’s split the difference and look at some three-million-year-old fossils. The most famous fossil of this vintage is ‘Lucy’, classified by her discoverer in Ethiopia, Donald Johanson, as Australopithecus afarensis. Unfortunately we have only fragments of Lucy’s cranium, but her lower jaw is unusually well preserved. She was small by modern standards, although n
ot as small as Homo floresiensis, the tiny creature the newspapers have irritatingly dubbed ‘the Hobbit’, which died out tantalizingly recently on the island of Flores in Indonesia. Lucy’s skeleton is complete enough to suggest that she walked upright on the ground, but probably also sought refuge in trees, where she was an agile climber. There is good evidence that the bones attributed to Lucy really did all come from a single individual. The same is not true of the so-called ‘First Family’, a collection of bones from at least thirteen individuals, similar to Lucy and of approximately the same vintage, who somehow became buried together, also in Ethiopia. The fragments from Lucy and from the First Family give a good idea of what Australopithecus afarensis looked like, but it is hard to make an authentically complete reconstruction from pieces of many different individuals. Fortunately, a rather complete skull known as AL 444-2 (right) was discovered in 1992 in the same area of Ethiopia, and this confirmed the tentative reconstructions that had previously been made.
The conclusion from studies of Lucy and her kind is that they had brains about the same size as chimpanzees’ but, unlike chimpanzees, they walked upright on their hind legs, as we do – the second of our three hypothetical scenarios. ‘Lucys’ were a bit like upright-walking chimps. Their bipedality is dramatically confirmed by the poignantly evocative set of footprints discovered by Mary Leakey in fossilized volcanic ash. These are further south, at Laetoli in Tanzania, and they are older than Lucy and AL 444-2: about 3.6 million years. They are usually attributed to a pair of Australopithecus afarensis walking together (hand in hand?) but what matters is that, by 3.6 million years ago, an erect ape walked the Earth, on two feet which were pretty much like ours although its brain was the size of a chimpanzee’s.
AL 444-2
It seems quite likely that the species we call Australopithecus afarensis– Lucy’s species – included our ancestors of three million years ago. Other fossils have been placed in different species of the same genus, and it is virtually certain that our ancestors were members of that genus. The first Australopithecine to be discovered, and the type specimen of the genus, was the so-called Taung Child. At the age of three and a half the Taung Child was eaten by an eagle. The evidence is that damage marks to the eye sockets of the fossil are identical to marks made by modern eagles on modern monkeys as they rip out their eyes. Poor little Taung Child, shrieking on the wind as you were borne aloft by the aquiline fury, you would have found no comfort in your destined fame, two and a half million years on, as the type specimen of Australopithecus africanus. Poor Taung mother, weeping in the Pliocene.
The type specimen is the first individual of a new species to be named and officially given the virgin label in a museum. Theoretically, later finds are compared against the type specimen to see if they match. The Taung Child was discovered and given brand new genus and species names by the South African anthropologist Raymond Dart in 1924.
What’s the difference between ‘species’ and ‘genus’? Let’s get the question swiftly out of the way, before proceeding. Genus is the more inclusive division. A species belongs within a genus, and often it shares the genus with other species. Homo sapiens and Homo erectus are two species within the genus Homo. Australopithecus africanus and Australopithecus afarensis are two species within the genus Australopithecus. The Latin name of an animal or plant always includes a generic name (with an initial capital letter) followed by a specific name (without a capital letter). Both names are written in italics. Sometimes there is an additional sub-specific name, which follows the specific name, as in, for example, Homo sapiens neanderthalensis. Taxonomists often dispute names. Many, for example, would speak of Homo neanderthalensis not Homo sapiens neanderthalensis, elevating Neanderthal man from sub-species to species status. Generic names and specific names are also often disputed, and often change with successive revisions in the scientific literature. Paranthropus boisei has been, in its time, Zinjanthropus boisei and Australopithecus boisei,* and is still often referred to, informally, as a robust Australopithecine – as opposed to the two ‘gracile’ (slender) species of Australopithecus mentioned above. One of the main messages of this chapter concerns the somewhat arbitrary nature of zoological classification.
Raymond Dart, then, gave the name Australopithecus to the Taung Child, the type specimen of the genus, and we have been stuck with this depressingly unimaginative name for our ancestor ever since. It simply means ‘southern ape’. Nothing to do with Australia, which just means ‘southern country’. You’d think Dart might have thought of a more imaginative name for such an important genus. He might even have guessed that other members of the genus would later be discovered north of the equator.
Slightly older than the Taung Child, one of the most beautifully preserved skulls we have, although lacking a lower jaw, is called ‘Mrs Ples’. Mrs Ples, who may actually have been a small male rather than a large female, obtained ‘her’ nickname because she was originally classified in the genus Plesianthropus. This means ‘nearly human’, which is a better name than ‘southern ape’. One might have hoped that, when later taxonomists decided that Mrs Ples and her kind were really of the same genus as the Taung Child, Plesianthropus would have become the name for all of them. Unfortunately, the rules of zoological nomenclature are strict to the point of pedantry. Priority of naming takes precedence over sense and suitability. ‘Southern ape’ might be a lousy name but no matter: it predates the much more sensible Plesianthropus and we seem to be stuck with it, unless . . . I’m still mischievously hoping somebody will uncover, in a dusty drawer in a South African museum, a long-forgotten fossil, clearly the same kind as Mrs Ples and the Taung Child, but bearing the scrawled label, ‘Hemianthropus type specimen, 1920’. At a stroke, all the museums in the world would immediately have to relabel their Australopithecus specimens and casts, and all books and articles on hominid prehistory would have to follow suit. Wordprocessing programs across the world would work overtime sniffing out any occurrences of Australopithecus and replacing them with Hemianthropus. I can’t think of any other case where international rules are potent enough to dictate a worldwide and backdated change of language overnight.
‘Mrs Ples’
Now for my next important point about allegedly missing links and the arbitrariness of names. Obviously, when Mrs Ples’s name was changed from Plesianthropus to Australopithecus, nothing changed in the real world at all. Presumably nobody would be tempted to think anything else. But consider a similar case where a fossil is re-examined and moved, for anatomical reasons, from one genus to another. Or where its generic status is disputed – and this very frequently happens – by rival anthropologists. It is, after all, essential to the logic of evolution that there must have existed individuals sitting exactly on the borderline between two genera, say Australopithecus and Homo. It is easy to look at Mrs Ples and a modern Homo sapiens skull and say, yes, there is no doubt these two skulls belong in different genera. If we assume, as almost every anthropologist today accepts, that all members of the genus Homo are descended from ancestors belonging to the genus we call Australopithecus, it necessarily follows that, somewhere along the chain of descent from one species to the other, there must have been at least one individual who sat exactly on the borderline. This is an important point, so let me stay with it a little longer.
KNM ER 1813
KNM ER 1470
Bearing in mind the shape of Mrs Ples’s skull as a representative of Australopithecus africanus 2.6 million years ago, have a look at the top skull opposite, called KNM ER 1813. Then look at the one underneath it, called KNM ER 1470. Both are dated at approximately 1.9 million years ago, and both are placed by most authorities in the genus Homo. Today, 1813 is classified as Homo habilis, but it wasn’t always. Until recently, 1470 was too, but there is now a move afoot to reclassify it as Homo rudolfensis. Once again, see how fickle and transitory our names are. But no matter: both have an apparently agreed foothold in the genus Homo. The obvious difference from Mrs Ples and her kind is th
at she had a more forward-protruding face and a smaller brain-case. In both respects, 1813 and 1470 seem more human, Mrs Ples more ‘ape-like’.
Now look at the skull below, called ‘Twiggy’. Twiggy is also normally classified nowadays as Homo habilis. But her forward-pointing muzzle has more of a suggestion of Mrs Ples about it than of 1470 or 1813. You will perhaps not be surprised to be told that Twiggy has been placed by some anthropologists in the genus Australopithecus and by other anthropologists in Homo. In fact, each of these three fossils has been, at various times, classified as Homo habilis and as Australopithecus habilis. As I have already noted, some authorities at some times have given 1470 a different specific name, changing habilis to rudolfensis. And, to cap it all, the specific name rudolfensis has been fastened to both generic names, Australopithecus and Homo. In summary, these three fossils have been variously called, by different authorities at different times, the following range of names:
‘Twiggy’
KNM ER 1813:
Australopithecus habilis, Homo habilis
KNM ER 1470:
Australopithecus habilis, Homo habilis,
Australopithecus rudolfensis, Homo rudolfensis
OH 24 (‘Twiggy’):
Australopithecus habilis, Homo habilis
Should such a confusion of names shake our confidence in evolutionary science? Quite the contrary. It is exactly what we should expect, given that these creatures are all evolutionary intermediates, links that were formerly missing but are missing no longer. We should be positively worried if there were no intermediates so close to borderlines as to be difficult to classify. Indeed, on the evolutionary view, the conferring of discrete names should actually become impossible if only the fossil record were more complete. In one way, it is fortunate that fossils are so rare. If we had a continuous and unbroken fossil record, the granting of distinct names to species and genera would become impossible, or at least very problematical. It is a fair conclusion that the predominant source of discord among palaeoanthropologists – whether such and such a fossil belongs in this species/genus or that – is deeply and interestingly futile.