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 not 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.
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 subspecies 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.
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.
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 that 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:
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.
Hold in your head the hypothetical notion that we might, by some fluke, have been blessed with a continuous fossil record of all evolutionary change, with no links missing at all. Now look at the four Latin names that have been applied to 1470. On the face of it, the change from habilis to rudolfensis would seem to be a smaller change than the one from Australopithecus to Homo. Two species within a genus are more like each other than two genera. Aren't they? Isn't that the whole basis for the distinction between the genus level (say Homo or Pan as alternative genera of African apes) and the species level (say troglodytes or paniscus within the chimpanzees) in the hierarchy of classification? Well, yes, that is right when we are classifying modern animals, which can be thought of as the tips of the twigs on the evolutionary tree, with their antecedents on the inside of the tree's crown all comfortably dead and out of the way. Naturally, those twigs that join each other further back (further into the interior of the tree's crown) will tend to be less alike than those whose junction (more recent common ancestor) is nearer the tips. The system works, as long as we don't try to classify the dead antecedents. But as soon as we include our hypothetically complete fossil record, all the neat separations break down. Discrete names become, as a general rule, impossible to apply. We can easily see this if we walk steadily backwards through time, much as we did with the rabbits in Chapter 2.
As we trace the ancestry of modern Homo sapiens backwards, there must come a time when the difference from living people is sufficiently great to deserve a different specific name, say Homo ergaster. Yet, every step of the way, individuals were presumably sufficiently similar to their parents and their children to be placed in the same species. Now we go back further, tracing the ancestry of Homo ergaster, and there must come a time when we reach individuals who are sufficiently different from
KNM ER 1470:
KNM ER 1813:
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