The human line separated from the chimpanzee line some 5 million years ago or a little more, according to dates derived from molecular "clocks". The earlier members of the human lineage, all of them entirely African, are lumped together as "australopithecines", named for the genus Australopithecus but including other genera too. Later members are placed together in the genus Homo.
Australopithecines have small cranial capacities (about 350 to 550 cc), large faces, jaws and cheek teeth, and the arrangement of the teeth in the jaws (dental arcade) tends to be rectangular. Where the postcranial skeleton is known, the ribcage is funnel-shaped (narrow at the top, expanding downwards), the hipbones a very wide and flaring, and the legs are short (leg:arm ratio intermediate between chimpanzee and human).The feet are basically bipedal and resemble humans, but the phalanges (toe-bones) are more curved. Fossils of the genus Homo have larger cranial capacities (510 cc upward), usually smaller faces, jaws and cheek teeth, and the dental arcades are parabolic. Except in the most primitive members the ribcage, where known, is barrel-shaped, the hipbones do not flare as much and are more curved, the legs are long, and the feet are fully modern.
|A afarensis||A africanus||A garhi||Early Homo|
|Molar & premolar size||moderate||moderate to large||huge||moderate|
|Anterior upper premolar||asymmetrical||more oval||more oval||more oval|
|Tooth enamel thickness||fairly thick||thick||thick||thick|
|Dental arcade shape||rectangular||rectangular||rectangular||parabolic|
|Anterior depth of palate||shallow||varies||shallow||deep|
|Diastema in upper jaw||common||absent||present||rare|
|Anterior pillars on face||no||yes||no||no|
|Supraorbital structure||thin bar||thin bar||thin bar||torus|
|Cranial capacity||343-500||428-ca 515||450||510-752|
As a typical bang-up-to-the-minute biologist, I adopt a cladistic attitude to taxonomy: a family or genus is an evolutionary lineage. I place humans, chimpanzees, gorillas and orangutans together in the family Hominidae; so "hominid", a term still all too often used to mean "in the human line", actually refers to other living Great Apes too. At most, humans can be separated from other Great Apes as a tribe, Hominini, so fossils on the human side of the divide are "hominins". Anthropologists as a crew are always about 10 years behind other biologists, so it will probably be quite a while yet before textbooks of human evolution stop using "hominids" in the old sense.
Among the australopithecines, the earliest member is Ardipithecus ramidus, which is about 4.4 million years (ma) old and presents a quite distinct set of traits. The other distinctive clade represents the "robust" or "nutcracker", Paranthropus species, a distinct lineage which can be traced over a million and a half years from 2.5 to about 1 Ma The others are for the moment (for want of a decent cladistic model, really) lumped into the genus Australopithecus, which contains - or did until early this year - at least 4 species:
The indications are that the early hominins were as diverse as any other group of large mammals. Among all the diversity, however, there must have been some actual ancestors and, human nature being what it is, everyone is obsessed with trying to deduce which, if any, of the fossil species might have filled this role. About all we can say so far about the ancestral possibilities of A anamensis is that it is in the right place at the right time and has no specialized bits of anatomy that would exclude it from having been an ancestor. A afarensis seems pretty primitive all around, but of course is more derived in the human direction than A anamensis. So, a plausible sequence begins to emerge. But what of A africanus?
Opinions have been rather divided about Australopithecus africanus. It is later in time than A afarensis and earlier than the first Homo, H habilis, so it fills the time gap; but it has seemed to be in the wrong place. Maybe our ancestors evolved in East Africa, moved south, and then moved back again in time to become Homo (though of course they may have existed in East Africa too but we just haven´t found any yet). But the differences from A afarensis to H habilis seem mostly to be pointing in the wrong direction. On the one hand A africanus had a larger cranial capacity on average, the lower premolars were wider (in A afarensis they were often narrow and fairly apelike), and the dental arcade sometimes tended to be more parabolic. On the other hand it had larger, broader molars and premolars but somewhat smaller front teeth, and a heavily built-up facial skeleton with what one specialist, Yoel Rak, has called "anterior pillars" - prominent bony thickenings alongside the snout and nasal aperture. If A africanus was ancestral to Homo, these last features would have been developed then lost again - a transition we try to avoid in deriving ancestor-descendant lineages.
Well-preserved specimens of Homo appear at around 2 Ma in East Africa, mainly at Olduvai Gorge (Tanzania), where Homo habilis occurs, and at Koobi Fora (Kenya), where 2 species are present, a habilis-like species and the larger Homo rudolfensis. Both, especially H rudolfensis, have large molars, but the premolars are less expanded than in A africanus. The cranial capacity is 510-680 cc in H habilis and about 750 in H rudolfensis. The postcranial skeleton in H habilis, at least, is every bit as primitive as in australopithecines (it is "well known" that the legs are even relatively shorter than in "Lucy", but Asfaw and others  point out that the evidence actually will not sustain this conclusion; this was shown earlier by Korey ). A couple of hundred thousand years after these 2 early Homo species appeared, the first more modern-looking species, Homo ergaster with its long legs, shortened forearms, short face, prominent nose and beetle-brows, and a cranial capacity over 800 cc, appears in the record and is well on the way to becoming us.
The early Homo-bearing beds also have stone tools. Chimpanzees modify grass stems, branches and other perishable material, and they use stones to crack nuts but do not modify the stone. Presumably australopithecines did at least as well as chimpanzees, but not until Homo are there signs that stone was deliberately modified to form tools.
Where, then, did Homo spring from? There has been a big gap in the record before 2 Ma - back to 2.5, if we think that A africanus was the ancestral stock, or to 2.9 if we reject A africanus and take it back to A afarensis. (A related question, where did Paranthropus spring from, has now gone some way to being answered by the discovery, in the mid-80s, of "the Black Skull", from 2.5-ma deposits at Lomekwi, west of Lake Turkana. This specimen is beautifully intermediate between A afarensis and the later (1-2 Ma) Paranthropus specimens we find at Koobi Fora, Olduvai and so on). Until this year, there were just a few suggestive scraps:
The Uraha mandible and Hadar maxilla are early Homo, there is no disagreement about this. The Chemeron temporal and Sts 19 are much more controversial. Even if we narrow it down to just the first two, we come to the interesting conclusion that by 2.3 Ma two species already seem to be in existence, the same two species that we find in the 2 Ma deposits at Koobi Fora
And now, and now... hot off the presses ... a paper by Asfaw and others (Nature 1999 Apr 23; 284:629-5) describes a new species which they think "is descended from Australopithecus afarensis and is a candidate ancestor for early Homo". The new species is Australopithecus garhi from Bouri, on the Middle Awash River in Ethiopia. The age is 2.5 Ma, and the remains are associated with large antelope remains with cut-marks on them, apparently from stone tools. The primitive stone tools themselves were found not at Bouri itself but at the nearby, contemporaneous site of Gona.
The type specimen of Australopithecus garhi is a partial cranium. From nearby sites, and perhaps belonging to the same species or perhaps not, come several postcranial bones including a partial skeleton, a fragment of a second cranium, and 2 mandibles (one fairly complete). The specific name, garhi, means "surprise" in the Afar language, and a bit surprising it is, too. It is basically australopithecine, with a small cranial capacity (450 cc), rectangular or slightly diverging dental arcade, and very prognathous face. It lacks the anterior pillars of Australopithecus africanus, and it even has a gap (diastema) between the lateral incisor and the canine, a primitive feature seen in A afarensis but not in A africanus. From the photos, it looks very like A afarensis, but the authors point out some more "advanced" features like the premolar shape and the more anteriorly placed malar (cheekbone) root. Like many australopithecines, including some A afarensis, it has a sagittal crest for anchoring large temporal (chewing) muscles. But what is astounding about the specimen are the huge premolars and molars. The canine, for example, is larger than any other hominin, the anterior premolar is larger than any except for some specimens of Paranthropus boisei (the East African "nutcracker" species), and the second molar is larger than any Homo, though within the range of A africanus.
About the mandible, Asfaw and colleagues say little, except that its morphology would be compatible with belonging to the same species. The stone tools might have been made by A garhi, or they might not. As for the postcranial bones, the authors are careful to explain, they too need not belong to the same species. There could be one species that left its head in the deposits and another that left its postcranial skeleton there (and of course either or neither of them might have made the stone tools). But for what it is worth, and it is worth a good deal, Asfaw and colleagues give a brief description and an interesting diagram of the limb bone proportions. The femur-to-humerus ratio was like Homo ergaster and modern humans (long femur, short "Lucy"-sized humerus), but the forearm (radius and ulna)-to-humerus ratio was long like a chimpanzee or, for that matter, like "Lucy".
Radius as % of humerus
|Humerofemoral index: |
Humerus as % of femur
|Pan paniscus (Bonobo)||91.9||97.8 ± 2.1|
|A.afarensis ("Lucy" skeleton)||90.7||84.6 ± 2.8|
|Bouri (perhaps A.garhi)||97.9||ca. 70.4|
|H.habilis (OH 62)||[79.5-93.2]||[94.3 ± 7.7]|
|Homo sapiens (African)||79.6 ± 2.5||73.3 ± 1.7|
What are we to make of it? One, 2 or 3 species? What we have is
Suppose Australopithecus garhi made the tools and was the ancestor of Homo. Where do the 4 early Homo specimens presumed older than 2 Ma fit in? The Bouri cranium lacks a base, so that prevents direct comparisons with both Sts19 and the Chemeron temporal. Asfaw and colleagues do not describe the Bouri-region mandibles, so that (for the moment) excludes comparisons with Uraha. But the Hadar maxilla is definitely different from the one found at Bouri. In fact, it could be lost among the Olduvai maxillae, more than 300 000 years later. So, if A garhi is ancestral to Homo, either there was a rapid change in maxillary morphology in the intervening 200 000 years, or else the Bouri specimen is a late survivor of its species. We must not exclude a speeding-up of evolutionary rates, nor must we fall into the trap of assuming anagenesis (evolution without branching).
It´s an exciting time to be alive if you´re interested in human evolution. New countries are getting onto the paleoanthropological map: India, Syria, Eritrea, Chad, Malawi, and Portugal. Every new fossil fulfils certain expectations but opens up a whole barrel of new research questions. Fossil discoveries are matched by new discoveries of just how human our nearest living relatives are. And the press is avid for them all, as well it might be. Keep on your (bipedal) toes; if you miss this week´s reports you might already be out-of-date.