This article was originally published in the May/June 1999 issue of Reports of the National Center for Science Education, by Colin Groves.
It is republished here by permission of the NCSE and Colin Groves.
Dr. Colin Groves is a paleoanthropologist and Professor of Biological Anthropology at the Australian National University.
The human line separated from the chimpanzee line some 5 million years ago or a little more, according to molecular clock dates. The earlier members of the human lineage, all of them purely 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 550cc), large faces, jaws and cheekteeth, and the dental arcades tend 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, the legs are short (leg/arm ratio intermediate between chimpanzee and human) and the feet are basically bipedal and resemble humans, but the phalanges (toe-bones) are more curved. Homo have larger cranial capacities (510 cc upward), usually smaller faces, jaws and cheekteeth, and the dental arcades are parabolic; except for 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.
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 ten 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, Ardipithecus ramidus, 4.4 million years old, is quite distinct; and the "robusts" or "nutcrackers", Paranthropus species, form a quite 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 is allotted - or was, until early this year - at least four species:
1. Australopithecus anamensis, 3.9 to 4.1 ma, from Kanapoi and Allia Bay, Lake Turkana district, northwestern Kenya. Though only recently described, this species is represented by quite a range of remains.
2. Australopithecus bahrelghazali, about the same age, from Koro Toro in Chad, the only australopithecine known from western Africa. This one is known so far only by a single jaw.
3. Australopithecus afarensis, known from Fejej in Ethiopia; about 4 ma, Laetoli in Tanzania, 3.5 to 3.75 ma; and Hadar in Ethiopia, 3.3 to 2.9 ma. These sites cover a wide area in space and time, and not everyone is convinced that they all belong to a single species. Laetoli has over 20 fossil individuals (mainly jaws and teeth), and some important fossil footprints, while the extremely rich deposits at Hadar include a collection called "The First Family" and the very famous partial skeleton, "Lucy".
4. Australopithecus africanus, the earliest described species, from South Africa; it has long known from the sites of Taung, Sterkfontein and Makapansgat, and recently specimens have begun to be excavated at other sites in the Sterkfontein Valley (Drimolen and Gladysvale). Until very recently no absolute ages for these South African sites seemed possible, but they were dated by comparing their mammal faunas with those from dated sites in East Africa; these comparisons suggested dates of 2.5 to 3 ma. Very recently, attempts have been made to apply Electron Spin Resonance dating to them, and the results so far seem consistent with the faunal inferences.
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 A. anamensis all we can say so far is that it is in the right place at the right time and has no specialised bits of anatomy that would exclude it from having been an ancestor. A. afarensis seems pretty primitive all round, but of course is more derived in the human direction than A. anamensis. 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. Either our ancestors evolved in East Africa, moved south, and then moved back again in time to become Homo; though of course it may have existed in East Africa too but we just haven’t found any yet. But its differences from A. afarensis seemed mostly to be pointing in the wrong direction: on the one hand it 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.
Well-preserved specimens of Homo appear at around 2 ma in East Africa, mainly Olduvai Gorge (Tanzania), where Homo habilis occurs, and Koobi Fora (Kenya), where two 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 Australopithecus africanus; the cranial capacity is 510-680cc in H.habilis and about 750 in H. rudolfensis; and 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 et al. (1999) point out that the evidence actually will not sustain this conclusion). A couple of hundred thousand years after these two early Homo species have appeared, the first more modern-looking species, Homo ergaster with its long legs, shortened forearms, short face, prominent nose and beetle-brows, with cranial capacity over 800cc, 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 use stones to crack nuts, but do not modify them. 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, a specimen beautifully intermediate between Australopithecus 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 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 press, a paper by Asfaw, White, Lovejoy, Latimer, Simpson and Suwa, in Nature, 284:629-635 (for 23 April, 1999), describing 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; the site is Bouri, on the Middle Awash River in Ethiopia; the age is 2.5 ma; the remains are associated with large antelope remains with cut-marks on them, apparently from stone tools; and primitive stone tools 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 skeletons, a fragment of a second cranium, and two 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 it is 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 et al. 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 et al. 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".
What are we to make of it? One, two or three species? What we have is:
On balance, the evidence favours the single-species interpretation, but until we find associated parts we must be cautious. Especially because of those vast teeth; it has been argued by McHenry, Tobias and others that megadontia (big-toothedness) is the primitive condition so that the teeth of early Homo had to get smaller, but that the putative ancestor of Homo had the biggest teeth of the lot - that was entirely unexpected.
Suppose Australopithecus garhi made the tools and was the ancestor of Homo. Where do the four presumed >2 ma early Homo specimens fit in? The Bouri cranium lacks a base, so that cuts out comparisons with both Sts19 and the Chemeron temporal. Asfaw et al. 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 from 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 palaeoanthropological map: India, Syria, Eritrea, Chad, Malawi, 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.
Copyright © Colin Groves, 1999
This page is part of the Fossil Hominids FAQ at the talk.origins Archive.
Home Page |
Illustrations | What's New | Feedback | Search | Links | Fiction
Copyright © Jim Foley || Email me