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by Colin Groves |
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by Colin Groves |
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by Colin Groves Australian National University, Canberra Reprinted with kind permission of the author from Reports of the National Center for Science Education May/June, 1999, Vol. 19, No. 3 INTRODUCTION 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 are 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. Table 1: Brief Comparison of Austalopithecus with early Homo fossils (based on Asfaw, et al, 1999)
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. THE AUSTRALOPITHECINES 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 as A. anamensis. This species is recovered from Koro Toro in Chad and represents the only australopithecine known from western Africa. A. bahrelghazali is known so far only by a single jaw. 3. Australopithecus afarensis is well known from Fejej in Ethiopia; about 4 Ma, Laetoli in Tanzania, 3.5 to 3.7S 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 been known from the sites of Taung, Sterkfontein and Makapansgat, and new excavations have recently begun 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 sites in East Africa that could be dated. 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 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 later moved back again 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. EARLY HOMO 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: 2. A maxilla from 2.3-ma levels at Hadar. This is very clearly Homo, less prognathous ("snouty") than an australopithecine, with a fairly parabolic dental arcade and no anterior pillars. Its smaller teeth resemble Homo habilis. From the same level come stone tools. 3. A temporal bone fragment, mainly the glenoid fossa (where the jaw fits into the skull), from 2.4 Ma deposits at Chemeron in Kenya The glenoid fossa is deep and Homo- (rather than Australopithecus-) like. It appears to be placed further under the braincase suggesting that the brain had expanded above and out over the side of the joint. 4. Finally, a basicranial specimen (Sts 19) from Sterkfontein, found in amongst the Australopithecus africanus remains, has quite a number of Homo-like details of the form of the ear region, all of which distinguish it from any australopithecine. In comparable parts, in fact, it is quite like the Chemeron temporal. 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. ENTER THE BOURI HOMININ - OR SHOULD THAT BE HOMININS? Table 2: Body 2Proportions
of Some Important Fossils (Compared With Modern
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 pre-molar 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.Êafricanus. 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". What are we to make of it? One, 2 or 3 species? What we have
is · - limb bones intermediate in proportion between A. afarensis and H. ergaster; and · - the earliest stone tools so far discovered. On balance, the evidence favors 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 ought to get smaller. Indeed, we do observe a decrease in tooth size in the emerging hominins, except for these specimens from Bouri. That the putative ancestor of Homo (based on derived condition of skull and limb bones) 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 4 early Homo specimens presumed older than 2 Ma fit in? The Bouri cranium lacks a base, so that prevents direct comparisons with both Stsl9 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 fulfills 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. AUTHOR'S ADDRESS FURTHER READING De Heinzelin J, Clark JD, White T, Hart W, Renne P, WoldeGabriel G, Beyene Y, Vrba E. 1999. Environment and behavior of 2.5--million-year-old Bouri hominids. .Science 199; 284:625-9. 2. What is human-like about the early
hominin locomotor skeleton, and what can and cannot be said
about them: McHenry HM. Tempo and mode in human evolution. Proceedings of the National Academy of Sciences USA 1994; 91:678() 6. 3. An alternative interpretation of Australopithecus
afarensis and A. anamensis: 4 The candidates for earliest Homo (2.3 to 2.5 Ma): Hadar: Chemeron: Sterkfontein: |
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