Early Hominids: Phylogeny and Chronology

Phylogeny and Chronology

Between 8 million and 4 million years ago

Fossils of Sahelanthropus tchadensis (6-7 million years) and Orrorin tugenensis (6 million years), discovered in 2001 and 2000 respectively, are still a matter of debate.

The discoverers of Orrorin tugenensis claim the fossils represent the real ancestor of modern humans and that the other early hominids (e.g., Australopithecus and Paranthropus) are side branches. They base their claim on their assessment that this hominid was bipedal (2 million years earlier than previously thought) and exhibited expressions of certain traits that were more modern than those of other early hominids. Other authorities disagree with this analysis and some question whether this form is even a hominid. At this point, there is too little information to do more than mention these two new finds of hominids. As new data come in, however, a major part of our story could change.

Fossils of Ardipithecus ramidus (4.4 million years ago) were different enough from any found previously to warrant creating a new hominid genus. Although the evidence from the foramen magnum indicates that they were bipedal, conclusive evidence from legs, pelvis and feet remain somewhat enigmatic. There might be some consensus that A. ramidus represent a side branch of the hominid family.

Between 4 million and 2 million years ago

Australopithecus anamensis (4.2-3.8 million years ago) exhibit mixture of primitive (large canine teeth, parallel tooth rows) and derived (vertical root of canine, thicker tooth enamel) features, with evidence of bipedalism. There appears to be some consensus that this may represent the ancestor of all later hominids.

The next species is well established and its nature is generally agreed upon: Australopithecus afarensis (4-3 million years ago). There is no doubt that A. afarensis were bipeds. This form seems to still remain our best candidate for the species that gave rise to subsequent hominids.

At the same time lived a second species of hominid in Chad: Australopithecus bahrelghazali (3.5-3 million years ago). It suggests that early hominids were more widely spread on the African continent than previously thought. Yet full acceptance of this classification and the implications of the fossil await further study.

Another fossil species contemporaneous with A. afarensis existed in East Africa: Kenyanthropus platyops (3.5 million years ago). The fossils show a combination of features unlike that of any other forms: brain size, dentition, details of nasal region resemble genus Australopithecus; flat face, cheek area, brow ridges resemble later hominids. This set of traits led its discoverers to give it not only a new species name but a new genus name as well. Some authorities have suggested that this new form may be a better common ancestor for Homo than A. afarensis. More evidence and more examples with the same set of features, however, are needed to even establish that these fossils do represent a whole new taxonomy.

Little changed from A. afarensis to the next species: A. africanus: same body size and shape, and same brain size. There are a few differences, however: canine teeth are smaller, no gap in tooth row, tooth row more rounded (more human-like).

We may consider A. africanus as a continuation of A. afarensis, more widely distributed in southern and possibly eastern Africa and showing some evolutionary changes. It should be noted that this interpretation is not agreed upon by all investigators and remains hypothetical.

Fossils found at Bouri in Ethiopia led investigators to designate a new species: A. garhi (2.5 million years ago). Intriguing mixture of features: several features of teeth resemble early Homo; whereas molars are unusually larger, even larger than the southern African robust australopithecines.

The evolutionary relationship of A. garhi to other hominids is still a matter of debate. Its discoverers feel it is descended from A. afarensis and is a direct ancestor to Homo. Other disagree. Clearly, more evidence is needed to interpret these specimens more precisely, but they do show the extent of variation among hominids during this period.

Two distinctly different types of hominid appear between 2 and 3 million years ago: robust australopithecines (Paranthropus) and early Homo (Homo habilis).

The first type retains the chimpanzee-sized brains and small bodies of Australopithecus, but has evolved a notable robusticity in the areas of the skull involved with chewing: this is the group of robust australopithecines (A. boisei, A. robustus, A. aethiopicus).

• The Australopithecines diet seems to have consisted for the most part of plant foods, although A. afarensis, A. africanus and A. garhi may have consumed limited amounts of animal protein as well;
• Later Australopithecines (A. boisei and robustus) evolved into more specialized "grinding machines" as their jaws became markedly larger, while their brain size did not.

The second new hominid genus that appeared about 2.5 million years ago is the one to which modern humans belong, Homo.

• A Consideration of brain size relative to body size clearly indicates that Homo habilis had undergone enlargement of the brain far in excess of values predicted on the basis of body size alone. This means that there was a marked advance in information-processing capacity over that of Australopithecines;
• Although H. habilis had teeth that are large by modern standard, they are smaller in relation to the size of the skull than those of Australopithecines. Major brain-size increase and tooth-size reduction are important trends in the evolution of the genus Homo, but not of Australopithecines;
• From the standpoint of anatomy alone, it has long been recognized that either A. afarensis or A. africanus constitute a good ancestor for the genus Homo, and it now seems clear that the body of Homo habilis had changed little from that of either species. Precisely which of the two species gave rise to H. habilis is vigorously debated. Whether H. habilis is descended from A. afarensis, A. africanus, both of them, or neither of them, is still a matter of debate. It is also possible that none of the known australopithecines is our ancestor. The discoveries of Sahelanthropus tchadensis, Orrorin tugenensis, and A. anamensis are so recent that it is hard to say what effect they will have on current theories.

What might have caused the branching that founded the new forms of robust australopithecines (Paranthropus) and Homo? What caused the extinction, around the same time (between 2-3 million years ago) of genus Australopithecus? Finally, what might have caused the extinction of Paranthropus about 1 million years ago?

No certainty in answering these questions. But the environmental conditions at the time might hold some clues. Increased environmental variability, starting about 6 million years ago and continuing through time and resulting in a series of newly emerging and diverse habitats, may have initially promoted different adaptations among hominid populations, as seen in the branching that gave rise to the robust hominids and to Homo.

And if the degree of the environmental fluctuations continued to increase, this may have put such pressure on the hominid adaptive responses that those groups less able to cope eventually became extinct. Unable to survive well enough to perpetuate themselves in the face of decreasing resources (e.g., Paranthropus, who were specialized vegetarians) these now-extinct hominids were possibly out-competed for space and resources by the better adapted hominids, a phenomenon known as competitive exclusion.

In this case, only the adaptive response that included an increase in brain size, with its concomitant increase in ability to understand and manipulate the environment, proved successful in the long run.

Hominoid, Hominid, Human

The traditional view has been to recognize three families of hominoid: the Hylobatidae (Asian lesser apes: gibbons and siamangs), the Pongidae, and the Hominidae.

• The Pongidae include the African great apes, including gorillas, chimpanzees and the Asian orangutans;
• The Hominidae include living humans and typically fossil apes that possess a suite of characteristics such as bipedalism, reduced canine size, and increasing brain size (e.g., australopithecines).

The emergence of hominoids

Hominoids are Late Miocene (15-5 million years ago) primates that share a small number of postcranial features with living apes and humans:

• no tail;
• pelvis lacks bony expansion;
• elbow similar to that of modern apes;
• somewhat larger brains in relationship to body size than similarly sized monkeys.

When is a hominoid also a hominid?

When we say that Sahelanthropus tchadensis is the earliest hominid, we mean that it is the oldest fossil that is classified with humans in the family Hominidae. The rationale for including Sahelanthropus tchadensis in the Hominidae is based on similarities in shared derived characters that distinguish humans from other living primates.

There are three categories of traits that separate hominids from contemporary apes:

• bipedalism;
• much larger brain in relation to body size;
• dentition and musculature.

To be classified as a hominid, a Late Miocene primate (hominoid) must display at least some of these characteristics. Sahelanthropus tchadensis is bipedal, and shares many dental features with modern humans. However, the brain of Sahelanthropus tchadensis was no bigger than that of contemporary chimpanzees. As a consequence, this fossil is included in the same family (Hominidae) as modern humans, but not in the same genus.

Traits defining early Homo

Early Homo (e.g., Homo habilis) is distinctly different from any of the earliest hominids, including the australopithecines, and similar to us in the following ways:

• brain size is substantially bigger than that of any of the earliest hominids, including the australopithecines;
• teeth are smaller, enamel thinner, and the dental arcade is more parabolic than is found in the earliest hominids, including the australopithecines;
• skulls are more rounded; the face is smaller and protrudes less, and the jaw muscles are reduced compared with earliest hominids, including the australopithecines.