# Introduction to Paleoanthropology/Hominids Early 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.

