The emergence of humans
The narratives of human
evolution
are oft-told and highly contentious. There are major disagreements in
the field about whether human evolution is more like a branching tree or
a crooked stick, depending partly on how many
species
one recognizes. Interpretations of almost every new find will be sure
to find opposition among other experts. Disputes often center on diet
and
habitat,
and whether a given animal could walk bipedally or was fully upright.
What can we really tell about human evolution from our current
understanding of the phylogenetic relations of
hominids and the sequence of evolution of their traits?
To begin with, let's take a step back. Although the evolution of
hominid features is sometimes put in the framework of "apes vs. humans,"
the fact is that humans
are apes, just as they are primates and
mammals. A glance at the evogram shows why. The other apes — chimp,
bonobo, gorilla, orangutan, gibbon — would not form a natural,
monophyletic group (i.e., a group that includes
all the descendants of a
common ancestor)
— if humans were excluded. Humans share many traits with other apes,
and those other "apes" (i.e., non-human apes) don't have unique features
that set them apart from humans. Humans have some features that are
uniquely our own, but so do gorillas, chimps, and the rest. Hominid
evolution should not be read as a march to human-ness (even if it often
appears that way from narratives of human evolution). Students should
be aware that there is not a dichotomy between humans and apes. Humans
are a kind of ape.
Virtually all systematists and taxonomists agree that we should only
give names to monophyletic groups. However, this evogram shows that
this guideline is not always followed. For an example, consider
Australopithecus. On the evogram you can see a series of forms, from just after
Ardipithecus to just before
Homo in the branching order, that are all called
Australopithecus. (Even
Paranthropus is often considered an australopithecine.) But as these
taxa appear on the evogram,
"Australopithecus" is not a natural group, because it is not monophyletic: some forms, such as
A. africanus, are found to be closer to humans than
A. afarensis and others. Beyond
afarensis, for example, all other
Australopithecus and
Homo
share "enlarged cheek teeth and jaws," because they have a more recent
common ancestor. Eventually, several of these forms will have to have
new
genus
names if we want to name only monophyletic groups. Students should
avoid thinking of "australopithecines" as a natural group with uniquely
evolved traits that link its members together and set it apart from
Homo. Instead they should focus on the pattern of shared traits among these species and the
Homo clade, recognizing that each species in this
lineage gains more and more features that are shared by
Homo.
In popular fiction and movies, the concept of the wild "ape-man" is
often that of a tree-living, vine-swinging throwback like Tarzan.
However, the pantheon of hominids is much richer than this, as the
evogram shows with forms as different as
Paranthropus and
Ardipithecus
shows. For example, imagine going back in time to the common ancestor
of humans and chimps (including bonobos). What did that common ancestor
look like? In the
Origin of Species Darwin noted that the
extinct common ancestor of two living forms should not be expected to
look like a perfect intermediate between them. Rather, it could look
more like one branch or the other branch, or something else entirely.
Did the common ancestor of humans and chimps conform to the ape-man
myth and live in the trees, swinging from vines? To answer this, we
have to focus not only on anatomy but on behavior, and we have to do it
in a phylogenetic context. Apes such as the gibbon and orangutan, which
are more distantly related to humans, are largely arboreal (i.e.,
tree-living). The more closely related apes such as the gorilla and
chimps are relatively terrestrial, although they can still climb trees.
The feet of the first hominids have a considerable opposition of the
big toe to the others but relatively flat feet, as arboreal apes
generally do. But other features of their skeleton, such as the
position of the foramen magnum underneath the skull, the vertically
shortened and laterally flaring hips, and the larger head of the femur,
suggest that they were not just mainly terrestrial but habitually
bipedal, unlike their knuckle-walking relatives. Most evidence suggests
that the hominid lineage retained some of the anatomical features
related to arboreal life and quadrupedal gait even after it had evolved a
more terrestrial lifestyle and a bipedal gait. There is no
fossil record of these behaviors, but the balance of the available evidence supports the
hypothesis that the hominid ancestor was terrestrial and bipedal.
Much discussion in human paleontology surrounds the evolution of a
bipedal, upright stance. When and why did this occur? One thing to
keep in mind is that "bipedal" and "upright" are not equivalent terms.
An animal can be bipedal without having a vertical backbone (think
T. rex).
It seems clear from the fossil record of hominids that habitual
bipedality preceded the evolution of a recurved spine and upright
stance. Other changes in the gait, such as how the relatively "splayed"
gait of chimps evolved into the gait of humans, who put one foot
directly in front of the other, involve studying the hip joint, the
femur, and the foot. The famous Laetoli footprints attributed to
Australopithecus afarensis are bipedal, but they are still relatively splayed compared to the tracks of living humans.
Another extremely interesting feature in hominid evolution is the
degree of sexual dimorphism (i.e., physical differences between the
sexes) in different species. Sexual dimorphism is linked to features of
sociality and mate competition in many sorts of animals. To understand
the evolution of this feature in humans, which have relatively low
sexual dimorphism, we need to consider the other apes, in which sexual
dimorphism tends to be moderate to high (with exceptions). We don't
have sufficient evidence about
Sahelanthropus,
Orrorin, and
Ardipithecus to understand much about sex differences in these species, but we do know that
A. afarensis
had relatively high sexual dimorphism: the males were considerably
larger than the females. The difference seems to have been less in
A. africanus,
Paranthropus, and most of the
Homo lineage. The evolutionary explanation for
A. afarensis'
dimorphism is not entirely clear. The larger males may have used their
size to attract females and/or repel rivals, which would fit with an
explanation based on sexual selection. Or the males and females may
have been differently sized because they played different roles in their
groups, the males hunting and gathering and the females caring for the
young. Darwin thought that this differentiation of the sexes may have
played a critical role in human evolution, but we simply do not know
much about the role of this feature in
A. afarensis. Some, all, or none of these functions may have been in play.
We do know that by the time the animals known as
Homo evolved,
they could make tools, and their hands were well suited for complex
manipulations. These features were eventually accompanied by the
reduction of the lower face, particularly the jaws and teeth, the
recession of the brow, the enlargement of the brain, the evolution of a
more erect posture, and the evolution of a limb more
adapted
for extended walking and running (along with the loss of arboreally
oriented features). The evogram shows the hypothesized order of
acquisition of these traits. Yet each of the
Homo species was
unique in its own way, so human evolution should not be seen as a simple
linear progression of improvement toward our own present-day form.
