Bipedal locomotion may have been an adaptation to living in a mixed woodland
and grassland environment. It has been suggested that bipedalism was
selected for because it made it easier to see long distances when moving
over areas covered with tall grasses. This would have been a useful
advantage in scavenging for food and watching for big cats and other
predators in open environments. An upright posture also potentially
helps to dissipate excess body heat and reduces the absorption of heat from
the sun because less skin has a direct exposure to ultra violet radiation
during the hottest times of the day. There is evidence suggesting that
bipedal animals usually can walk greater distances because less energy is
expended with their longer strides. This would be useful for
scavenging for food throughout vast areas. However, the legs of
bipedal animals need to be sturdy enough to support at least 2.5
times their body weight while running. Over many generations, early
hominin legs grew longer and much stronger than their arms. Their
feet became longer and developed arches for more efficient support of their bodies. In
addition, their hands became more adept at carrying and manipulating objects
such as tools and food. It also made it easier to hold babies and to
tend to their needs. These adaptations to walking bipedally on the
ground made it progressively more difficult to climb and travel through the
canopies of trees. However, they obviously provided many other natural
selection advantages.
Walking Tall--a comparison between human and chimpanzee skeletons
This link takes you to a
video at an external website. To return here, you must
click the "back" button on your
browser program. (length
= 56 secs) |
Baby Steps: Learning to Walk, The Hominid Way--the evolution of
bipedalism
among our hominid ancestors. This link takes you to an
audio file at an
external website. To return here, you must click the "back" button on your
browser program. (length
= 7 mins, 46 secs) |
Chimpanzees and all of
the other apes have longer arms than legs and lack arches on the bottoms of
their feet. In addition, their big toes are divergent from their other
toes much like human thumbs.
While the late
australopithecines were similar to humans anatomically below the neck, their heads were significantly
different from ours in several key features. Their adult brain size was about 1/3 that of
people today. As a result,
the widest part of the skull of these early hominins was below the brain case. For modern
humans, it usually is in the temple region. Early hominin faces were large relative to
the size of their brain cases. They had comparatively big molar teeth
with thick enamel. By comparison, their front teeth were small.
They had large jaws, and powerful jaw muscles. The size and shape of these muscles is indicated by flaring
zygomatic
arches
,
or cheek bones, behind which the major jaw muscles pass and the presence of a
sagittal crest
,
which is a jaw muscle attachment ridge of bone on top of the skull in the robust species
(paranthropoids). In
modern humans, the jaw muscles are much smaller and attach onto the skull in
the temple region. From the side view, early hominin faces were
concave or dish-shaped and projecting forward at the bottom due to their
relatively small brain cases and huge teeth and
jaws. In contrast, our teeth and jaws are relatively small, and our faces are nearly vertical.
 |
|
Paranthropus boisei
|
Modern human |
NOTE: When the lower portion of the face
markedly projects forward (as in the case of the early hominins), it is
known as prognathism.
Australopithecine and other early hominin
fossils have been found only in Africa. The majority of them were
discovered in East and South Africa. However, some also were found
in Chad, which is located in North Central Africa. Current evidence
indicates that there were as many as 12 species of early hominins between 6
and 1.5 million
years ago, but they did not all live at the same time. The following
species are the most widely accepted ones:
| 1. |
Australopithecus anamensis |
| 2. |
Australopithecus afarensis |
| 3. |
Australopithecus africanus |
| 4. |
Paranthropus
aethiopicus (or
Australopithecus
aethiopicus) |
| 5. |
Paranthropus
boisei
(or Australopithecus boisei) |
| 6. |
Paranthropus
robustus
(or Australopithecus
robustus) |
The fossil record of early hominins is being added to
by new important discoveries almost every year. As a result, it is not
yet clear how many species of them actually existed nor is it
certain what their evolutionary relationship was to each other. However,
the broad outlines of this complex evolutionary history are already known and
are summarized here. To see a more complete listing of proposed species
of early hominins and their immediate ancestors, select the button below.
It would be helpful to have a printout of this table in order to understand
the discussion of the early hominins that follows.
Australopithecine
Species
Australopithecus
anamensis may have
been the earliest australopithecine species. They lived about 4.2-3.9
million years ago in East Africa. Unfortunately, little is known about
them due to the scarcity of their fossils and the fact that the ones that
have been found are highly fragmentary. This species apparently was
descended from
Ardipithecus
ramidus, which lived
around 4.4 million years ago, or an even earlier ape/hominin transitional species near the beginning of the Pliocene Epoch.
Anamensis was bipedal but may still have been an efficient
tree climber. The shapes of the arm and leg bones indicate that it was
bipedal. The canine teeth are relatively large compared to later
australopithecines and humans. The alignment of teeth in the jaw is
somewhat rectangular, reminiscent of apes, rather than like the modern human
parabolic dental arch (like the McDonald's golden arches sign).
Anamensis remains have been found in what had been woodlands around
lakes. Their diets were apparently mainly vegetarian with an emphasis
on fruits and nuts.
Australopithecus
afarensis lived about
3.7-3.0
million years ago in East Africa. Skeletally, they
were still somewhat transitional from earlier ape species. This can be seen in their
legs which were relatively shorter than those of the later australopithecines
and humans.
Afarensis also had slender curved fingers reminiscent of chimpanzees.
Because of these anatomical characteristics, it has been suggested that
they were less efficient bipeds and more efficient tree climbers than the
later australopithecines. Afarensis
canine teeth were
relatively large and pointed, reminiscent of apes. They projected
somewhat beyond their other teeth but not as much as in chimpanzees.
Some of the male afarensis had small sagittal
crests.
|
 |
|
 |
|
Australopithecus afarensis
(Lucy) |
|
Australopithecus afarensis
(reconstructed appearance) |
|
|
|
 |
|
|
Kenyanthropus
platyops
(reconstructed appearance) |
|
Tim White
and some other paleoanthropologists believe that there was considerable
physical variation within the species Australopithecus afarensis.
They suggest that the recently discovered fossils classified as
Kenyanthropus platyops
(3.5-3.2 million years ago)
was a variant form of afarensis but with somewhat smaller teeth.
White discounts the flattened face of platyops as being due to the
deformation of the bones by ground pressure after death. Its
discoverer, Meave Leakey,
disagrees. She believes that platyops was a separate species
and that it was more likely to have been the progenitor of humans.
Additional hominin fossils from the
crucial time period of 4-3 million years ago must be discovered to
conclusively determine the place of platyops in our evolution.
|
|
 |
| Australopithecus africanus |
Australopithecus africanus lived about 3.3-2.5 million years ago in South
and East Africa.
Skeletally,
they were less ape-like than earlier species of australopithecines but were still usually
small and light in frame like afarensis. However, the teeth of africanus
were in some ways more like humans than like afarensis.
Specifically, the front teeth of africanus were relatively large like ours and
their canine teeth did not project beyond the others. Microscopic wear patterns on
africanus
teeth suggest a diet consisting of relatively soft foods, which very likely
included some meat along with plants. This does not necessarily imply
efficient hunting skills. More likely, they obtained
meat by scavenging what remained on the abandoned corpses of large animals killed by lions
and other predators. It is possible that they also did some hunting of
small animals in much the same inefficient manner of chimpanzees today.
They probably ate insects and eggs as well.
The classification of
Australopithecus garhi
is still very problematical. This Ethiopian fossil has been dated to
2.5 million years ago, which makes it contemporaneous with late africanus.
Largely for that reason, some paleoanthropologists have suggested that
garhi is a variant of africanus. However, several features
of the head of garhi look more like a holdover from the older
afarensis species. On the other hand, the relative lengths of the
arms and legs of garhi are more reminiscent of the first humans.
The discovery of butchered animal bones with garhi suggests that
their diet included at least some meat, as was the case with africanus.
Paranthropoid
Species
The
australopithecines have been referred to collectively as
gracile
species
(literally "gracefully slender") of early hominins. Most of them were relatively small, slender, and delicate boned compared to
the somewhat more muscular, robust
species (paranthropoids) that mostly came later. However, this is not always
a reliable descriptive distinction because the range of variation in
physical appearance of the two groups of species overlaps.
Subsequently, some individual graciles were bigger than some of the robust
ones. However, the robust species shared some characteristics of their
heads that dramatically show that they had diverged from the evolutionary
line that would become humans. They had larger faces and jaws accompanied by pronounced sagittal
crests (in the case of males). They also had much larger back teeth
(premolars and molars) and
smaller front ones (incisors) compared to gracile australopithecines and
early humans who were alive at the same time.
 |
|
 |
 |
|
Australopithecus
(gracile body) |
Paranthropus
(robust body) |
|
Paranthropus
teeth
(upper) |
human teeth (lower) |
Little is known about
Paranthropus
aethiopicus
(the "black skull") other than it apparently was one of
the earliest robust species--it lived about 2.5 million years ago. So far,
this species has been found only in East Africa. Since it had a smaller brain than
the other robust species and it was early, aethiopicus is thought to
be a transitional form from one of the gracile species that came before. It had an
unusually large sagittal
crest (shown below).
Paranthropus robustus
was a South African robust species that lived about 2.0-1.4
million years ago. They had strong jaws and very large molar and
premolar teeth with thick enamel. Males also had pronounced sagittal
crests, though not as large as the species listed next.
Paranthropus
boisei was a super-robust East African species that lived about 2.0-1.4
million years ago. They tended to be more massive and beefy-looking even than
Paranthropus robustus. Male boisei were
especially muscular. Like their South African cousins, robustus, they
had prominent sagittal crests and very large grinding teeth with thick enamel.
These
teeth would have been capable of cracking hard nuts and dry seeds.
However, such food items may not have been important in their diet.
Microscopic analysis of dental wear patterns and carbon isotope analysis of
teeth indicate that what boisei predominantly
ate was soft foods such as grasses, leaves, roots, and possibly even meat.
 |
|
 |
|
 |
|
Paranthropus aethiopicus |
Paranthropus
robustus |
Paranthropus boisei |
Early Hominin Body Size
The early
hominins were significantly smaller on average than modern humans.
Adult male australopithecines were usually only about 4.3-4.9 feet tall and
weighed around 88-108 pounds. Females
were much smaller and less muscular. They were usually 3.4-4.1 feet tall
and weighed only 64-75 pounds. This is greater
sexual dimorphism
than is found in
human populations today. In some australopithecine species, sexual
dimorphism may have been nearly as great as among the great apes.
Female gorillas weigh about 61% that of males, while modern human females are about 83% the weight of
males.
|
|
AVERAGE WEIGHT |
AVERAGE STATURE |
|
|
|
|
SPECIES |
males
|
females
|
females as
% of males |
males
|
females
|
females as
% of males |
|
|
|
|
|
|
|
|
Australopithecus afarensis |
92 lbs
(42 kg) |
64 lbs
(29 kg) |
64% |
4 ft 11 in
(151 cm) |
3 ft 5 in
(105 cm) |
70% |
|
Australopithecus africanus |
90 lbs
(41 kg) |
66 lbs
(30 kg) |
73% |
4 ft 6 in
(138 cm) |
3 ft 9 in
(115 cm) |
83% |
|
Paranthropus robustus |
119 lbs
(54 kg) |
88 lbs
(40 kg) |
74% |
3 ft 9 in
(114 cm) |
3 ft 3 in
(99 cm) |
87% |
|
Paranthropus boisei |
108 lbs
(49 kg) |
75 lbs
(34 kg) |
69% |
5 ft 4 in
(137 cm) |
4 ft 1 in
(124 cm) |
91% |
earliest humans
(Homo habilis) |
114 lbs
(52 kg) |
70 lbs
(32 kg) |
61% |
5 ft 2 in
(157 cm) |
4 ft 1 in
(125 cm) |
79% |
modern humans
1
(Homo sapiens) |
144 lbs
(65 kg) |
119 lbs
(54 kg) |
83% |
5 ft 9 in
(175 cm) |
5 ft 3 in
(161 cm) |
92% |
|
1 The relatively low weight and
height of modern humans shown here is a rough average of all people
around the globe. Some populations are significantly
bigger (e.g., many Europeans and Africans).
(Source: H. M. McHenry, "How
Big Were Early Hominids?", Evolutionary Anthropology 1
[1992] p. 18;
and
"What Does It Mean To
Be Human? Human Evolution Evidence" [2010}, Smithsonian
National
Museum of Natural History) |
Possible Evolutionary
Links
There
has been a gap in the fossil hominin record for the crucial period before 4.2
million years ago when Australopithecus anamensis appeared. New
discoveries are now beginning to fill in the missing picture of evolution
leading to the australopithecines at that early time.
Beginning
in 1992, Tim White and several of his Ethiopian colleagues found fossils of
what may be the immediate ancestor of the australopithecines at the Aramis
site in the Middle Awash region of Northern Ethiopia. The teeth of these
very early fossils seem to have been transitional between apes and
Australopithecus anamensis. Among the living apes, they were most
similar to chimpanzees, however, they were not apes as we usually think of
them today. These Aramis fossils date to about 4.4 million years ago and may represent the first stage
in the evolution of bipedalism. Because of their primitiveness, White
has given them a new genus and species designation (Ardipithecus ramidus
,
nicknamed "Ardi")
rather than include them with australopithecines.
Based on the time frame, body shape, and dentition
similarities, it is reasonable to conclude that some of the early hominin species were
ancestors of our genus Homo. Most likely, some of the
australopithecines
(shown as red in the diagram below) were in our line of evolution, but the later
paranthropoids (blue
below) were
not. The first humans (Homo habilis
)
were contemporaries of the paranthropoids. As a result, they could not be our
ancestors. However, it is likely that Australopithecus afarensis
and Australopithecus africanus were in our evolutionary line.
Australopithecus
garhi and/or Australopitheus sediba
may also have been our
ancestors, though more evidence is needed to settle this question.
We have not yet been
able to extract DNA from the bones of any australopithecine for comparison
with modern human DNA. When we can do this, it is almost certain that
we will discover many of their genes still in us today.
Beginning
around 2.5
million years ago or a bit earlier, there was a major forking in the
evolutionary path of hominins. The australopithecines diverged into at
least two very different evolutionary directions. One led to the
paranthropoids and a genetic dead-end by about 1.4
million years ago. The other led to the first humans. It is
likely that these diverging evolutionary paths were the result of exploiting
different environmental opportunities. Coinciding with this hominin
divergence was a shift in the global climate to progressively cooler
conditions and frequently fluctuating environments. In
East and South Africa, where most of the early hominins apparently lived,
dry grasslands expanded at the expense of woodlands and forests. It
has been suggested that the adaptive radiation that led to humans and
paranthropoids is connected with this change in the environment.
The early human line succeeded by learning how to exploit new kinds of
habitats for food. It is likely that climate instability selected for
their adaptability.
Becoming Human: Part 1----NOVA episode about the
complex evolutionary picture of early
hominins
that is emerging from the fossil record. This
link takes you to a video at an external website. To return
here, you must click the "back"
button on your browser program. (length = 51 mins,
57 secs) |
NEWS:
John Novembre et.al.
reported in the October 1, 2007 issue of Nature Genetics that human
saliva has significantly more of the enzyme amylase compared to chimpanzees.
Amylase breaks down starches into glucose which can be readily used by the
cells of the body. With more amylase, humans get more useable calories
from starchy vegetable foods such as tubers, corms, and bulbs. The
authors suggest that this would have been a distinct advantage for early
humans because these foods are readily available. They believe that
natural selection favored additional copies of the gene responsible for
amylase production (AMY1) in our early hominin ancestors but not in apes.
