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. 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 since less surface area has a direct exposure. There
is evidence suggesting that bipedal
animals usually can walk and run 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 became longer
and much stronger than their arms. Their feet 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.
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) |
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 teeth with thick
enamel, large jaws, and
powerful jaw muscles. The size and shape of these muscles is indicated by flaring
zygomatic
arches
and the presence of a
sagittal crest
,
which was a jaw muscle attachment area in the robust species. 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 large teeth and
jaws. In contrast, our jaws are relatively small and our faces are nearly vertical.
 |
|
|
Australopithecus boisei
|
Modern human |
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
recently 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. |
Australopithecus
aethiopicus (or
Paranthropus
aethiopicus) |
| 5. |
Australopithecus
boisei
(or Paranthropus boisei) |
| 6. |
Australopithecus
robustus
(or Paranthropus
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.
Early 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 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).
|
|
 |
Australopithecus afarensis
(Lucy) |
|
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.
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. 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 possibly 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 meat along with plants. This does not necessarily imply
efficient hunting skills. More likely, they obtained much of their
meat by scavenging what remained on the corpses of 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.
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.
Late Australopithecine
Species
The
early australopithecines have been referred to collectively as
gracile
species
(literally "gracefully slender"). Most of them were relatively small, slender, and delicate boned compared to
the somewhat more muscular, robust
species of
australopithecines 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. The late australopithecines, which were
all robust species, had larger jaws accompanied by pronounced sagittal
crests in the case of males. They also had much larger back teeth and
smaller front ones compared to those of the early humans who were present at
that time.
 |
|
 |
|
gracile form |
robust form |
|
robust australopithecine teeth
|
Little is known about
Australopithecus 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).
Australopithecus robustus was a South African robust species that lived
about 2.0-1.4 million years ago. Their teeth appear to have
been adapted to eating tough, fibrous vegetable foods. This is indicated by
their strong jaws and very
large molar and premolar teeth with thick enamel. Microscopic wear patterns on their
teeth indicate that they regularly ate foods such as hard nuts and seeds.
Males also had pronounced sagittal crests, though not as large as the next
species listed here.
Australopithecus
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
Australopithecus robustus. While they were only a few
inches taller, they averaged 20 pounds heavier. Male boisei were
especially muscular. Like their South African cousins, robustus, they
had prominent sagittal crests and very large grinding teeth with thick enamel. This suggests a similar
diet of hard to chew vegetable foods.
 |
|
 |
|
 |
Australopithecus aethiopicus
(the "black skull") |
Australopithecus robustus |
Australopithecus boisei
(formerly known as
Zinjanthropus) |
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. Apparently, 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.
|
|
BODY
WEIGHT |
STATURE |
|
|
|
|
SPECIES |
males
|
females
|
females as
% of males |
males
|
females
|
females as
% of males |
|
|
|
|
|
|
|
|
Australopithecus afarensis 1 |
99 lbs
(45 kg) |
64 lbs
(29 kg) |
64% |
59 in
(151 cm) |
41 in
(105 cm) |
70% |
|
Australopithecus africanus |
90 lbs
(41 kg) |
66 lbs
(30 kg) |
73% |
54 in
(138 cm) |
45 in
(115 cm) |
83% |
|
Australopithecus robustus |
88 lbs
(40 kg) |
70 lbs
(32 kg) |
80% |
52 in
(132 cm) |
43 in
(110 cm) |
83% |
|
Australopithecus boisei |
108 lbs
(49 kg) |
75 lbs
(34 kg) |
69% |
54 in
(137 cm) |
49 in
(124 cm) |
91% |
earliest humans
(Homo habilis) |
114 lbs
(52 kg) |
70 lbs
(32 kg) |
61% |
62 in
(157 cm) |
49 in
(125 cm) |
79% |
modern humans
2
(Homo sapiens) |
144 lbs
(65 kg) |
119 lbs
(54 kg) |
83% |
69 in
(175 cm) |
63 in
(161 cm) |
92% |
|
Source: H. M. McHenry, "How
Big Were Early Hominids?", Evolutionary Anthropology 1
[1992] p. 18.
1 Afarensis
may have been somewhat less sexually dimorphic than indicated by McHenry's data
presented here (Phillip Reno et al., "Sexual Dimorphism in
Australopithecus afarensis was Similar
to that of
Modern Humans, Proceedings of the National Academy of Sciences
[2003] pp. 9404-9409).
2 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., Europeans and Africans). |
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
)
rather than include them with australopithecines.
Based on 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 early australopithecines
(shown in red below) were in our line of evolution, but the later robust ones (blue
below) were
not. The first humans (Homo habilis
)
were contemporaries of the late australopithecines. As a result, they could not be our
ancestors. However, it is likely that Australopithecus afarensis
and possibly even early Australopithecus africanus, Australopithecus garhi,
or Kenyanthropus platyops
were in our
evolutionary line.
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 robust australopithecines 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 cooler conditions. 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 robust australopithecines is connected with this change in the
environment.
Search for the First Human--Donald Johanson talks 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 = 2 mins, 10 secs)
Note:
the
Orrorin
referred to in the video is
Orrorin tugenensis,
a possible
ancestor of the australopithecines
that lived about 6 million years ago.
|
NOTE:
This
is not the only possible model of early hominin evolutionary links that has
been suggested in recent years. Some paleoanthropologists have proposed that
neither africanus nor afarensis were ancestral to
the robust australopithecines.
In other words, the division between robust and gracile forms occurred earlier, perhaps at
the time of anamensis or before. It is also possible that humans descended from
anamensis
through a still unknown intermediate gracile species instead of afarensis.
On-going research will very likely sort out the relationships between the various hominin
species in the near future.
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 in our early hominin ancestors but not in apes.
