Explore the basic information on locations, age, cranium before returning
to this page to look at the general trends. This trends segment covers a
variety of topics and is sort of an overview of hominid evolution up to
and including early Homo and Homo
erectus.
Perspective of Environmental Changes between 3.0 and 2.0 million years ago |
The development of bipedal walking was relatively well developed by the
time of A. afarensis about 3.75 million years ago. Bipedality became
most efficient in Homo Erectus by 1.6 million years ago.
With development of bipedal walking, A. afarensis had a relatively modern foot and pelvis, but the arms remaind long and the legs short. The characteristics of the hand and foot suggest that A. afarensis may have been adapted to an arboreal lifeway, but had the ability to walk upright when on the ground. Bipedal walking is more efficient that knucklewalking or quadrupedal movement in terms of the amount energy it requires to move around. It also enables hominids to move greater distances for less energy. Therefore, while a number of significant physical changes would have been required to become bipedal and these early hominids may have been somewhat less efficient in walking than later ones, there would have been advantages to this new form of positional behavior. One major advantage would have been the freedom of the hands from locomotion. The hands could therefore be used in other ways. For example, the hands could be used to make and use tools as well as for carrying objects. The bipedal posture may have improved the ability of A. afarensis to throw, dig, club and carry objects such as food , tools or children. Further advantages may have arisen as a result of the social dimension of food sharing if Owen Lovejoy is correct. The formation of a cooperative group would have had dramatic implications for these early hominids in terms of their cultural and physcial development.
The trend for short legs relative to arm
length continued until H. erectus. According to Alan Walker, H.
erectus with its relatively longer legs, had perfected the mechanics
of bipedality . The initial focus on increasing the efficiency of bipedality
can be seen in the discovery of a 90 percent complete skeleton on the west
side of Lake Turkana (next image). This skeleton tells us that H. erectus
was much taller than previous or contemporary hominids. 
Conservatively, this 12 year boy would
have stood about six feet in height had he reached adulthood. The mechanics
of the lower back, pelvis, femur head, and femur may have been better than
ours is today.
The increased length of the legs would
have improved the efficiency of bipedal walking. Increased distances could
be covered for the same calorie output.
Except for minor differences, the postcranial
skeleton of Homo erectus is nearly identical to ours. Evidence does suggest
that the mechanics of H. erectus bipedality were better than ours
today.
It should also be noted that the wall of a H. erectus femur was about
twice as thick as that of modern Homo sapiens. This would have added
strength of running over the savanna.
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This H. erectus skeleton is 1.6
million years old. Richard Leakey challenges anyone to deny this is a "human"
skeleton. Note the long legs and modern pelvis. The mechanics for bipedality
may have been better at this time than they are today. Alan Walker believes
that this young boy would have stood as tall as 6 foot had he matured. Walker
also believes the long head of the femur and the articulation with the pelvis
meant that he was more efficient at walking and running than we are today.
The cranial capacity of this 12 year old boy was about half ours. The 800
cc capacity is typical of H. erectus at this early stage of development.
He may have had a skeleton similar to ours, but his facial appearance and
brain capacity was different. 
There are two basic directions in the evolution
of hominid dentition. The first is for larger back teeth and is associated
with A. robustus and A. boisei who specialized their diet
towards vegetation. Coupled with the large molars, massive muscular structures
to aid chewing developed in these early hominids. These hominids shared
the ability for bipedal walking. The reasons for the development of bipedalism
in these early hominids would have been the same as for later hominids.
They shared the freedom of the hand from locomotion, as well as the ability
of the trunk to be controlled on the hindlimbs during bipedal postures.
Therefore, their hands could be used in many other activities besides walking.
The other direction in the evolution of the dentition of hominids was for
smaller teeth and a parabolic shape to the palate. The trend for smaller
teeth was one which began with early Homo and continued through to
modern Homo sapiens. Despite a reduction in the size of dentition
for H. erectus, their teeth were still larger than ours. The decrease
in the size of the teeth is thought to be related of a diet that included
a wide variety of foods. There was less emphasis on plant foods that required
heavy chewing.
Every animal has its niche, a space that it fills and a role it plays in
an ecosystem. It is likely that early Homo straddled the carnivore
and herbivore niches. Niches can shift over time as various environmental
factors change. Hominids may have been forced shift from time to time. Their
diurnal, mobile, mixed ground-tree scavenging niche may have been well suited
through flexibility. Dry season scavenging and wet season foraging may have
created a highly flexible adaptive niche for early hominids. This probably
provided early Homo with a distinct advantage as different types
of hominids competed with each other for survival.
A. boisei represents
a specialized direction for hominids. The massive face, large premolars
and molars, and sagittal crest each mark this specialized form. This "human
cuisinart" was a hominid for the same reasons that we are. Yet, the
specialized adaptation for chewing may have led to extinction. The role
of competition with H. erectus can be explored as a reason for the
failure of this specialized hominid.
The east African and south African robust australopithecines probably shared
similar habitats and behavior. During the period that the robusts existed,
spanning from at least 2.5 m.y.a. to around 1.0 m. y. a., the climate changed
regularly with fluctuating cold and warm spells on a global scale. After
a million years ago, temperatures dropped steadily. This sharp cooling trend
may have impact the robusts' ability to survive in several ways. First,
its likely they did not possess the fire-bearing capabilities of homo erectus.
Second, they were dependent on a highly specialized plant diet that may
have undergone modification in a cooler climate; H. erectus enjoyed
a much more varied and flexible diet. This may have been a key to survival.
Even without climate change, herbivores have an evolutionary disadvantage.
They need to eat more to get the same amount of nutrition that carnivores
or omnivores do from their richer diet. The robust australopithecines may
have found their dry habitats offering less and less to eat over the millennia,
especially if their hard, tough foods were of low quality.
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It is now clear that bipedality developed before larger brain capacity. It is equally clear that A. africanus is earlier than A. robustus in southern Africa. The face of A. africanus is not as massive as the robust forms, and there is no sagittal crest. In overall size, the back teeth of A. africanus are still larger than those of modern humans but not as large as those for A. robustus or A. boisei. The brain size is relatively small (440 cc.). This is only slightly larger than that for a modern chimpanzee ( 282 to 500, avg. 385 cc.). The average cranial capacity for H. habilis is 660 cc., which is 50 percent larger than that of A. africanus. The cranial capacity for H. habilis ranges from 509 to 810 cc. This overlaps with the range of the early H. erectus (804 to 900 cc.; average 851 cc.) . Later H. erectus were to have cranial capacities near 1200 cc. with still later archaic Homo sapiens having capacities over 1600 cc.
There are obvious advantages to increased
brain capacity. As an evolutionary trend for one branch of the hominids,
the increasing size of the brain meant greater behavioral flexibility to
adapt to different environmental circumstances. These would include the
ability to manufacture more complex tools, construct sturdier shelters,
and use fire for warmth, protection and cooking. It also was easier to transmit
innovative behaviors from one generation to the next.
Large brains also have costs. A longer gestation period and extension of
the period of care for the infant following birth is required. Much of the
development of the human brain occurs after the birth. There is a period
of critical care for a human infant for a period of one to two years after
birth. The growth of the brain requires a high level of energy intake for
the mother not only during the longer gestation (pregnancy) but for a relatively
extended period following. Those hominids with greater brain size and effective
energy intake for infant development and care may have been better able
to survive and reproduce. Therefore, the trend for increased brain capacity
may have been coupled with a trend for increased nuitritional intake. (Read the following on "growing
young".)
At the same time that we recognize that there was a trend for greater brain
size in some hominids, there was another group of hominids that did not
develop a larger brain. This is the A. robustus and A. boisei
division. There is very little difference between even the gracile and robust
forms of australopithecines. The average for the graciles (A. africanus)
is about 442 cc. while the robust forms (A. robustus and A. boisei)
have an average of 516 cc. Given the greater body size of the robust forms,
this is an insignificant difference. If there is a continuum between forms
such as represented by WT17000 and the later A. boisei, it is possible to
look at very little change in brain capacity for one line of the hominids
over nearly a two million year period.
The development of a "thinking machine" as Alan Walker has put
it is a significant trend that followed the development of bipedality. The
success of a generalized hominid in contrast to a more specialized one should
be considered as you evaluate trends in hominid natural history.
Increasing brain size can be correlated with complex development and an
increased relience on tools. With increased ability to adapt by means of
manipulating the environment with technology, H. erectus appears
to be successful in living in a number of different environments. One must
continue to focus on the adaptive processes underlieing hominid evolution.
Homo clearly is part of an
adaptive process. It is possible
to think of it in terms of evolution experimenting with different types
of hominids to see which one was best fit for long term su