©1
Return to: Human evolution index page
Upright walker:
About six or seven million years ago there were no bipedal creatures resembling humanity. The earth was rich with diverse life forms, all subject to and shaped by natural forces and not the slightest signs of the intelligence and creativity that typifies humanity. Planet earth flourished without humanities' interfering nature as it had for billions of years. God awareness had not yet emerged upon earth in any form that we can understand. As stated in the Islamic Holy Quraan, there was a time when humans did not exist.
A trend toward larger brain size, a primate
characteristic, started with the australopiths. Food, its gathering and
eating influences an animal's
evolution (Reader, 1988). Primate species that eat higher-quality, more
widely dispersed foods, generally have a larger brain. Brain
enlargement probably arose in response to changes in foods (=
habitat). Two features that separate a human from an ape are
bipedal locomotion and a larger brain volume. The largest
known brain volume of the gorilla is 650 cc, and the smallest known in
humans are 855 cubic centimeters.
Brain volumes of
Australopithecus , the most primitive possible human ancestor
identified, ranges from 435 to 650 cc (other say 413 to 530 cc), well
within the gorilla and chimpanzee range. However, the part of the brain
responsible for our greater mental dexterity than other primates, the
cerebral cortex, is well developed in A.
africanus (Dart, 1925)
compared to other great apes. By three million years ago, this creature
already had expanded frontal lobes more human-like than that of any
living primate (Falk, 2001). Scientists include species within
the Genus "Homo" and thus as ancestors of humanity on this basis. The
"cerebral Rubicon" proposed is a cranial volume of 750 cc. Fossils
called Java Man (850 cc) and Peking (915 to 1225 cc) are both included
within the genus Homo. Homo habilis
, the first species of our genus,
had a brain volume of between 600 and 800 cubic centimeters (Wilson,
1992). Homo rudolfensis with
a brain volume from about 775 cc to 900 cc, lived around the same time
as H. habilis (Mayr, 2001).
Our upright method of walking evolved before
significant brain
enlargement occurred. In north-eastern Tanzania, at Laetoli is the
oldest evidence of a bipedal
(upright walking) hominid. These human-like footprints are found with
guinea fowl and other bird prints, antelope and gazelle, giraffe,
elephant, rhinoceros, pig, hyena, baboon, carnivores, hares, and
three-toed horse (Leakey and Hay, 1979). 
Dated at 3.5 to
3.6 million
years old, the prints are preserved in a volcanic ash bed (Reader,
1988). This is 1.6 million years before the oldest known tool use by
early hominids! Some trails are 50 metres long and are entirely human,
with a well developed arch to the foot! This creature's big toe
appeared quite long, resembling the mobile toe of the chimpanzee, but
the absence of any hand marks showed that the creature walked upright.
Its stride suggests that the larger individual stood at 140 centimeters
and the smaller at 120 centimeters. Some foot bones of such a creature
have been found in South Africa (Sterkfontein caves). This creature's
had an ankle bone adapted for upright walking and is almost identical
to the ankle bone of a modern human. The big toe angles away from the
foot and the joints suggesting that it was highly mobile (Tobias,
1995). Scientists place these with Australopithecus (the "southern
ape"), a hominid that lived in East and South Africa between 4.4
and 1.4 million years ago ("
1.4
to 4 lines ago ") (Avers, 1989). Sterkfontein would have been a
steamy tropical jungle at the time the "Little Foot" entered Southern
Africa.
Bipedalism is a major biological adaptation (Leakey, 1994). One question is whether the fossil record is complete enough and allows enough time for the evolution of Australopithecus sp. into Homo sp. Researchers can argue that some of the fossil features suggest that the Australopiths are not the direct ancestors of the genus Homo, but that they share a common ancestor. Some say there is too much biological discontinuity for the two creatures to be directly related. Others say that bipedalism requires important changes to the bones, muscle arrangement and limb movement. Any difference, such as that of the inner ear bones (see details below), may be an evolutionary vestige of the animal's past. Researchers need more fossils, especially of Homo habilis , H. rudolfensis and the earlier species, such as Sahelanthropus tchadensis, to resolve this problem.
Australopithecus species
((now referred to as australopiths (Wood, 2000)))Australopithecus was a bipedal ape with a small brain (450 cubic centimeters) (Washburn, 1978), and had massive, thickly enamelled molar teeth adapted to a diet of tough plant material. Its chest is funnel-shaped, with long arms and short legs as in apes. Bipedalism arose before the evolution of intelligence or large brain volumes associated with humans, a proposal that Haeckel presented in the 1860's (Reader, 1988). They may not have walked in the human manner but loped along similar to a chimpanzee. A naturally upright stance is further confirmed by foramen magnum and occipital condyles being set far forward and the neck muscles attach low down on the back. There is also a prominent mastoid process , necessary for balance in upright walking. Bones of the inner ear of Australopithecus resemble those of the chimpanzees and gorillas more than humans, suggesting that either, these creatures were very early on the path of bipedalism or were not fully bipedal. Only in Homo erectus is the inner ear structure the same as in modern humans, showing that the creature definitely walked and ran exclusively on two feet (Horgan, 1994).
About 2.5 million years ago, several species of Australopithecus ranged in Africa from Ethiopia to South Africa. Today scientists recognise numerous Australopithecine species (now called australopiths). A. robustus , is a heavily built creature, represented by specimens from southern and east Africa and existed 2.2 to 1.2 (a narrower 1.9 to 1.5 million years ago (Mya) by other estimates (Lemonick & Dorfman, 1999)) ( see timeline image ). A. africanus , the "gracile" species represented by the Taung specimen, lived 2.3 to 3 (or a wider 3.5 to 2) million years ago and A. afarensis , (e.g. "Lucy:) also a fine boned species, existing about 2.9 to 3.6 Mya (Lemonick & Dorfman, 1999) and is represented by fossils found in Tanzania and Ethiopia (A. aethiopicus). Sometimes they include another species A. boisei in the classification (see skull ), of which fossils dated at 1.6 to 2.5 million years (1.4 to 2.3 mya by other estimates (Lemonick & Dorfman, 1999)) old have been found, but others say A. boisei is the same species as A. robustus (1.5 to 1.9 MYA). A. boisei was similar to robustus , but with even more massive (some molars being up to 2 cm across) face and cheek teeth (Mayr, 2001). The brain size is very similar to robustus , about 520 to 530 cc. A few experts consider boisei and robustus to be variants of the same species. A few people add other species, such as A. aethiopicus , (2.3 to 2.8 (or 2.6) Mya) from northern Kenya (usually included within A. boisei, as its ancestor). They have called another fossil group from Chad, the most western distribution of Australopiths, A. bahrelghazali , (3 to 3.5 Mya) (Gore, 1997) and another Ethiopian species A. garhi (2.5 MYA) (Lemonick & Dorfman, 1999). Most of the australopith males were between four and five feet tall and the females up to four feet tall.
Ardipithecus ramidus
The story of bipedalism and Australopithecus begins with the
earliest known biped, Ardipithecus
ramidus. This period covers the end of the Miocene epoch (5 to
23 Mya), the Pliocene epoch (1.8 to 5 Mya), the Pleistocene epoch
(10,000
to 1.8 Mya) and up to today (Holocene epoch (zero to 10,000)). A
few fossil fragments from Kenya are between 5.5 and
4.5 million
years old (Bilsborough, 1992).
Since the chimp (Pan) and
human lineages have split we have 4 recognised genera: Ardipithecus,
Australopithecus, Paranthropus and Homo.
Remains of a creature in Ethiopia (Middle
Awash/Aramis) , dated as living from 5.8 to 4.4 million years
ago, called Ardipithecus ramidus (4.5
and 4.3 Myr ) (
Australopithecus
ramidus (White et al., 1994)) and Ardipithecus
kadabba (5.2–5.8 Myr), are probably ancestral to
Australopithecus afarensis and subsequently the long line leading
to humanity (and so the oldest hominid
found to date). 
Tim White of Berkeley, the anatomist working
some
individuals, says, "Ramidus is the first species this side of our
common ancestor with chimpanzees." Its similarities to
chimpanzees includes the thinner tooth enamel and smaller molars of a
fruit and vegetable eater, while its lower canines and upper premolars
display hominid traits. All apes have large canines, while human
canines are small.
A. ramidus was a forest dweller that does appear to have
been bipedal due to the forward placement of the foramen magnum. It may
have lived a partly arboreal existence. Five to six million years ago,
the Middle Awash region was deep rift valley with active volcanoes. The
region would also have been showered by pulses of thick and hot
volcanic ashes from these volcanoes. Ardipithecus lived in a high
in elevation that was relatively cool, wet, and forested. Fossils of
more than 60 mammal species have been found associated with the new
hominid, including primitive elephants, rhinos, horses, rats, woodland
forest antelope and monkeys (White et al., 1994).
Australopithecus anamensis
Fossil remains of
Australopithecus anamensis ,
have been found at Kanapoi and Allia Bay on the East African Rift
System. This Australopithecine species is about 4.2 to 3.9 million
years old. It was a bipedal ape and possible
ancestor to A.
afarensis (Gore, 1997). The dentition of A. anamensis
is ape-like, with large canines, yet it was definitely a bipedal
hominid,
as seen from its tibia. A. anamansis was a small
brained, biped (walked on two legs) with big
teeth that fits into the one million-year
gap between the earlier Ardipithecus and Australopithecus
afarensis (which includes the famous fossil skeleton known as
Lucy). A. anamansis fossils
are anatomically intermediate between the earlier species
Ardipithecus ramidus and the
later species Australopithecus
afarensis. Fossils of
this species have been found in Ethiopia (Middle Awash area in the Afar
desert of eastern Ethiopia) and Kenya. The Ethiopian fossils, at 4.1
milion years old represent a fossil continuity between
Australopithecines and the earlier Ardipithecus
species.
Hundreds of mammal fossils also were found in the region, indicating a woodland habitat with colobus monkeys, kudus, pigs, birds and rodents, and carnivores such as hyenas and big cats. This wooded habitat type persisted over a long period in this part of the Afar and was favored by early hominids between 4 and 6 million years ago.
A. afarensis
A. afarensis (3.9 to 3.0 Mya) (Eastern african species) precedes A. africanus (Southern African species) (3 to 2 Mya) by at least 500,000 years. Some scientists say that A. afarensis is probably the ancestor to every hominid fossil ever found. Fossil finds cover a period at least 1,000,000 years (3.8-2.8 Mya), suggesting a very long lived species. They became extinct about three million years ago (Avers, 1989). According to once scheme, this species is most likely the ancestor to two divergent lines:
- Australopithecus afarensis (3.6 to 3.0 Mya) to A. africanus , to A. aethiopicus (2.6 Ma) to P. boisei (2.3 to 1.2 Ma) species and/or P. robustus lineage;
- A route to humanity via A. garhi, then H. habilis , H. ergaster and H. erectus .
These researchers believe that the human line (called Hominines or of the Genus Homo) diverged about 4 million years ago from the line that evolved into A. afarensis and Homo habilis. Until the discovery of Sahelanthropus tchadensis , there was no fossil record to support this (Parker, 1992) (Avers, 1989). Now, we have a possible ancestor to H. rudolfensis at nearly 7 million years ago. There is an increasing amount of circumstantial evidence in support of this view:
[1] Both Australopithecus and Homo sp. are bipedal apes as shown by the central position foramen magnum (the hole at the base of the skull for the spinal column), typical of an upright posture. This unusual feature leads to the two species being grouped together. Australopith members had much longer and curved toes (phalanges) and fingers. They also had a cranially orientated shoulder joint and other features of the arms, typical of tree climbers. As well as the foot, and foramen magnum, we see bipedalism in the shape of the pelvis and the angle between the thighbone and the knee (Leakey, 1994).
[2] A. robustus and A. africanus both had teeth different to the apes. Apes have teeth with pointed cusps as adaptations to a diet of soft fruit and other vegetation. The Australopiths have teeth flattened into grinding surfaces. This suggests a diet different to apes and consisting of tougher foods such as roots, nuts and hard fruits. Furthermore, the Australopiths did not have canines, as are found in other apes but not the Homo sp. Such dentition brings the classification of the Australopiths closer to the genus Homo. It is likely that the Australopiths ate mostly plant matter.
[3] In the genus Homo, the inner ear structure is identical to Homo sapiens sapiens (modern humans), while the Australopith's inner ear semicircular canals resemble those of apes.
[4] Australopith's pattern of growth and development was ape-like while early Homo erectus showed development midway between the ape and modern human (Leakey, 1994). As the brain capacity exceeds 770 cubic centimeters, serious constraints are encountered, as the pelvic opening becomes too small. A solution is found in human children being born earlier and with brains one third the adult size. Apes are born with brains half that of the adult size. This results in human babies being born more helpless.
[5] Australopith tooth development with age is ape-like, while H. erectus and the Neanderthals followed the human pattern.
[6] Australopiths have a conical shaped rib cage, similar to apes and tree climbers, not barrel shaped as in humans. Early Homo species must have been physically active, while this is not so with the Australopiths. A Homo erectus skeleton, called Turkana boy (KNM-WT 15000) (Brown et al. , 1985), was 1.64 to 1.68 metres tall and would have been powerfully muscled. Australopithecus would not have been able to breathe as we do when running. This places Homo as a runner and Australopithecus as a less active animal. To support this we find that Australopiths have a typical ape-like heavy build for their height, while early Homo species had the more agile build of humans.
[7] With the greater activity of Homo species as compared to the Australopiths, the blood vessel structure draining the brain of the Homo line is much more conducive to cooling than the Australopiths. An active animal has to remain cool and dissipate heat and its physiology has to adapt to this.
Fossil evidence such as a perfect knee joint shows that A. afarensis walked upright up to 3.6 million years ago. Bipedalism provides a competitive advantage as it is more energy efficient than quadrupedal locomotion. It also frees the hands for other activities (Chiras, 1994). The footbones of Australopiths are similar to those of the human foot, but have ape features that persisted with H. habilis . Its foot has long, curved toes, while the forelimbs are proportionally longer than in the genus Homo. A. afarensis has many cranial features reminiscent of our ape ancestry, such as a forward protruding (prognathic) face, a "U-shaped" (see AL 200-1) palate (with the cheek teeth parallel in rows to each other similar to an ape) and not the parabolic shape of a modern human's jaw teeth.
Studies of the mechanics of operation of the skeletal frame, its limits and potential, known as biomechanics have revealed some interesting information. Australopithecus ' articular ball of the hip joint exerts about half the pressure on the joint that a human's does, as it is closer to its centre of gravity (view of Australopithecus afrcanus pelvis, compared to human's. ) ©1 . This is a mechanical advantage, in that the stresses of bipedalism better distribute throughout the pelvis. It permits a smaller ball joint (femoral head) and a longer femoral neck, joining the ball to the femur. Muscles that operate the hip have a more efficient lever arm by this design. The long femoral neck of the Australopiths may have inhibited walking for any but short periods. It is possible that the evolution of a larger brain and thus the skull of humans put demands upon the structure of the female pelvic opening. Natural selection would have selected for large birth canals. A broadening of the hips (an instinctively attractive feature!) accompanied by the shortening of the femoral neck would allow a larger birth canal and the necessary loss in efficiency of the pelvis from a biomechanical point of view.
A female A. afarensis , "Lucy", (20 years old), with wisdom teeth , stood only 107 to 122 centimeters high. Its teeth showed a mixture of modern and primitive features. As with apes such as the chimpanzee, there is a distinct gap between the canines and incisors, the palate was shallow and the teeth rows were parallel rather than curved. An Australopith brain is barely larger than that of an ape of similar body size, such as a chimpanzee, yet it walked upright . Even at this early stage the brain structure differed from that of the apes, having a greater cerebral height, and more human-like frontal and temporal lobes. There is also evidence of the Broca's area, a part of the brain necessary for speech.
Lucy was less than one metre tall and probably weighed 27 to 30 kg, while some males topped 1.5 m and weighed up to 50 kg. This suggests that these creatures displayed sexual dimorphism. (Human males and females differ by an average of 13 cm.) Females appear to have been better at tree climbing while, the males were better at walking - a really odd differentiation. It's curved fingers were part of a creature that still climbed trees as a normal part of its life. Australopithecus ' erect bipedalism was probably not as developed, or identical to the perfection shown in Homo sapiens . Its upper torso possessed distinctly arboreal features and its leg's distinctly bipedal as expected from an ape newly on the bipedal road. Lucy's skeleton was different from earlier primates. Her knees were able to lock, her femur slanted inward and her large toe was in line with her other toes, allowing her to walk upright (see AL 129-1a/b) . Another feature that identifies bipedalism is the foramen magnum. This is the opening at the base of the skull through which the spinal cord passes. It is at the far back of the skull in quadrupeds (four-legged-walking) such as dogs, and is more centrally located in bipeds such as humans. In Australopithecus the foramen magnum is not quite central, so this creature may have walked with a stooped posture (Avers, 1989).
A. africanus
It is possibile that A. africanus and A afarensis ( see species time chart ) share a common ancestor and that A. africanus is the ancestor of H. habilis (Berger, et al , 1998) via A. garhi ( see above ) . This hypothesis arises from the incongruity that A. africanus, although a more recent species, had more ape-like longer arms and shorter legs that A. afarensis , from which it was supposed to have evolved. Further corroboration is found in the comparison of the thigh bone and arm lengths of Lucy (A. afarensis) and Homo habilis (and the discovery of A. garhi). The thigh bones are the same length, while the arms of H. habilis are longer. A. africanus also shares other features with Homo habilis . When compared to A. afarensis , it has smaller canine teeth, a larger brain and a shorter face. This raises the possibility that A. afarensis is not the ancestor of H. habilis , but more fossils are needed to confirm this. Lee Berger, the proponent of this idea even suggests that bipedalism may have evolved twice, one lineage in East Africa and another in southern Africa where a mix of habitats existed. Wear patterns on the teeth of A. africanus are similar to that of modern primates that eat leaves and fruit (Gore, 1997).
A. africanus represents the
gracile
fom of Australopith. All finds from South Africa are of around 2.4
to 3
MYA.
Compared to robust Paranthropus species, their faces are more lightly
built and dished-out, lacking a sagittal crest. Their cranial capacity
of was about 450 cc.
A. garhi
Australopithecus garhi may represent an evolutionary link between the genera of Australopithecus and Homo (Asfaw et al 1999). It is probably descended from A. afarensis . It is associated with world's earliest stone tools found in the same general locality (Bouri, in the Middle Awash area in the Afar desert of Ethiopia) and dated, at 2.5 mya, and excavated antelope fossils with cutmarks made by stone tools and some opened by hammerstones (Sanders, 1999). However, the extremely large size of its teeth, especially the rear ones, and a primitive skull morphology make it unlikely to be a human ancestor according to other paleonologists.
A. garhi has larger
postcanine dentition than A.
afarensis. A. garhi
lacks the dental,
facial, and cranial features shared by A. aethiopicus, A. robustus,
and A. boisei. Australopithecus garhi
is distinguished
from A. africanus and other
early Homo species by its
primitive frontal, facial, palatal, and subnasal morphology (Asfaw
et al 1999).
Paranthropus species
After 2.5 mya, fossil finds show the Australopiths diverged in both east and south African populations into a line of “gracile” forms represented by A. africanus and a line “robust” forms as a different genus, Paranthropus (P. aethiopicus, P. robustus, P. boisei). The robust forms had larger cheek teeth (molars and premolars).
A.
robustus ,
A.
aethiopicus and A. boisei had a gorilla-like bony
crest down the midline of the skull, serving to anchor the
enormous jaw
muscles needed to chew their fibrous diet (Wilson, 1992). This feature
makes them look very ape-like. These robust forms were called "megadont " in that they had huge, broad
cheek teeth with thick enamel (compare A.
afarensis and P. boisei
teeth from Asfaw,
et al (1999)). Their incisor teeth relatively small. The rear teeth
were designed for the stresses of heavy chewing and the associated
skull morphology supports this observation. Their large zygomatic
arches allowed the passage of large chewing muscles, while the large
sagittal crest provided a large area to anchor these muscles to the
skull. These are adaptations to chewing tough, fibrous foods.They also
have very heavy brows making them
look far from human. Anatomists group these three together as the genus
Paranthropus and
consider them an offshoot
diverging from the ancestral line leading to humanity (Aiello &
Dean, 1990). Scientists have not decided which Paranthropus
species is closer to the ancestral human line. P. robustus ,
from South Africa (Swartkrans and Kromdraai) is the least
'human-looking' species, but has a human pattern of tooth development,
while the tooth pattern of the other two species is distinctly
ape-like. These species, P. boisei
(Tanzania, Olduvai gorge)
and P. aethiopicus ,
may represent evolutionary dead ends
that separated from the Homo line at sometime.
P. robustus
P. robustus could be descended from the southern gracile A. africanus, in which case, the genus name Paranthropus would be invalid (see Paranthropus phylogeny). It lived from 2 million to 1.2 million years ago (others say 1.8 to 1.6 mya). This animal was around 1.1 - 1.3 metres tall, weighing between 40 and 80 kilograms and with a brain volume of 530 cc. It has a robust skull and grinding teeth. Its teeth indicate a diet of mostly coarse, tough food requiring a lot of chewing. It may have used tools. Fossils of this species are found in southern Africa.
P. aethiopicus
An example of this species is the skull KNM WT 17000, representing a
distinct robust species, Paranthropus
aethiopicus, that lived in eastern Africa 2.6 to 2.3 million
years ago.
It is possibly descended from A.
afarensis, and ancestral to P.
boisei, but
could be an evolutionary dead end. This species is known from a
fossil called the Black Skull and a few other specimens attributed to
the same species. It has an unexplained mixture of primitive and
advanced traits and a very small brain size at 410 cc. Parts of the
skull are primitive and resemble A.
afarensis. Other characteristics, like the massiveness of the
face, jaws and the largest sagittal crest in any known hominid, place
it close to A. boisei.
P. boisei
P. boisei (was Zinjanthropus boisei) persisted in east Africa from 2.1 until 1.2 mya. Fossils of P. boisei dated around 2 to 1.5 million years ago, are found with two species of Homo at Koobi Fora in Kenya. The exact identity of the two Homo species present is uncertain. Some classify them as Homo habilis and Homo rudolfensis (Groves, 1994). They have a larger brain and smaller teeth than Australopithecus and may be a single species (Gore, 1997). As anthropologists refine the study of these fossils, they are creating new species. Paranthropus walkeri is now seen as the link between australopiths and P. boisei (Groves, 1994) and P. crassidens from Swartkrans. This animal was around 1.2 - 1.4 metres tall, weighing between 40 and 80 kilograms and with a brain volume of 410 to 530 cc.
Kenyanthropus platyops
Another
species of hominid, Kenyanthropus platyops (WT
40000) is dated
at between 3.2 and 3.5 million years old. This places this creature in
the same time period as A. afarensis . It has a flat face and
small teeth. Its place
in the human family
tree has yet to be decided. Leakey et al (2001) see K. platyops
as distinct from the australopithecines, sharing features with KNM-ER
1470 (Homo rudolfensis). It is possible that it is the ancestor
of Homo, while the australopiths are a side-branch that are not
ancestral to Homo. Tim White (2003) believes that the fossil
skull is severely distorted
and cannot be reliably identified. He says that it may be a
Kenyan version of Australopithecus afarensis. (Image of K. platyops and H. rudolfensis from Lieberman
(2001)).
The features that distinguish K. platyops from A.
afarensis include primitive traits like small ear holes and
advanced traits like a relatively flat face and small molars. K.
platyops has morphological similarities with Homo rudolfensis ( KNM ER 1470) or Homo
habilis, species that lived as many as 1 million years later.
Similarities between these species might be explained in terms of convergent evolution, or by the
possibility of a direct ancestral line between K. platyops and H.
rudolfensis or H. habilis. Some scientists have placed H.
rudolfensis specimens in the genus Kenyanthropus. Others
ignore Kenyanthropus as a genus altogether, placing K.
platyops specimens into the genus Australopithecus, saying
that the species is not different enough to warrant its own genus.
Orrorin tugensis
Another bipedal hominid was discovered in 2000
by French anthropologists and named Orrorin
tugenensis. At 6 million years old O. tugenensis
("Millennium man") is one of the oldest proposed human ancestors to
date (Lemonick and Dorfman, 2001). It was estimate to be 1.2 metres
tall and lived in a well-forested habitat. It has smaller molars and
thicker enamel than the Australopithecine (now
called australopiths) teeth, a characteristic of all later
hominids. Richmond and Jungers of Stony Brook University published
their findings in the March 21, 2008 issue of "Science" showing that
the overall shape profile of the bone closely
resembles early human fossils from about three to two million years
ago; reflecting a bipedal animal. Biomechanically, O. tugenensis probably walked
differently than modern humans.
The physical mechanism of upright walking remained the same from O. tugenensis to Australopithecus. About two million
years ago, an environmental shift that occurred
within East Africa, changing forests into grasslands, forced hominins
to rely even more on walking as they looked for food. This added new
selective pressures on bipedal walking leading to a dramatic change in
hip structure. Longer-distance walking became necessary, giving humans
the capabilities of today, with long striding legs and a very different
kind of body shape compared to O.
tugenensis or Australopithecus.
Sahelanthropus tchadensis
Another fascinating
find is a male hominid skull from Chad,
with a preliminary dating of 6 to 7 million years old, making it the
oldest hominid fossil found, is nicknamed Toumai ( "hope of
life" in the local Goran language).
Sahelanthropus
tchadensis has a neck attachment to the skull from below, a
feature of a bipedal animal, forcing a review of the origins of
bipedalism. As no absolute date
could be established, scientists had to use other fossils, such as pigs
and elephants, found with the skull and that had been dated at other
sites, to get an age estimate. This creature has a face
similar to Homo habilis
of 2 million years ago according to Dr. Daniel E. Lieberman, a Harvard
palaeontologist. The Lucy fossil of 3.2 million years ago is very
chimp-like by comparison! This suggests that the australopithecines
(now called australopiths) my turn out to be a side branch outside the
human ancestral line. Its face is "tall" with a massive brow
ridge, while the mid-face is short (in the superoinferior dimension),
being less prognathic than either Pan or
Australopithecus (Brunet, et al. 2002). The
teeth of the new fossils are taxonomically distinctive enough to assign
the fossils to a new species and genus (Wood, 2002). Its brain volume
was chimp-like at 320 to 380 cc. The site where S. tchadensis
was found (TM 266) had a rich aquatic fauna of fish, crocodiles and
amphibious mammals, alongside gallery forest and savannah animals such
as primates, rodents, elephants, equids and bovids. This and
sedimentological evidence, suggests that S. tchadensis lived
close to a lake, and a sandy desert. The fauna suggests a
biochronological age between 6 and 7 million years. This fossil could
be close to the common ancestor of humans and chimpanzees.
The current evolutionary scenario emerging ( view timeline
image ) is that, the original fossils identified as Homo habilis were as mix of
species. However, the specimens described were too variable to
belong to a single species. Larger-brained specimens have been renamed
as H. rudolfensis (Mayr,
2001). Homo habilis is now
considered to have evolved from the Australopithecines, but not to have
evolved on to become Homo sapiens.
It is even possible that tool use atributed to H. habilis, was really that of H. rudolfensis.
Sahelanthropus tchadensis
may have been bipedal in its walking habits, but as bones from its legs
and feet have not yet been found, this cannot yet be verified.
African Origins:
Australopithecus africanus lived in association with many African mammals. To the East of the Great Rift Valley is found the most ancient fossils of this species. This ape did not use fire or tools. Palaeontologists attribute tools found with fossil remains of Australopithecus sp . to Homo habilis or H. rudolfensis and the earliest tools known to date are about two million years old. Australopiths were about 1.5 metres tall (Wilson, 1992) and lived near rivers, streams and lakes in highland areas periodically inundated with volcanic ash from nearby eruptions. Fossil remains of crocodiles, flamingos, and Tilapia are commonly found with these hominid fossils. Environmental conditions fluctuated between wet and dry during this period. Evidence for this is from fossil pollen of vegetation that was present, the animal life found in each level and the wind-blown, or water-borne materials.
Yves Coppens, a palaeontologist, did some important detective work on the site of our origins. His proposal is founded strongly upon geology, molecular biochemistry (molecular biology), palaeontology and ecology. His is the type of reasoning that emerges once we accept that we are evolved animals. Molecular biologists have established that humanity is closely related to the chimpanzee and that lines such as the Orangutan are very distantly related to humanity. With palaeontologists, they came to a "prehistoric compromise", agreeing that the lineage leading to humanity separated from the chimpanzee lineage seven and a half million years ago.
An objection was that if Pan , the chimpanzee was our ancestor, why were no such fossils found with Australopithecus in eastern Africa. Hominid fossils (family Hominidae) as old as seven or eight million years have been found in Ethiopia, Kenya and Tanzania, but no chimpanzee or gorilla ancestors (family Panidae) occur in the same area. Molecular biology, biochemistry and cytogenetics all show that humans and chimpanzees evolved from a common ancestor. A solution to this puzzle came from vertebrate distribution maps and geological history. Africa's Rift Valley runs from north to south. Vegetation and climate differ dramatically on each side of this rift. To the west are woods and forest where chimpanzees are found and to the east are grasslands where only hominid fossils are found.
The hominid and chimpanzee ancestor lived in a homogeneous biogeographical province, a massive lush forest in central Africa. Roughly twenty million years ago, the Rift valley started to divide equatorial Africa due to east-west stresses in the tectonic plates (Maslin, 1994). According to geophysicists, this tectonic activity created the 2000 metre high Kenyan and Ethiopian domes. Rainfall patterns began to alter and rainforest, woodlands, shrub and grasslands developed in close association. As the rift subsided into a large valley, a process evolving over ten million years, the Eden of this ancestral species split into two. This new geological structure was a natural barrier that altered the weather patterns, with the west remaining humid and moist. Eastern Africa became organised into a seasonal monsoon and according to palaeoclimatologists, progressively drier. Within the unchanged western forests and woodlands the chimpanzee evolved from this common ancestor. On the east the change in climate subjected this ancestor to different selective pressures and a more drastic change in this creature's habitat and niche. Forest gave way to open savannah. Palaeontologists note a change of species to what they call Ethiopian fauna, eight to ten million years ago. This was accompanied by a decline in the number of ape species from more than 20.
Along the African Rift System, which runs from the Red Sea in the north to Lake Malawi in the South a diverse habitat evolved, with ponds, small streams, woodlands, savannah plains, and rocky valleys. Along this valley and extending down to Sterkfontein and Taung in Southern Africa, are the many fossil sites for the Australopiths and early Homo species. Natural selection driven by climatic and vegetation changes favoured a less arboreal way of life, suited to the bipedal savannah ape, Australopithecus . As food was more dispersed bipedal apes could forage more successfully . As with much species formation, the genetic divergence of our ancestor from the line leading to the chimpanzee, arose through geographic isolation caused by the formation of the Great Rift Valley. The process was also driven by a shift toward prolonged and seasonally more arid conditions, opening up grasslands in place of forests after 2.8 million years ago (deMenocal, 1995). The chimpanzee ancestor remained in the available forest habitat and became the species if chimps we find today. Another lineage from this common ancestor adapted to the more open grasslands, experiencing completely different selective pressures, evolving to occupy a comletely different habitat - open graslands.
As an example of other evolutionary trends that follow through from Australopiths, we find that there is strong sexual dimorphism in the Australopiths, similar to what exists in gorillas and orangutans today. There is good evidence of large sex differences in A. afarensis (Leutenegger, 1995). This is clearly seen in the jaws, skulls and the thigh bone. A pattern of sexual dimorphism continues through to Homo habilis and Homo erectus , but is reduced in archaic Homo sapiens and Neanderthals. Its reduction in modern humans is due to females getting larger and not males getting smaller. This difference seems to vary with environmental and nutritional conditions. Dutch women of 100 years ago were about 10 centimeters shorter than males. Less that 2 percent of males reached 180 cm then. Today the average Dutchman is 183 cm tall and 14 cm taller than the average woman (Usher, 1996).
We do not have the fossil remains of our ancestors of between 5.5 and 10 million years ago. There is no fossil record for the African apes (Gorilla and Chimpanzee) due to their living in moist tropical forests where decomposition is rapid, preventing fossilization. It follows that there is a limit to how far back in time hominid fossils will be found, as we originated in a similar habitat. Molecular comparison of living primates suggests that the African apes, the Chimpanzee and Gorilla, split from the hominid line between 6 and 8 million years ago (Mya) (Aiella & Dean, 1990).
Robust Australopiths were vegetarians, with heavy, specialised teeth. Such animals would not have much use for complex tools. This diet doomed them to extinction as the evolutionary pressures of this route forced greater and greater morphological specialisation. We see a trend in the increasingly specialised characteristics of the line, leading to a branch yielding A. africanus about 2.5 million years ago, then to A. robustus and then A. boisei , without selecting for a larger brain and finally becoming extinct. Their facial architecture becomes increasingly bulky through time, selected for greater chewing forces to handle their herbivorous diet. Their adaptive solution to selective pressures was morphological instead of behavioural. Herbivores typically lack the intellectual challenges faced by predators and do not need a cooperative culture.
Tools?
Australopiths did not shape stone tools, but if they used wooden tools such as a stick, no fossil evidence would remain. Our closest living relative, the chimpanzee, uses tools spontaneously, both in the wild and in nature. Wooden tools will not fossilize and chimpanzees do not shape stone tools for use. Chimpanzees use many different tools, such as "dipping" for ants or termites using a stick, using a leaf as a sponge and using a stick as a weapon. Weapon use, shown mostly by males, involved sticks and stones that they flailed, clubbed and threw at other chimpanzees, humans and baboons. In one experiment, a chimpanzee attacked a stuffed leopard, using a stick as a club. It hit the leopard with enough force to break a live leopard's back! (Jolly, 1972). Researchers list 19 habitual patterns of tool-use (McGrew, 1993). More than 34 populations of free-ranging, wild chimpanzees have different repertoires of tool use. These chimpanzees learn by imitation, so that even in the wild there are population differences in tool-using traditions (Gibson, 1993). Technology transfer even occurred, where a chimpanzee moved from one community to another and introduced the use of twigs to catch carpenter ants (Miller, 1995).
Chimpanzees of the Tai Forest are impressive. A mother was observed showing her five-year old child the necessary spatial orientation needed to hammer open a nut ( Coula edulis and Panda oleosa and three less common species) with a tool. The juvenile then attempted, copying the mother. To open a nut, a mother chimpanzee carries some nuts to a root, which she uses as an anvil. She places a nut in a depression on the root and hits it with a hammer tool - a wooden club or stone. Wooden sticks are prepared in various ways before use, such as shortening the stick to a suitable length. The harder the nut, the heavier and harder the hammer. Panda oleosa , is the hardest nut, so Chimps transport tools in foraging for this nut (Boesch, 1993). Rare stone tools are more often transported. Chimps remember up to five different tool locations and go to the closest, usually from a distance of at least 100m. Here we see how a specific diet, based on a single tree species, led to unique behavioural adaptations in the chimpanzee.
Mother chimpanzees also gave their child a good hammer. She would leave nuts near the anvil or put the nut on the anvil and leave the tool nearby, so providing an easy incentive for the young animal to try to crack the nut. Active teaching even took place. Panda oleosa nuts are the hardest and most difficult to open. They have three kernels independently embedded in a hard wooden shell. A partly opened nut had to be carefully and skilfully repositioned to get each kernel without smashing them. A young male succeeded in removing one kernel but then incorrectly placed the nut for the attempt to get at the second kernel. The mother quickly picked up the nut, cleaned the anvil, and replaced the nut correctly. The youth then succeeded in opening the second kernel.
In another episode, a young female was having trouble opening nuts with an irregularly shaped tool. She changed her position about 14 times and her grip on the tool about 40 times. She also tried moving the nut to a new position and rotating the nut to hit it on a different position. Having exhausted all the imaginable variables over an eight minute period, her mother, who had been resting, joined her. With the daughter sitting in front, she slowly rotated the hammer to the best grip, taking about one minute to do this and then opened 10 nuts. The daughter ate all the contents and then took the hammer. She then opened four nuts in 15 minutes, so still had difficulties, but always held the hammer in the position shown to her. At each difficulty she whimpered for attention and threw a tantrum after unsuccessfully trying to open the fifth nut for three minutes.
The nut-cracking techniques of this population of chimpanzees take 10 years of practice to master, so such tool-use could only develop where the nutritional rewards are high to compensate for the long period of learning required. This is a long period of trial for a consistent or perceived reward. Nuts can be cracked with the chimpanzees own teeth, but tool use permits individual chimpanzees to process more nuts than they can consume. Mother chimpanzees that master this nut cracking technique, provide the excess, up to 1000 calories per day in nuts, to their young until they reach about eight years of age. This gives the young extra and valuable nutrition and the leisure time to practice and develop their own tool-using skills.
In captivity, chimpanzees also use tools. When researchers hung food in their enclosure, they acted as anthropoid engineers, stacking up crates and using a pole to reach suspended food. Chimps solved the problem without help, using available materials. In this way, chimpanzees are the only nonhuman species in nature to associate and use different tools to solve problems (Mc Grew, 1993). A chimpanzee in the wild became the dominant male through his use of empty kerosene cans that he banged together to intimidate larger males (Miller, 1995).
Human capabilities far outreach this behaviour, but the behaviour in a nonhuman provides credence to the possibility of crude tool use in our most ancient ancestor. This social and technological complex in the chimpanzee provides a window to our most ancient beginnings. Jane Goodall noted a wide complex of chimpanzee behaviour during 35 years of study. This included meat eating, toolmaking, deliberate planning, warfare, cannibalism, the formation of coalitions, consortship, adoption of non related chimps, exploitation of social situations, the use of medicinal plants, and technology transfer (Miller, 1995).
From an evolutionary point of view there is great advantage in obtaining an excess of a highly valuable food resource. Nut cracking provided a net benefit of 3800 calories per day during the nut seasons (Boesch, 1993). A population of chimpanzees could develop in two specific directions. One could rely on its teeth to crack nuts and another on its ability to learn a transmitted skill. Natural selection would act on these two traits - "favouring" tooth jaw muscle development in the one and mental dexterity in the other. Nut cracking behaviour using tools was first reported by American missionaries working in West Africa (only found in Liberia and Ivory coast) in 1843, so there is little chance that humans somehow transmitted this behaviour to the chimpanzees (Hewes, 1993).
Another significant component to this chimpanzee nut cracking it that they would leave stone tools that worked well as nut cracking hammers and anvils at the tree base. They would revisit and use the same tool when the nuts in that oil-palm tree ripened. These chimps had a "mental map" of both where the oil-palm trees and their stone tools were. When another chimp has taken the tool, the returning chimp would search for its missing tool in the vicinity where it was last found. A chimp would take the tool with him to the next tree, so would have to remember at which tree the tool was last discarded (Potts, 1996). With this behaviour of bringing rocks directly to the food source, natural selection comes into play on various features:
- The tool replaces teeth as a food processing organ, so selection will favour dexterity in tool use.
- The chimp that better remembers when trees ripen and where the trees and tools are will fine life "easier"; a selective advantage.
Australopithecus's jaws have reduced canines and incisors and large molars and premolars. Their teeth show heavy wear. Its a strong projecting face and long zygomatic arch concentrates chewing power over the cheek teeth. With such a specialised chewing apparatus and large jaw muscles, they probably ate tough seeds or nuts. The tooth wear of the gracile forms is similar to that of fruit eaters. Such a diet would not necessarily select for tool use in this small brained ape but it may have behaved like its chimpanzee relative. This dentition is just as likely to reflect a root eating animal, which would come down from the trees to dig for roots in the soil. Sand on the roots eaten would explain some of the tooth chipping found in robustus. Researchers have established that thin-enameled cheek teeth and large canines are adaptations to the slicing and chewing of soft plant foods. Such foods would be common in the tropical forests, but not the domain of Australopithecus. Heavy chewing is required to crush and grind tough plant foods. As an adaptation to this diet, large, thick-enameled molars and reduced canines evolved. Ramipithecines and hominids have such dentition and lived in nonforest habitats providing such food, but the former is not the direct ancestor of the latter.
At the threat of danger Australopithecus could flee to the trees to escape as it had no other form of defense, being slow running and lacking a biting defense or claws. It may have used some type of stick as a pick to dig in the soil for carbohydrate rich roots. Australopith hands do not suit digging, but would serve for tool manipulation as in chimpanzees. Its long arms and stooped posture would have suited this mode of life. It may have even been nocturnal, foraging at night and retreating to tree roosts to eat its pickings during the day. As the search for food was spent on the ground and trees were only used as a refuge, it is not difficult to imagine the evolution of a flat footed upright walking creature that migrated long distances to get its food and had long powerful arms to haul itself up into trees in times of flight. The difference between this creature and the baboon or chimpanzee would have been that Australopithecus had a more specialised and restricted diet reflected in its herbivore-like dentition.
By 1.9 million years ago, a new behavioural system is evident from remains at fossil sites. Natural selection has opened the door to a new behavioural strategy, and one finds "extensive evidence" of stone and food transport to a processing site. This is the beginnings of the human "home base" and the foundation for human society. The animal parts at these sites show cut marks, indicating definite tool use in a carnivorous context. Potts (1996) calls this "a turning point in the behaviour of early toolmakers". Stone was transported from up to nearly 10 kilometres away to the foraging range. Repeated visits to these sited led to the archaeological sites excavated today.
Return to: Human evolution index page. GOTO to next chapter: Homo habilis & Homo erectus
Nature' Holism is a self funded research project. Please help with this progress by advertising, sponsoring or donating now. See more details on sponsorship or advertising. PayPal allows free, easy, secure and safe transactions. They will not charge you any fee.
Donate any amount to keep this site and research progressing.
