Brain capacity & teeth
Scientists have discovered a
specific gene which responsible for the increase in the size of the brain when
compared the humans and apes. This gene includes in the humans and it stops the
production of N-glycolylneuramine
acid. And this gene has entered the human evolutionary line as a result of
a mutation 2.7 million years ago. While it is presumed that the
australopithecines lacked this gene, there is no direct
evidence.Early transitional humans had
brains that on average were about 35% larger than those of Australopithecus
africanus. In fact, it is beginning with Homo habilis that our
ancestors finally had brains that were consistently bigger than those of the
great apes.
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As the early human cranium, or brain
case, began to enlarge in response to increased brain size, the mouth became
smaller. In comparison to the australopithecines, the early humans had
smaller teeth, especially the molars and premolars. This suggests that
they mostly ate softer foods. An analysis of the wear patterns on their
teeth indicates that they had diverse diets that included a wide range of
plants and meat.
The differences between australopithecines
and early humans are most noticeable in the head. Humans developed
significantly larger brains and relatively smaller faces with progressively
smaller teeth and jaws. In addition, humans became ever more proficient
in developing cultural technologies to aid in their survival, while the
australopithecines did not.
The absolute sizes of
cheek teeth expand through successively younger species of australopithecine,
from the oldest, A. anamensis (428 mm2), to A. afarensis (460 mm2), A.
africanus (516 mm2), P. robustus (588 mm2), and P. boisei (756 mm2). The trend
is reversed in successively younger species of Homo, from H. rudolfensis (572 mm2) to H.
habilis (478 mm2), H. ergaster (377 mm2), and Homo sapiens (334 mm2). The absolute size of
the cheek teeth is correlated to the morphology of much of the skull,
including mandibular corpus robusticity, position and robusticity of the zygomatic arches,
attachment areas and buttressing for the chewing muscles, and many features of
the face. H. rudolfensis resembles some of the australopithecines in retaining
absolutely larger cheek teeth and related features (Wood 1991), but when
scaled to body weight, its teeth are relatively much smaller than any
australopithecine.
The relative size of
the cheek teeth can be estimated by comparing postcanine tooth area with body
size. This can be done by comparing postcranial dimensions of associated
skeletons with cheek-tooth size, but there are few specimens. Although
there are methodological problems, it is heuristically interesting to
compare tooth area directly with estimated body weight to find a measure of relative
tooth size. Both modern chimps and humans are slightly below this
average and have a value of 0.9. The australopithecine species expand through time
from the earliest, A. anamensis, with 1.4, to A. afarensis with 1.7, A. africanus with
2.0, P. robustus with 2.2, and P. boisei with 2.7. This trend is reversed in the Homo
lineage. The earliest species of Homo show some reduction from late Australopithecus
(H. habilis has an MQ of 1.9 and H. rudolfensis one of 1.5). The values
for H. rudolfensis depend on the assumption that the large hindlimbs of Area 103
at Koobi Fora belong to that species and thereby provide valid body weight
estimates. Attempts have been made to estimate body weight directly from the
skull and these range between 46 and 54 kg for the skull of H. rudolfensis.
These are slightly smaller than the 60-kg estimate derived from the postcranium
of the presumptive male of that species. The lower body weight estimates would
raise the MQ value slightly.Because of its large
body size, the relative size of the cheek teeth of H. ergaster is the same as that of
modern humans.
Posture & skeleton
The striking similarities in
appearance between the human genus Homo and our ancestors, the genus Australopithecus,
is sufficient reason to place us both into the same biological tribe (Hominini).
Both genera are bipedal and habitually upright in posture. Humans have
been somewhat more efficient at this mode of locomotion. Like
australopithecines, early humans were light in frame and relatively
short. They were only about 3 ft. 4 in. to 4 ft. 5 in. tall (100-235 cm)
and weighed around 70 pounds (32 kg) The evolution of larger bodies
occurred later in human evolution.
Transformations of forelimb
Though the evidence from the hand is
incomplete, forelimb as a whole shows modifications between Australopithecus
C habilis and later Homo: The former ones had big, robust
arms and the latter was relatively petite. There are no associated limb bones
of A. anamensis, The humor of A. afarensis is exceptionally robust & forearms appear to be very long relative to humeral
length, A. garhi also has a long
forearm relative to humeral length.Which makes a ratio of 98, more similar to
chimps than modern tropical people (76–79) or H. ergaster (80).The limbs
of A. africanus are too fragmentary , & forelimb length is probably greater relative
to hindlimb length than is true for modern people . Joint breadths of the
forelimbs are much larger than expected from human proportions relative to the
joints of the hindlimb .In fact, A. africanus had relatively larger
forelimb breadths than did A. afarensis . Relatively large forelimbs
characterize H. habilis as well. This is certainly true comparing shaft
breadths, and according to some , but not all, is probably true of estimated
humeral and femoral lengths.
As noted above, the
size of the arm relative to the forearm in the H. ergaster skeletion is very
human-like and not at all similar to any species of Australopithecus
. This is a conspicuous change and adds weight to the
argument favoring a dramatic alteration in locomotor behavior
between the
australopithecines C H. habilis and later Homo. Both the
humerusto-femur length index
(74%) and the ulnar-to-humeral length ratio (85%) of this specimen are
human-like. Other partial skeletons of H. ergaster confirm the
observation that forelimbs
dramatically decreased in relative size.
Shoulders and Trunks
What many would
consider climbing features are also retained in the shoulder and trunk of Australopithecus
C H. habilis but not in later Homo. The shoulder joint appears to be
directed more superiorly in A. afarensis, but this appearance may
not be related to locomotor behavior.The thorax of A.
afarensis is distinctly pongid-like in its funnel shape , but the thorax is
barrel shaped in H. ergaster and later humans.
Perhaps the more pongid shape of th A. afarensis thorax is simply an
artifact of its wider hips, but it is also interpreted as an indication that
this species’ back muscles were specially adapted to climbing.
Hips
Hips transform
dramatically between Australopithecus and Homo. Here, the fossil sample includes a
rich collection of pelvic and femoral specimens, including those that we argue
belong to H. rudolfensis. The pelvic girdles show key bipedal adaptations, such as
shortening of the pelvic blades and anterior rotation of the sacrum. The big
alterations from the ponged condition resulted from changes in the morphogenesis of
the limb. Still, there are conspicuous
differences between Australopithecus
and Homo that are important but harder to explain in terms of
genetic alterations.The most obvious
change from Australopithecus to Homo is in relative hip joint size. But there were interesting changes between the
hips of early Homo and later Homo as
well. Changes in the pattern of gait explain most of the changes in pelvic
morphology between the last common ancestor of African apes and humans, but
changes within the human lineage also involve birth. The shortening of the
pelvic blades to make bipedalism possible reduced the front-to back dimension of the
birth canal. This may or may not have
affected the birth process of small-brained
australopithecines, but it became a painful reality to Homo. It probably explains the
difference between early and late Homo hips.
Unfortunately, no
pelvic remains are known for H. habilis except for a very eroded sacrum.
Something can be discerned about the H. habilis hip on the basis of its
femoral shafts. Analyses using engineering
principles show interesting contrasts between the proximal femora attributed to
australopithecines C H. habilis and H. rudolfensis C later species of archaic Homo.
The ratio of medio lateral bending strength to the
anteroposterior bending strength is much higher in H. rudolfensis and later species of
archaic Homo than in the australopithecines. The one femoral shaft that can
definitely be attributed to H. habilis, O.H. 62, is australopithecine-like in this regard. Ruff
(1995) provides one explanation for this difference that involves femoral neck length
and the shape of the pelvic inlet. In australopithecine hips,
long femoral necks
compensate for the high hip-joint reaction forces generated by the abductor
muscles. These high forces are due to the relatively wide mediolateral dimension of the
birth canal. In H. rudolfensis and later archaic members of the genus Homo,
the hip-joint reaction force increased, as indicated by relatively large joints, and so
did the mediolateral strain of the femoral shafts. This implies, according to
Ruff (1995), that early Homo retained the platypelloid pelvic outlet of Australopithecus
and compensated by increasing the abductor force and
mediolateral strain
on the femoral shafts. Only by Middle Pleistocene times did the rounder pelvic
inlet typical of modern humans evolve, a change that was made possible by the rotation
during birth of the infant’s head.
Femoral Length
It is known with
certainty that relative to humeral length, the femur of A. afarensis was short and that of H. ergaster was long. There is less
certainty about relative femoral length in other early hominid species
because of the fragmentary nature of the fossils, but enough is preserved to
indicate that A. africanus and H.
habilis also had relatively short femora. Associated fore- and hindlimbs
from the Hata beds of Ethiopia’s
Middle Awash probably belong to A. garhi and appear to show femoral lengthening relative
to humeral length. Relative to radial length,
however, the length of this femur is intermediate between humans and apes.
Legs
The tibia and fibula
of the australopithecines C H. habilis are variable, decidedly more human-like than
ape-like, but there remains a debate as to the precise kinematics of the
knee and ankle. These elements are variable in modern human populations, but
all relevant specimens of australopithecines C H. habilis show the key
adaptations to bipedalism, particularly a horizontally oriented talar facet.
Feet
There are numerous
primitive features reported from the pedal remains of A.afarensis, including
relatively long and curved toes and the lack of side-to-side widening of the
dorsal region of the metatarsal heads. Primitive features have also been
emphasized in the description of foot remains from Member 2 of Sterkfontein
that might belong to A. africanus . The primitive qualities of the
Olduvai Hominid 8 foot have been noted, and this foot probably belongs qto H.
habilis. Unfortunately, there are no foot specimens that can be attributed
to H. rudolfensis and only a few scraps
to H. ergaster. There is evidence that the toes of H.
ergaster were shorter and less curved than those of A. afarensis. One of the partial
skeletons of H. ergaster.
Body Size
The body size of Homo habilis
was not significantly larger than the early hominins that preceded them.
Likewise, the arms of habilis and their australopithecine ancestors were
relatively long compared to ours. The modern human body size and limb
proportions began to appear with the next species in our evolution--Homo
erectus. Until the appearance
of H. rudolfensis, the male averages are small by modern human standards
(37–51 kg) and female averages are tiny (29–37 kg). By 1.95 mya, modern-sized
hindlimbs appear in the record. Although it is still uncertain what isolated limb
bones belong to H. rudolfensis, by 1.8 mya there are partial associated skeletons
of H. ergaster that are from big-bodied individuals. What is
particularly striking is the apparent increase in the size of the H. ergaster
female compared with that seen in earlier species of hominid.
For the purposes of
this review, we assumed that H. habilis, H. rudolfensis,H. ergaster, H. ergaster,
and all later species of Homo are monophyletic relative to species of australopithecines.
When scaled to body size, they all share two distinctive and fundamentally
important characteristics not found in combination in any other hominid species: All
species of Homo have both a relatively reduced masticatory system and an expanded
brain.
References :
http://www.uic.edu/classes/bios/bios104/mike/humanevolution/
http://www.massey.ac.nz/~alock/175316new/lecture_notes/lecture_12/humanevollect.html
http://www.modernhumanorigins.net/habilis.html
References :
http://www.uic.edu/classes/bios/bios104/mike/humanevolution/
http://www.massey.ac.nz/~alock/175316new/lecture_notes/lecture_12/humanevollect.html
http://www.modernhumanorigins.net/habilis.html
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