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Similarities between apes and humans, and the implications for human evolution

The anatomical evidence - both fossil and contemporary - demonstrates that australopithecines and chimpanzees share a geologically recent common ancestor and that Homo sapiens are descendants of the evolutionary branch that began with the divergence of the australopiths.

The anatomical characteristics that link the australopiths to Homo, and show their intermediate form between modern humans and the last common ancestor between humans and chimpanzees, include:
* The canines of the australopiths do not project much further forward in relation to the other teeth than they do in Homo;
* Australopith canines also show a decrease in sexual size differences over time - the more recent forms are more like the condition of modern humans;
* Tooth enamel progresses to a more Homo-like thickness over time;
* Wear patterns on australopith teeth suggest a "crushing" action, similar to that of Homo;
* The cranial capacity of the australopiths increases to a capacity range approaching that of early Homo;
* The australopith foramen magnum, which allows the spinal cord to connect with the base of the brain, is located more toward the base of the skull than in apes, yet not completely under the skull, as in Homo but excluding the robust australopiths (also known as Paranthropus) where it was just as in Homo; and
* The features of the tibiae (orientation angle, thickness and internal structure) shared by australopiths and Homo reflect the demands placed on their bodies by bipedalism.

The anatomical similarities between chimpanzees and anatomically modern humans (Homo sapiens) can be summarized as follows:
* In both species, the rib cage is broad from side to side and shallow from front to back; the rib cage extends back beyond the vertebral column
* Both have a dorsally-placed scapula and shoulder joints facing outward to the side, giving humans a mobile shoulder joint; a hangover from our arboreal ancestry; and
* The positioning and angle of the humeral shaft and humeral head and other joints in the forelimb are the same in both species.

Table 1 below summaries the similarities and differences between chimpanzees, australopiths and modern humans as a result of millions of years of evolution.

Modern chimpanzees Australopiths Modern humans
Canines larger and project out from tooth row Canines slightly larger, but non-projecting Canines of similar size to other teeth and non-projecting
Extended canine size determined by sexual dimorphism Moderate canine size determined by sexual dimorphism Minimal canine size determined by sexual dimorphism
Thin tooth enamel Moderate tooth enamel Thick tooth enamel
Dental wear pattern shows grinding action Dental wear pattern shows crushing action Dental wear pattern shows crushing action
Cranial capacity average 400 cc Cranial capacity 350 - 540 cc Cranial Capacity > 1000 cc
Foramen magnum opens toward rear of skull Foramen magnum opens between rear and base of skull Foramen magnum opens at base of skull
Tibiae thin and angled Tibiae thicker and straighter Tibiae thick and straight
Rib cage broad and extends past vertebral column Rib cage broad and extends past vertebral column Rib cage broad and extends past vertebral column
Scapulae on the back, shoulder joints oriented to the sides Scapulae on the back, shoulder joints oriented to the sides Scapulae on the back, shoulder joints oriented to the sides

It is also worthwhile noting Bernard Wood and Brian Richmond's (2000) summary of comparative limb morphology: "The substantial differences between the lower limbs of modern humans and apes are largely attributable to the bipedal locomotion of the former. The most striking difference is the great absolute and relative length of modern human lower limbs that increases stride length and thus the speed of bipedal walking. Because the lower limbs support the body during bipedal gait, the acetabulum, femoral head and other lower limb joints are relatively larger in humans. Modern human femora are distinctive in that they show the valgus condition (i.e. they converge towards the knees), thus helping to position the feet closer to the midline."

In addition, recent reviews of the available anatomical (Shoshani et al. 1996) and genetic evidence (Ruvolo 1995, 1997; Wise et al. 1997) have convincingly re-affirmed yet again the theory that apes and anatomically modern humans share a common ancestry. The DNA sequences of human chromosomes 2 and 4 have been completely analysed and were published in the April 7 (2005) issue of Nature, reinforcing the conclusions reached by previous studies. In essence, the chimp chromosomes 2a and 2b fused to form the human chromosome 2. Previous comparisons between the chimpanzee and human genomes and other known genomes have yielded a gene which appears to be functional only in chimpanzees and humans. This gene is suggested to make a protein in the brain and testicals. Furthermore, geneticists have analysed the differences in the amino acid sequences of protein and in the base sequences of DNA from apes and humans; the results have yielded a divergence time-frame of 5-8 million years ago.

A good case study is that of the Dikika child, first published in 2006 and found by the Dikika Research Project members in Ethiopia. The sandstone sediment in which the child was found was deposited on a subaerial delta plain; the age of the Sidi Hakoma Member ranges from 3.31 - 3.35 million years. When combined with the lack of pre-weathering of the anatomical remains, these factors indicate that the child was buried in a flood. The initial anatomical report (Alemseged et al. 2006) is the accumulation of five years of painstaking cleaning and examination. Most of the postcranial remains were covered by sandstone matrix, together with the cranium's midface, left temporal bone and the cranial base. Analysis of the skeletal elements places the child firmly within the known range for Australopithecus afarensis and distinguish them from modern gorillas and chimpanzees. CT-scans were used to model the age using an African ape model. The resulting 3 years of age would not differ by more than a year and a half if a modern human model had been used instead. Given that A. afarensis is a hominin, the utilisation of an African ape model is a prudent measure for a minimum age. The tibiae has a sharper anterior border and its muscle attachment orientations resemble that of modern humans'. In addition, also like modern humans, the side of the upper part of the shaft is a little concave, becoming convex towards the back. The ape-like scapula, the longer phalanges and the reconstructed environmental settings (woodland with a nearby plain) all combine to reinvigorate investigations into the mobility patterns of A. afarensis. The basis of these investigations will likely be that A. afarensis combined a form of arboreal behaviour with habitual bipedalism, as reflected by the more derived lower body which would have been under heavy selection pressures. As more of the sandstone matrix is removed from the skeletal remains and further analysis is made possible (hopefully including isotope analysis), expect to see the new models of hominin growth, dietary and evolutionary patterns emerging over the coming years. These will yield valuable insights which can then be tested against the known and future anatomical remains of both A. afarensis and other hominin species.

These models and predictions are based upon detailed anatomical evidence and do not use any pseudoscientific stance as either the starting point or the given conclusion. Those who back such fundamentalist works display a profound ignorance of basic 1st year scientific methods, which places a huge question mark over the reliability of their own published and presented works. Creationist works, and those who support such efforts, have no basis whatsoever in any scientific procedure and basic plain scientific reality.


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