Science, Just Science

24 April 2007

The Evolution Of Man

Filed under: Science — Kyuuketsuki @ 12:48 pm

Introduction

There exists significant evidence for the evolution of man extending back over 5 million years (and further to man’s ancestors) most of which is based on fossil remains, the geological layer in which the remains were found, and fossils of other species found around them. The fossil record shows a steady development towards modern man.

Discussion

The following is a list of the various hominids fossils:

Ardipithecus ramidus
Dated at 4.4 million years this is the oldest of the known hominid species (White et al. 1994; Wood 1994). Most of the remains are skull fragments but indirect evidence suggests that it may have been bipedal and around 122 cm (4’0″) tall. Other fossils found with ramidus indicate that it may have been a forest dweller. (White et al. have since discovered a skeleton which is 45% complete, but have not yet published on it.)

Australopithecus anamensis
Named in 1995 these consist of 9 fossils found in Allia Bay, Kenya (Leakey et al. 1995). Anamensis is between 3.9 and 4.2 million years old and has a mixture of primitive features in the skull, and advanced features in the body. The teeth and jaws are very similar to those of older fossil apes. A partial tibia (the larger of the two lower leg bones) is strong evidence of bipedality, and a lower humerus (the upper arm bone) is extremely humanlike.

Australopithecus afarensis
Australopithecus afarensis, between 3.0 and 3.9 million years old, had an apelike face with a low forehead, a bony ridge over the eyes, a flat nose, and no chin. They had protruding jaws with large back teeth. Cranial capacity varied from about 375 to 550 cc. And the skull is similar to that of a chimpanzee, except for the more humanlike teeth (canines are much smaller than those of modern apes but larger & more pointed than those of humans and shape of the jaw is shaped part-way between ape and human. However their pelvis and leg bones far more closely resemble those of modern man leaving no doubt that they were bipedal (pref. walking). The finger and toe bones are curved and proportionally longer than in humans, but the hands are similar to humans in most other details (Johanson and Edey 1981). Most scientists consider this evidence that afarensis was still partially adapted to climbing in trees, others consider it evolutionary baggage.

Australopithecus africanus
Australopithecus africanus, 2 to 3 million years old is similar to afarensis, and was also bipedal, but body size was slightly greater. Brain size may also have been slightly larger, ranging between 420 and 500 cc. This is a little larger than chimp brains (despite a similar body size), but still not advanced in the areas necessary for speech. The back teeth were a little bigger than in afarensis. Although the teeth and jaws of africanus are much larger than those of humans, they are far more similar to human teeth than to those of apes (Johanson and Edey 1981). The shape of the jaw is now fully parabolic, like that of humans, and the size of the canine teeth is further reduced compared to afarensis.

Note:
Australopithecus afarensis and africanus are known as gracile australopithecine’s, because of their relatively lighter build, especially in the skull and teeth. Despite this, they were still more robust than modern humans.

Australopithecus aethiopicus
Australopithecus aethiopicus, 2.3 to 2.6 million years old, is known from one major specimen, the Black Skull discovered by Alan Walker, and a few other minor specimens which are thought to belong to the same species. Aethiopicus may be an ancestor of robustus and boisei and its brain size is very small, at 410 cc. Parts of the skull, particularly the hind portions, are primitive and resemble afarensis but others (massive face, the jaws and single tooth found and the largest sagittal crest in any known hominid) are more reminiscent of Australopithecus boisei (Leakey and Lewin 1992).

Australopithecus robustus
Similar to A. africanus, A. robustus had larger and more robust skull and teeth. It existed between 1.5 and 2 million years ago. The massive face is flat or dished, with no forehead and large brow ridges. It has relatively small front teeth, but massive grinding teeth in a large lower jaw. The average brain size is about 530 cc. Bones excavated with robustus skeletons indicate that they may have been used as digging tools.

Australopithecus boisei (was Zinjanthropus boisei)
Australopithecus boisei, 1.1 to 2.1 million years old, was similar to robustus, but the face and cheek teeth were even more massive, some molars being up to 2 cm across. The brain size (530 cc) is very similar to robustus and a few experts consider boisei and robustus to be variants of the same species.

Note:
Australopithecus aethiopicus, robustus and boisei are known as robust australopithecine’s, because their skulls in particular are more heavily built.

Homo habilis (“Handy Man”)
Homo habilis, 1.15 to 2.4 million years old, has been found with evidence of tools. It is very similar to the australopithecine’s in many ways e.g. a primitive face but projecting less than in A. africanus. The back teeth are smaller, but still considerably larger than in modern humans. The average brain size, at 650 cc (varies between 500 and 800 cc), is considerably larger than in australopithecine’s and is also more humanlike. The bulge of Broca’s area, essential for speech, is visible in one habilis brain cast, and indicates it was possibly capable of rudimentary speech. Habilis is thought to have been about 127 cm (5’0″) tall, and about 45 kg (100 lb.) in weight, although females may have been smaller.

Note:
Habilis has been a controversial species. Some scientists have not accepted it, believing that all habilis specimens should be assigned to either the australopithecine’s or Homo erectus. Many now believe that habilis combines specimens from at least two different Homo species.

Homo erectus
Homo erectus, 300,000 to 1.8 million years old, has protruding jaws with large molars, no chin, thick brow ridges, a long low skull and a significantly larger brain (early specimens averaging around 900 cc, late around 1100 cc – Leakey 1994). Some (Asian) have a sagittal crest and the skeleton is more robust than those of modern humans, implying greater strength. Body proportions vary, the Turkana Boy being tall and slender (like modern humans from the same area) whilst the few limb bones found of Peking Man indicate a shorter, sturdier build. Study of the Turkana Boy indicates that Homo erectus may have been more efficient at walking than modern humans, whose skeletons have had to adapt to allow for the birth of larger-brained infants (Willis 1989). The evidence indicates that Homo habilis and the australopithecines all lived in Africa, but erectus was wide-ranging (Africa, Asia, and Europe). There is evidence that erectus probably used fire, and their stone tools are more sophisticated than those of habilis.

Homo sapiens (archaic)
Archaic forms of Homo sapiens first appear about 500,000 years ago and are so termed because the skulls are diverse and have features of both Homo erectus and modern humans. The brain size is larger than erectus and smaller than most modern humans, averaging about 1200 cc, and the skull is more rounded than in erectus. The skeleton and teeth tend to be less robust than erectus but more so than modern humans with many still featuring large brow ridges, receding foreheads and chins. There is no clear dividing line between late erectus and archaic sapiens, and many fossils between 200,000 and 500,000 years ago are difficult to classify as one or the other.

Homo sapiens neanderthalensis (was Homo neanderthalensis)
Neanderthal man, between 30,000 and 230,000 years old, has an average brain size slightly larger than that of modern humans (about 1450 cc) but it was probably correlated with their greater bulk. The brain case however is longer and lower than that of modern humans, with a marked bulge at the back of the skull and, like erectus, they had a protruding jaw and receding forehead. The chin was usually weak and the mid-facial area protrudes (a feature that is not found in erectus or sapiens) which may have been an adaptation to cold. Neanderthals mostly lived in cold climates, and their body proportions are similar to those of modern cold-adapted peoples: short and solid, with short limbs (males averaged about 168 cm in height). Their bones are thick & heavy, showing signs of powerful muscle attachments, and Neanderthals would have been extraordinarily strong by modern standards … their skeletons show that they endured brutally hard lives. A large number of tools and weapons have been found, indicating greater advancement than Homo erectus. Neanderthals were formidable hunters, and are the first people known to have buried their dead, with the oldest known burial site being about 100,000 years old. They are found throughout Europe and the Middle East and Western European Neanderthals usually have a more robust form: “classic Neanderthals” (Trinkaus and Shipman 1992; Trinkaus and Howells 1979; Gore 1996).

Homo sapiens sapiens (modern)
Modern forms of Homo sapiens first appear about 120,000 years ago (some scientists argue it is more recent).

Modern humans have an average brain size of about 1350 cc. The forehead rises sharply, eyebrow ridges are very small or more usually absent, the chin is prominent, and the skeleton is very gracile. About 40,000 years ago, with the appearance of the Cro-Magnon culture, tool kits started becoming markedly more sophisticated, using a wider variety of raw materials such as bone and antler, and containing new implements for making clothing, engraving and sculpting. Fine artwork, in the form of decorated tools, beads, ivory carvings of humans and animals, clay figurines, musical instruments, and spectacular cave paintings appeared over the next 20,000 years. (Leakey 1994)

Conclusion

Even within the last 100,000 years, the long-term trends towards smaller molars and decreased robustness can be discerned. The face, jaw and teeth of Mesolithic humans (about 10,000 years ago) are about 10% more robust than ours. Upper Palaeolithic humans (about 30,000 years ago) are about 20 to 30% more robust than the modern condition in Europe and Asia. These are considered modern humans, although they are sometimes termed “primitive”. Interestingly, some modern humans (aboriginal Australians) have tooth sizes more typical of archaic sapiens. The smallest tooth sizes are found in those areas where food-processing techniques have been used for the longest time. This is a probable example of natural selection that has occurred within the last 10,000 years (Brace 1983).

References

  • “Evolution of Man”, (Author Unknown).
  • Hominid Species”, Jim Foley (1997)
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