Question

1. Hominin Skulls

Explain the cranial features that distinguish the different hominin species that we covered this semester. When you make comparisons (e.g. smaller, larger, lower, higher, etc...) then be sure to explain which species you are comparing it to. Your answer will need to clearly describe and explain enough features for each to show that you would be able to correctly identify the skulls these species:

1. Australopithecus afarensis
   2)  Homo habilis
   3)  Homo erectus
   4)  Homo heidelbergensis
   5)  Homo neanderthalensis
   6)  Homo sapiens

Answer 1

1) Australopithecus afarensis

Brain

• brain was small, averaging approximately 430 cubic centimetres and comprised about 1.3% of their body weight

Skull

• many cranial features were quite ape-like, including a low, sloping forehead, a projecting face, and prominent brow ridges above the eyes.
• unlike most modern apes, this species did not have a deep groove lying behind its brow ridge and the spinal cord emerged from the central part of the skull base rather than from the back.
• males had a bony ridge (a sagittal crest) on top of their skull for the attachment of enormous jaw muscles. In this species, the crest was very short and located toward the rear of the skull.
• a small hyoid bone (which helps anchor the tongue and voice box) found in a juvenile specimen suggests A. afarensis had a chimp-like voice box
• semi-circular ear canal similar in shape to African apes and A. africanus, suggesting this species was not as fast or agile on two legs as modern humans

Jaws and teeth

• jaws and teeth were intermediate between those of humans and apes:
• jaws were relatively long and narrow. In the lower jaw, the teeth were arranged in rows that were slightly wider apart at the back than at the front. In the upper jaw, the placement of the last molar results in tooth rows that curve in at the back.
• front incisor teeth were quite wide.
• canine teeth were pointed and were longer than the other teeth. Canine size was intermediate between that of apes and humans. Like apes, males had much larger canines than females.
• a gap (diastema) was often present between the canines and adjacent teeth. This ape-like feature occurred between the canines and incisors in the upper jaw, and between the canines and premolars of the lower jaw.
• premolar teeth in the lower jaw had ape-like cusps (bumps on the chewing surface). The front premolar tended to have one large cusp (ape-like) rather than two equal-sized cusps as in humans.
• back molar teeth were moderate in size and were human-like in having a ‘Y-5’ pattern. That is, they had five cusps arranged so that the grooves between the cusps form a Y-shape.

2) Homo habilis: The differences between Homo habilis and Australopithecus are, Homo habilis possesses a greater cranial capacity(610 to about 800cc), it has reduced prognathism- a flatter face and a shorter tooth row.

Brain

• brain averaged 610 cubic centimetres in size, representing 1.7 per cent of their body weight. This was a significant increase compared to australopithecine brains.

Skull

• brain case had become fuller and more rounded due to expansion of the brain
• beginnings of a slight forehead were appearing
• face had a small, arched brow ridge and was smaller and shorter than those of earlier ancestors
• hole for the spinal cord was located in the centre of the skull base, showing that this species walked on two legs
• facial projection was reduced compared with earlier species

Jaws and teeth

• jaw was smaller than those found in the earlier australopithecines
• teeth were arranged in a more rounded arc like those of modern humans
• teeth had become smaller and more human-like, although the incisors were still relatively large

3) Homo erectus:  H. erectus coexisted in East Africa with several other early human species including Homo rudolfensis, Homo habilis, and Paranthropus boisei. Sometimes they were even found at the same fossil sites.

Brain

• showed an increase in size over earlier species and averaged about 1050 cubic centimetres
• structure of the brain was similar to that of modern humans

Skull

• face was large with a low, sloping forehead, a massive brow ridge and a broad, flat nose
• skull was broad and long with sharp angles at the rear, unlike the curve found in modern humans
• bones of the skull were very thick and formed a small central ridge, known as a midline keel, along the top of the skull

Jaws and teeth

• jaw was large and thick without a pointed chin
• molar teeth had large roots but were decreasing toward a more modern size

4) Homo heidelbergensis: Homo heidelbergensis fossils tend to have features that are intermediate between those of Homo ergaster and either Homo neanderthalensis or Homo sapiens.

Brain

• brain was large, averaging approximately 1250 cubic centimetres in size, representing 1.9% of their body weight
• frontal and parietal lobes of the brain were enlarged and may indicate an increase in brain complexity

Skull

• small post-orbital constriction behind the eye sockets.
• a moderate, double arched brow ridge and a short, sloping forehead lay above the eyes. The brow ridge was more arched than that of the earlier species, Homo ergaster. The sloping forehead resembled those found in earlier species rather than the vertical foreheads of modern humans.
• nasal opening was relatively wide

Jaws and teeth

• jaws were shorter than those of earlier species resulting in a face with only a slight projection
• some members of this species possessed a gap, called the retromolar space, behind the third molars (or wisdom teeth) at the back of the jaw. Others had only a tiny gap or no gap.
• lower jaw was strongly built for the attachment of strong chewing muscles
• as with earlier species, the lower jaw did not have a protruding, pointed chin
• teeth were arranged in the jaw so that they formed a parabolic shape (curved at the front then splayed out toward the back)
• teeth were smaller than those of earlier species but were larger than those of modern humans

5) Homo neanderthalensis

Brain

• brain size was larger than the average modern human brain and averaged 1500 cubic centimetres. This is expected, as Neanderthals were generally heavier and more muscular than modern humans. People that live in cold climates also tend to have larger brains than those living in warm climates.

Skull

• distinctive skull shape that was long and low, with a rounded brain case
• back of the skull had a bulge called the occipital bun and a depression (the suprainiac fossa) for the attachment of strong neck muscles
• thick but rounded brow ridge lay under a relatively flat and receding forehead
• mid-face region showed a characteristic forward projection (this resulted in a face that looked like it had been ‘pulled’ forward by the nose)
• orbits (eye sockets) were large and rounded
• nose was broad and very large

Jaws and teeth

• jaws were larger and more robust than those of modern humans and had a gap called the retromolar space, behind the third molars (wisdom teeth) at the back of the jaw.
• jaw lacked the projecting bony chin that is found in Homo sapiens.
• teeth were larger than those of modern humans.

6) Homo sapiens

Brain

• Homo sapiens living today have an average brain size of about 1350 cubic centimetres which makes-up 2.2% of our body weight. Early Homo sapiens, however, had slightly larger brains at nearly 1500 cubic centimetres.

Skull

• modern Homo sapiens skulls have a short base and a high braincase. Unlike other species of Homo, the skull is broadest at the top. The fuller braincase also results in almost no post-orbital constriction or narrowing behind the eye sockets
• back of the skull is rounded and indicates a reduction in neck muscles
• face is reasonably small with a projecting nose bone
• brow ridge is limited and the forehead is tall
• orbits (eye sockets) are square rather than round

Jaws and teeth

• jaws are short which result in an almost vertical face
• usually no gap (retromolar space) between the last molar teeth and the jaw bone
• jaws are lightly built and have a protruding bony chin for added strength. Homo sapiens is the only species to have a protruding chin.
• shortened jaw has affected the arrangement of the teeth within the jaw. They are now arranged in a parabolic shape in which the side rows of teeth splay outwards rather than remain parallel as in our earliest long jawed ancestors.
• teeth are relatively small compared with earlier species. This is especially noticeable in the front incisor and canine teeth.
• front premolar teeth in the lower jaw have two equal-sized cusps (bumps on the chewing surface)