Hominin
the group consisting of modern humans, extinct human species and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus and Ardipithecus).
Hominid
the group consisting of all modern and extinct Great Apes (that is, modern humans, chimpanzees, bonobos, gorillas and orang-utans . . . plus all their immediate ancestors).
Sahelanthropus
Oldest hominin
Approx. 7 MYA (Miocene)
Chad
Sahelanthropus tchadensis mosaic of primitive and advanced features
Small brain case
Most teeth (not canines) are chimp-like
Fairly flat nose and prominent brow ridge (until now) only seen in Homo
Probably ancestor to Orrorin
Possibly very close to the time of the chimpanzee–human divergence
Few specimens are known, other than the partial skull nicknamed ‘Toumaï’
May have been bipedal but more bones (from legs) need to be found first to confirm this
Orrorin
- Kenya
- 6.1-5.7 million years (Ma)
- Small teeth relative to its body size.
- Its dentition differs from that found in Australopithecus in that its cheek teeth are smaller and less elongated mesio-distally and from Ardipithecus in that its enamel is thicker.
- The dentition differs from both these species in the presence of mesial groove on the upper canines. The canines are ape-like but reduced, like those found in Miocene apes and female chimpanzees.
- Orrorin had small post-canines and was microdont like modern humans, whereas robust Australopithecenes were megadont.
Bipedalism in Orrorin
Femur – spherical head, rotated anteriorly neck is elongated and oval in section lesser trochanter protrudes medially. While this suggest that Orrorin was bipedal, the rest of the postcranium indicates it climbed trees. While the proximal phalanx is curved, the distal pollical phalanx is of human proportions and has thus been associated with toolmaking, but should probably be associated with grasping abilities useful for tree-climbing in this context. Very possibly filling a niche suited to something somewhere between a human-chimp in terms of its locomotion.
Ardipithecus
- Ethiopia
- 5.6 - 4.4 MYA (Late Miocene and Early Pliocene)
- Originally described as one of the earliest ancestors of humans after they diverged from the main ape lineage, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate.
- Two fossil species are described:
- A. kadabba, dated to approximately 5.6 million years ago (late Miocene)
- A. ramidus, which lived about 4.4 million years ago (early Pliocene)
- Behavioral analysis showed that Ardipithecus could be very similar to chimpanzees, indicating that the early human ancestors were very chimpanzee-like in behaviour.
Paranthropus
Para- “beside” -anthropos “human”
- Ca. 2.7 Mya
- AKA ‘robust australopithecines’
- Bipedal hominids probably/possibly descended from ‘gracile’ australopithecine hominids (Australopithecus)
- Three species described:
- P. aethiopicus
- P. boisei
- P. robustus
Robust craniodental features
Zygomatic arches Mandible Maxilla Sagittal crest No transverse cranial crests (cf. gorillas) Broad, well-enameled teeth
Why Bipedalism?
- Most energy efficient way to travel long distances.
- Carrying capacity.
- Reduction of overall heat stress – facilitates heat loss through convection (by exposing body to air currents. Only humans (that we know of) have sweat glands that produce moisture to cool the body.
- Allows for better vision in open environments and defensive action by freeing hands to throw objects (predators and competitors).
Energy Efficiency
Human walking is about 75% less costly than both quadrupedal and bipedal walking in chimpanzees.
Some hypotheses have been supported that bipedalism increased the energetic efficiency of travel and that this was an important factor in the origin of bipedal locomotion.
Humans save more energy than quadrupeds when walking but not when running. Human running is 75% less efficient than human walking. A study helped to prove that walking of living hominin bipeds is noticeably more efficient than walking of living hominin quadrupeds, but the costs of quadruped and bipedal travel (over shorter distances) are the same.
