Chapter 10, Early Hominid Origins and Evolution: The Roots of Humanity
What is a Hominid?
Bipedal Locomotion: Getting Around on Two Feet
Evolved before large brain size
Walking on two limbs (with associated skeletal changes)
Origin or Bipedalism: Three Hypotheses
Hypothesis 1: Darwin’s Hunting Hypothesis
Bipedalism freed the hands for carrying weapons.
Intelligence increased; size of canines diminished.
Tool production & use essential for development of human intelligence.
New evidence shows this not to be the case.
Hypothesis 2: Rodman and McHenry’s Patchy Forest Hypothesis
Human origins and bipedalism related to the greater efficiency in some habitats of moving on two limbs rather than four.
Bipedalism arose in areas where forests were fragmented and food resources also
As forests fragmented, bipedalism freed the hands to pick up food allowing for both
tree and ground food resources to be exploited.
Freeing the hands was important in allowing males to assist females more
efficiently in procuring food; thus “provisioning” acted as a form of sexual
competition among males for females.
Birth spacing would also be reduced, since females would have to move around
less and would have access to greater food resources.
Lovejoy’s hypothesis makes the argument for monogamous fathers.
Bipedality had its benefits and costs” An evolutionary tradeoff
Bipedalism was a clear advantage to early humans.
It allows for a better view of the horizon, but also exposes the individual to predators.
Walking along with lifting or carrying heavy loads can cause back injuries.
The circulatory system also faces a greater burdern.
Loss of Sexual Dimorphism
Many primate species are highly sexually dimorphic, with males larger than females.
Some scientists see little sexual dimorphism in early hominids; therefore males would have
been more cooperative and less competitive.
Who Were the First Hominids?
The Pre-Australopithecines: Sahelanthropus tchadensis
Located in central Africa and dated to 7–6 mya
Brain size (cranial capacity) of 350 cc
Foramen magnum indicative of likely bipedality
Close to pongid/hominid divergence
The Pre-Australopithecines: Orrorin tugenensis
Located near Lake Turkana and dated to 6 mya
Femurs indicative of bipedalism
Curved hand phalanx, suggesting time spent in trees
Nonhoning chewing complex
Lived in a forest
The Pre-Australopithecines: Ardipithecus kadabba and Ardipithecus ramidus
Located at Aramis and dated to 5.8–4.4 mya
Partial skeleton, other bones, and teeth
Variation in tooth wear; possessed thin enamel
Lived in a forest
The Pre-Australopithecines” Australopithecus anamensis
Located at Lake Turkana and Ethiopia and dated to 4 mya
Physically somewhat similar to Ardipithecus
Large canines, parallel tooth rows, different cusp pattern on lower first premolar
The Australopithecines (4–1 mya) : Australopithecus afarensis (3.6–3.0 mya)
Located in Laetoli and Hadar (found first in Hadar) and dated to 3.6–3.0 mya
Bipedal but with shorter legs, resulting in a slightly different stride
Cranial capacity of 430 cc
At Laetoli, assemblages include hominids as well as footprints of three hominids
Lived in varied habitats
The Australopithecines (4–1 mya): Australopithecus (Kenyanthropus) platyops (3.5 mya)
Found at Lake Turkana and dated to 3.5 mya
Evolution and Extinction of the Australo-pithecines: Australopithecus aethiopicus and Australopithecus boisei
Found in several locations in Africa
A. aethiopicus dates to 2.5 mya and had a cranial capacity of 410 cc
A. boisei dates to 2.3–1.2 mya and had a cranial capacity of 510 cc
Sagittal crest for attachment of chewing muscles
Dietary focus on harder foods
Evolutionary dead end
Evolution and Extinction of the Australo-pithecines: Australopithecus africanus
Larger teeth than A. afarensis
Brain size of 450 cc
Evolution and Extinction of the Australopithecines: Australopithecus robustus
Found in South Africa and dated to 2 mya
Similar to East African forms
Brain size of 530 cc
Australopithecus garhi (2.5 mya): First Maker and User of Tools
Located in Ethiopia and dated to 2.5 mya
Bones, teeth, partial skeleton, and a skull
More humanlike humerus-to-femur ratio
Cranial capacity of 450 cc
Probable ancestor of Homo
Associated mammal bones with cutmarks, leading to conclusion that A. garhi made
Oldowan tools, the earliest stone tools.
Stone tools long associated with meat consumption; some wear may indicate tools were
also used for digging in ground.