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From Hominoid to Hominin
During the Miocene, the Earth’s temperature began to fall. This global cooling caused two important changes in the climate of
the African tropics.
o The total amount of rain that fell each year declined.
o Rainfall became more seasonal, so there were several months each year when no rain fell.
As the tropical regions of Africa became drier, moist tropical forests shrank and woodlands and grasslands expanded.
Changes brought about through generations of natural selection allowed a few species to move down from the trees, out of the
rain forests, and into the woodlands and savannahs.
Six m.y.a, climate and geographic change led to the evolution of hominins.
o Cooling, less rain, rain seasonality, woodland and savannah spread.
These creatures were different from any of the Miocene apes in two ways.
o First, and most important, they walked upright. This shift to bipedal locomotion led to major morphological changes
in their bodies and new food became available.
o Second, some of the hominin species began to exploit new savannah and woodland habitats, and new kinds of food
As a result, of the hominin chewing apparatus-including many features of the teeth, jaws, and skull-
What is Unique about Us?
We walk on two legs (bipedalism).
We have small canines and large molars with thick enamel.
We have large brains in relation to our body size.
We have very slow life histories and a long juvenile period.
We talk, and have elaborate symbolic culture.
What It Takes to Be a Biped
o Foramen magnum position
o Lumbar lordosis
o Short and stout in a biped
o Abductor mechanism
o Long femoral neck
o Cortical bone distribution
o Bicondylar angle
Foot and ankle
o Nongrasping big toe
Between 7-5 m.y.a, the common ancestor of chimps and humans roamed the earth.
Sahelanthropus tchadensis, discovered in 2002, I the earliest known hominin, living around 7-6 m.y.a. It has both derived and
At the Beginning: Sahelanthropus
Genetic data indicate that the last common ancestor of humans and chimpanzees lived between 7- 5 m.y.a.
Is the earliest known hominin, has a surprising mix of derived and primitive features.
The fossil material consists of a nearly complete cranium (skull minus the lower bone), four partial mandibles and four teeth.
This fossil shook up the paleontological community because of its age and location. Represents the oldest known hominin.
6-7 million years old.
o Foramen magnum, the hole in the skull through which the spinal cord passes, is located under the skull, rather than
at the back-> suggests bipedalism.
o Chimpanzee-sized brain.
o Small canines, upper canines not sharpened against the lower premolar, and the enamel is thicker.
o Flat face; large brow ridge-> associated with hominins who date to later than 2 m.y.a.
Orrorin tugenensis, discovered in 2001 by Kenyan-French team, is also similar to both chimps (e.g., incisors, canines, one
premolar) and humans (e.g., small maloras, thick enamel, thigh bones).
At the Beginning: Orrorin
6 million years old
Mix of woodland and savannah
o Femur suggests bipedalism
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