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C O U R S E 
Human Evolution: Past and Future
John Hawks, University of Wisconsin
C O U R S E   L E C T U R E 
The Place of Ardipithecus
Notes taken on September 6, 2015 by Edward Tanguay
important in the early evolution of the hominids
not every paleontologist agrees if Ardipithecus is connected to homo sapien lineage or not
but it sheds light into what the common ancestor of humans, chimpanzees and bonobos
a genus of hominids that lives between 5.5 and 4.4 million years ago
homo sapiens probably shared a common ancestor to chimpanzees and bonobos somewhere between 10 and 7 millions years ago
might have been one of the earliest ancestors of humans after they diverged from the main ape lineage
fossil discoveries
1994 Ardipithecus ramidus
2009 Ardi
a small-brained 50-kilogram
remains include most of the skull and teeth, as well as the pelvis, hands, and feet
4.4 million years ago
3.9 million years
a million years older than Lucy (3.2 million years ago)
during the course of the Pliocene (5 Ma - 2 Ma)
hominids began to diversify
we find more and more different kinds of them
they became successful
after 5 Ma we start to find much evidence of various kinds of hominids
but before 5 Ma it is hard to find fossils of hominids intact enough for us to differentiate them from apes that may or may not be closely linked to us
Sahelanthropus tchadensis
a skull and mandible
an extinct Hominini species that is dated to about 7 million years ago
possibly close to the time of the chimpanzee–human divergence
unclear whether it can be regarded as a member of the Hominini tribe
not known definitively whether Sahelanthropus tchadensis was indeed bipedal
has skull features which aligns it to later hominids in many ways
skull has a hole at the base, how the spinal cord enters
in humans, who walk upright, that hole is slightly forward
in chimpanzees and gorillas, the hole is posterior
where this hole is in the skull is an indicator of the posture of the animal
Orrorin Tugenensis
estimated at 6.1 to 5.7 Ma
found in Kenya
the femur has features which align them with other hominids and humans
Ardipithecus is unique because we have a large part of the skeleton and lower body
base of skull has hole toward the front, indicating that it walked on two feet
organizing for a vertical posture has consequences throughout the cranial base
therefore we can argue that Sahelanthropus and Ardipithecus were probably a member of the hominid lineage
teeth of Ardipithecus
the story of teeth is a relatively complicated story when we consider early hominids
chimpanzees, bonobos and gorillas all have thinly enameled and relatively small teeth for their body size
humans also have thinly enameled and relatively small teeth for our body size
but when we look at early hominids who are very close to humans in terms of their locomotive anatomy, their teeth have thick molar enable and quite large molars and premolars in many cases
early hominids underwent an expansion of the teeth
but the ancestral state of humans, chimpanzees, apes and orangutans
almost every ape fossil that we have from the Miocene Period (23-5 Ma), have relatively thicker molar enamel then what is found in modern chimpanzees
it looks like we came from a heritage where the teeth were quite generalized, a little bit thicker and bigger for body size of living chimpanzees but about the same thickness and size of living orangutans
the early hominids represent an expansion of that
Ardipithecus teeth are intermediate in terms of molar thickness and molar size
and so Ardipithecus could well represent our ancestral state
it was similar to other apes that we see who lived in the Miocene
does have one feature of its teeth that align it with hominids
canines in other primates are usually larger than in humans and in early hominids
our canines are barely larger than our incisors
have functions in primates in sexual display
large difference in size between males and females
used for tearing meat and thick-rinded fruits
early hominids lose this function
a dentition that reflects more masticatory power, i.e. much greater chewing
Ardipithecus hands and feet are fundamentally not like other hominids
very long fingers with small finger tips and short thumbs
good for suspending and climbing in trees
not good for opposibility, which later hominids had
more of a climber than a tool maker
very long and curving toes
except for the first one which was substantially shorter than our big toe
opposed the other toes, therefore created a grasping foot
good for climbing in trees, not good for bipedality
an effective quadraped in the trees and on the ground
but interestingly not very much like living chimpanzees and gorillas
living chimps and gorillas are knuckle walkers
can't hyperextend their palms to walk on their palms
most primates are palm walkers, not knuckle walkers
Ardipithecus was not a knuckle walker
suggests that knuckle walking arose separately in chimpanzees and gorillas
common ancestor of human-chimpanzee-bonobo was an effective quadruped but not a specialized knuckle-walker
the first case in fossil record in which we have a pelvis to look at
we see changes that align them with the pelvis of humans
placed her hamstring muscle origin in a place that gave its legs effective leverage in a quadrupedel posture
humans are not as good as this
sprinters in blocks to start a race
not a very natural poster for humans
Ardipithecus was well-suited for this
differs from gorillas and chimps
chimpanzee, bonobo, and gorilla lower backs are strong and stiff
humans are different
short ilium that curves around the front of the body
move our legs forward in a forward motion while walking
our lower lumbar spine is movable and we move it in our gait
Ardipithecus have ilia that is intermediate in form
Ardipithecus is interesting because it presents a mosaic of information across the different parts of the skeleton
head and teeth with some similarities to other hominids
pelvis with some similarities to other hominids
but arms, legs and hands which look like other primates, other Miocene apes, yet was not a knuckle walker
Ardipithecus may or may not be in the human ancestor lineage