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Ardipithecus

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important in the early evolution of the hominids

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not every paleontologist agrees if Ardipithecus is connected to homo sapien lineage or not

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but it sheds light into what the common ancestor of humans, chimpanzees and bonobos

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a genus of hominids that lives between 5.5 and 4.4 million years ago

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homo sapiens probably shared a common ancestor to chimpanzees and bonobos somewhere between 10 and 7 millions years ago

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might have been one of the earliest ancestors of humans after they diverged from the main ape lineage

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fossil discoveries

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1994 Ardipithecus ramidus

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2009 Ardi

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a small-brained 50-kilogram

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remains include most of the skull and teeth, as well as the pelvis, hands, and feet

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4.4 million years ago

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3.9 million years

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a million years older than Lucy (3.2 million years ago)

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during the course of the Pliocene (5 Ma - 2 Ma)

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hominids began to diversify

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we find more and more different kinds of them

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they became successful

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after 5 Ma we start to find much evidence of various kinds of hominids

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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

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Sahelanthropus tchadensis

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a skull and mandible

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an extinct Hominini species that is dated to about 7 million years ago

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possibly close to the time of the chimpanzee–human divergence

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unclear whether it can be regarded as a member of the Hominini tribe

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not known definitively whether Sahelanthropus tchadensis was indeed bipedal

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has skull features which aligns it to later hominids in many ways

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skull has a hole at the base, how the spinal cord enters

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in humans, who walk upright, that hole is slightly forward

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in chimpanzees and gorillas, the hole is posterior

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where this hole is in the skull is an indicator of the posture of the animal

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Orrorin Tugenensis

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estimated at 6.1 to 5.7 Ma

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found in Kenya

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the femur has features which align them with other hominids and humans

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Ardipithecus is unique because we have a large part of the skeleton and lower body

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base of skull has hole toward the front, indicating that it walked on two feet

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organizing for a vertical posture has consequences throughout the cranial base

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therefore we can argue that Sahelanthropus and Ardipithecus were probably a member of the hominid lineage

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teeth of Ardipithecus

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the story of teeth is a relatively complicated story when we consider early hominids

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chimpanzees, bonobos and gorillas all have thinly enameled and relatively small teeth for their body size

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humans also have thinly enameled and relatively small teeth for our body size

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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

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early hominids underwent an expansion of the teeth

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but the ancestral state of humans, chimpanzees, apes and orangutans

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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

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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

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the early hominids represent an expansion of that

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Ardipithecus teeth are intermediate in terms of molar thickness and molar size

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and so Ardipithecus could well represent our ancestral state

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it was similar to other apes that we see who lived in the Miocene

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does have one feature of its teeth that align it with hominids

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canines in other primates are usually larger than in humans and in early hominids

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our canines are barely larger than our incisors

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have functions in primates in sexual display

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large difference in size between males and females

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used for tearing meat and thick-rinded fruits

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early hominids lose this function

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a dentition that reflects more masticatory power, i.e. much greater chewing

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Ardipithecus hands and feet are fundamentally not like other hominids

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hands

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very long fingers with small finger tips and short thumbs

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good for suspending and climbing in trees

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not good for opposibility, which later hominids had

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more of a climber than a tool maker

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feet

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very long and curving toes

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except for the first one which was substantially shorter than our big toe

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opposed the other toes, therefore created a grasping foot

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good for climbing in trees, not good for bipedality

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an effective quadraped in the trees and on the ground

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but interestingly not very much like living chimpanzees and gorillas

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living chimps and gorillas are knuckle walkers

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can't hyperextend their palms to walk on their palms

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most primates are palm walkers, not knuckle walkers

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Ardipithecus was not a knuckle walker

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suggests that knuckle walking arose separately in chimpanzees and gorillas

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common ancestor of human-chimpanzee-bonobo was an effective quadruped but not a specialized knuckle-walker

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pelvis

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the first case in fossil record in which we have a pelvis to look at

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lucy

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we see changes that align them with the pelvis of humans

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ischium

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long

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placed her hamstring muscle origin in a place that gave its legs effective leverage in a quadrupedel posture

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humans are not as good as this

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sprinters in blocks to start a race

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not a very natural poster for humans

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Ardipithecus was well-suited for this

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ilium

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differs from gorillas and chimps

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chimpanzee, bonobo, and gorilla lower backs are strong and stiff

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humans are different

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short ilium that curves around the front of the body

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move our legs forward in a forward motion while walking

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our lower lumbar spine is movable and we move it in our gait

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Ardipithecus have ilia that is intermediate in form

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Ardipithecus is interesting because it presents a mosaic of information across the different parts of the skeleton

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head and teeth with some similarities to other hominids

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pelvis with some similarities to other hominids

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but arms, legs and hands which look like other primates, other Miocene apes, yet was not a knuckle walker

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Ardipithecus may or may not be in the human ancestor lineage