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refers to the posterior (i.e., back) of the head; the bone located at the back and base of the cranium.
a relative term referring to the part of the mouth that is closer to the crown of the tooth; the opposite of distal.
relationship between the upper and lower portions of the teeth.
Old World monkey
primates that are in the superfamily Cercopithecoidea whose natural habitats are in all regions of the world, except Central and South America.

a tool industry that approximately dates from 2.5 million years ago to 250,000 years ago; characterized by simple choppers and flakes that represent the oldest types of stone tools used mainly for food procurement and processing. The choppers are often very crude and have only a few flakes removed. The flakes were once thought to have been byproducts of the manufacture of the choppers, but are now believed to have also been used for cutting.

an African paleontological and archaeological site located in northern Tanzania in the Great Rift Valley. This site is well known for the work done by Louis Leakey starting in the 1950s and continued by his family. Olduvai has produced fossils of hominins such as Australopithecus bosei and Homo habilis.
olecranon-ulnar index

the ratio of the length of the olecranon process of the ulna to the length of the ulna from the center of the trochlear notch to the distal head, multiplied by 100.


an epoch in the Cenozoic era on the geologic time scale that approximately dates from 34 million years ago to 24 million years ago; marked by the expansion of terrestrial mammals and the diversification of primates. The Oligocene is the third epoch of the Tertiary period.


a dietary category that describes animals with no specialized diet and will eat any edible and/or nutritional foodstuffs (i.e., plants and animals).

open environment
an geographic area with little to no canopy of foliage allowing a substantial amount of light to reach the ground.
opposable thumb
a thumb that is able to rotate along its long axis and can grip objects.
a cavity in the face that protects the eyes and its associated muscles and nerves.
orbital convergence
in primate evolution, the realignment of the orbits from the lateral sides of the face to orbits on the anterior portion of the face.
in taxonomic classification, all members of the same Family but only certain members of the the same Class.
a period in the Paleozoic era on the geologic time scale that approximately dates from 500 million years ago to 430 million years ago; marked by prevalence of trilobites and many shallow inland seas.
a functional unit of animal or plant life. An organism can consist of one living cell, such as algae, or many living cells, such as a single human being.
[muscle origin] the point from which a muscle arises and is not involved in the movement or action of the muscle.
original horizontality
the belief that smaller sediments are usually deposited in a horizontal layer. Therefore, if a stratum is tilted geologists are able to calculate the declination (i.e., degree of tilt or distrubance) from the horizontal.
Orrorin tugenensis

an extinct fossil hominin species that existed in Africa approximately 6 million years ago; fossil remains found at Tugen Hills, Kenya, include portions of a femur that exhibits evidence of bipedalism (i.e, a bicondylar angle).

os coxae
[syn. pelvis, hip bone, or innominate] [sing. os coxa] formed of two halves, each consisting of three fused bones: ilium, ischium, and pubis.
a cell that produces or deposits bone tissue by depositing protein.
a cell that erodes bone tissue.
Out of Africa Hypothesis

a hypothesis that states that an ancestral form to modern Homo sapiens evolved in Africa about 1 million years ago and migrated to the continents of Asia and Europe between 50,000 and 60,000 years ago. As human populations began to radiate into other parts of the globe, they underwent separate adaptive pressures resulting in at least three distinct near-modern human forms that were later replaced by modern H. sapiens radiating from Africa. The Out of Africa model suggests little to no gene flow between modern H. sapiens and the archaic forms H. sapiens replaced.

outgroup comparison
the assessment of one group's features to another in order to establish evolutionary relationships.
oxygen-isotope ratio
ratio of stable 16Oxygen and 18Oxygen isotopes found in fossils; can be used to determine the temperature of past environments in which the fossil was found.