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formed by the fusion of five sacral vertebrae; the part of the vertebral column that forms the most dorsal part of the pelvic girdle, in between the os coxae.
sagittal crest

a protrusion made of bone at the joint between the parietal bones to which the temporalis muscles attach; common in many larger ape species and some fossil hominins.

Sahelanthropus tchadensis

an extinct hominin species that existed in Africa approximately 7 to 6 million years ago; fossil remains found at Toros-Menalla, locality TM 266, in Chad, suggest that Sahelanthropus tchadensis may have been bipedal; this find also led to the reevaluation of the chimp-human divergence time frame.

[alt. savannah] an environment usually located near woodlands that consists of grasses, a few trees, and shrubs; vegetation can also be located below the ground in the form of tubers and roots; strong seasonality and reduced rainfall; animal life not as diverse as in other environments, but typically inhabited by a high number of herbivores, carnivores and insects. Baboons and humans are the only primates know to inhabit savannas.

a behavior that allows animals to incorporate meat into the diet, but one that relies on finding the carcasses of animals that died of natural causes or were killed by other predators, rather than engaging in active predation.

a process that studies the natural world and tests the accuracy of the results. Science's primary utility comes from its self-correcting nature, as erroneous ideas are discarded.
scientific law
a scientific statement or description of a fact that has been supported to be true by all available means of the scientific method. A scientific law explains behaviors or phenomena when specific conditions are applied.
scientific method
the process used by scientist to answer or clarify scientific inquiries. The process begins with the formation of a hypothesis based upon observation, followed by further systematic observations and experiments related to the phenomenon in question. These experiments and observations are conducted several times to establish whether the results are repeatable. The repeatable results are then used to support or disprove the initial hypothesis.
the dominant tool used during the Middle Paleolithic mainly used for scraping; includes the notched scraper, side-scraper, steep-sided scraper, and core-scrapper.
to remain in or near a home range.
sedimentary rock
when clays, silts, or sands compact over long periods of time, eventually forming into a hard element.
describes a region with low annual rainfall and predominantly short vegetation, such as shrubs and grasses.
sexual dimorphism

the condition in which the males and females of a particular species each have different forms of the same feature or characteristic (e.g., body size, canine size). In those primate species that display marked sexual body size dimorphism, the males are larger than the females. In some species, such as gorillas and orangutans, the males are twice as large as the females (i.e., a 2:1 difference in mass).

sexual selection

a form of natural selection where fitness is determined by an individual's ability to compete for mates.

shared derived character
a new trait common between evolutionary related groups or individuals.
the third period in the Paleozoic era on the geologic time scale that approximately dates from 440 million years ago to 400 million years ago; marked by presence of large coral reefs, massive evolution of fish, and the first appearance of terrestrial plants.
refers to monkeys, apes, and humans.
single-species hypothesis
the theory that only one hominin species has lived at any one time. The idea behind this theory is that all hominins would have occupied the same ecological niche and therefore could not have existed together. Later discoveries have proved the single-species hypothesis to be false.
sister group
a group that shares the same common ancestor with another.

an accumulation of artifacts or fossils at a single location that is limited in spatial and stratigraphic extent. Some sites represent the activities of a single event (e.g., the butchery of a single animal) while other sites represent multiple activity cycles carried out by many individuals over time. Interpreting what sorts of activities are represented at a site is one of the primary goals of an archaeologist.

the protective and supportive structure of an organism constructed of bone and cartilage.
the bony portions of the head, including the cranium and the mandible.
the evolution of a new species from an ancestral species; the foundation of macroevolution.
the basic unit of biodiversity. Individuals of a species are reproductively isolated, meaning that they cannot produce viable offspring with a mate of a different species. Taxonomic classification uses the binomial nomenclature: where the names of organisms start with a capitalized genus name (i.e., Homo for humans) and a specific epithet for the species name (i.e., sapiens for humans). Therefore, the proper species name for humans is Homo sapiens.

A geologic deposit that reslts from calcite precipitations in a wet cave.

round stones which may have been shaped by smoothing. An example of spheroids are bolas, rounded stones used as throwing weights in weapons that snare game by entangling the legs.
stabilizing selection
when the extreme variants are selected against, increasing the mean.
a period in which a species undergoes little to no morphological or behavioral changes; evolutionary stability.
statistical inference
an inference that is replicable and uniform.
statistical significance
a decision about validity, usually an estimate of the probability that the predictions were fulfilled by chance events.
stone tool
tools made of stone that enabled hominins to exploit a broader range of activities such as food processing. Through time the stone tools used by hominins became more complex and standardized and new manufacturing techniques were invented along the way.
stone tool technological Mode I
simple unstandardized flakes, cores and choppers; produced during the Lower Paleolithic. The Oldowan tool industry is an example of Mode I stone tools.
stone tool technological Mode II
standardized flakes and other artifacts as a result of direct percussion techniques; typically associated with Homo erectus; produced during the Lower Paleolithic. The Acheulean tool industry is an example of Mode II stone tools.
stone tool technological Mode III
standardized blades and burins; produced during the Middle Paleolithic; typically associated with Homo neanderthalensis and possibly modern H. sapiens. The Mousterian tool industry is an example of Mode III stone tools.
stone tool technological Mode IV
standardized blades and burins; produced during the Upper Paleolithic.
stone tool technological Mode V
the production of microliths; produced during the Upper Paleolithic.
stone tool technological Mode VI
the production of polished (ground) stone tools and pottery and a wide variety of lithics, with a continued emphasis on blades; produced during the upper Paleolithic.
stone tool technological Modes
a system of classifying stone tool technology into six modes based on the complexity of the tool manufacturing technique; formulated by J.G. D. Clark in 1969.
the study of strata, or the layers of sediments deposits onto the ground, and the fossils that are found within these layers.

a relative term used for bipedal hominins (those that travel on two legs) to describe features on the trunk that are closer to the head; the opposite of inferior. The term cranial is a synonym commonly used when referring to quadrupedal animals.

an extra element.
the idea that the top layer of sediments is younger than the sedimentarly layer below it.

rotary motion that moves the thumb from the medial position to a lateral position; the opposite of pronation.

suspensory behavior

a form of positional behavior in which the body of an organism is supported by their arms or legs holding on to a branch above them; the tail may also be used in suspensory posture.

a joint between two bones that are securely connected and are separated by fibrous tissue. The joints between most of the bones of the skull are sutures.
an object, behavioral act, or language that stands for something else and carries some codified meaning.
sympatric speciation
the evolution of a daughter species due to changes within a subset of the parent population; in this sense, the parent and daughter populations are not geographically isolated from one another.
when the home range of two different species overlaps.
a type of joint that is cartilaginous.
a shared primitive trait.
a shared derived trait.

the science of classification; the organization of organisms according to their evolutionary relationships.