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Yet even this seminal K-Ar dating study was plagued by the seemingly insurmountable problem of contamination.The principal materials for dating East Africa hominid sites are volcanic ashes, yet many of these ashes are not deposited as primary air fall (Greek for ash).Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.
Detailed studies of these fossils provide new insights into human evolution, such as the origin of locomotion and cultural activity, and the evolution of the brain, among many other complex features that have come to define humanity.
Even during the time this manuscript was written, new hominid discoveries in Ethiopia and Kenya were announced that trace our earliest ancestors further back into the Pliocene.
Chronometric dates are given in numbers of years since or before the beginning of some calendar system. That point can be in the past, the present, or the future.
Scientists who use chronometric dating methods usually prefer to count years from the present.
They do not tell us when an event happened in years before the present. This usually means that there is a 2/3 probability that the actual date of the event was within the range of 300-380 B. The 2/3 probability is what statisticians refer to as one standard deviation.
For example, in comparing the cross-section of the series of geological strata illustrated below, level 4 is older than 5 because it is below it. At a higher probability, the plus or minus factor will be larger.The ages assigned to these fossils have been obtained through radiometric dating of volcanic rocks interbedded with the fossiliferous sediments.Such numerical calibrations are crucial to understanding rates and timing of evolutionary change.Rather, most are reworked by stream action and are redeposited into the sedimentary environment.In the process of reworking, these ashes can pick up pre-existing detrital grains that, by definition, are older than the juvenile ash.The basis for stratigraphy seems quite intuitive today, but its applications were no less than earth-shattering to archaeological theory.For example, JJA Worsaae used this law to prove the Three Age System.During the latter half of this century anthropological surveys in East Africa have made significant contributions to understanding how the human species has evolved.In the past two decades, particularly, discoveries of our fossil ancestors have been made in unprecedented numbers and diversity.For detailed information about how seriation works, see Seriation: A Step by Step Description.Seriation is thought to be the first application of statistics in archaeology. The most famous seriation study was probably Deetz and Dethlefsen's study Death's Head, Cherub, Urn and Willow, on changing styles on gravestones in New England cemeteries.