Chronology and dating methods

Having an accurate time scale is a crucial aspect of reconstructing how anatomical and behavioral characteristics of early hominids evolved.

Researchers who are interested in knowing the age of particular hominid fossils and/or artifacts have options that fall into two basic categories:

• Relative dating methods
• Chronometric dating methods

Relative dating methods

Relative dating methods allow one to determine if an object is earlier than, later than, or contemporary with some other object. It does not, however, allow one to independently assign an accurate estimation of the age of an object as expressed in years. The most common relative dating method is stratigraphy. Other methods include fluorine dating, nitrogen dating, association with bones of extinct fauna, association with certain pollen profiles, association with geological features such as beaches, terraces and river meanders, and the establishment of cultural seriations. Cultural seriations are based on typologies, in which artifacts that are numerous across a wide variety of sites and over time, like pottery or stone tools. If archaeologists know how pottery styles, glazes, and techniques have changed over time they can date sites based on the ratio of different kinds of pottery. This also works with stone tools which are found abundantly at different sites and across long periods of time.

Principle of stratigraphy

Stratigraphic dating is based on the principle of depositional superposition of layers of sediments called strata. This principle presumes that the oldest layer of a stratigraphic sequence will be on the bottom and the most recent, or youngest, will be on the top. The earliest-known hominids in East Africa are often found in very specific stratigraphic contexts that have implications for their relative dating. These strata are often most visible in canyons or gorges which are good sites to find and identify fossils. Understanding the geologic history of an area and the different strata is important to interpreting and understanding archaeological findings.

Chronometric dating methods

The majority of chronometric dating methods are radiometric, which means they involve measuring the radioactive decay of a certain chemical isotope. They are called chronometric because they allow one to make a very accurate scientific estimate of the date of an object as expressed in years. They do not, however, give "absolute" dates because they merely provide a statistical probability that a given date falls within a certain range of age expressed in years. Chronometric methods include radiocarbon, potassium-argon, fission-track, and thermoluminescence.

The most commonly used chronometic method is radiocarbon analysis. It measures the decay of radioactive carbon (14C) that has been absorbed from the atmosphere by a plant or animal prior to its death. Once the organism dies, the Carbon-14 begins to decay at an extremely predictable rate. Radioactive carbon has a half-life of approximately 5,730 years which means that every 5,730 years, half of the carbon-14 will have decayed. This number is usually written as a range, with plus or minus 40 years (1 standard deviation of error) and the theoretical absolute limit of this method is 80,000 years ago, although the practical limit is close to 50,000 years ago. Because the pool of radioactive carbon in the atmosphere (a result of bombardment of nitrogen by neutrons from cosmic radiation) has not been constant through time, calibration curves based on dendrochronology (tree ring dating) and glacial ice cores, are now used to adjust radiocarbon years to calendrical years.

The development of Atomic Absorption Mass Spectrometry in recent years, a technique that allows one to count the individual atoms of 14C remaining in a sample instead of measuring the radioactive decay of the 14C, has considerably broadened the applicability of radiocarbon dating because it is now possible to date much smaller samples, as small as a grain of rice, for example.

Dendrochronology is another archaeological dating technique in which tree rings are used to date pieces of wood to the exact year in which they were cut down. In areas in which scientists have tree rings sequences that reach back thousands of years, they can examine the patterns of rings in the wood and determine when the wood was cut down. This works better in temperate areas that have more distinct growing seasons (and thus rings) and relatively long-lived tree species to provide a baseline.

Methods of dating in archaeology

Techniques of recovery include:

• Surveys
• Excavations

Types of archaeological remains include:

• Perishable: plant remains, animal bones, wooden artifacts, basketry, and other easily degradable objects
• Nonperishable materials: stone tools, pottery, rocks used for structures.

Data collection and analysis is oriented to answer questions of subsistence, mobility or settlement patterns, and economy.

Methods in physical anthropology

Data collections based on study of hard tissues (bones and teeth), usually the only remains left of earlier populations, which include:

• Identification of bones/Which part of skeleton is represented?
• Measurement of the cranium and other elements of a skeleton. Carefully defined landmarks are established on the cranium, as well as on the long bones, to facilitate standardization of measurements.
• Superficial examination of bone for any marks (for instance, cutmarks)
• Further examination using specific techniques:
  • X-ray to identify evidence of disease and trauma in bones
  • DNA extraction to determine genetic affiliations