118 CHAPTER 5 | Field Methods in Archaeology and Paleoanthropology
of rings with those in the trunk of a tree of known age, ar-
chaeologists can date the archaeological material.
Dendrochronology is applicable only to wooden ob-
jects. Furthermore, it can be used only in regions in which
trees of great age, such as the giant sequoias and the bris-
tlecone pine, are known to grow. Radiocarbon dating of
wood from bristlecone pines dated by dendrochronology
allows scientists to correct the carbon 14 dates so as to
bring them into agreement with calendar dates.
Potassium-argon dating, another commonly used
method of absolute dating, is based on a technique similar
to that of radiocarbon analysis. Following intense heating,
as from a volcanic eruption, radioactive potassium decays
at a known rate to form argon—any previously existing
argon having been released by the heating of the molten
lava. The half-life of radioactive potassium is 1.3 billion
years. Deposits that are millions of years old can now be
dated by measuring the ratio of potassium to argon in a
given rock.
Volcanic debris at various localities in East Africa is
routinely dated by potassium-argon analysis, indicating
when the volcanic eruption occurred. If fossils or artifacts
are found sandwiched between layers of volcanic ash, as
they are at Olduvai and other sites in East Africa, they can
be dated with some precision. As with radiocarbon dates,
there are limits to that precision, and potassium-argon
dates are always stated with a plus-or-minus margin of
error attached. The precision of this method is limited to
time periods older than about 200,000 years ago.
Though radiocarbon and potassium-argon methods
are extremely valuable, neither technique works well dur-
ing the time period dating from about 50,000 years ago to
about 200,000 years ago. Because this same time period
happens to be very important in human evolutionary his-
tory, scientists have developed a number of other methods
to obtain accurate dates during this critical period.
One such method, amino acid racemization, is based
on the fact that amino acids trapped in organic materi-
als gradually change, or racemize, after death, from left-
handed forms to right-handed forms. Thus the ratio of
left- to right-handed forms should indicate the specimen’s
age. Unfortunately, in substances like bone, moisture and
acids in the soil can leach out the amino acids, thereby
introducing a serious source of error. However, ostrich
eggshells have proved immune to this problem, the amino
acids being so effectively locked up in a tight mineral ma-
trix that they are preserved for thousands of years. Because
ostrich eggs were widely used as food and the shells were
used as containers in Africa and the Middle East, they
provide a powerful means of dating sites of the later parts
of the Old Stone Age (Paleolithic), between 40,000 and
180,000 years ago.5,730 years (11,460 years total), half of the remaining
amount will also decay to nitrogen 14 so that only one
quarter of the original amount of^14 C will be present. Thus
the age of an organic substance such as charcoal, wood,
shell, or bone can be measured through determining
the changing proportion of^14 C relative to the amount of
stable^12 C.
Though scientists can measure the amount of radioac-
tive carbon left in even a few milligrams of a given organic
substance of a recent specimen, the amount of carbon 14
present in remains from the distant past is so small that
accurate detection is difficult. The radiocarbon method
can adequately date organic materials up to about 50,000
years old, but dating older material is far less reliable.
Of course, one has to be sure that the organic remains
were truly contemporaneous with the archaeological mate-
rials. For example, charcoal found on a site may have gotten
there from a recent forest fire rather than a more ancient
activity, or wood found at a site may have been retrieved by
the people who lived there from some older context.
Because there is always a certain amount of error in-
volved, radiocarbon dates (like all chronometric dating
methods) are not as absolute as is sometimes thought. This
is why any stated date always has a plus-or-minus (±) factor
attached to it corresponding to one standard deviation above
and below the mean value. For example, a date of 5,200 ±
120 years ago means that there is about a 2 out of 3 chance
(or a 67 percent chance) that the true date falls somewhere
between 5,080 and 5,320 radiocarbon years ago. The quali-
fication “radiocarbon years” is used because radiocarbon
years are not precisely equivalent to calendar years.
The discovery that radiocarbon years are not precisely
equivalent to calendar years was made possible by another
method of absolute dating: dendrochronology (derived
from dendron, a Greek word meaning “tree”). Originally
devised for dating Pueblo Indian sites in the North Amer-
ican Southwest, this method is based on the fact that in
the right kind of climate, trees add one (and only one) new
growth ring to their trunks every year. The rings vary in
thickness, depending upon the amount of rainfall received
in a year, so that climatic fluctuation is registered in the
growth ring. By taking a sample of wood, such as a beam
from a Pueblo Indian house, and by comparing its pattern
dendrochronology In archaeology and paleoanthropology, a
technique of chronometric dating based on the number of rings
of growth found in a tree trunk.
potassium-argon dating In archaeology and paleoanthro-
pology, a technique of chronometric dating that measures the
ratio of radioactive potassium to argon in volcanic debris asso-
ciated with human remains.