Evolution And History

Dating the Past 117

culture areas, series have even been developed for particu-

lar styles of pottery.

Similar inferences are made with animal or faunal se-

ries. For example, very early North American Indian sites

have yielded the remains of mastodons and mammoths—

animals now extinct—and on this basis the sites can be

dated to a time before these animals died out, roughly

10,000 years ago. For dating some of the earliest African

fossils in human evolution, faunal series have been devel-

oped in regions where accurate chronometric dates can be

established. These series can then be used to establish rel-

ative sequences in other regions. Similar series have been

established for plants, particularly using grains of pollen.

This approach has become known as palynology. The

kind of pollen found in any geologic stratum depends on

the kind of vegetation that existed at the time that stratum

was deposited. A site or locality can therefore be dated by

determining what kind of pollen was found associated

with it. In addition, palynology also helps to reconstruct

the environments in which pre historic people lived.

Chronometric Dating

Chronometric dating methods apply chemistry and phys-

ics to calculate the ages of physical and cultural remains.

Several methods use naturally occurring radioactive ele-

ments that are present either in the remains themselves or

in the surrounding soil.

One of the most widely used methods of absolute dat-

ing is radiocarbon dating. This method uses the fact that

while they are alive, all organisms absorb radioactive car-

bon (known as carbon 14 or^14 C) as well as ordinary car-

bon 12 (^12 C) in proportions identical to those found in the

atmosphere. Absorption of^14 C ceases at the time of death,

and the ratio between the two forms of carbon begins to

change as the unstable radioactive element^14 C begins

to “decay.” Each radioactive element decays, or trans-

forms into a stable nonradioactive form, at a specific rate.

The amount of time it takes for one half of the material

originally present to decay is expressed as the “half-life.” In

the case of^14 C, it takes 5,730 years for half of the amount

of^14 C present to decay to stable nitrogen 14. In another

method is that the amount of naturally occurring fluorine

is not constant but varies from region to region, making

it difficult to validate cross-site comparisons of fluorine

values. This method was vital for exposing the infamous

Piltdown hoax in England, in which a human skull and

orangutan jaw were placed together in the earth as false

evidence for an early human ancestor (see Chapter 7).

Relative dating can also be done by seriation, a method

of establishing sequences of plant, animal, or even cultural

remains. With seriation, the order of appearance of a suc-

cession (or series) of plants, animals, or artifacts provides

relative dates for a site based on a series established in

another area. An example of seriation based on cultural

artifacts is the Stone–Bronze–Iron Age series used by pre-

historians (see Chapter 11). Within a given region, sites

containing artifacts made of iron are generally more recent

than sites containing only stone tools. In well-investigated

seriation In archaeology and paleoanthropology, a technique

for relative dating based on putting groups of objects into a

sequence in relation to one another.

palynology In archaeology and paleoanthropology, a tech-

nique of relative dating based on changes in fossil pollen over

time.

radiocarbon dating In archaeology and paleoanthropology,

a technique of chronometric dating based on measuring the

amount of radioactive carbon (^14 C ) left in organic materials

found in archaeological sites.

Some ancient societies devised precise ways of recording dates that

archaeologists have been able to correlate with our own calendar.

Here is the tomb of an important ruler, Siyaj Chan K’awil II, at the

ancient Maya city of Tikal. The glyphs painted on the wall give the

date of the burial in the Maya calendar, which is the same as March

18, AD 457, in the Gregorian calendar.

© University of Pennsylvania Museum CX 61–4–123