Evolution And History

Chance and Study of the Past 119

Chance and Study

of the Past

Archaeological and fossil records are imperfect. Chance

circumstances of preservation have determined what has

and what has not survived the ravages of time. Thus the

biology and culture of our ancestors are reconstructed

on the basis of incomplete and possibly unrepresentative

samples of physical and cultural remains. This is further

compounded by the role that chance continues to play in

the discovery of prehistoric remains. Remains may come

to light due to factors ranging from changing sea level,

vegetation, or even a local government’s decision to build

a highway.

Electron spin resonance, which measures the num-
ber of trapped electrons in bone, and thermolumines-
cence, which measures the amount of light emitted from
a specimen when heated to high temperatures, are two
additional methods that have been developed to fill in
prehistoric time gaps. Dates derived from these two
methods changed the interpretation of key sites in pres-
ent-day Israel vital for reconstructing human origins
(see Chapters 8 and 9).
A few other chronometric techniques rely on the
element uranium. Fission track dating, for example, counts
radiation damage tracks on mineral crystals. Like amino
acid racemization, all these methods have problems: They
are complicated and tend to be expensive, many can be
carried out only on specific kinds of materials, and some
are so new that their reliability is not yet unequivocally
established. It is for these reasons that they have not been
as widely used as radiocarbon and potassium-argon dating
techniques.
Paleomagnetic reversals contribute another interesting
dimension to absolute dating methodologies by providing
a method to cross-check dates (Figure 5.6). This method is
based on the shifting magnetic pole of the earth—the same
force that controls the orientation of a compass needle.
Today, a compass points to the north because we are in a
period defined as the geomagnetic “normal.” Over the past
several million years, there have been extended periods of
time during which the magnetic field of the earth pulled
toward the South Pole. Geologists call these periods geo-
magnetic reversals. Iron particles in stones will be oriented
into positions determined by the dominant magnetic pole
at the time of their formation, allowing scientists to derive
broad ranges of dates for them. Human evolutionary his-
tory contains a geomagnetic reversal starting 5.2 million
years ago that ended 3.4 million years ago, followed by a
normal period until 2.6 million years ago; then a second
reversal began, lasting until about 700,000 years ago when
the present normal period began. This paleomagnetic
sequence can be used to date sites to either normal or
reversed periods and can be correlated with a variety of
other dating methods to cross-check their accuracy.
Establishment of dates for human physical and cul-
tural remains is a vital part of understanding our past. For
example, as paleoanthropologists reconstruct human evo-
lutionary history and the movement of the genus Homo
out of Africa, dates determine the story told by the bones.
In the next chapters we will see that many of the theories
about human origins are dependent upon dates. Similarly,
as archaeologists dig up material culture, interpretations
of the movement and interactions of past peoples depend
on dating methods to provide a sequence to the cultural
remains.

0.0

Millions

of years ago

Magnetic–

reversal

time scale

Magnetic

polarity

of lava

Brunhes

normal

epoch

Matuyama

reversal

epoch

Gauss

normal

epoch

Events

1.0

2.0

3.0

0.5

1.5

2.5

3.5

Figure 5.6 Scientists have documented a geomagnetic

polarity time scale in which the changes in the earth’s

magnetic force—to north or south—have been calibrated. This

geomagnetic time scale provides scientists with opportunities

to cross-check other dating methods.