When paleoanthropologists refer to time in the past, it is done with either
relative or chronometric dates. Relative dates give the time of an event with
reference to another event that is not worldwide in scale. They tell us simply that
one thing is older or younger than another. They do not tell us when an event
happened in years before the present. 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. This does not tell us how many years ago these strata were laid
down. It only tells us how old each is relative to the others.
Also, we cannot assume that level 4 in one site is from the same time period
as level 4 at a site in another location.
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| Relative dating by relating geological strata |
In contrast,
chronometric
dates place events in their chronological
position with reference to a universal time scale such as a calendar.
All events
given the same chronometric date will actually be contemporaneous
.
Chronometric dates are given in numbers of years
since or before the beginning of some calendar system. For instance, 3000 B.C. was
3000 years before the starting point in the Gregorian
calendar, which is the one that the U.S. and most other nations use today.
Likewise, 2005 A.D. is 2005 years after the zero starting point. The A.D. and B.C.
following Gregorian calendar dates are overtly Christian references. A.D. stands for anno domini ("the year of our Lord" in Latin--the once presumed birth
date of Jesus) and B.C. stands for "before Christ." Israel and some other
nations substitute B.C.E. (before the Common
Era) and C.E. (the Common
Era).
Many different calendar systems have been created around the world. They all have in common the fact that they have a starting point from which one can count forward and backward. 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. With this system, an event that occurred in 1000 B.C. would be written 2950 B.P. (or 2950 years "before the present"). By convention, 1950 A.D. is considered the present. That is because 1950 was the year that the first universal chronometric dating method, radiocarbon dating, became readily available for use.
Chronometric dates are just close approximations of the true age of a fossil or geological deposit. This is due to the finite limits of the dating techniques and the materials being dated. Most chronometric dates are given with a plus or minus factor. For instance, 340 B.P ± 40. 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.P. The 2/3 probability is what statisticians refer to as one standard deviation. At a higher probability, the plus or minus factor will be larger. The fact that chronometric dates are only approximations of the actual age does not mean that they should be distrusted. To the contrary, modern scientific techniques for chronometrically dating samples are highly reliable.
Whenever possible, paleoanthropologists obtain many samples from an ancient site to be tested with a variety of dating techniques. In this way, the chronological placement of a fossil can be more dependable. A date for an object, such as a human bone, is also more dependable when the bone itself is dated rather than something else physically associated with it in the same geological strata.