The Record of Time: Relative Techniques

In the past, relative dating methods often were the only ones available to paleoanthropologists. As a result, it was difficult to chronologically compare fossils from different parts of the world. However, relative methods are still very useful for relating finds from the same or nearby sites with similar geological histories.

The oldest and the simplest relative dating method is stratigraphy

, or stratigraphic dating. It is based on the principle of superposition

, which is that if there are layers of deposits, those laid down first will be on the bottom and those laid down last will be on the top. This principle is logical and straightforward. However, geological strata are not always found to be in a neat chronological order. Wind and water erode strata and some areas are uplifted or even tilted. These processes result in geological unconformities

, or breaks in the original stratigraphic sequence. In addition, people and other animals dig holes, resulting in a mixing of material from different strata (as shown below).

All of these processes confuse the stratigraphic record. In many cases, however, it is possible to reconstruct the original sequence of strata so that they can be used for relative dating. For instance, if we find a fossil bone below the strata 3 rock level shown in the illustration above, we assume that the animal most likely lived at a time before that layer was formed. However, we must be careful to note whether or not the fossil comes from the mixed strata zone of the filled in hole.

When two objects are found in the same strata of a site, it is usually assumed that they date to the same time period. This is an application of the principle of association. However, the assumption of contemporaneity

may not always be correct. This is due to the fact that one or both of the objects may have been moved or redeposited into a different location. In other words, they may no longer be in their primary context.

When the bones of our early ancestors are found in the same geological strata as those of other animals that are known to have lived only during a specific time period in the past, we assume that these ancestors must also have come from that time. This is referred to as dating by association with index fossils, or biostratigraphy

. Elephants, horses, pigs, rodents, and some monkey species have been used as index fossils because they underwent relatively rapid evolutionary changes that are identifiable in their teeth and other skeletal parts. Their bones also were frequently found in association with our human and primate ancestors.

When bones, teeth, or antlers are found at a site, fluorine analysis can be used to tell us whether or not the animals they were from actually lived at about the same time. This relative dating method is based on the fact that there are specific progressive chemical changes in skeletal remains that result from burial underground. As time passes, the organic components of bone (mostly fats and proteins) are lost primarily through bacterial action. Since these components contain nitrogen, there is a progressive loss of that element. At the same time, percolating ground water deposits trace amounts of fluorine and other elements, such as uranium, into the bone. As a result, the amount of fluorine and other trace elements progressively increase. If the bones of two animals are buried at the same time in the same site, they should have the same relative amount of nitrogen and fluorine. If they do not, they most likely come from different eras, despite the fact that they were found in association with each other.

Changes in the amounts
of fluorine and nitrogen
over time in a buried bone

Fluorine analysis can be used only as a relative dating method because the rate of decay and the amount of dissolved minerals in the ground water varies from site to site. In other words, the biochemical clock that this method relies on runs at a different rate in different environments. Fluorine analysis is primarily used for verifying whether or not two fossils in the same strata at a site were in fact contemporaneous. If not, then at least one of them must be physically out of context.

A good example of the value of fluorine analysis was in bringing to light the Piltdown Man hoax. In 1912, Charles Dawson, an amateur paleontologist, found what was thought to be an early human skull and jaw in the Piltdown gravel deposits of England. Because it had an ape-like jaw and was found in association with the bones of extinct animals, this "Piltdown Man" was also believed to be a very ancient human. It was popularly referred to as "the missing link" in human evolution. In 1949, the Piltdown bones were finally tested for fluorine content by Kenneth Oakley and the fraud became apparent. After reexamining the strata at the Piltdown site, the evidence of a hoax was published in 1953. This was verified through the use of X-ray fluorescence examination. The skull and jaw clearly were not from the same time period. The jaw was likely to have come from a modern young adult orangutan. It had been cleverly carved to fit the skull and stained to look ancient. In addition, the associated bones from extinct animals had much older fluorine and nitrogen ratios than either the jaw or the human skull. Unfortunately, by 1953 Charles Dawson and all of the other people involved with the Piltdown Man discovery and analysis had died, so we do not know for sure who was responsible for the hoax.

Piltdown Man Hoax--an explanation of one of the greatest frauds in the history of science
This link takes you to an audio file at an external website. To return here, you can click
the "back" button on your browser program. (length = 7 mins, 24 secs)

NOTE: The perpetrator of the Piltdown Man hoax may have been uncovered. In the mid 1990's, an old tool kit was found in a dusty backroom of the Natural History Museum in London. It contained tools and chemicals that could have been used to modify the ape jaw and human skull to give them the appearance of great antiquity. The owner of this kit was Martin Hinton, curator of zoology at the time of the hoax. We will never know whether he was really the perpetrator, but he is now the leading candidate. Another candidate was a famous practical joker who lived near the Piltdown gravel deposit during the early 20th century. That man was a medical doctor by training and therefore had the necessary skills to carry off the hoax. He was Sir Arthur Conan Doyle, the author of the Sherlock Holmes detective stories.

The three relative dating methods described here are not the only ones available. Archaeologists also regularly employ geochronology

, artifact time markers, seriation

, and pollen analysis. However, stratigraphy, biostratigraphy, and fluorine analysis are the ones that paleoanthropologists are most likely to use for relatively dating our early fossil ancestors.