Ostrich eggshells are among the most commonly found objects at archeological sites in Africa. Primitive humans used the eggs as food and the shells for water containers, bowls and beads.
Now, archeologists may find them even more valuable for understanding the history of early humans.
A team of researchers headed by paleoanthropologist Allison S. Brooks of George Washington University and geophysicist P. Edward Hare of the Carnegie Institution of Washington reported recently in the journal Science that the slow decomposition of proteins in the eggshells can be used to date archeological sites, particularly in the range of 40,000 to 100,000 years ago--a period for which there are few usable dating techniques.
Preliminary studies with ostrich-eggshell dating at two African sites indicate that modern humans were present earlier than had previously been believed.
One particular advantage of the new technique is that it is inexpensive, so that a large number of objects at a site can be dated to determine if artifacts from different time periods have been jumbled together by weather, flooding or excavations--a problem that occurs frequently. "That is one of the things I am most excited about," Brooks said.
Eggshell dating relies on a physical process called amino acid racemization. Amino acids are the individual molecules from which proteins are formed. In nature, virtually all amino acids are present in only one of two possible spatial configurations, called D and L, that have a three-dimensional relationship to each other, similar to the relationship of the right hand to the left.
After an animal's death, the L amino acids will slowly undergo a chemical change to the D-form until a 50-50 mixture of the two forms is present. The rate of this change is constant at any temperature, so by measuring the ratio of D and L amino acids, researchers can determine how long ago the organism died.
Researchers such as Jeffrey Bada of the Scripps Oceanographic Institution in La Jolla originally tried to use this technique to date human and animal bones, but they encountered great difficulties. Indigenous amino acids are readily leached out of the bones by water, and the bones are easily contaminated by amino acids from other sources. Hence, the technique produces unreliable dates and it has been largely discredited.
The Science paper thus represents a rebirth of sorts for the technique.
The chief advantage of the eggshells is that the protein they contained is locked into position by the mineral portion of the shell, so it does not easily leach out and contaminants do not easily get in.
Hare and his colleagues have done a series of tests with contemporary ostrich eggshells, heating them in the absence of water and determining the rate at which one amino acid, called L-isoleucine, is converted into its D analog. They found that this racemization reaction proceeds at a steady, predictable rate that can be used for dating as long as it is compared to materials of known age at a given site.
In the tropics, Brooks and Hare report, the technique can be used to date objects as old as 200,000 years. In cooler climates, such as China, it can be used on objects 1 million years old.
The team has already been using the technique at African sites. At Ashango in Zaire, for example, researchers have excavated skeletal fragments "which are essentially similar to skeletons of modern East Africans, so they were thought to be fairly recent," Brooks said. Dating of eggshells found with them, however, shows that the bones are at least 25,000 years old.
Anthropologist Gifford H. Miller of the University of Colorado in Boulder, who is also a member of the team, has been studying the remains of unquestionably modern humans taken from the Border Cave on the east coast of South Africa. He found that the eggshells from the site were about 77,000 years old, providing strong additional evidence that modern humans originated in Africa, not on some other continent as some researchers believe.
Although ostriches are most common in Africa, ostrich eggs have also been found in China and Mongolia and would be useful for dating there. Moreover, the research team has found that every other type of bird egg they have studied gives similar results, so those types of shells can be used as well. They are now beginning to study owl eggs, which are found at many archeological sites in Northern Europe.
Other archeologists are beginning to recognize the importance of the eggshells, Miller said. "When I started talking to archeologists, I learned that the damned stuff's everywhere," but that people were in the habit of tossing it aside as uninteresting. Now, he said, "they are getting keyed in and saving it."
DATING TECHNIQUES FOR ARTIFACTS
Carbon-14 dating is good for assesing the age of items from the present to 40,000 years ago. Another commonly used technique, potassium-argon dating, is good for artifacts 300,000 years old or older. Dating of ostrich eggshells can fill the gap between and is important because that is a period when modern humans were first emerging. It is particularly useful in the range of 40,000 to 100,000 years ago--a period for which there are few usable dating techniques. In tropic climates, it can be used to date objects as old as 200,000 years. In cooler climates, it can be used on objects 1 million years old.