Showing Their Age

Dating the Fossils and Artifacts that Mark the Great Human Migration

Cheryl Carlin

Archaeological finds worldwide have helped researchers to fill out the story of human evolution and migration. An essential piece of information in this research is the age of the fossils and artifacts. How do scientists determine their ages? Here are more details on a few of the methods used to date objects discussed in "The Great Human Migration" (Smithsonian, July 2008):

Coprolites, Paisley 5 Mile Point Caves, Oregon
Age: ~14,300 years old
Method: radiocarbon dating

In a cave in Oregon, archaeologists found bones, plant remains and coprolites—fossilized feces. DNA remaining in the coprolites indicated their human origin but not their age. For that, the scientists looked to the carbon contained within the ancient dung.

By definition, every atom of a given element has a specific number of protons in its nucleus. The element carbon has six protons, for example. But the number of neutrons in the nucleus can vary. These different forms of an element—called isotopes—are inherently stable or unstable. The latter are called radioactive isotopes, and over time they will decay, giving off particles (neutrons or protons) and energy (radiation) and therefore turn into another isotope or element. They do this at a constant rate called an isotope's "half-life".

Most carbon comes in the stable forms of carbon-12 (six protons, six neutrons) or carbon-13, but a very small amount (about 0.0000000001%) exists as the radioactive carbon-14 (six protons, eight neutrons). Living plants and animals take up carbon-14 along with the other carbon isotopes, but when they die and their metabolic functions cease, they stop absorbing carbon. Over time, the carbon-14 decays into nitrogen-14; half will do so after about 5,730 years (this is the isotope's half-life). After about 60,000 years, all of the carbon-14 will be gone.

Anything that was once part of a living object—such as charcoal, wood, bone, pollen or the coprolites found in Oregon—can be sent to a lab where scientists measure how much carbon-14 is left. Because they know how much there would have been in the atmosphere and, therefore, how much someone would have absorbed when alive, they can calculate how long it has been since death or deposition. The coprolites averaged about 14,300 years old and are some of the oldest human remains in the Americas.

Hominid skulls, Herto, Ethiopia
Age: ~154,000 to 160,000 years old
Method: argon-argon dating

A team of scientists digging in Ethiopia in 1997 found stone tools, the fossil remains of several animal species, including hippopotamuses, and three hominid skulls. How old were they? The organic remains were too old for carbon-14 dating, so the team turned to another method.

Radiocarbon dating works well for some archaeological finds, but it has limitations: it can be used to date only organic materials less than about 60,000 years old. However, there are other radioactive isotopes that can be used to date non-organic materials (such as rocks) and older materials (up to billions of years old).

One of these radioisotopes is potassium-40, which is found in volcanic rock. After the volcanic rock cools off, its potassium-40 decays into argon-40 with a 1.25-billion-year half-life. It is possible to measure the ratio of potassium-40 to argon-40 and estimate a rock's age, but this method is imprecise. However, scientists discovered in the 1960s that they could irradiate a rock sample with neutrons and thereby convert the potassium-40 to argon-39, an isotope not normally found in nature and easier to measure. Though more intricate, this process yields more precise dates. For example, scientists at the University of California at Berkeley were able to date samples from the 79 A.D. eruption of the volcano Vesuvius to within seven years of the event.

Because the hominid skulls and other artifacts found at Herto could not be directly dated—the organic material had long since been fossilized—the researchers instead performed their analysis on volcanic rock that was embedded in the sandstone near the fossils. The rock was about 154,000 to 160,000 years old, making the skulls the oldest Homo sapiens remains yet to be found.

Engraved ocher stones, Blombos Cave, South Africa
Age: ~77,000 years old
Method: thermoluminescence dating

An excavation of a seaside cave in South Africa revealed two objects that were clearly manmade—pieces of ocher stone etched with a crisscross pattern. Neither the stones nor the rock in which they were buried were volcanic in origin, though, so the researchers chose another method for determining their age: thermoluminescence.

As in argon-argon dating, the thermoluminescence clock also begins with the last time that a rock was heated to a high temperature. The extreme heat eliminates electrons stored in certain crystals—such as quartz and feldspar—within the rock. Over time, the crystals trap electrons produced by trace amounts of radioactive atoms found in the environment. By reheating the rock, scientists can release the stored energy, which is given off as light and called "thermoluminescence." The intensity of the light indicates how long it has been since the rock was last heated.

Like the Herto skulls, the ages of the carved ocher stones from Blombos Cave could not be directly determined. However, in the same rock layer as the ochers were pieces of burnt stone, which were likely the same age as the ochers and ideal for thermoluminescence dating. The burnt stone, it was revealed, was about 77,000 years old, which made the ochers some of the oldest pieces of abstract design to be discovered.