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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 :. 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 six protons, six neutrons or carbon, but a very small amount about 0.
Living plants and animals take up carbon along with the other carbon isotopes, but when they die and their metabolic functions cease, they stop absorbing carbon. Over time, the carbon decays into nitrogen; half will do so after about 5, years this is the isotope's half-life. After about 60, years, all of the carbon 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 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, years old and are some of the oldest human remains in the Americas. A team of scientists digging in Ethiopia in 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 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, 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, which is found in volcanic rock. After the volcanic rock cools off, its potassium decays into argon with a 1. It is possible to measure the ratio of potassium to argon and estimate a rock's age, but this method is imprecise. However, scientists discovered in the s that they could irradiate a rock sample with neutrons and thereby convert the potassium to argon, 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. 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 , to , years old, making the skulls the oldest Homo sapiens remains yet to be found. 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. 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, years old, which made the ochers some of the oldest pieces of abstract design to be discovered. The ability to precisely date, or identify the age of an object, can teach us when Earth formed, help reveal past climates and tell us how early humans lived. So how do scientists do it? Radiocarbon dating is the most common method by far, according to experts. This method involves measuring quantities of carbon, a radioactive carbon isotope — or version of an atom with a different number of neutrons.
Carbon is ubiquitous in the environment. After it forms high up in the atmosphere, plants breathe it in and animals breathe it out, said Thomas Higham, an archaeologist and radiocarbon dating specialist at the University of Oxford in England.
Related: What's the oldest living thing alive today? While the most common form of carbon has six neutrons, carbon has two extra. That makes the isotope heavier and much less stable than the most common carbon form. So after thousands of years, carbon eventually breaks down. One of its neutrons splits into a proton and an electron.
While the electron escapes, the proton remains part of the atom. With one less neutron and one more proton, the isotope decays into nitrogen. When living things die, they stop taking in carbon and the amount that's left in their body starts the slow process of radioactive decay.
Scientists know how long it takes for half of a given quantity of carbon to decay — a length of time called a half-life. That allows them to measure the age of an organic piece of matter — whether that's an animal skin or skeleton, ash or a tree ring — by measuring the ratio of carbon to carbon left in it and comparing that quantity to the carbon half-life.
The half-life of carbon is 5, years, making it ideal for scientists who want to study the last 50, years of history. For older objects, scientists don't use carbon as a measure of age. Instead, they often look to radioactive isotopes of other elements present in the environment. For the world's oldest objects, uranium - thorium - lead dating is the most useful method. While radiocarbon dating is useful only for materials that were once alive, scientists can use uranium-thorium-lead dating to measure the age of objects such as rocks.
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