Which of the following techniques is a more precise method of determining the age of a fossil?

As plants and animals die, their remains are sometimes preserved in Earth’s rock record as fossils. Fossils can provide clues to how plants and animals lived in the past – what they looked like, what they ate, what environments they lived in, and how they evolved and went extinct. For hundreds of millions of years, the remains of organisms (as well as tracks, trails, and burrows – called trace fossils) were the majority of the clues left behind in Earth’s fossil record.

About 3 million years ago, a new type of clue appeared in the rock layers of eastern Africa – objects made by our hominin ancestors. Hominins began to live their lives in a different way, utilizing tools made of stone in their day-to-day activities.  Sharp stone tools allowed hominins to cut wood more easily or strip meat from bones. Other tools may have helped them forage for plant foods or hunt and kill animals. Using tools can also leave marks on bones, which may be preserved. Tools and butchery-marked bones are traces of human behavior, and they are also key elements in the study of human evolution. These objects make up the earliest archeological record, which is studied in tandem with the fossil record to piece together hominin life and evolution. While there is a rich fossil and archeological record, the process of preserving ancient remains can only occur under very specific conditions. As the record is incomplete, the scientific search continues to uncover more and more ancient remains.

To best understand how the clues in the archeological and fossil record fit together to reveal the story of the evolution of humans, we need to know the age in Earth’s history when each clue was left. Thanks to the hard work of many scientists, a multitude of techniques are available to date the amount of time since an object entered the geological record. These techniques can be divided into two main categories: relative dating and absolute dating. The first section of this page explores relative dating techniques relying on geological principles. The second section discusses how the physical and chemical properties of elements can provide more precise ages.

RELATIVE DATING

Some archeological and fossil sites do not contain any materials that are suitable for the most precise absolute dating methods (discussed later). For these types of sites, scientists rely on relative dating methods to get an approximate idea of the age of objects found there. Relative dating is the ability to determine that one thing is older or younger than another. Relative dating methods are based on certain basic principles of geology that govern how rock layers are formed on Earth’s surface.

Sedimentary rocks are made of tiny particles that are transported by natural agents (like wind and water) and laid down in different environments, forming one layer after another. Each layer is a stratum, and multiple layers on top of one another are called strata. Stratigraphy is the study of these layers to reconstruct the sequence of certain aspects of ancient landscapes and environments over time. In 1669, scientist Nicolaus Steno proposed a set of “Principles of Stratigraphy” that are fundamental to all relative dating techniques. These principles are key to establishing the order in which strata were formed. Determining this order, and where artifacts and fossils occur within the sequence, is the basis of relative dating.

Principles of Stratigraphy

1.  Principle of Superposition: For sedimentary rocks, strata on the bottom of a sequence are older and were deposited before any strata on top of them. The sequence allows scientists to label layers from oldest to youngest. Fossils and artifacts found in those layers can then be understood as older or younger in time. 

2.  Principle of Original Horizontality: Layers of sedimentary rock were originally deposited horizontally – parallel to the ground. Strongly tilted rocks did not start that way. They were affected by geological processes that occurred after the layers were originally deposited. Identifying tilted and folded rocks assists scientists in putting the sequence of events in order.

3.  Principle of Lateral Continuity: Sedimentary rock layers are originally continuous in all directions, but may be broken up or displaced by later events. This can happen when a river or stream erodes a portion of the rock layers. This can also happen when faulting occurs, causing displacement of rock units. Layers of sediment do not extend indefinitely; the limits are controlled by the amount and type of sediment and the size and shape of the area where sediments are deposited. Nonetheless, rock layers that look identical but are now separated by a valley or other erosional feature can be assumed to have originally been continuous and thus the same age.

4.  Principle of Cross Cutting Relationships: Sedimentary layers that cut across other layers are younger than the layers that are cut. This observation helps scientists identify interruptions in the sequence of events and place those events in their correct order. In the diagram below, layer “H” cuts into layers D, E, F, and G and is therefore the youngest.

Biostratigraphy/Biochronology, and Index Fossils

Fossils have been used to define geological periods and their durations. A large change in the plants and animals is required to identify a new geological period. Most of the geological periods scientists have named were ended by a major extinction event or replacement of a large number of species. As a result, geological periods and smaller units of geological time typically have a characteristic set of fossil species. These fossils can then be used to compare the ages of different geological units. To further constrain the age of sequences, scientists rely on index fossils. Index fossils are specific plants or animals that are characteristic of a particular span of geologic time, and can be used to date the sediments in which they are found. Index fossils must have both a limited time range and wide geographic distribution. Sediments that were deposited far apart but contain the same index fossil species are interpreted to represent the same limited time.

When scientists first discovered the famous Australopithecus afarensis hominin fossil skeleton nicknamed “Lucy” in Ethiopia, they initially estimated her age using the principles of biostratigraphy: correlating fossil discoveries of other animals in strata over a large area. Three extinct suid (pig) species, which had been previously dated at other sites, were found in the same layer as her skeleton. They were Nyanzachoerus kanamensis, which occurred 5.1 – 2.4 million years ago; Kolpochoerus afarensis, which occurred 3.5 – 2.9 million years ago; and Notochoerus euilus, which occurred 3.8 – 1.8 million years ago. The only time interval in which all three species lived is between 3.5 and 2.9 million years ago. Using later advances in absolute dating techniques, we now know that Lucy’s skeleton is around 3.18 million years old – a result that places her directly within the predicted age range based on biostratigraphy.

Paleomagnetism and Magnetostratigraphy

This method involves measuring magnetic particles in strata to determine the orientation of Earth’s magnetic field. There are two separate definitions of the concept of “north”. The first is true geographic north, which is located at the North Pole. The second is magnetic north, which shifts its location based on fluctuations in Earth’s magnetic field. So, at any given time, a compass might not point to geographic north; it points to wherever magnetic north is located. The current location of the magnetic north pole is near Ellesmere Island in northern Canada.

Earth’s magnetic field also undergoes shifts that are much larger. These rare events take place slowly and are known as magnetic reversals. During a magnetic reversal, the position of magnetic north shifts to the southern hemisphere of the planet. If a magnetic reversal occurred today, the magnetic north pole would eventually switch to near the geographic south pole, and compasses would begin to point south. Such reversals happen frequently enough to be useful in geologic dating. Researchers have determined the dates when these reversals happened. The most recent magnetic reversal occurred approximately 780,000 years ago.

Scientists are able to record the change in Earth’s magnetic field over time. Iron-rich magnetic minerals “float” freely in molten rock and orient themselves to Earth’s magnetic field like compass needles. At the time when the molten rock cools and becomes solid, those magnetic minerals become locked into position within the rock layer. These rocks are now a record of the direction (polarity) of Earth’s magnetic field at the time when they formed. Any rock layer containing iron can have its magnetically-aligned particles locked in at the time when the rock was formed.

Scientists can study a long sequence of strata and see how the magnetic polarity of the iron minerals within the rock has changed throughout that sequence. This pattern can be compared to the well-established worldwide polarity record, which is the entire history of large flips in Earth’s magnetic field. Once they figure out which general part of that history they have, scientists can determine the time range of the rock and its contents. This is particularly useful in groups of strata. Geologists typically do not use a single stratigraphic layer in paleomagnetic dating, because you need multiple layers to find the back and forth pattern of flipping of Earth’s magnetic field. Fossils of a South African hominin, Australopithecus sediba, were able to be dated using this method because the fossils were found embedded in a stratum very close to one of these magnetic reversals.

Tephrochronology/Tephrostratigraphy

Tephrochronology is the dating of volcanic eruptions and other events by studying layers of tephra.  Tephra refers to the products of volcanic eruptions: lava, ash, pumice, and volcanic rock debris. All of these products contain volcanic glass. The chemical composition of this glass material is unique to each eruption, like a fingerprint. This means that geologic layers containing this glass material can be linked to specific eruptions at specific times and locations.

Tephrostratigraphy analyzes these chemical fingerprints and compares them across space. Rocks with the same fingerprint in different places can be traced to the same eruption. If scientists find a layer of volcanic ash with a known date on one side of a valley and also find a layer of ash with the same chemical fingerprint somewhere else in the valley, they can assume these layers were laid down at the same time.

Scientists use the Principle of Superposition discussed earlier for this dating technique as well. When excavating a site containing hominin fossils or artifacts, layers of volcanic ash can sometimes be dated (see Absolute Dating section below) above and below where these ancient remains are found. This method allows scientists to determine the age range for the site: it cannot be younger than the top ash layer and it cannot be older than the bottom ash layer.

ABSOLUTE DATING

Relative dating methods provide “older-to-younger” sequences or approximate age brackets. Absolute dating methods are ways of estimating a specific chronological age in years. These age estimates are subject to margins of error – a statistic expressing the degree of precision of the estimate. All absolute dating methods have margins of error, and these vary depending upon the method used and factors associated with the material dated. Absolute dating methods are the first choice for geologic dating if the appropriate materials are available to date. These methods work with certain types of geologic materials, and they can be used to provide direct age measurements of fossils, archeological remains, or the layers associated with these finds. This section will explore some of these methods in more detail, focusing on those most commonly used in human evolution research.

Radiometric Dating

To establish the absolute age of a fossil or artifact, scientists can use a type of natural “clock” as a basis to determine the date it was formed. A clock records time at a fixed rate. Radioactive materials also decay at a fixed rate that can be measured in a laboratory. Geologists commonly use radiometric dating methods based on the natural radioactive decay of certain elements such as uranium, potassium, and carbon as reliable methods to date ancient events.

Atoms are composed of three basic building blocks: protons, neutrons, and electrons. The protons and neutrons make up most of the mass of the atom (found in the nucleus), and electrons orbit the nucleus. For each element in the periodic table, the number of protons is constant while the number of neutrons and electrons can vary. Isotopes are variations of a chemical element. Each variation has the same number of protons, but a different number of neutrons. Each isotope is identified by the sum of the protons and neutrons within an atom. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C).

Most isotopes found on Earth are stable, meaning they do not change their composition of protons and neutrons regardless of time or environmental conditions. Some isotopes, however, have an unstable nucleus and are radioactive. Radioactive decay changes an unstable isotope of an element to a stable one. The unstable isotope spontaneously emits energy through radiation that changes its number of protons, neutrons, or both. The atomic nucleus that decays is called the parent isotope, and the product of the decay is called the daughter isotope.

Radiometric dating entails measuring the ratio of parent and daughter isotopes in a radioactive sample. These samples must be organic matter (i.e., wood, bones, and shells) or certain minerals and geologic material that contain radioactive isotopes. The rate of decay for many radioactive isotopes has been measured; neither heat, pressure, gravity, nor other variables change the rate of decay.

Radioactive decay is measured in half-lives. A half-life is the amount of time that it takes for half of the parent isotope to decay into daughter isotopes. When the quantities of the parent and daughter isotopes are equal, one half-life has occurred. If the half-life of an isotope is known, the amount of the parent and daughter isotopes can be measured and the amount of time since the radioactive decay began can be calculated. Different elements’ isotopes are useful for different age ranges due to variations in their half-life length

Radiocarbon (Carbon-14) Dating

Carbon has three isotopes: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). 12C and 13C are stable isotopes, and do not function as change-over-time indicators for radiometric dating. 14C, however, is unstable. With a half-life of 5730 years, radiocarbon dating is one of the most widely used radiometric dating techniques. 14C is generated in the atmosphere when cosmic radiation bombardment creates neutrons that interact with nitrogen atoms, ejecting a proton from the nucleus to create a carbon atom with 8 neutrons (14C). 14C is then incorporated into some of the molecules of carbon dioxide (CO2) in the air. During photosynthesis, plants take in CO2 and use it to build their tissue. 14C is passed through the plants to the animals (and humans) that eat them. When an organism dies, it stops taking in 14C and the concentration of 14C in its body begins to decrease through radioactive decay. Knowing the half-life of 14C, the age of dead plant or animal tissue can be calculated by measuring the amount of 14C left in a sample. Critically, to use this dating method the sample must be organic – it must contain carbon and have once been alive. Because the half-life of 14C is short (by geologic standards), the age range within which this method is useful is between 50 and 50,000 years old. Beyond 50,000 years old, the amount of 14C left in the sample will be too small to measure accurately. Luckily, there are methods available to scientists that allow the dating of materials older than the age limit of radiocarbon dating.

40Potassium-40Argon Dating

Potassium-argon (40K-40Ar) dating 1 is a radiometric dating method that relies on the radioactive decay of an unstable isotope of potassium into a stable isotope of argon. Potassium is a common element found in many minerals. It is also a major component of certain types of volcanic materials. In these materials, 40K decays into 40Ar (a gas), which is trapped within the mineral crystals as the materials cool. The daughter isotope 40Ar then begins to accumulate. The ratio between the two isotopes in a mineral sample is used to calculate the time since the mineral began to trap the 40Ar. The half-life of this process is 1.3 billion years and is much longer than the decay of 14C. Because of this, the age range over which this method can be applied is also longer, between 100,000 years old and the age of the Earth (4.6+ billion years).

An important revolution in absolute dating for human evolution research was the introduction of single-crystal 40Ar-39Ar dating. This modified method uses a very narrow laser to heat a single crystal within a sample in order to extract the argon isotopes for measurement and age calculation. This approach permits the researcher to identify and exclude contaminant grains from previous eruptions of different ages. Only crystals of the most recent eruption are included in the dating analysis. This approach greatly increases the dating precision and accuracy of the layer that produced the volcanic sample. The ability to identify a single eruption is very advantageous in places where volcanoes frequently erupt and thus may mix material from past eruptions. This method has been used to constrain the ages of several notable sites and finds, including the site of Olorgesailie, Kenya and the regions where the “Lucy” skeleton in Ethiopia and the “Turkana Boy” skeleton in Kenya were found.

Together, 40K-40Ar and 40Ar-39Ar are extremely useful methods to date fossils and archaeological sites relevant to human origins in eastern Africa, because that area of the world has been highly active volcanically for millions of years. The widespread presence of volcanic materials throughout the landscape makes it possible to use these methods to date many of the important hominin sites in this region.

1  40K-40Ar dating requires splitting samples into two for separate K and Ar measurements. This process results in sizeable error margins in the measurement. An update to 40K-40Ar dating was developed in order to reduce this error. This updated method, 40Ar-39Ar dating, requires only one sample and uses a single measurement of argon isotopes. The aforementioned steps are carried out, but an additional process is introduced which relies on neutron irradiation from a nuclear reactor to convert 39K (stable) into 39Ar (unstable). A standard reference material of known age is irradiated at the same time as the unknown samples, making it possible to use a single measurement of argon isotopes to calculate the 40K/40Ar ratio and obtain an age.

Uranium Series Dating

This method is one of a family of methods that use multiple, different unstable uranium isotopes that decay into stable lead isotopes by different chemical pathways. The most relevant for human evolution research is the decay pathway beginning with Uranium-238 (238U), which decays to Lead-206 (206Pb). Unlike the many other radioactive elements, uranium requires multiple steps to decay into lead due to its massive atomic weight. 206Pb is the final step in this decay process because it is stable. This multiple-decay process means that the half-life of uranium series is long, allowing scientists to date very old materials such as the Canadian Acasta Gneiss (the world’s oldest known rock) which was estimated to be 4.03 billion years old using uranium-lead dating.

Uranium-lead dating is similar to other radiometric methods in that the end product (206Pb) is stable. However, the intermediate decay steps to get to that stable end product are useful for dating as well. The methods in this series calculate ages differently from other radiometric methods, because their daughter isotopes are unstable. The most commonly used of this series is the 234U-230Th (uranium-thorium) pathway. Thorium is not soluble in water, so geologic material formed from flowing water (like caves) do not usually contain any thorium. In contrast, uranium is water soluble and becomes incorporated into geologic material. As time passes, unstable 234U decays to 230Th; this process has a half-life of 245,000 years. However, 230Th is also radioactive (with a half-life 75,000 years), so instead of accumulating indefinitely, it also begins to decay. Eventually a balance between decay and accumulation of these isotopes is reached, which allows a calculation of the date of the sample. Uranium series dating is especially useful in regions that are not volcanically active such as South Africa and western Europe. It is also particularly useful in cave sites, because uranium is frequently introduced into caves through slow-flowing water.

Non-Radiometric Dating

There are many absolute dating methods that rely on some process other than radioactive decay. There are many natural “clocks” that have varying degrees of reliability and use. The following section introduces a few of these techniques that are most commonly applied in human evolution research.

Trapped Electron Dating

Trapped electron dating methods measure the amount of radiation (sunlight, heat, etc.) received by an object. These methods only work on materials that are crystalline, meaning they have a lattice-like atomic arrangement. All crystalline structures have imperfections caused by missing atoms or the presence of impurities in the structure. When exposed to radiation from the environment, electrons in the structure absorb energy, detach from the nucleus of their atom, and become “trapped” in these lattice imperfections and begin to accumulate.

When the material is subjected to sunlight or other high heat, the trapped electrons are released. If the material is buried, it begins to accumulate trapped electrons. Scientists can later expose the material to heat or light in the lab, which again releases the trapped electrons. Instead of indicating when the material was formed, this release shows researchers how much time has passed since the material was last exposed to heat or light. This method is useful for dating events such as the burial of an object, firing of pottery, or heat treatment of stone tools. It is critical to be able to separate when a material (a rock, for example) was formed versus when it was altered and buried (after heat treatment as a stone tool).

There are two main trapped electron dating methods, which are discussed below:

1.  Thermoluminescence (TL)

Thermoluminescence is used to date crystalline minerals to the time of their last heating event in the past. This method is useful for ceramics (pottery) and sediments that were exposed to a very significant amount of sunlight. As radiation from the environment is constantly bombarding minerals, energized electrons start to become trapped within defects of the crystal lattice.

As noted above, an input of energy such as heat or light is required to free these trapped electrons. The accumulation of trapped electrons occurs at a measurable rate proportional to the radiation received from a specimen’s immediate environment. When a specimen is reheated, the trapped energy is released in the form of light (luminescence) as the electrons escape.

The amount of light produced can be measured in a laboratory setting. Because this accumulation of trapped electrons begins with the formation of the crystal structure, thermoluminescence can date crystalline materials to when they formed or to the last time the materials were exposed to light. For ceramics this is either the moment they are fired or the last time the ceramics were exposed to the sun as they were buried, which can be distinguished by the degree of purge in the electron traps.

2.  Optically Stimulated Luminescence (OSL)

Optically stimulated luminescence detects when sediments were last exposed to lower levels of light than required for thermoluminescence dating. Certain minerals within sediments (such as quartz) store energy in the form of radiation at a known, constant rate. When these minerals are in the ground, electrons from radioactive elements get trapped in the defects of their crystalline structures. If the minerals are exposed to sufficiently high levels of radiation (such as sunlight), that exposure causes vibrations in the mineral lattices. A portion of the trapped electrons from the radioactive elements are freed, and that released energy is measured and used to calculate the date when the mineral was last exposed to that level of sunlight (i.e. the approximate date of burial).

OTHER METHODS RELEVANT TO HUMAN  EVOLUTION RESEARCH

Fission Track Dating

Fission track dating is based on the same principles as uranium-lead dating, but the “daughter” product that is measured is not an element, but rather the damage made within a crystal. Because uranium is such an unstable element, the nucleus is capable of spontaneous fission, which means forcefully splitting the nucleus into two fragments of similar mass. This event is so powerful that it can leave “tracks” of damage in the crystal in which the uranium is trapped. Scientists can submerge this crystal in acid and make these tracks visible for analysis under a microscope. The number of tracks that they count can be compared against the uranium content within the sample itself to calculate the age of the crystal. This method is typically applied to rocks that show the tracks well, such as zircons.

Electron Spin Resonance (ESR)

This technique was introduced in the 1970’s to date recently-formed materials that cannot be dated using the radiocarbon method. It can be applied to organic materials such as tooth enamel and shell. This makes this technique useful because teeth are the most common part of the skeleton found in the fossil record.

Tooth enamel is primarily composed of the mineral hydroxyapatite, which possesses two energy states: the non-excited state and the excited state. Natural geological radiation can transfer electrons between these states. The ratio of electrons trapped in both states is proportional to the duration of irradiation (i.e. the amount of time buried), which in turn gives an accurate age of the tooth.

Amino Acid Racemization

This method was introduced in the mid 1980’s and refined throughout the 1990’s as an attempt to expand the variety of dating methods of use for biogenic materials. Amino acids can exist in two different mirror-image forms (L and D type) that can be differentiated using polarized light. Living organisms only have L-type amino acids. When an organism dies, amino acids can flip (“racemize”) between L and D-types; the L-type changes to the D-type at a steady rate until there are an equal number of L and D types. The ratio of the two types in an organic sample can be used to estimate the time passed since death.

Obsidian Hydration

Obsidian is a volcanic glass that has been used by ancient and modern humans to make very high-quality, sharp stone tools and weapons. Obsidian undergoes a process called mineral hydration: when fractured, the material begins to absorb water from the air or environment at a relatively regular rate. This absorption forms a layer on the surface of the obsidian. The thickness of that layer can be measured and correlated to the amount of time that has passed since the obsidian was fractured. Archaeologists can use this method to date the manufacture of a stone tool. 

SUMMARY

Dating methods are a cornerstone of studying the past, and are a good example of how multiple kinds of science work together – e.g., geology, chemistry, physics, and statistics. Developing and refining dating methods has been a critical component of human evolution research, and has provided numerous insights into the timeline of our past. From the most ancient of our relatives to historical innovations of our own species, dating methods have helped scientists to understand the sites and events relevant to human evolution. Some of these events covered on this website are highlighted in the figure below.

If you would like to learn more, we recommend visiting these two websites:

1. The Dating Rocks and Fossils Using Geological Methods article in Nature's excellent Scitable series of online articles in the Nature Education Knowledge Project.

2. University of California, Berkeley Museum of Paleontology's Understanding Deep Time online resource. This is an informational tour in which students gain a basic understanding of geologic time, the evidence for events in Earth’s history, relative and absolute dating techniques, and the significance of the Geologic Time Scale.

The text and illustrations on this page were developed primarily by Kim Foecke, with contributions from Kevin Takashita-Bynum, and edited by Rick Potts, Briana Pobiner, and Jennifer Clark. We owe thanks to several educators (Nikki Chambers, John Mead, Wes McCoy, and Mark Terry) and Hall of Human Origins Volunteers (Ben Gorton, Jurate Landwehr, Carol Schremp, Dave Wrausmann) who also provided comments and suggestions.