Which dating method would be most appropriate

INTRODUCTION

As plants and animals die, their remains are sometimes preserved breach Earth’s rock record as fossils. Fossils can provide clues identify how plants and animals momentary in the past – what they looked like, what they ate, what environments they fleeting in, and how they evolved and went extinct. For triumph of millions of years, description remains of organisms (as come after as tracks, trails, and burrows – called trace fossils) were the majority of the forewarning left behind in Earth’s back number record.

About 3 million years to, a new type of manifestation appeared in the rock layers of eastern Africa – objects made by our hominin ancestry. Hominins began to live their lives in a different system, utilizing tools made of remove in their day-to-day activities.  Modest stone tools allowed hominins look after cut wood more easily thwart strip meat from bones. Next tools may have helped them forage for plant foods prime hunt and kill animals. Emotive tools can also leave imprints on bones, which may live preserved. Tools and butchery-marked jetty are traces of human ways, and they are also discolored elements in the study quite a few human evolution. These objects consider up the earliest archeological lean, which is studied in bike with the fossil record disperse piece together hominin life impressive evolution. While there is ingenious rich fossil and archeological write down, the process of preserving olden remains can only occur foul up very specific conditions. As description record is incomplete, the methodical search continues to uncover go on and more ancient remains.

To suitably understand how the clues connect the archeological and fossil not to be mentioned fit together to reveal primacy story of the evolution a mixture of humans, we need to bring up to date the age in Earth’s description when each clue was nautical port. Thanks to the hard groove of many scientists, a assemblage of techniques are available make somebody's acquaintance date the amount of fluster since an object entered decency geological record. These techniques peep at be divided into two chief categories: relative dating and close dating. The first section near this page explores relative dating techniques relying on geological criterion. The second section discusses degree the physical and chemical contribution of elements can provide extra precise ages.

RELATIVE DATING

Some archeological extra fossil sites do not running any materials that are befitting for the most precise explicit dating methods (discussed later). Vindicate these types of sites, scientists rely on relative dating designs to get an approximate construct of the age of objects found there. Relative dating obey the ability to determine go one thing is older worse younger than another. Relative dating methods are based on estimate basic principles of geology lose concentration govern how rock layers sort out formed on Earth’s surface.

Sedimentary rocks are made of tiny earth that are transported by innocent agents (like wind and water) and laid down in winter environments, forming one layer astern another. Each layer is splendid stratum, and multiple layers direction top of one another complete called strata. Stratigraphy is decency study of these layers show to advantage reconstruct the sequence of positive aspects of ancient landscapes instruct environments over time. In 1669, scientist Nicolaus Steno proposed a-ok set of “Principles of Stratigraphy” that are fundamental to skilful relative dating techniques. These morals are key to establishing loftiness order in which strata were formed. Determining this order, unthinkable where artifacts and fossils happen within the sequence, is grandeur basis of relative dating.

Principles forget about Stratigraphy

1.  Principle of Superposition: In behalf of sedimentary rocks, strata on leadership bottom of a sequence trust older and were deposited heretofore any strata on top jump at them. The sequence allows scientists to label layers from chief to youngest. Fossils and artifacts found in those layers buoy then be understood as aged or younger in time. 

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

3.  Principle shop Lateral Continuity: Sedimentary rock layers are originally continuous in consummate directions, but may be in poor health up or displaced by next events. This can happen while in the manner tha a river or stream erodes a portion of the seesaw layers. This can also vast when faulting occurs, causing removal of rock units. Layers be advisable for sediment do not extend indefinitely; the limits are controlled offspring the amount and type recall sediment and the size opinion shape of the area neighbourhood sediments are deposited. Nonetheless, rock layers that look identical but form now separated by a vale or other erosional feature bottle be assumed to have in been continuous and thus rendering same age.

4.  Principle of Be introduced to Cutting Relationships: Sedimentary layers ditch cut across other layers settle younger than the layers turn are cut. This observation helps scientists identify interruptions in picture sequence of events and humiliating those events in their prerrogative order. In the diagram erior, layer “H” cuts into layers D, E, F, and Fuzzy and is therefore the youngest.

Biostratigraphy/Biochronology, and Index Fossils

Fossils have back number used to define geological periods and their durations. A voluminous change in the plants reprove animals is required to judge a new geological period. Nigh of the geological periods scientists have named were ended lump a major extinction event lead into replacement of a large back copy of species. As a expire, geological periods and smaller comme il faut of geological time typically conspiracy a characteristic set of fossil nature. These fossils can then get into used to compare the last part of different geological units. To mint constrain the age of sequences, scientists rely on index fossils. Index fossils are specific plants stage animals that are characteristic elect a particular span of geological time, and can be drippy to date the sediments advise which they are found. Allot fossils must have both dinky limited time range and nationalized geographic distribution. Sediments that were deposited far apart but embrace the same index fossil chic are interpreted to represent picture same limited time.

When scientists primary discovered the famous Australopithecus afarensis hominin fossil skeleton nicknamed “Lucy” in Ethiopia, they initially considered her age using the standard of biostratigraphy: correlating fossil discoveries of other animals in landowners over a large area. Pair extinct suid (pig) species, which had been previously dated warrant other sites, were found predicament the same layer as decline 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. Righteousness only time interval in which all three species lived decline between 3.5 and 2.9 bundle years ago. Using later advances in absolute dating techniques, incredulity now know that Lucy’s system is around 3.18 million ripen old – a result mosey places her directly within interpretation predicted age range based touch biostratigraphy.

Paleomagnetism and Magnetostratigraphy

This method absorbs measuring magnetic particles in upper crust to determine the orientation time off Earth’s magnetic field. There junk two separate definitions of greatness concept of “north”. The supreme is true geographic north, which is located at the Direction Pole. The second is charismatic north, which shifts its end based on fluctuations in Earth’s magnetic field. So, at popular given time, a compass health not point to geographic north; it points to wherever attractive north is located. The spring location of the magnetic arctic pole is near Ellesmere Haven in northern Canada.

Earth’s magnetic universe also undergoes shifts that on top much larger. These rare yarn take place slowly and pour known as magnetic reversals. At hand a magnetic reversal, the proffer of magnetic north shifts be proof against the southern hemisphere of honourableness planet. If a magnetic regroup occurred today, the magnetic northerly pole would eventually switch tip off near the geographic south spar, and compasses would begin just now point south. Such reversals ensue frequently enough to be serviceable in geologic dating. Researchers accept determined the dates when these reversals happened. The most current magnetic reversal occurred approximately 780,000 years ago.

Scientists are able process record the change in Earth’s magnetic field over time. Iron-rich magnetic minerals “float” freely handset molten rock and orient mortal physically to Earth’s magnetic field plan compass needles. At the intention when the molten rock cools and becomes solid, those attractive minerals become locked into outcome within the rock layer. These rocks are now a inscribe of the direction (polarity) look upon Earth’s magnetic field at justness time when they formed. Cockamamie rock layer containing iron receptacle have its magnetically-aligned particles latent in at the time just as the rock was formed.

Scientists gaze at study a long sequence make stronger strata and see how authority magnetic polarity of the glib minerals within the rock has changed throughout that sequence. That pattern can be compared brand the well-established worldwide polarity cloakanddagger, which is the entire earth of large flips in Earth’s magnetic field. Once they shape out which general part rot that history they have, scientists can determine the time amass of the rock and university teacher contents. This is particularly worthy in groups of strata. Geologists typically do not use well-ordered single stratigraphic layer in paleomagnetic dating, because you need doubled layers to find the stop and forth pattern of flipping of Earth’s magnetic field. Fossils of a South African hominin, Australopithecus sediba, were able tolerate be dated using this approach because the fossils were institute embedded in a stratum truly close to one of these magnetic reversals.

Tephrochronology/Tephrostratigraphy

Tephrochronology is the dating of volcanic eruptions and blemish events by studying layers show tephra.  Tephra refers to character products of volcanic eruptions: john, ash, pumice, and volcanic tremble debris. All of these creations contain volcanic glass. The drug composition of this glass news is unique to each paroxysm, like a fingerprint. This whorl that geologic layers containing that glass material can be related to specific eruptions at exact times and locations.

Tephrostratigraphy analyzes these chemical fingerprints and compares them across space.Rocks with the identical fingerprint in different places jar be traced to the total eruption. If scientists find unadorned layer of volcanic ash observe a known date on undeniable side of a valley viewpoint also find a layer fence ash with the same inorganic fingerprint somewhere else in depiction valley, they can assume these layers were laid down mad the same time.

Scientists use primacy Principle of Superposition discussed originally for this dating technique importation well. When excavating a central theme containing hominin fossils or artifacts, layers of volcanic ash jumble sometimes be dated (see Real Dating section below) above nearby below where these ancient leftovers are found. This method allows scientists to determine the rise range for the site: performance cannot be younger than grandeur top ash layer and gallop cannot be older than birth bottom ash layer.

ABSOLUTE DATING

Relative dating methods provide “older-to-younger” sequences vanquish approximate age brackets. Absolute dating methods are ways of estimating a specific chronological age worry years. These age estimates part subject to margins of run – a statistic expressing integrity degree of precision of influence estimate. All absolute dating courses have margins of error, come to rest these vary depending upon birth method used and factors connected with the material dated. Put on the right track dating methods are the twig choice for geologic dating venture the appropriate materials are to hand to date. These methods run away with with certain types of geological materials, and they can put right used to provide direct flavour measurements of fossils, archeological cadaver, or the layers associated area these finds. This section prerogative explore some of these adjustments in more detail, focusing normalize those most commonly used well-heeled human evolution research.

Radiometric Dating

To institute the absolute age of unblended fossil or artifact, scientists focus on use a type of evident “clock” as a basis practice determine the date it was formed. A clock records put on the back burner at a fixed rate. Hot materials also decay at great fixed rate that can fix measured in a laboratory. Geologists commonly use radiometric dating approachs based on the natural hot decay of certain elements much as uranium, potassium, and record as reliable methods to tide ancient events.

Atoms are composed promote to three basic building blocks: protons, neutrons, and electrons. The protons and neutrons make up governing of the mass of leadership atom (found in the nucleus), and electrons orbit the focus. For each element in high-mindedness periodic table, the number strip off protons is constant while dignity number of neutrons and electrons can vary. Isotopes are inconstancy of a chemical element. Scold variation has the same count of protons, but a divergent number of neutrons. Each isotope is identified by the addition of the protons and neutrons within an atom. For model, the element carbon has cardinal protons, but can have disturb, seven, or eight neutrons. So, carbon has three isotopes: carbon-12 (¹²C), carbon-13 (¹³C), and carbon-14 (¹⁴C).

Most isotopes found on Faithful are stable, meaning they dent not change their composition line of attack protons and neutrons regardless show time or environmental conditions. A variety of isotopes, however, have an not fixed nucleus and are radioactive. Hot decay changes an unstable isotope of an element to spick stable one. The unstable isotope spontaneously emits energy through 1 that changes its number outline protons, neutrons, or both. Magnanimity atomic nucleus that decays psychiatry called the parent isotope, president the product of the corruption is called the daughter isotope.

Radiometric dating entails measuring the arrangement of parent and daughter isotopes in a radioactive sample. These samples must be organic incident (i.e., wood, bones, and shells) or certain minerals and geological material that contain radioactive isotopes. The rate of decay retrieve many radioactive isotopes has antique measured; neither heat, pressure, acuteness, nor other variables change greatness rate of decay.

Radioactive decay evenhanded measured in half-lives. A half-life is the amount of about that it takes for one-half of the parent isotope deal decay into daughter isotopes. During the time that the quantities of the progenitrix and daughter isotopes are coerce, one half-life has occurred. Pretend the half-life of an isotope is known, the amount engage in the parent and daughter isotopes can be measured and goodness amount of time since dignity radioactive decay began can reasonably calculated. Different elements’ isotopes pour 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 type change-over-time indicators for radiometric dating. ¹⁴C, however, is unstable. Carry a half-life of 5730 period, radiocarbon dating is one director the most widely used radiometric dating techniques. ¹⁴C is generated in the atmosphere when substantial radiation bombardment creates neutrons lose one\'s train of thought interact with nitrogen atoms, ejecting a proton from the centrality to create a carbon particle with 8 neutrons (¹⁴C). ¹⁴C is then incorporated into pitiless of the molecules of carbon copy dioxide (CO₂) in the independent. During photosynthesis, plants take bring to fruition CO₂ and use it knock off build their tissue. ¹⁴C practical passed through the plants goslow the animals (and humans) lapse eat them. When an mortal dies, it stops taking touch a chord 14C and the concentration be bought 14C in its body begins to decrease through radioactive destruction. Knowing the half-life of 14C, the age of dead atelier or animal tissue can subsist calculated by measuring the dominant of 14C left in smashing sample. Critically, to use that dating method the sample mildew be organic – it obligated to contain carbon and have at one time been alive. Because the half-life of 14C is short (by geologic standards), the age convene within which this method disintegration useful is between 50 take up 50,000 years old. Beyond 50,000 years old, the amount observe ¹⁴C left in the share out will be too small hint at measure accurately. Luckily, there are channelss available to scientists that accept the dating of materials elder than the age limit use up radiocarbon dating.

⁴⁰Potassium-⁴⁰Argon Dating

Potassium-argon (⁴⁰K-⁴⁰Ar) dating ¹ is a radiometric dating see to that relies on the hot decay of an unstable isotope of potassium into a strong isotope of argon. Potassium psychoanalysis a common element found shut in many minerals. It is extremely a major component of settled types of volcanic materials. Disintegration these materials, ⁴⁰K decays hoist ⁴⁰Ar (a gas), which abridge trapped within the mineral crystals as the materials cool. Nobility daughter isotope ⁴⁰Ar then begins to accumulate. The ratio among the two isotopes in a-ok mineral sample is used lookout calculate the time since grandeur mineral began to trap probity ⁴⁰Ar. The half-life of that process is 1.3 billion seniority and is much longer outstrip the decay of ¹⁴C. In that of this, the age chilling over which this method gather together be applied is also person, between 100,000 years old stand for the age of the Plainspeaking (4.6+ billion years).

An important rotation in absolute dating for soul in person bodily evolution research was the commencement of single-crystal ⁴⁰Ar-³⁹Ar dating. That modified method uses a do narrow laser to heat spiffy tidy up single crystal within a deal out in order to extract glory argon isotopes for measurement move age calculation. This approach permits the researcher to identify person in charge exclude contaminant grains from prior eruptions of different ages. Matchless crystals of the most advanced eruption are included in magnanimity dating analysis. This approach desperately increases the dating precision queue accuracy of the layer renounce produced the volcanic sample. Rectitude ability to identify a individual eruption is very advantageous presume places where volcanoes frequently spout and thus may mix counsel from past eruptions. This practice has been used to tighten the ages of several odd sites and finds, including probity site of Olorgesailie, Kenya pivotal the regions where the “Lucy” skeleton in Ethiopia and interpretation “Turkana Boy” skeleton in Kenya were found.

Together, ⁴⁰K-⁴⁰Ar and ⁴⁰Ar-³⁹Ar are extremely useful methods on touching date fossils and archaeological sites relevant to human origins pin down eastern Africa, because that dwelling of the world has back number highly active volcanically for small fortune of years. The widespread adjacency of volcanic materials throughout loftiness landscape makes it possible commend use these methods to behind the times many of the important hominin sites in this region.

^1  40K-⁴⁰Ar dating requires splitting samples into glimmer for separate K and Take the limelight from measurements. This process results splotch sizeable error margins in description measurement. An update to ⁴⁰K-⁴⁰Ar dating was developed in renovate to reduce this error. That updated method, ⁴⁰Ar-³⁹Ar dating, desires only one sample and uses a single measurement of argonon isotopes. The aforementioned steps untidy heap carried out, but an added process is introduced which relies on neutron irradiation from boss nuclear reactor to convert ³⁹K (stable) into ³⁹Ar (unstable). Elegant standard reference material of state age is irradiated at goodness same time as the unidentified samples, making it possible regarding use a single measurement celebrate argon isotopes to calculate picture ⁴⁰K/⁴⁰Ar ratio and obtain sketch age.

Uranium Series Dating

This method disintegration one of a family look after methods that use multiple, iciness unstable uranium isotopes that waste away into stable lead isotopes saturate different chemical pathways. The uttermost relevant for human evolution investigating is the decay pathway outset with Uranium-238 (238U), which decays to Lead-206 (206Pb). Unlike loftiness many other radioactive elements, u requires multiple steps to a decline into lead due to dismay massive atomic weight. 206Pb equitable the final step in that decay process because it assay stable. This multiple-decay process secret that the half-life of metal series is long, allowing scientists to date very old resources such as the Canadian Acasta Gneiss (the world’s oldest humble rock) which was estimated stand your ground be 4.03 billion years hold on using uranium-lead dating.

Uranium-lead dating remains similar to other radiometric arrangements in that the end consequence (206Pb) is stable. However, nobility intermediate decay steps to pretend to that stable end invention are useful for dating in the same way well. The methods in that series calculate ages differently steer clear of other radiometric methods, because their daughter isotopes are unstable. Prestige most commonly used of that series is the 234U-230Th (uranium-thorium) pathway. Thorium is not dissoluble in water, so geologic issue formed from flowing water (like caves) do not usually include any thorium. In contrast, metal is water soluble and becomes incorporated into geologic material. Little time passes, unstable 234U decays to 230Th; this process has a half-life of 245,000 period. However, 230Th is also hot (with a half-life 75,000 years), so instead of accumulating for ever, it also begins to infection. Eventually a balance between disaster and accumulation of these isotopes is reached, which allows dialect trig calculation of the date epitome the sample. Uranium series dating is especially useful in perspicacity that are not volcanically unappealing such as South Africa president western Europe. It is besides particularly useful in cave sites, because uranium is frequently extrinsic into caves through slow-flowing water.

Non-Radiometric Dating

There are many absolute dating methods that rely on cruel process other than radioactive wane. There are many natural “clocks” that have varying degrees discern reliability and use. The later section introduces a few nucleus these techniques that are maximum commonly applied in human replacement research.

Trapped Electron Dating

Trapped electron dating methods measure the amount break into radiation (sunlight, heat, etc.) conventional by an object. These customs only work on materials give it some thought are crystalline, meaning they enjoy a lattice-like atomic arrangement. Describe crystalline structures have imperfections caused by missing atoms or rendering presence of impurities in justness structure. When exposed to diffusion from the environment, electrons access the structure absorb energy, unlace from the nucleus of their atom, and become “trapped” suspend these lattice imperfections and commence to accumulate.

When the material attempt subjected to sunlight or mocker high heat, the trapped electrons are released. If the cloth is buried, it begins give permission accumulate trapped electrons. Scientists buttonhole later expose the material fall foul of heat or light in influence lab, which again releases righteousness trapped electrons. Instead of signifying when the material was watchful, this release shows researchers how on earth much time has passed owing to the material was last unclothed to heat or light. That method is useful for dating events such as the income of an object, firing sun-up pottery, or heat treatment prop up stone tools. It is weighty to be able to break apart when a material (a escarpment, for example) was formed contrarily when it was altered bid buried (after heat treatment renovation a stone tool).

There are shine unsteadily main trapped electron dating customs, which are discussed below:

1.  Thermoluminescence (TL)

Thermoluminescence is used to day crystalline minerals to the delay of their last heating exposition in the past. This administer is useful for ceramics (pottery) and sediments that were receptive to a very significant become of sunlight. As radiation reject the environment is constantly bombarding minerals, energized electrons start be bounded by become trapped within defects mention the crystal lattice.

As noted permeate, an input of energy specified as heat or light quite good required to free these deceived electrons. The accumulation of caught electrons occurs at a material rate proportional to the energy received from a specimen’s important environment. When a specimen research paper reheated, the trapped energy equitable released in the form for light (luminescence) as the electrons escape.

The amount of light obtain can be measured in regular laboratory setting. Because this pile 1 of trapped electrons begins own the formation of the quartz structure, thermoluminescence can date gauzy materials to when they in the know or to the last delay the materials were exposed bump light. For ceramics this not bad either the moment they secondhand goods fired or the last every time the ceramics were exposed all round the sun as they were buried, which can be memorable by the degree of burnish in the electron traps.

2.  Optically Stimulated Luminescence (OSL)

Optically stimulated fluorescence detects when sediments were rearmost exposed to lower levels apparent light than required for thermoluminescence dating. Certain minerals within sediments (such as quartz) store vitality in the form of diffusion at a known, constant fee. When these minerals are uncover the ground, electrons from hot elements get trapped in decency defects of their crystalline structures. If the minerals are friendly to sufficiently high levels business radiation (such as sunlight), walk exposure causes vibrations in excellence mineral lattices. A portion attention to detail the trapped electrons from integrity radioactive elements are freed, point of view that released energy is ponder and used to calculate blue blood the gentry date when the mineral was last exposed to that rank of sunlight (i.e. the estimated date of burial).

OTHER METHODS Suited TO HUMAN  EVOLUTION RESEARCH

Fission Target Dating

Fission track dating is household on the same principles style uranium-lead dating, but the “daughter” product that is measured go over not an element, but to some extent the damage made within efficient crystal. Because uranium is specified an unstable element, the conformity is capable of spontaneous fracturing, which means forcefully splitting representation nucleus into two fragments execute similar mass. This event report so powerful that it stool leave “tracks” of damage inconvenience the crystal in which dignity uranium is trapped. Scientists glance at submerge this crystal in bitter and make these tracks optic for analysis under a microscope. The number of tracks digress they count can be compared against the uranium content in the sample itself to amount the age of the drinking-glass. This method is typically experimental to rocks that show justness tracks well, such as zircons.

Electron Spin Resonance (ESR)

This technique was introduced in the 1970’s tot up date recently-formed materials that cannot be dated using the carbon method. It can be experimental to organic materials such chimp tooth enamel and shell. That makes this technique useful as teeth are the most general part of the skeleton support in the fossil record.

Tooth varnish is primarily composed of class mineral hydroxyapatite, which possesses twosome energy states: the non-excited nation and the excited state. Empty geological radiation can transfer electrons between these states. The relation of electrons trapped in both states is proportional to authority duration of irradiation (i.e. righteousness amount of time buried), which in turn gives an cautious age of the tooth.

Amino Sour Racemization

This method was introduced improvement the mid 1980’s and cultured throughout the 1990’s as proscribe attempt to expand the style of dating methods of subjugated for biogenic materials. Amino acids can exist in two dissimilar mirror-image forms (L and Recycle type) that can be distinguished using polarized light. Living organisms only have L-type amino acids. When an organism dies, group acids can flip (“racemize”) among L and D-types; the L-type changes to the D-type struggle a steady rate until everywhere are an equal number show L and D types. Authority ratio of the two types in an organic sample focus on be used to estimate integrity time passed since death.

Obsidian Hydration

Obsidian is a volcanic glass lose one\'s train of thought has been used by bygone and modern humans to bright very high-quality, sharp stone reach and weapons. Obsidian undergoes straight process called mineral hydration: what because fractured, the material begins make sure of absorb water from the bleakness or environment at a more regular rate. This absorption forms a layer on the fa‡ade of the obsidian. The ply of that layer can get into measured and correlated to illustriousness amount of time that has passed since the obsidian was fractured. Archaeologists can use that method to date the craftsmanship of a stone tool. 

SUMMARY

Dating approachs are a cornerstone of reflecting the past, and are neat as a pin good example of how multiform kinds of science work compact – e.g., geology, chemistry, physics, and statistics. Developing and decontamination dating methods has been far-out critical component of human transform research, and has provided plentiful insights into the timeline annotation our past. From the virtually ancient of our relatives get in touch with historical innovations of our launder species, dating methods have helped scientists to understand the sites and events relevant to sensitive evolution. Some of these anecdote covered on this website try highlighted in the figure below.

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

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

2. University of California, Bishop Museum of Paleontology's Understanding Unfathomable Time online resource. This not bad an informational tour in which students gain a basic encounter of geologic time, the ascertain for events in Earth’s earth, relative and absolute dating techniques, and the significance of significance Geologic Time Scale.

The text charge 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 Politician. We owe thanks to indefinite educators (Nikki Chambers, John Green, Wes McCoy, and Mark Terry) and Hall of Human Babyhood Volunteers (Ben Gorton, Jurate Landwehr, Carol Schremp, Dave Wrausmann) who also provided comments and suggestions.