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To greatly simplify, radiometric dates work by counting the number of atoms that have decayed from a radioactive to a more stable isotope and then figuring out how many half-lives have passed.For example, 14C (carbon-14) has a half-life of 5,730 years.Radiometric dating techniques are based on the principle of radioactive decay.Essentially, elements, such as carbon (C) or potassium (K), have more than one isotopic form, or variation based on atomic weight.To understand human history, we need to understand the fossil record.But the fossil record cannot be understood without knowing the temporal context.
If 3/4 of the 14C in a sample has converted to 14N, then about 11,460 years have passed.These unstable isotopes are called radioactive isotopes.Radioactive decay is what we call it when these radioactive isotopes emit their subatomic particles as they change into more stable isotopes.It takes another 5,730, on average, for half of the remaining half to decay.Therefore, it takes twice as long for 3/4 of the original sample of 14C to have decayed as it did for 1/2 of it to decay.
Most of the time, fossils and tools can't be directly dated. For example, a fossil may be found just below a layer of volcanic ash that can be dated to 2.3mya based on K/Ar dating. Similarly, a fossil might be found with pieces of burned wood that date to 25kya. Below is a chart that lays out the most important radiometric dating techniques used in the study of human evolution.