Biology (Year 12) - Evidence for Evolution
What is fossil dating? Several questions are posed regarding life on earth. How old are fossils? Which organisms existed first? Which organisms lived together? Answers to these questions are only possible if we can determine the age of fossils in the record. A combination of comparative (relative) and absolute aging techniques are used to estimate fossils and remains' age. Comparative Dating
Also known as relative dating, it is used to determine the age of rock layers or strata or fossil in rock-based by those nearby. Sedimentary rock is composed of weathered material such as gravel, sand and mud. Overtime, deposited sediments form defined layers that consolidate into sequences of sedimentary rock. These layers are called strata and a section showing the successive layers of sedimentary rock is called stratification. The principle of superposition can be applied in fossil dating, acknowledging that the oldest fossils are at the bottom layers and youngest at top, but it does not provide an actual age for the fossils.
Absolute dating, also known as chronometric dating, assigns numerical age in years to fossils and rocks based on the physical and chemical properties of materials in rock. There are three types: 1. Radio-metric dating: Based on the predictable rates of decay of naturally occurring radioactive isotopes which are present in rock or fossil. Isotopes are elements that have the same atomic number (protons) but a different mass (electrons). Some isotopes have unstable nuclei and emit radioactive energy at a measurable rate. The half-life of an isotope is time taken for half radioactive atoms to decay, and it can measure the time taken for the original amount to decay to the present amount (carbon-14 dating). Carbon dating is only accurate for samples up to 70,000 years old as its half-life is 5730 years. Carbon dating is generally not applied to fossils, but the rock in which fossils are found. 2. Electron spin resonance: A new method of dating that measures the properties of electrons in crystals of minerals. Electrons are collected and become trapped within the crystal lattice. The level of magnetism can be measured to give the electron spin reading, and radiation can be divided by the background dose rate to determine the age. 3. Luminescence techniques: These are used in Australia, and they measure the characteristics of minerals in rocks via light emitted when the minerals are heated. It is important to note that it cannot estimate the absolute age of sedimentary rock.