Here’s a handy geological timeline for the different eras that mark prehistory:
Palaeontologists are frequently called upon to date fossil specimens according to this timeline, and may do so with a convincing degree of accuracy. There in fact exists great controversy over the exact dates and even the science of dating methodologies.
The major fossil dating methodologies include:
1. Amino acid dating. Organisms contain protein, which is made up of amino acids. After death, the amino acids undergo racemization, an inversion of a chemical and optical property with their molecules rotating the plane of polarized light. The time taken to reach eventual equilibrium, given the temperature in a region and the chemical makeup of a species, provides an estimate of the age of the specimen.
2. Paleomagnetism. Sediments with iron minerals in them have a natural remnant magnetism that is aligned with the Earth’s magnetic field. The Earth’s magnetic field reverses its polarity periods, and the age of a fossil may be cross-checked with the remnant magnetic polarity of its rock strata.
3. Radioactive dating. Organisms stop absorbing carbon after they die. The residual carbon comprise of 3 isotopes: Carbon 12, Carbon 13, and Carbon 14, which then start to decay gradually. The rate of decay of each of these atoms decays at a fixed but different rate (e.g. Constant for Carbon 12 vs. 5,730-year half life for Carbon 14). Measuring the ratio of the remaining Carbon 14 against the Carbon 12 isotopes helps to estimate the age of a specimen, but only up to 50,000 years ago – due to the limited half-life of carbon. A combination of other isotopes (e.g. rubidium to strontium, for specimens older than 100mn years, or potassium to argon, for specimens under 60mn years old) is used to cross-verify the dating estimates.
4. Relative dating. This most intuitive and straightforward method involves observing how certain species in the stratigraphic record do not overlap with those found in alternate sediment layers. This tends to be the most commonly-used approach, and anchor species in each period of geological time e.g. Trilobites in the Cambrian, the Stegosaurus in the Jurassic, or Pterosaurs and T-Rexes in Cretaceous act as markers for the neighbour fossils.
Even an expert combination of these methodologies gives at best, an estimate accurate to a few degrees of confidence. It is important to note the impact of the sheer quantum length of time on accuracy. For instance, a date attached to a ~200mn year old specimen, to even 99% confidence level, translates to a +/- 2mn-year difference. In comparison, us Homo sapiens have barely been around some 120,000 years!