The earliest evidence of writing anywhere in the world only goes back about 5000 years.Paleoanthropologists frequently need chronometric dating systems that can date things that are many thousands or even millions of years older.The L-amino acids change to D-amino acids more or less steadily following death. As a result, remains of organisms that died long ago will have more D-amino acids than ones that died recently.Aspartic acid (one of the 20 amino acids) is usually extracted from samples for this dating technique.The L- and D-amino acid ratios are determined by gas and liquid chromatography..Likewise, it can occur in molten rock from a volcano.These orientations are later measured by geologists and sometime historical land movement and deformation can be deduced.Paleomagnetism has played a significant role in understanding and confirming the theories of plate tectonics, magnetic pole reversals, and mid oceanic ridge seafloor spreading.
Despite this limitation, paleoanthropologists have found dating by association with polar reversals to be a helpful additional method of tracking the evolution of our fossil ancestors in regions such as East Africa where there has been frequent volcanic activity leaving clear thermoremnant magnetic evidence.
The time between reversals has ranged between less than 100,000 to tens of millions of years with an average of about 300,000 years. It takes 1,000-8,000 years for one to be completed. It is thought that as a reversal approaches, the earth's magnetic field weakens.
It has been gradually weakening for the last 250 years.
A rock samples magnetic direction and intensity is a record of the earth’s magnetic field at the time the rock formed.
Paleomagnetic measurements are magnetic measurements of rocks.