Here of some of the well-tested methods of dating used in the study of early humans:

Potassium-argon dating, Argon-argon dating, Carbon-14 (or Radiocarbon), and Uranium series: All of these methods measure the amount of radioactive decay of chemical elements; the decay occurs in a consistent manner, like a clock, over long periods of time.

Thermo-Luminescence, Optically Stimulated Luminescence, and Electron Spin Resonance: All of these methods measure the amount of electrons that get absorbed and trapped inside a rock or tooth over time.

Paleomagnetism: This method compares the direction of the magnetic particles in layers of sediment to the known worldwide shifts in Earth’s magnetic field, which have well-established dates using other dating methods.

Biochronology: Since animal species change over time, the fauna can be arranged from younger to older. At some sites, animal fossils can be dated precisely by one of these other methods. For sites that cannot be readily dated, the animal species found there can be compared to well-dated species from other sites. In this way, sites that do not have radioactive or other materials for dating can be given a reliable age estimate.

Molecular clock: This method compares the amount of genetic difference between living organisms and computes an age based on well-tested rates of genetic mutation over time. Since genetic material (like DNA) decays rapidly, the molecular clock method can’t date very old fossils. It’s mainly useful for figuring out how long ago living species or populations shared a common ancestor, based on their DNA.

	Years Before Present	Milestones	Dating Method
	4-7 Million	Humans and Chimps diverge	Molecular genetic clock
	By 4 Million	Bipedal walking becomes well developed	Argon dating
	2.6 Million	Oldest stone tool	Argon dating, Paleomagnetic dating
	1.8 Million	Homo erectus expands out of Africa	Argon dating, Paleomagnatic dating
	800,00 – 200,000	Rapid period of brain expansion	Argon and U-series dating, Paleomagnetic dating
	250,000 30,000	Neanderthals emerge, then become extinct. Homo Sapiens emerge in Africa, then expands to other continents. Symbolic culture begins to flourish	TL, ESR, C-14 dating, Molecular clock
	12,000 – 10,000	Origins of agriculture	C-14 dating
	4,500	Origins of writing, state, societies, civilization: Sumer & Egypt	C-14 dating

Sources:

http://humanorigins.si.edu/evidence/dating