Biologists estimate how long ago species separated from a common ancestor using a technique called

The answer is molecular clocks.

Molecular clocks use rates of mutation to measure evolutionary time. This technique is based on the fact that mutations add up at a fairly constant rate in the DNA of species that evolved from a common ancestor. The more mutations that happened in each lineage, the greater is the differences between these lineages and vice versa.

The truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey