The sequence of amino acids in a protein (say haemoglobin) from an elephant is compared to the sequence of amino acids extracted from a mammoth preserved in permafrost from 30,000 years ago. Scientists count the number of differences. They assume this rate of change is constant. They can then project this lineage back further.
Scientists can then compare different organisms and where the number of differences between their projected ancestors intersect, i.e. zero sequence differences, they assume a common ancestor.
6 million years ago scientists say there was zero difference between the protein’s amino acids or DNA base sequences of humans and chimpazees ancestors.