This section from 'The Blind Watchmaker' by Richard Dawkins has really answered questions for me:
The genetic dictionary has 64 DNA words of 3 letters each. The language appears to be arbitrary n the same sense as a human language is arbitrary (there is nothing intrinsic in the sound of the word "house", for instance, which suggests to the listerner any attribute of a dwelling).
'Given this, it is a fact of great significance that every living thing, no matter how diefferent from others in external appearance it may be, 'speaks' almost exactly the same language at the level of the genes.
'The genetic code is universal. I regard this as near-conclusive proof that all organisma are descended from a single common ancestor.
'Molcular biology suddenly opened a new treasure chest of resemblances to add to the meagre list offered by anatomy and embryology.
'Though all living things share the same dictionary they don't all make the same sentences with their shared dictionary. This offers us the opportunity to work out varying degrees of cousinship.
Closely similar protein or DNA sentences can be assumed to come from close cousins, more different sentences from more distnt cousins.
We can measure exactly how many steps separate one animal from another. ..
The molecular clock allows us to estimate, not just which pairs of animals have the most recent common ancestors, but also approximately when those common ancestors lived.
We can use the molecular clock to date branch points in the evolutionary tree.
DNA sequences are the gospel documents of all life, and we have learned to decipher them.
Chimpanzees and hmans share more than 99% of their genes.
The last common ancestor of humans and chimps lived perhaps as recently as 5 million years ago, dfinitely more recently than the common ancestor of chimps and orangutans, and perhaps 30 million years more recently than the common ancestor of chimps and monkeys.
(Taken from pps. 263 and 270,271)