In his new book on the history of the alphabet, David Sacks says, "Follow any letter back through time and you glimpse the extraordinary commerce of language across centuries."
"It is extraordinary how far and how clearly we can see back to the origins of our letters. English took its alphabet from Latin (as did many a language that the Romans never heard spoken, from Polish to Zulu to Indonesian). Latin itself was written with letters copied from the utterly dissimilar Etruscan language, a tongue still largely unintelligible to us. A few centuries before this happened, the Etruscans had appropriated the Greek alphabet, even though, again, the languages had little in common. And the Greeks had taken their letters, with minor adaptations, from the Phoenicians, though the two peoples' languages were "as different as Arabic and English".
"The Romans began using the Etruscan H when transliterating Greek words containing sounds without a Roman equivalent. So the Greek theta, representing a sound foreign to Latin, became TH. So too phi, khi and rho became PH, CH and RH, and originally represented breathy sounds that were distinct from F, K and R. Thus we have philosophy, chrome and rhetoric. A thousand years later the Normans used H to eradicate the non-Roman letters of Anglo-Saxon. The letter yogh, representing the breathy English 'g' or 'y' sound was to be GH. So, too, we got TH for the 'th' sound in 'then' and CH for the 'ch' in 'cheese'."