Look at the alphabet: it is made up of 26 symbols, each one representing a specific sound made in human speech. Each one of these symbols has a history, having evolved over thousands of years. Earlier incarnations did not represent sounds but were actually pictures, depicting their subject. Still others were symbols for ideas.
Sometime in our prehistory, people began to communicate using visuals. The first were simple drawings of the concrete objects that existed around the writer. These are called pictographs.
The more people got used to writing, the more they found they needed to express. They needed to be able to express ideas and abstractions. So the symbols originally meant to describe a literal object came to have multiple associations or meanings. The symbol for “ox” came to mean “food” as well. Often, symbols would be combined to signify ideas.
The major disadvantage to this system is that the user ends up with thousands of characters, making it hard to learn and write the language.
The Phoenicians developed a simplified writing system around 1200 B.C.E. wherein the symbols would instead represent sound. This cut the necessary number of symbols down to a much more manageable amount. These letters could be written quickly and were much easier to learn.
The Phoenicians were traders and merchants, in need of a simple way of tracking their commerce. Their alphabet, consisting of 20 simple marks, accomplished this with aplomb.
This alphabet had no vowels, and was the ancestor to not only ours, but to that of Hebrew and Arabic. The Phoenicians read from right to left.
Around 800 B.C.E., the Greeks adopted the Phoenician alphabet. They saw this form of writing as a means to preserving the knowledge they held so dear. They added vowels, reversed the orientation of reading and standardized the letterforms. By 403 B.C.E., the Greeks had a cleaned-up alphabet of all capital letters.
Early capital letters were often inscribed into stone or marble. They were thus designed with as few curved strokes as possible, to simplify the process.
The Romans adopted the Greek alphabet and reformulated some letters. They added F and Q right off the bat, and gave the letters simplified names, much like ours have today. They also had only capital letters for a long time.
At the time, the reed pen was in use, a tool which is held at an angle (or cant) to the page. This is what creates the variation in the line, and it had a great deal of influence on the formation of the lower case letters to come.
Small letters came out of writing the capital letters (or majuscule) with a pen. At first, only a few were used, but soon enough a whole set of these minuscules were created. The more writing was used, the more people wanted to economize on space—parchment did not come cheap, after all. The small letters meant sa person could write all the more content per page.
There were two popular forms of writing in Europe: the Gothic or Black Letter form of Germany and the Humanistic hand of Italy. Gutenberg used the Black Letter form as models for the type he cast in the 1400s. The Humanistic hand is the ancestor to our current lower case letters.
The Greeks and Romans had no punctuation—the words either ran together or were separated by a dot or slash. The rules of grammar and punctuation came to be formalized with the advent of printing.