As an extension, what are the original origins of the numbers? I cans see how 1 to 3 were just lines of that number, and 0 is pretty simple, but what about the others?
The symbols (+) for addition, (-) for subtraction (=) date from the 15th and 16th century. The signs for + and - were used by Johannes Widmann in the late 15th century. They first appear in print in a book he wrote about accounting. You can see a reproduction here. In the book, they denoted surplus (+) or deficit (-) rather than operations, so he used them to distinguish between positive and negative quantities. However, Cajori writes that Widmann also used the symbols to signify the operations in other places.
The symbol for equality is due to the Welshman Robert Recorde in his 1557 book The Whetstone of Witte (math book titles used to be so much better). You can find the book here. You can see the part where he introduces the symbol here. He explains that he chose two parallel lines because "no two things could be more equal".
As you can see, these are relatively recent and certainly much more so than the need to express addition and equality. A complete survey of the symbols that were used before would be take a long time (take a look at the second chapter in Cajori - referenced below). Very often, operations would simply be explained in words, and only the numerical results expressed with symbols. Often, addition would be denoted simply by juxtaposition (putting the numbers next to each other).
The numbers we use now came from Arabic numerals around the 10th century. The Arabic numerals were themselves based on Hindu numerals (you can see a comparison here). Apart from speculation, I don't think there is a definitive explanation for the shape of the numerals above 4 (which evolved from a cross).
The classic reference for all this is Cajori (A History of Mathematical Notations) which, although a bit dated, contains beautiful illustrations. You can also look at Boyer and Merzbach's History of Mathematics.
It should be noted that the Persians had pretty much figured out the algebra you learn in High-school before they had come up with symbols to represent the equations. Al-Khwarizmi was the first to publish the quadratic formula in the form of a long, thorny, worded description, with no actual numbers. Accompanying it was the method of completing the square, which I didn't realize until now actually involved drawing, and then completing, a square.
