Merging two row cells and repeating down a column (not down a row).

I have an excel file of contact names. The first name is in one cell (Cell A) and the last name is in a separate cell (Cell B). They are in the same row. I have over 1600 contact names (or rows) like this. I want the first and last names to be merged with one another in just one cell. How do I do this in the quickest and simplest way?