Barricelli’s numeric organisms
Before Core Wars, or the computer game Darwin, just about as soon as a computer was built that could run the program, a rather obscure Italian scientist named Barricelli, did pioneering experiments in digital life. While his work remains largely unknown, it does demonstrate how people have been working on evolving computer code from the very earliest days of computers.
Here’s a link to the talk where I first heard of Barricelli.
Here’s a page that includes an implementation of his work, including source code.
And here’s an article about Barricelli and his work.