For some reason, yesterday I was obsessed with artificial life. It started out on Friday night when I was trying to sleep but found myself thinking about things that are entertaining but don’t help me sleep. (This happens to me a lot. In one particularly insomnia-provoking bout a few years ago, I figured out how long division works as a way of proving that 0.9999999… equals one, and discovered that that is true for the highest digit in base other than base ten.)

In this case, in the morning I searched online to find something interesting enough that I won’t have to write my own to satisfy my curiosity. Right now I barely have enough time to read about other people’s software, let alone write my own. Unfortunately the field is so diverse that I just might have to write my own someday. Not because the world needs more artificial life, but just because.

Artifical life consists of three things: a genome; a method for mutation and “natural” selection, a universe, which provides the domain where the life forms reside, as well as the physical laws of that universe. (The body of an artifical creature can be thought of as the union of the genome with the universe.) All three of these things are incredibly broad in their possibilities.

At one extreme is framstics, which is a research project (as well as shareware and other stuff) which models systems that could be physically possible in our world. Think of it as a construction kit, not unlike an Erector Set or Lego Mindstorms, that exists entirely in software. There are sticks, muscles, neurons, and sensors, all of which are modeled on real-world things. For the genome, they have several options. One is a low-level language which simply describes body parts and connections between them, much like a blueprint or circuit diagram. The rest are all intended to be more realistic or interesting genomes. They wrote a paper which describes in engrossing detail something I regard as completely obvious (and which makes artificial life compelling to me): the genome you choose has a profound impact on the organisms you get. In the low-level Framsticks language, a simple mutation (changing one letter in the genome) is likely to result in a useless change. Languages which have symbols for repetition or recursion allow simple mutations to yield symmetrical bodies or repeating patterns.

On the topic of natural selection, the defining quality of artifical life is that there is mutation (and optionally gene sharing, a.k.a. sexual reproduction) and culling of the less fit. In some cases it’s more like breeding, where the computer chooses the most fit organisms each generation according to an explicit “fitness function”– for example, choosing the fastest or the tallest organisms. That is analagous to breeding. The option is to build reproduction into the organisms and limit the resources so that some of them die off (or worse, none die and your computer gets swamped calculating the actions of random, unevolved– and therefore uninteresting– critters.) The former case has practical applications– a genetic algoritm has yielded a patentable circuit design– but the latter strikes me as more entertaining. Framsticks supports both modes.

Even though framsticks is a complete toolkit which includes great eye candy and enough complexity to keep you busy for years, I’m somewhat more fascinated by a little program called Evolve. It is in every way the opposite of Framsticks. It’s a one-person project. The genome is simple computer code, a simple version of Forth. So it’s not unlike what I write for a living. The universe is a grid, like a chess board. (The technical term for this sort of grid universe is a cellular automaton, the most famous of which is Conway’s Game of Life.)

What makes Evolve so fascinating is it’s so simple and comprehensible to a programmer like me.  Its author has learned a lot about what sort of languages and mutations yield intersting results.  Are these things modelling plants or animals or what?  Do they have brains, or is it all just DNA?  In this case, it’s the latter, and the DNA contains commands like “MOVE”, “EAT”, “GROW” and “MAKE-SPORE”.  It is far more like a computer program than a life form, and yet it is all the more compelling because it is so unlike earthly life.

Finally, this isn’t artifical life, but it’s the inspiration for a lot of artificial life sims.  Core Wars.   First described in a popular Scientific American article, Core Wars is a battle to the death between two computer programs in a simulated machine.  The assembly language for the Core Wars computer (called Redcode) is simple enough that the article suggests running it as a pen-and-paper game.  (Or you could write to the author to get the computer code; this was before the Internet.)

Until yesterday, I’d heard of Core Wars but never looked into it, since I tend to think of assembly language as difficult.  Which it is, if you’re trying to write something complicated like a web server.  But for this sort of thing, it’s simple and approachable– not to mention the best language for doing the sort of mischief that other languages try to keep you from doing.  Self-modifying code in particular, which is the core of the game:  modify your opponent before it modifies you.