Each creature is represented as a letter. The higher the lifeform is, the higher its letter. The universe starts with A's and B's randomly scattered about the screen (each letter is a different color to make it more interesting). A's are special and don't move and don't ever die. An A takes a certain amount of time to mature after being eaten, and when it is immature it cannot be eaten and is represented as a lowercase "a" instead of "A". Other creatures can move one square in any direction and always move towards the food. Creatures can eat one or two letters below them, and do not eat their own kind. If creatures do not eat often enough, they starve. When a creature eats a certain amount, it can reproduce. It will have a litter. Usually all its offspring will be the same letter as it. However, sometimes the creatures mutate, and can mutate up to three letters above themselves.

So, for example, the simulation starts with scattered A's (inanimate) and a few B's, which are the only animate creatures the universe starts with. The B's head towards the A's and start eating them. (B's can only eat A's because there is nothing below A) The B's begin to reproduce. At some point there will be mutations. B's can mutate into C's, D's, and E's. If the B mutates into a C, the C turns around and eats the B's. C's can also eat A's, according to the rules. If the mutation is instead a D, the D eats the B's and C's. However, if the mutation is an E, the E protects the B's by eating all the C's and D's (the harmful mutations of B). C's have a similar hierarchy, only D and E are harmful and F protects them. It continues up the scale. G's protect D's, H's protect E's, I's protect F's, etc. The letters naturally form vast hierachies and large advanced "agricultural societies" based on the rules of the game. If all the B's and C's die (because of an outbreak of D's, for example) the universe has no way the get new energy input and everything but the A's quickly die. The lifetime of a universe varies from 200 cycles to 1400 cycles in its current incarnation. The highest letter I have ever seen created with default options is an O.