complex system of a given mass (Fig. 1). Its dimension is energy per time per mass (erg s-1 g
-1). Let us illustrate it with some examples (Chaisson 2003, 96). A star has a value ~1, planets
~102, plants ~103, humans ~104 and their brain ~105, current microprocessors ~1010.

According to this metric, complexity has risen at a rate faster than exponential in recent times [1.20]. We might add along this complexity increase, the hypothesis that there is a tendency to do ever more, requiring ever less energy, time and space; a phenomenon also called ephemeralization (Fuller 1969; Heylighen 2007), or "Space-Time Energy Matter" (STEM) compression (Smart 2008). This means that complex systems are increasingly localized in space, accelerated in time, and dense in energy and matter flows. 

In Tomas Ray's simulation *Tierra* (Ray 1991), digital life competes for CPU time,

![0_image_0.png](0_image_0.png)

which is analogous to energy in the organic world. The analogue of memory is the spatial resource. The agents thus compete for fundamental properties of computers (CPU time, memory) analogous to fundamental physical properties of our universe. This design is certainly one of the key reasons for the impressive growth of complexity observed in this simulation. 