Videos for the paper
Evolving Sims's Creatures for Bipedal Gait

By Amin Azarbadegan, Frank Broz and Chrystopher L. Nehaniv

School of Computer Science, University of Hertfordshire©, UK, 2010


Karl Sims innovated a new way of evolving virtual artificial agents which generates their morphology and behaviour simultaneously in a simulated physical environment (see the PDF version of his two papers, both published in 1994: "Evolving virtual creatures" and "Evolving 3D morphology and behavior by competition").

However, the evolution of two-legged creatures is not observed in the Sims's experiments nor in their replication in other works, as the selection pressure was not towards any particular gait; rather, it was merely focused on efficient movement. Our hypothesis is that biases in morphology that encourage limb specialisation, combined with rewards for successful locomotion and carrying at the same time and realistic, physics-based penalties for falling, would lead to creatures capable of bipedal locomotion.

The model used in these experiments is an extension of the model used by Thomas Miconi. The simulation environment is adapted from his simulation code, an open-source implementation based on Sims's research. Thanks to his generosity this code and some compilation guide are available on his personal page.

You can also obtain our modified and altered version of the source code here. For compiling this code, please notice that you will need ODE installed on your machine, and you should make suitable changes in the makefile to correspond to your machine paths, also uncomment there some lines related to compiling of the random generator. We have compiled the code only with ODE 0.5 on a Debian distribution of GNU/Linux. Also notice that after compilation there will be four executable files: evolve1 to evolve4. It is due to a parallelism trick to improve performance, and in order to genetic algorithm proceeds, you will need to run all four files.

The Galloper

This creature performs an excellent balance during hopping and travels in high speed, although the balance does not seem robust enough. In one galloping cycle, the front foot is used for jumping and the back foot absorbs the falling impact. This causes damages which are illustrated by darker limb colours in the simulation.

The upright morphology is apparent with the head (coloured magenta) on the top and the limbs (depicted in green or black) at the bottom. The torso (white block) seems to have become vestigial but it might help the balance. In the later generations, however, it is the main cause of damage penalties, since the feet are exempt from damage but the torso can get slight contacts with the ground, which are damaging in high sensitivities of later generations.


The Crawl-Stepper

Even though this morphology is not completely bipedal, the behaviour is very similar to walking, with an oddly enough stepping gait which takes advantage of only one limb. The creature shows dynamic balance, and while it is slower than the galloper, it is more robust. It stands on one horizontal limb (pelvis) and swings the upper body, which causes two sides of the pelvis periodically rise and fall in a stepping-like behaviour.