In an essay in the May 14 2009 issue of Nature en led "Is Free Will an Illusion" (the illusion reference is to Daniel Wegner) Heisenberg says that the debate on free will has focused on humans and ‘conscious free will’. Yet when it comes to understanding how we initiate behaviour, we can learn a lot by looking at animals. Although we do not credit animals with anything like the consciousness in humans, researchers have found that animal behaviour is not as involuntary as it may appear. The idea that animals act only in response to external stimuli has long been abandoned, and it is well established that they initiate behaviour on the basis of their internal states, as we do.
(Nature, vol. 459, 2009, p.164)
Heisenberg argues for some randomness even in unicellular bacteria, followed by more lawful behaviors such as moving toward food.
Evidence of randomly generated action — action that is distinct from reaction because it does not depend upon external stimuli — can be found in unicellular organisms. Take the way the bacterium Escherichia coli moves. It has a flagellum that can rotate around its longitudinal axis in either direction: one way drives the bacterium forward, the other causes it to tumble at random so that it ends up facing in a new direction ready for the next phase of forward motion. This ‘random walk’ can be modulated by sensory receptors, enabling the bacterium to find food and the right temperature.