It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as "Nimbia", found in the Twitya formation, and older rocks dating to in Kazakhstan, possibly displaying the most complex morphology of the time.