Here’s a small excerpt from a Science Daily article that might help you better visualize the relation between front-loading and terraforming:
During the Proterozoic, oxygen levels in the atmosphere rose in two phases: first ranging from 2.5 to 2 billion years ago, called the Great Oxidation Event, when atmospheric oxygen rose from trace amounts to about 10% of the present-day value. Single-celled organisms grew larger during this time and acquired cell structures called mitochondria, the so-called “powerhouses” of cells, which burn oxygen to yield energy. The second phase of oxygen rise occurred between about 1 billion and 540 million years ago and brought oxygen levels to near present levels. This time intervals is marked by the earliest fossils of multi-celled organisms and climaxed with the spectacular increase of fossil diversity known as the “Cambrian Explosion.”
Notice that the two most significant evolutionary events – the origin of eukarya and origin of metazoa – were both coupled to rising oxygen levels. Oxygen, of course, is a biological output. This understanding can lead us to a tantalizing shift in perspective – the evolution of the complex eukaryotic cell, needed to make something as complex as animal, and the evolution of metazoa itself, were not the gradual accumulation of incremental complexity stretched out over deep time. Instead, the origin of these two novel states was nudged into existence, as least in part, due to rising oxygen levels which in turn had a biological origin.