Grasping the Depth of Time as a First Step in Understanding Evolution

One of the most powerful limits to the human imagination is our inability to grasp, in a truly intuitive way, the depths of terrestrial and cosmological time. That inability is hardly surprising because our own lives are so very short in comparison. It's hard enough to come to terms with the brief scale of human history. But the difficulty of comprehending what time is on an evolutionary scale, I think, is a major impediment to understanding evolution.
It's been approximately 3.5 billion years since primeval life first originated on this planet. That is not an unimaginable number in itself, if you're thinking of simple, discrete units like dollars or grains of sand. But 3.5 billion years of biological history is different. All those years have really passed, moment by moment, one by one. They encompass an actual, already lived reality, encompassing all the lives of all the organisms that have come and gone in that time. That expanse of time defines the realm of biological possibility in which life in its extraordinary diversity has evolved. It is time that has allowed the making of us. [Thanks, Tom P]