There is one fact about the origin of life that is reasonably certain. Whenever and wherever it happened, it started a very long time ago, so long ago that it is extremely difficult to form any realistic idea of such vast stretches of time. The shortness of human life necessarily limits the span of direct personal recollection. Human culture has given us the illusion that our memories go further back than that. Before writing was invented, the experience of earlier generations, embodied in stories, myths and moral precepts to guide behavior, was passed down verbally. Writing has made more precise and more extensive the transmission of such information. Even so, we have difficulty in contemplating steadily the march of history. Our minds are not built to deal comfortably with periods as long as hundreds or thousands of years. Yet when we come to consider the origin of life, the time scales we must deal with make the whole span of human history seem but the blink of an eyelid. There is no simple way to adjust one's thinking to such vast stretches of time. One can only construct an impression of it from indirect and incomplete descriptions, just as a blind man laboriously builds up, by touch and sound, a picture of his immediate surroundings. The customary way to provide a convenient framework for one's thoughts is to compare the age of the universe with the length of a single earthly day. Perhaps a better comparison, along the same lines, would be to equate the age of our earth with a single week. On such a scale the age of the universe, since the Big Bang, would be about two or three weeks. The oldest macroscopic fossils (those from the start of the Cambrian period) would have been alive just one day ago. Modern man would have appeared in the last ten seconds and agriculture in the last one or two. Odysseus would have lived only half a second before the present time. Even this comparison hardly makes the longer time scale comprehensible to us, perhaps the most vivid method is to compare time to the lines of print themselves. Let us make a 200-page book equal in length to the time from the start of the Cambrian to the present; that is, about 600 million years. Then each full page will represent roughly 3 million years, each line about ninety thousand years and each letter or small space about fifteen hundred years. The origin of the earth would be about seven books ago and the origin of the universe (which has been dated only approximately) ten or so books before that. Almost the whole of recorded human history would be covered by the last two or three letters of the book. If you now turn back the pages of the book, slowly reading one letter at a time―remember, each letter is fifteen hundred years―then this may convey to you something of the immense stretches of time. On this scale the span of your own life would be less than the width of a comma.