They cannot be found in living memory or in the annals of our species

I am fond of lists, particularly lists that establish perspective or some form of comparison. For example, I find it intriguing that the average healthy adult human can survive about three minutes without breathing, three days without water, and three weeks without food. Why I find this intriguing, I cannot answer.

In Carl Sagan's Shadows of Forgotten Ancestors, there is such a list on page 4. Indirectly he is speaking of the issue of Deep Time and the human incapacity to comprehend relatively longer periods of time in with any reasonable sense of scale.

Had life and humans first come to be hundreds or even thousands of years ago, we might know most of what's important about our past. There might be very little of significance about our history that's hidden from us. Our reach might extend easily to the beginning, But instead, our species is hundreds of thousands of years old, the genus Homo millions of years old, and life about 4 billion years old. Our written records carry us only a millionth of the way back to the origins of life. Our beginnings, the key events in our early development, are not readily accessible to us. They cannot be found in living memory or in the annals of our species. Our time-depth is pathetically, disturbingly shallow. The overwhelming majority of our ancestors are wholly unknown to us. They have no names, no faces, no foibles. No family anecdotes attach to them. They are unreclaimable, lost to us forever. We don't know them from Adam. If an ancestor of yours of a hundred generations ago - never mind a thousand or ten thousand - came up to you on the street with open arms, or just tapped you on the shoulder, would you return the greeting? Would you call the authorities?