Robert M. May said this quote

“ One way of emphasizing the singularity of the recent past is [..] to observe that the total number of humans ever to have lived is estimated at around (a bit less than) 100 billion. One of Walt Whitman's poems has a memorable image—thinking of all past people lined up in orderly columns behind those living—‘row upon row rise the phantoms behind us’. Actually, looking over our shoulder, we would see only around 15 rows. ”