From what is presently known, Homo sapiens—the modern form of man—has existed on earth for approximately a hundred thousand years in numbers large enough to constitute a population. Barring catastrophic accidents, it can be expected that man will continue living on earth for many millions of years. Using a somewhat fanciful kind of arithmetic, it can be calculated from these figures that the present age of humanity corresponds to very early childhood in the life of a human being. Pursuing still further the same far-fetched comparison, reading and writing were invented a year ago; Plato, the Parthenon, Christ, date from but a few months; experimental science is just a few weeks old, and electricity a few days; mankind will not reach puberty for another hundred thousand years. In this perspective, it is natural that so far mankind should have been chiefly concerned with becoming aware of the world of matter, listening to fairy tales, and fighting for pleasure or out of anger. The meaning of life, the problems of man and of society, become dominant preoccupations only later during development. As mankind outgrows childhood, the proper use of science may come to be not only to store food, build mechanical toys, and record allegories, myths, and fairy tales, but to understand, as well as possible, the nature of life and of man in order to give more meaning and value to human existence.
(René Dubos,The Torch of Life)
I'm not sure if this is the right place to ask, but I took this excerpt from an introductory philosophy book. What is this fanciful arithmetic? Why would one consider mankind at it's childhood and what would be an adult mankind?