Humanity is a new experiment on planet Earth. For most of its history, life on Earth was r

estricted to the sea. Living things began to populate the land slightly over 400 million years ago. And humans have existed for no more than 3 million years.

One way to represent the evolution of life is to compress the 4. 6-billion-year history of Earth into a 1-year-long film. In such a film, Earth forms as the film begins on January 1, and through all of January and February it cools and is cratered(变成坑状) and the first oceans form. But those oceans remain lifeless until sometime in March or early April, when the first living things develop. The 4-billion-year history of Precambrian (前寒武纪) evolution lasts until the film reaches mid-November, when primitive ocean life begins to evolve into complex organisms such as trilobites(三叶虫).

If we examine the land instead of the oceans, we find a lifeless waste. But once our film shows plant and animal life on the land, about November 28, evolution proceeds rapidly. Dinosaurs, for example, appear on about December 12 and vanish by Christmas Eve, as mammals (哺乳动物) and birds flourish.

Throughout the 1 -year-run of our film there are no humans, and even during the last days of the year as the mammals rise and dominate the landscape, there are no people. On the early evening of December 31, vaguely human forms move through the grasslands, and by late evening they begin making stone tools. The Stone Age lasts until about 11:45 p. m. , and the first signs of civilization, towns and cities, do not appear until 11:54 p. m. The Christian era begins only 14 seconds before the New Year, and the Declaration of Independence is signed with one second to spare.

In comparing all of Earth's geological evolution to one calendar year, the author shows

A．just how recently humanity has arrived on the scene

B．just how simple it is to understand the history of Earth

C．just how early humanity appears on planet Earth

D．just how difficult it is to understand the history of Earth