Final answer:
On a cosmic calendar imagining Earth's history as a 24-hour day, humans appear in the evening of December 31. Significant advancements like the alphabet and modern astronomy occur only in the final seconds of this cosmic year, underscoring the brevity of human history.
Step-by-step explanation:
Visualizing Earth's entire 4.6 billion-year history as a single 24-hour day helps put the origins and milestones of life into perspective. On this cosmic calendar, humans appear in the final moments of December 31. To be more precise, human origins are pinpointed to the evening of December 31, with significant human developments such as the invention of the alphabet and the beginnings of modern astronomy occurring in the final seconds of the cosmic year.
This figurative approach highlights just how brief human existence has been in contrast to the vast expanse of geological and cosmic time. It emphasizes our limited time studying the Earth and the stars, with the invention of the alphabet happening at the fiftieth second of 11:59 p.m., and the birth of modern astronomy in a split second before the New Year.