Final answer:
In 1900, the average human lifespan in the U.S. was 47 years, which increased to 77 years by 1999 due to advancements in public health and medicine.
Step-by-step explanation:
In 1900, the average human lifespan in the United States was about 47 years, but by 1999 it had increased to about 77 years. These gains are largely attributed to positive externalities in public health programs, medical advancements, improved living standards, and better healthcare. With these improvements, we've seen a significant rise in life expectancy from the turn of the 20th century to the turn of the 21st, marking an important change in the standard of living over the centuries.