Our understanding of what race is, and who belongs to which race, keeps shifting over time — even though people have usually been convinced that the racial divisions of their era are just scientific fact. Just look at how much the US Census has changed since its first use in 1790.
The Pew Research Project has put together an interactive infographic comparing each decade of racial categories in the census to the ones we use today. They've changed tremendously — from the "Free whites"/"All others"/"Slaves" categories of the 1790 census to the 19 categories, including "Other," available to Americans in 2010.
Americans tend to think of the four major racial groups in the US (in addition to Native Americans) as white, black, Hispanic, and Asian — but the infographic (as well as the full timeline) shows that those four groups evolved in very different ways.
In 2020, the way the census counts African Americans will change for the ninth time over just 24 censuses, when the census finally drops "Negro" from its term for African Americans — though it's still better than 1890, when census takers decided whether an African-American family was black, "mulatto," "quadroon," or "octoroon."
The census has had at least one category for Asian Americans since 1870, when it started counting "Chinese" residents separately. As immigration from Asia got more diverse, the census did as well; in 1920, there were five categories for Asian Americans.
Americans of Latin American descent, on the other hand, were categorized as white (with the exception of one census that counted "Mexicans") until 1970. But instead of adding one country at a time, the 1970 census added five different categories for Hispanics at once — recognizing a US population that was already diverse, and had simply been counted as "white" in earlier censuses.
Explore the infographic for yourself (use the drop-down menu to change the year displayed on the left):