In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Q: When did the area of Mexico City become very populated?, Context: Historically, and since pre-Hispanic times, the Valley of Anahuac has been one of the most densely populated areas in Mexico. When the Federal District was created in 1824, the urban area of Mexico City extended approximately to the area of today's Cuauhtémoc borough. At the beginning of the 20th century, the elites began migrating to the south and west and soon the small towns of Mixcoac and San Ángel were incorporated by the growing conurbation. According to the 1921 census, 54.78% of the city's population was considered Mestizo (Indigenous mixed with European), 22.79% considered European, and 18.74% considered Indigenous. This was the last Mexican Census which asked people to self-identify themselves with an heritage other than Amerindian. However, the census had the particularity that, unlike racial/ethnic census in other countries, it was focused in the perception of cultural heritage rather than in a racial perception, leading to a good number of white people to identify with "Mixed heritage" due cultural influence. In 1921, Mexico City had less than one million inhabitants.
A:
pre-Hispanic times