Literacy emerged as early as 8000 BCE (10 ka or 10,000 years ago), with script developing independently at least five times (Mesopotamia, Egypt, Indus, Mesoamerica, and China). Less than 1% of the population was illiterate, and confined to a small ruling elite.

Nearly all humans alive today, except potentially for those in isolated, uncontacted tribes, can read at least some characters. The gene for literacy should be not much older than 16 ka.

The earliest settlement of the Americas is 16-13 ka. The gene for literacy is at least as old as 16 ka, since it crossed over the Beringia (which submerged 10 ka).

Literacy has to be a major advantage for Fertility. However, taking a narrow view, the absence of literacy is currently correlated with higher fertility. This study, for example, shows that in Kerala, India, female literacy is 65%, but fertility is 3.4; in Rajasthan, it is 11% / 6.0. It's likely everyone in those regions can read the alphabet.

On a larger timescale, the literacy gene was aggressive. Even though in modernity, lower literacy is correlated with higher reproduction, the quick proliferation of literacy is proof of its contribution to fitness. It's also apparent, in general, how useful literacy is in modern society. Usefulness doesn't equal reproductive success in the short-term, but in the long-term, useful genes prevail.

"20,000 DNA variants explains 5 percent of the differences between children's reading abilities" (source)

The definition of literacy has also evolved since the emergence of literacy.