I’ve reformatted and added a bit more information to my page about Mayan languages. It remains a confusing subject, since there are a large number of languages that are more often than not mostly mutually unintellible, yet at the same time “with the exception of Waxtek, these Mayan languages have been in contact with one another for many centuries and often grade into one another” (Robert J. Sharer, The Ancient Maya).
In general, the timeline probably looks like this. The Huastecans (or Waxtekans) split away from the other language groups in the fourth millennium BCE (they are now a Mexicanized group centered around Veracruz). Later in the same millennium, Yucatek split away from the others. Next to split were the Cholan-Tzeltalan languages, and thereafter things get so complicated that it really requires a Mayan specialist to sort it all out.
If Proto-Mayan dates to around the first quarter of the third millennium as linguistic reconstructions suggest (although this is far from certain), that would make it contemporaneous with predynastic Egypt, the earliest civilizations of Sumeria, the first year of the Jewish calendar (3760 BCE) and the appearance of phonetic writing in the West (ca. 3500 BCE). It would long predate the semi-legendary Xia dynasty, the “First Dynasty” of China.
Based on reconstructions of the Proto-Mayan vocabulary, it appears that Mayan probably originated in the Guatemalan highlands. Some of the areas in which Proto-Mayan is rich are words related to maize agriculture (“with separate words for generic maize, the green ear, the mature ear, the cob, maize flour, maize dough, the tortilla, a toasted maize drink, the rindstone, and three terms for the increasingly fine grindings of maize,” again according to Sharer), ashes, weaving, and writing (implying that the Maya developed some form of writing quite early on).
Other Proto-Mayan words further suggest how ancient many aspects of present-day Maya culture may be. They include words for hammocks, beans, chili peppers, sharpening stones, mats, sandals, combs, and other features still commonly encountered by travelers in the Maya world.