The 16th of January 378 C.E. marked a turning point in ancient Maya history. On that day, foreigners arrived in the Maya city of Tikal—in what is now northern Guatemala—and Tikal's king died. Shortly thereafter, the son of the conquering king became Tikal's new ruler.Many archaeologists think these invaders came from Teotihuacan, a metropolis 1000 kilometers away, near what is now Mexico City, famed for its imposing pyramids and sweeping central avenue. But a new discovery in Tikal reveals Teotihuacan may have had an outpost in the Maya city long before possibly conquering it. That bolsters the idea that Teotihuacan’s...