Also who did the Mississippian peoples of Cahokia descend from?

The Cahokians are descended from the pre-Cahokian population of the region. A recent study (Reynolds et al, 2015) examined the DNA of Late Woodland and Early Mississippian populations at the nearby Schild Site, a short ways north of Cahokia itself. The analysis revealed no evidence for a notable migration of people into the region. Likewise, early Cahokia and affiliated sites like Pulcher and Washausen show a cultural continuation, rather than any abrupt cultural replacements during the Late Woodland period. Admittedly, Cahokia underwent major rennovations around 1050 CE as it transitioned into the cultural giant it's now famous for being, but that was an expression of a cultural shift that had started over a century before, as Cahokia shed its first-among-equals status and rose above all other communities in the Middle Mississippi. The pre-Mississippian societies of the area as sometimes known as the Late Bluff Culture, but that term isn't as widely accepted as it once was, so they're also referred to rather generically as Late Woodland cultures (a grab-bag term for post-Hopewell, non-Mississippian cultures) or Emergent Mississippian cultures. Emergent Mississippians sometimes include neighboring populations like the Coles Creek Culture in the lower Mississippi and the Plum Bayou Culture on the Arkansas River, which gave rise to the Plaquemine and Caddo Misssissippians, respectively. There were interactions between all three Emergent Mississippian cultures, and all informed the way of life now known as Mississippian and most famously exemplified by Cahokia.