I'm only a student, but I can tell you something related. The first evidence of Neanderthal burial was found in 1908 at La Chappelle-aux-Saints in France (POSTEL B., 2008 - Neandertal et la mort. Archéologia n°458 : 6-11). What was observed there was the body of a Neanderthal in a sort of fetal position that makes archaeologists believe that he was placed there. Now, this case may not have kicked off the practice, but it shows that the idea of burial was around at least 60,000 years ago (the age of the specimen's bones).
At another site in Shanidar, Iraq, 9 burials were found, all with similar ages (~60,000 years). Among these, an adult male with a withered arm and several fractures in the process of healing. The healing status of his bones implies that he was being kept around and not abandoned by the group (T. D. Stewart, The Restored Shanidar I Skull, Smithsonian Institution Annual Report for 1958, pp. 473-480, 1959).
Another burial at Shanidar was found to have traces of pollen around the bones, which hinted at flowers being placed (A. Leroi-Gourhan, Shanidar et ses fleurs, Paléorient, vol. 24, pp. 79-88, 1998).
So what I'm getting at is that the practice of burial happened at around 60,000 years ago. I have no data about when it became a ceremony, but there are some hints through the placement of bodies and flowers being used.
(I hope my sources are okay, first time contributing).

Edit: you can also find more information by searching for the aforementioned areas.