H. Denisova not an H. Erectus Lineage?

Both modern humans and neanderthals are believed to be human lineages of Homo heidelbergensis via Homo erectus. There is hot debate within the scientific community as to the origin of the Homo denisova species - and there is evidence that the Denisovan species is too old to be of the H. heidelbergensis lineage and that the Denisovan species of human may be a lineage of human not within the H. erectus evolutionary line (as modern humans and neanderthals are thought to be).