Source: TW
The development of a Hidden Markov model for population splits and rejoinings suggests that all modern sapiens are a 80:20% composite between two ancient Homo lineages that split from each other around 1.5 Mya! One of the lineages the (80% one) also gave rise to the Neanderthal-Denisovan lineage.
This suggests that the 20% lineage was probably erectus, which split at some point from the heidelbergensis lineage, which could be the 80% one.
An erectus population mated back into the 80% population after the Neanderthal-denisovan lineage diverged. They find evidence that this 20% genetics had negative effects against the 80% background and was selected against in genic regions.