Since it's a pre-print the whole paper is available.

Previous research has shown that modern Eurasians interbred with their Neanderthal and Denisovan predecessors. We show here that hundreds of thousands of years earlier, the ancestors of Neanderthals and Denisovans interbred with their own Eurasian predecessors—members of a “superarchaic” population that separated from other humans about 2 mya. The superarchaic population was large, with an effective size between 10 and 46 thousand individuals. We confirm previous findings that: (1) Denisovans also interbred with superarchaics, (2) Neanderthals and Denisovans separated early in the middle Pleistocene, (3) their ancestors endured a bottleneck of population size, and (4) the Neanderthal population was large at first but then declined in size. We provide qualified support for the view that (5) Neanderthals interbred with the ancestors of modern humans.

Author summary
We show that early in the middle Pleistocene, long before the expansion of modern humans into Eurasia, the “neandersovan” ancestors of Neanderthals and Denisovans undertook a very similar expansion. In both cases, an African population expanded into Eurasia, endured a narrow bottleneck of population size, interbred with indigeneous Eurasians, largely replaced them, and split into eastern and western sub-populations. In the earlier expansion, neandersovans interbred with a “superarchaic” population that had been separate since about 2 mya and may represent the original expansion of humans into Eurasia."