Remember the Denisovans: a sister lineage to Neandertals with a short fossil record, only composed of a few teeth and bone fragments from different levels of the Denisova Cave in Siberia – dated to between 50-250 thousand years (ka), and a mandible found in Karst Baishiya in the Tibetan Plateau – dated to c. 160 ka -, where Denisovan DNA has also been recovered from cave sediments in layers of 100 ka, 60 ka and possibly 45 ka.
From their DNA we know that Denisovans and Neandertals share a common ancestor, which in turn has a common ancestor with Homo sapiens. Denisovans also interbred with some Homo sapiens populations in East and South Asia, and this left traces in c. 5% of the genome of some present-day groups of Island Southeast Asia (ISEA), and notably of Papua New Guinea.
The question is: what causes the disparity between the lack of Denisovan fossils in ISEA, and the genetic evidence suggesting mixing events between modern humans and Denisovans in this region?
ISEA hosts a unique and rich fossil record of a variety of hominin groups, with a very open debate around the phylogenetic relationships between them:
- Homo erectus, present in Java from c. 1.49 million years ago (Ma) until 117~108 ka.
- H. floresiensis, an endemic species on Flores (Indonesia), which seems close to H. erectus, or alternatively to an even more archaic species that independently reached ISEA in a separate dispersal.
- H. luzonensis, another endemic species on Luzon (Philippines). Its fossils share similarities with various hominin taxa including Australopithecus, Asian H. erectus, H. floresiensis and H. sapiens.
- Although we lack of Denisovan remains in the region, the multiple distinct pulses of Denisovan admixture in contemporary human populations of ISEA, New Guinea and Australia suggest that when modern humans arrived around 50 ka, they probably found at least one Denisovan-related group across ISEA, as well as local groups of H. floresiensis and H. luzonensis: a variety of hominin populations!