Homo luzonensis and the role of homoplasy in the morphology of hominin insular species

Homo luzonensis lived during the upper Pleistocene in the northern Philippines, east of the Wallace line. The few specimens attributed to this species show a mosaic of plesiomorphies for the genus Homo and apomorphies found in upper Pleistocene Homo species. Plesiomorphic traits could result from a close phylogenetic relationship with Australopithecus or Homo habilis/rudolfensis or from insular evolution favouring reversals, possibly linked to a relationship with a more recent species like Homo erectus. We conducted a cladistic analysis based on dental and postcranial remains of adult H. luzonensis to test these hypotheses. The trees obtained indicate that both hypotheses are supported by different anatomical elements. The trees suggesting H. luzonensis is close to H. erectus (as a sister-taxon or as a close relative of H. erectus) are supported by dental characters and feature high retention and consistency indices. Moreover, H. luzonensis likely had reduced body size and postural/locomotor modifications, similar to other insular mammals with medium-to-large-sized ancestors. The tropical forest environment of Luzon Island may have favoured the selection of these features. Based on these results and our current knowledge of insular evolution, we suggest that H. luzonensis likely evolved from an Asian H. erectus population in an insular context.