The really interesting part of the paper comes when the author discuss the taxonomic attribution of the metatarsal. They compare it to various extant primates and show that it is a convincing Homo bone, aligning itself most closely with small-bodied populations, such as H. habilis or contemporary Philippine Negritos, the latter of which stand out as likely potential analogs of the hominin to whom the metatarsal belonged. That said, "the dimensions of the base of the bone and the section of the shaft are smaller, indicating peculiar proportions for the Callao metatarsal. At the mid-shaft, the shaft appears to be considerably smaller in the dorso-plantar direction than in the Negrito comparative sample. As shown by the reduced dimensions obtained for the dorso-plantar height and medio-lateral breadth of the proximal facet for the lateral cuneiform, the base is very small. It is the smallest of our sample, confirming the particular shape and proportions of the bone as seen from lateral and superior views" (Mijares et al. 2010: 8).
From Mijares et al. (2010: 7, Fig. 8, Copyright © 2010 Elsevier Ltd). The Callao specimen (A) is compared to H. sapiens (B) and H. habilis (C), and three non-human primates to the right.
The authors emphasize that the peculiarities of the Callao metatarsal are unique in the panorama of known foot bones attributed to various Pleistocene Homo. Provocatively, they point out that the dimensions of the H. floresiensis third metatarsal from Liang Bua (LB 1) are very close to those of the Callao specimen (Mijares et al. 2010: 9). While they present this comparison as speculative, the implications of the exercise are clear: they're asking whether something like H. floresiensis could have been present at Callao ca. 67 kya, although they do cover their bases by emphasizing that the closest analog small-bodied humans known in the region today are Negritos.
What's a bit puzzling is their repeated discussion that the Philippines are east of Wallace's line. While I know there's a bit of debate over this, I've always understood the Philippines as being located west of Wallace's line, on the Asian side of things. Mijares et al.'s argument that the Philippines are "beyond Wallace's Line in Island Southeast Asia" appear to be a further manner of potentially linking the Callao specimen to those from Flores.
In any case, as the authors conclude, the Callao third metatarsal "documents the presence of a hominin species on the island of Luzon as early as 67 ka, and is testimony to a capability to colonize new territories across open sea gaps. The Philippine specimen also indicates that Flores was not the only island in Wallacea to be occupied by hominins more than 50,000 years ago" (Mijares et al. 2010: 9). Regardless of the precise taxonomic affiliation of that bone, it indicates a great time depth for human presence in that part of the Old World, and provides some thought-provoking evidence that seafaring must have been part of the hominin behavioral range by that time, something that seems to potentially have been the case in other parts of East Asia at that time.
Mijares, A., Détroit, F., Piper, P., Grün, R., Bellwood, P., Aubert, M., Champion, G., Cuevas, N., De Leon, A., & Dizon, E. (2010). New evidence for a 67,000-year-old human presence at Callao Cave, Luzon, Philippines Journal of Human Evolution DOI: 10.1016/j.jhevol.2010.04.008