Named after a village in the area, Callao Cave is one of 300 limestone caves located in the municipality of Peñablanca, 24 kilometers northeast of Tuguegarao City, the capital of Cagayan province in the Philippines. The Callao Limestone formation is part of the Peñablanca Protected Landscape and Seascape along the western foothills of the Northern Sierra Madre Mountains [3][4][5][6].

Callao Cave is the largest and longest cave in the area, with seven chambers. There were previously nine reported chambers in the system, but two were sealed off by an earthquake in the 1980s. The length of the cave is 366 meters from the mouth to the innermost chamber, and the passages are 14 to 35 meters wide, with a ceiling height ranging from 10 to 45 meters [7][4][3].

Callao Cave was first excavated in 1979 and 1981 by the National Museum of the Philippines. During excavations, the team, led by Cuevas, recognized two cultural horizons: the upper horizon and the pre-ceramic horizon [7][8]. Archaeological excavations in the cave were resumed by a team led by Mijares from 2002 to 2020, producing finds from Late Pleistocene layers [9][5][10][11][1][12]. 

The Callao record revealed three periods of human occupation. The most recent is a Neolithic occupation from around 3,600 BP. Ceramics, flake tools, a spindle whorl, faunal remains, and human burials were found in this layer. Below this is a layer of volcanic sediment that marks a break in archaeological materials, which is followed by a second phase of occupation at 28,980 - 27,420 cal. BP. This layer contained hearths, charcoal and burnt sediments, and associated chert flake tools. The earliest occupational layer dates from 50,000 to 67,000 BP. It produced animal bones and remains of small-bodied hominins later classified as Homo luzonensis. The periods of human occupation were preceded and succeeded by long periods of human inactivity, implying that humans used the cave intermittently [10][6][1]. 

Homo luzonensis 

CCH1 (Callao Cave Hominin 1) is a hominin third metatarsal (MT3) discovered in Layer 14 of Callao Cave in 2007. Directly dated to 66.7±1 ka, this small-sized foot bone with unusual morphology is the oldest known human fossil in the Philippines [11]. In 2019, the Callao team announced that CCH1 and additional hominin fossils from subsequent excavations on the site belong to a new species Homo luzonensis, named after the Philippine island of Luzon [1].  

Homo luzonensis is described based on dental and postcranial remains (dated to at least 50-67 ka) from Callao Cave. This includes the metatarsal CCH1 and the 12 additional fossils that represent at least three individuals that were found in the same stratigraphic layer: the right P³-M³ belonging to a single individual (CCH6-a to CCH6-e, which serves as the holotype of H. luzonensis), the right P⁴ (CCH8), the right M³ (CCH9), two adult manual phalanges (CCH2 and CCH5), two adult pedal phalanges (CCH3 and CCH4), and a juvenile femoral shaft (CCH7) [1][13].  

The relatively small size and features of the H. luzonensis dental remains are similar to those of the Homo floresiensis, another small-bodied hominin found in Liang Bua Cave on the island of Flores in Indonesia. The H. floresiensis remains are penecontemporaneous to those of H. luzonensis [14][15][16][1][13]. According to a 2012 study [13] both H. luzonensis and H. floresiensis likely evolved from Homo erectus groups that dispersed in the various islands of the region and became isolated. Both species present anatomical traits that are either rare or absent elsewhere in the genus Homo but have commonalities with those of Australopithecus [1].

The curves and grooves of the Homo luzonensis fossils reveal an unexpected mix of both ancient and more advanced traits. For instance, the small size and relatively simple shapes of the teeth indicate a more ‘modern’ individual, whereas one upper premolar has three roots, a characteristic present in fewer than 3 percent of modern humans. On the other hand, one foot bone resembles those of the more ancient australopithecines [2].