Homo luzonensis: The Callao Cave Humans of the Philippines

Homo luzonensis is an extinct species of human named in 2019 from fossils found in Callao Cave on the island of Luzon in the Philippines. The remains, at least 50,000 to 67,000 years old, combine very small teeth with curved finger and toe bones reminiscent of far older human ancestors. Together with Homo floresiensis from Indonesia, the species shows that South-East Asia’s islands were home to unusual human populations before and around the time modern humans arrived.

Callao Cave

Callao Cave is a large limestone cave system in Peñablanca, in the province of Cagayan in northern Luzon. Its chambers are lit by openings in the roof, and the first chamber contains a small chapel, making it a well-known tourist site.

Archaeologists from the University of the Philippines, led by Armand Salvador Mijares, began excavating deeper layers in the cave in the early 2000s. In 2007 they found a human foot bone, a third metatarsal, in a layer containing animal bones, including deer and wild pig, some with cut marks.

The first find and its dating

The metatarsal was dated directly by uranium-series dating, which measures the decay of uranium absorbed by a bone after burial. It gave a minimum age of about 67,000 years. When the find was published in 2010, it was the oldest direct evidence of humans in the Philippines.

Its species was uncertain. The bone was small and slender, within the range of small-bodied modern humans but also similar in some ways to Homo habilis and Homo floresiensis. The team decided more fossils were needed.

Naming a new species

Further excavations in 2011 and 2015 recovered more remains from the same layer: seven teeth, two finger bones, two toe bones and part of a thigh bone. In all, there are 13 specimens from at least three individuals, including a juvenile. A tooth gave a minimum uranium-series age of about 50,000 years.

On 10 April 2019 a team led by Florent Détroit of the National Museum of Natural History in Paris, with Mijares and Philip Piper, published the finds in Nature and named the new species Homo luzonensis, after the island.

Unusual features

The species is defined by a mix of traits not found together in any other known human.

• Small, simple teeth: the molars are very small, smaller even than those of Homo floresiensis, while some premolars are relatively large and have two or three roots, a feature more common in older hominins.
• Curved fingers and toes: the finger and toe bones are curved, like those of Australopithecus, which lived in Africa more than 2 million years ago. This may indicate that climbing remained important.
• A distinctive foot: one toe bone has a joint shape unlike that of modern humans, perhaps reflecting a different way of walking.

The researchers stressed that body size cannot be estimated reliably from so few bones. The species’ brain size is unknown, since no skull has been found.

Origins and island crossings

Luzon was never connected to mainland Asia by land, even during the lowest sea levels of the Ice Ages. Any hominins who reached it had to cross the sea. In 2018 archaeologists reported a butchered rhinoceros skeleton with stone tools at Rizal in Kalinga province, also in northern Luzon, dated to about 709,000 years ago. This showed that some kind of hominin was on Luzon far earlier than Homo luzonensis, perhaps its ancestors.

One leading idea is that Homo luzonensis descended from Homo erectus or an even earlier species that crossed to the island, and then evolved in isolation. Island populations often change in body size and form over many generations, a process seen in many animals. Others suggest the fossils may represent a population related to known species such as Homo erectus or the Denisovans. No DNA has been recovered from the fossils, as tropical heat usually destroys it.

Debates and open questions

Some anthropologists question whether so few fossils, without a skull, are enough to define a new species. Others accept the name but note that the relationships with other hominins are unclear. When and why Homo luzonensis disappeared is also unknown. Modern humans reached the Philippines by around 47,000 years ago, as shown at Tabon Cave on Palawan, but no evidence yet shows whether the two groups met.

Timeline

• c. 709,000 years ago: Hominins butcher a rhinoceros at Kalinga, northern Luzon.
• At least c. 67,000 to 50,000 years ago: Homo luzonensis lives at Callao Cave (minimum ages).
• 2007: A hominin foot bone is found in Callao Cave.
• 2010: The foot bone is published with a minimum age of about 67,000 years.
• 2011 and 2015: Further excavations recover teeth and hand, foot and thigh bones.
• 10 April 2019: Homo luzonensis is named in Nature.

Why it matters

Homo luzonensis shows that human evolution in South-East Asia was more varied than once thought. It suggests early hominins crossed the sea to remote islands and evolved there in isolation, and it reminds researchers how much of the human family tree may still lie undiscovered.