A handful of teeth and limestone-encrusted hand, foot and leg bones dug out of a cave in the Philippines have been given their own branch on the human family tree.
The new species – Homo luzonensis – is described this week in the journal Nature, and is believed to have lived more than 50,000 years ago on Luzon, the largest island in the Philippines.
The discovery marks the second time that a species of early human has been found on islands in Southeast Asia that are separated from both the Asian and Australian continental shelves by deep-water straights.
The diminutive Homo floresiensis, discovered in 2004 on the Indonesian island of Flores, was the first.
[Although “the hobbit”, as it became known, is now accepted as a unique species, its seemingly bizarre mix of archaic and modern traits initially was met with great skepticism.]
H. luzonensis is also a mix of old and new, packaged in what was probably a small body.
In 2007, archaeologist Phillip Piper, then at the National Museum of the Philippines, was sorting through bones dug from an animal-rich layer nearly three metres below the surface of the Callao cave floor, some 300 kilometres north of Manila.
In among the bones of deer and extinct cattle was one from a hominin foot, in two parts.
“It was a pleasant shock,” says Piper, now at the Australian National University (ANU).
Subsequent digs have added 12 new pieces to the collection: seven teeth, two finger bones, two toe bones, and one broken thigh bone.
But “it could also be older”, says geochemist Rainer Grün from the ANU, and Australia’s Griffith University, who dated the remains. That’s because the uranium series dating method used only gives a minimum date estimate.
Indeed, a discovery in 2018 of stone tools and a dismembered rhinoceros found just 40 kilometres southwest of Callao Cave date to around 700,000 years ago.
Whether that was the work of H. luzonensis or some other early inhabitant of the island is not known.
Five of the H. luzonensis teeth were from a single individual, which allowed the Filipino, French and Australian research team to compare their dimensions to other early humans and to modern Homo sapiens.
The premolars, in particular, are unusual. They are large relative to the molars, and have two or three roots, a primitive feature more akin to ancient Australopithecines, which lived in Africa more than two million years ago, than to modern humans.
Overall, though, the teeth are tiny, says Piper, suggesting that the people were also small.
“It’s not a hard and fast rule, but teeth are generally reflective of overall body size,” he says.
Characteristics in the foot and hand bones are also somewhat of a throwback to more archaic hominid lineages. “The finger bones and toes bones are very Australopithecine-like,” says Piper.
Together, the features set H. luzonensis apart from anatomically modern humans, as well as other early human lineages including Homo erectus, which made it all the way to Java, on the edge of the Asian continental shelf.
“This is a very important discovery,” says palaeoanthropologist Darren Curnoe from the University of New South Wales, in Australia, who was not involved in the study.
“It does potentially rewrite, in a major way, our understanding of human origins in this part of the world.”
The ancient features of both H. floresiensis and H. luzonensis suggest that hominids may have dispersed out of Africa more than two million years ago.
Once they reached the islands of south-east Asia, they could have evolved into separate species on different islands, much as occurred with tortoises and other animals on the Galapagos Islands.
“You could have hominin species on all the different large islands in the Philippines,” says Piper. “It’s absolutely incredible to think about.”
However, Curnoe is not sure the find constitutes a new species. At least, not yet.
“Whilst I’m convinced that the material they’ve found is very unusual, I’m not convinced at the moment that there’s enough evidence to warrant a new species,” he says.
The team is now working on dating some of the animal bones found in the same layer as the hominin remains, to get a better idea of when these archaic people lived.
Excavations at the site also continue. The remains that have been found were likely washed into the cave, so the team is hopeful that excavations closer to the cave entrance could yield larger bones or even skull fragments, explains Piper.
Work on other islands of Southeast Asia may also help to fill out the increasingly complex picture of early human evolution in the region. So far, says Grün, “we’re just scratching the surface”.