Bones discovered in an island cave may be a new human species

The first chamber of Callao Cave on Luzon Island, in the Philippines, where the fossils of Homo luzonensis were discovered. Photo / Callao Cave Archaeology Project

Please welcome a possible new member to our band of upright apes: Homo luzonensis, whose teeth and bones were discovered in an island cave. The remains represent a new species, scientists concluded in a report published Wednesday in the journal Nature. They named it after Luzon, the island in the Philippines where the remains were found.

Our genus, the Homo in Homo sapiens, contains multitudes, including the thick-browed yet sophisticated Neanderthals and Homo erectus, a nearly 2 million-year-old species that may be our direct ancestor.

The first chamber of Callao Cave on Luzon Island, in the Philippines, where the fossils of Homo luzonensis were discovered. Photo / Callao Cave Archaeology Project

Homo luzonensis is the fourth peculiar and extinct human discovered in this century. Homo floresiensis, so small it was nicknamed "the hobbit," was found in Indonesia in 2004. Mysterious Denisovans, identified as a species based on a finger bone in 2010, lived in Siberia. Homo naledi skeletons, with strange mixes of modern and primitive features, were pulled out of an African cave in 2013.

Together, these newfound species show that human evolution was highly versatile, as groups adapted to unfamiliar conditions around the world. Modern humans were not alone - our close kin survived until fairly recently. And some of our co-inhabitants possibly embarked on long sea voyages, suggesting similar levels of intelligence.

"The evolution of our evolutionary group, Homo, is getting weirder and weirder," said paleoanthropologist Rick Potts, who directs the Smithsonian's Human Origins Program and was not involved with this research. Like Homo naledi, these fossils show a jumble of old and new traits, Potts said. Their particular combination suggests these humans were "unknown previously to science."

In 2007, Armand Mijares, an archaeologist at the University of Philippines, asked his colleague Philip Piper to examine animal bones Mijares dug out of Callao Cave in Luzon. The expansive cave yawns open above a river plain. One limestone chamber is so large it houses a Catholic chapel. A deposit of bones in the entrance chamber goes back to the Pleistocene epoch, which lasted from 2.6 million to 11,700 years ago.

Piper, a zooarchaeologist at Australia National University, set about cataloguing the animal remains. "On the second day I was working through them," he said, "I pulled out a human metatarsal." Piper immediately rang Mijares about the unexpected foot bone, exclaiming, as he recalled, "'Oh my God, we've got human bones in here!'"

Piper, Mijares and their team published a description of the foot bone in 2010. They knew it was the oldest human remain in the Philippines, dated to 67,000 years ago, based on the amount of the radioactive element uranium in the fossil. But the 2010 paper didn't address who walked on that foot. "We didn't know what it was at that time, except that it was human," Piper said.

A Homo luzonensis toe bone, showing the longitudinal curve. Photo / Callao Cave Archaeology Project

Mijares returned to Callao Cave and uncovered more remains in 2011 and 2015. All told, the scientists pulled a dozen fossilised parts from the cave - teeth, a thigh bone, finger bones and foot bones, representing three individuals. Attempts to extract DNA from the remains were unsuccessful.

The body parts are diminutive, suggesting Homo luzonensis grew no more than 4 feet tall. Its molars have modern shapes. The way its leg muscle attached to its thigh bone is "distinctively human," Potts said.

The bones in its hands and feet are curved, "spitting images" of the toes and finger bones that belonged to the very ancient Australopithecus, Piper said. These hominids, such as the 3-million-year-old Australopithecus afarensis Lucy, had digits well-suited for climbing.

This species lived at the same time as humans with modern anatomy, who first appeared in the fossil record 200,000 years ago (or perhaps as long as 350,000 years ago). "We continue to realise that few thousands of years back in time, H. sapiens was definitely not alone on Earth," said study author Florent Detriot, a paleoanthropologist at the National Museum of Natural History in Paris.

New York University anthropologist Susan Anton, an expert on Homo erectus, was skeptical the remains came from a new species. The study authors "don't have any heads," said Anton, who described herself as "somewhat conservative and somewhat of a lumper." ("Lumpers," unlike "splitters," are more reluctant to categorise fossils or living organisms as different species.) Skulls, rich in anthropological detail, would be more convincing, she said.

Named Homo luzonensis, the species stood at less than four feet tall, information which scientists have gleaned from the size of its teeth. Photo / Supplied

Each difference in bone or tooth shape is subtle. "That's one reason to be a little cautious about this," said University of Wisconsin at Madison's John Hawks, who helped lead the expedition to excavate Homo naledi bones and was not involved in this discovery. But those differences, taken in aggregate, make a "reasonable" case for a new species, he said.

The study authors do not know how Homo luzonensis reached the island, which at 109,000 square kilometres is the 15th largest island in the world.

Though these fossils are the oldest in the Philippines, evidence for habitation is even older; 700,000 years ago, ancient butchers on Luzon carved up a rhinoceros with stone tools. Which species did the butchering is unknown.

A few "mammal species you find on Luzon appear to have come from the mainland," Piper said. The Asian continent is 640 or more kilometres away through the Luzon strait. But in the Middle Pleistocene, when glacial sheets locked up vast amounts of water, sea levels dropped by as much as 121m, Piper said.

Random weather events, like tsunamis, may have swept people across a prehistoric sea as they clung to detritus, Anton suggested. "After climatic events or accidents, you can get particularly clever organisms surviving in places that they maybe shouldn't be," she said.

Hawks wondered if these humans deliberately crossed the ocean. "I would just say that when humans could see land or they could smell it or they knew the signs, that birds were coming from it, they sought it out," he said. "That's not a Homo sapiens trait. It's something our ancestors and extinct relatives had."

A previously unknown ancient ancestor has been discovered in the Philippines and could change what we know about human evolution. Photo / Supplied

It is also possible Luzon's first inhabitants meandered there from the Indonesian archipelago, hopping across island chains.

The cartoon version of evolution, in which a hunched ape becomes a tall and jaunty biped, suggests a journey with a destination. The reality is messier, particularly when species adapt to islands.

An island's confines can rapidly spark evolutionary change; Charles Darwin saw this in finches' beaks. Small animals enlarge, large animals shrink, slight contrasts grow pronounced. "Think about the Galapagos islands: Each of the islands has got its own species of tortoise," Piper said.

"Isolation plays games," Potts said. Homo floresiensis showed anthropologists that an island could be an "odd little laboratory of human evolution," he said. These bones reinforce that lesson.

"It's beginning to look like the evolutionary process is really fluid," Potts said. "And it's surprising that it is so fluid where each species of Homo may actually be a history or a record." The result is a fusion of the modern and ancient: molars that could be yours alongside toes with millions-year-old curves.

Is it possible other human species evolved on these island laboratories and that their fossils are waiting to be found? "I wouldn't be surprised," Piper said.

Fifteen years ago, Hawks said, anthropologists chalked up the worldwide success of Homo sapiens to our modern anatomy. These new discoveries, in far-flung corners, suggest exceptionalism is not built into our brains or skeletons.

"The archaeological record is now showing us that ancient human forms were much more adaptable, and I would say clever, than we imagined," Hawks said. "This isn't 'Flowers for Algernon,' where, suddenly, we're super smart and everyone else in the world is behind us." Scientists are now plumbing genomes for other clues to Homo sapiens' survival, looking at our metabolisms or resistance to disease. "I'd say the doors have opened, and we haven't figured out where they lead."