The history of human evolution might be getting yet another rewrite.
According to a study published today in the journal Nature, a vast, chambered cave in the Philippines holds the remains of a previously unknown, small-bodied population of early humans who lived some 67,000 years ago. If confirmed to be a distinct species, the newly-dubbed Homo luzonensis could add yet another branch to our ever-growing family tree.
Regardless of species designation, though, the presence of any ancient human remains on this remote island locale reaffirms the role Southeast Asia played in shaping our evolution—and suggests our ancestors weren’t quite as landlocked as once thought.
“This is a really welcome and exciting addition,” says Carol Ward, a biological anthropologist at the University of Missouri who was not involved in the study. “We now have evidence of [ancient humans] in a time and place that has been very poorly understood, and yet was undoubtedly important for the biology and evolution of our species.”
But Ward and others remain cautious about assigning an entirely new species to the remains, which contain just 13 fossils: a smattering of hand and foot bones, a few teeth, and a partial femur. Additionally, at least two adults and one child are jumbled up in the mix, muddying the picture of what a full skeleton might look like. “New fossils are always exciting,” Ward says. “But they always raise more questions than they answer.”
The first fossil in the collection—a petite, curiously curved foot bone—was unearthed in 2007 in Callao Cave on the Philippine island of Luzon. It clearly came from a member of the group known as hominins, which encompasses the human lineage after it split from the chimpanzee and bonobo line. But study author Armand Mijares, an archaeologist at the University of Philippines Diliman, was struck by the bone’s anatomy, which didn’t fall neatly within the bounds of any known human species.
No matter how peculiar, though, a single fossil isn’t enough to tell a story. So Mijares and his team dug deeper.
It was no easy task. In the years that followed, the team hacked its way through foot after foot of sticky, unyielding clay with little success. Finally, in 2011, they hit pay dirt: Clustered within a few square feet of soil were six tiny teeth and four bones from dainty-looking hands and feet. The fragmented remains of an adolescent’s femur followed soon after. Another excavation in 2015 revealed one final tooth.
Mijares credits the discoveries to his team’s patience and perseverance—but also to a tried-and-true Beach Boys soundtrack that now carries a bit of mythos among his colleagues. “Once the Beach Boys are playing, the fossils come out,” Mijares attests.
The remains were next shuttled to study author Florent Détroit, an anthropologist at France’s National Museum of Natural History in Paris. It was soon apparent to Détroit that the remains featured a puzzling mosaic of traits both modern and ancient. “Each of the features [of Homo luzonensis] corresponds to some hominin or another,” he says. “But the combination makes for something really unique. There’s no known species with this same suite of features.”
A couple of the bones from the feet, for instance, hint at a very diminutive stature—an adaptation that’s also been observed in Homo floresiensis, who had to cope with limited food availability on the Indonesian island of Flores. They’re also shaped in a way that bears a striking resemblance to our Australopithecus predecessors, who lived some 3 million years ago—a similarity that’s “extraordinary for something so young in geological age,” says William Harcourt-Smith, a physical anthropologist at Lehman College who was not involved in the study.
But it’s perhaps the teeth—seven upper jaw premolars and molars, five of which are from the same individual—that are most fascinating, says Kristin Krueger, a paleoanthropologist at Loyola University Chicago who was not involved in the study. While molars typically dwarf premolars in modern humans and many of our extinct relatives, the newly uncovered teeth are all fairly small and uniform, stacking hefty premolars up against molars that are fairly puny by modern standards. “Overall, the dental differences...present the strongest indication of a new species,” Krueger says. “I am cautiously optimistic.”
However, without a skull, mandible, or DNA evidence (which has been unrecoverable so far), it’s still possible these individuals are just unusual members of an already-characterized species like Homo floresiensis, says Ashley Hammond, a paleoanthropologist at the American Museum of Natural History who was not involved in the study. Only time—and more fossil evidence—will tell if Homo luzonensis lives up to its name, she says.
In the meantime, it might not be worth it to quibble over the finer points of species distinctions, which can carry a kind of social baggage, says Leslie Aiello, an anthropologist at the Wenner-Gren Foundation for Anthropological Research who was not involved in the study. Though these boundaries are often drawn on the basis of reproductive incompatibility, evidence for interbreeding between species has surfaced time and time again throughout human evolution. Even we Homo sapiens still carry traces of Neanderthal in our DNA.
“The natural world doesn’t care what labels we put on things,” says Jeremy DeSilva, a paleoanthropologist at Dartmouth University who was not involved in the study. “Species are much more fluid and interesting than the distinct groupings that we as humans arbitrarily put them in.”
Aiello would much rather spend her time thinking about the potentially “tantalizing” new narrative that’s surfaced so far. New species or not, these fossils point to the presence of early humans in a time and place that they were not previously known to occupy. “This changes the role of the Philippines in the human evolution debate,” Mijares says. And that’s a tremendous contribution in and of itself.
“This tells us that hominins could really get all over the place,” Hammond says. “To see them making it to a place where we weren’t really expecting much interchange…that’s pretty exciting.”
What’s not yet clear, though, is how these people got there, and when. Last year, a separate study identified stone tools and butchered animal remains on Luzon that date to 700,000 years ago. The discovery of these artifacts sparked many theories on their makers, ranging from seafaring Homo erectus clans (which is pretty unlikely) to castaways washed ashore in the wake of a catastrophic tsunami. For now, these more recent human fossils are the oldest in the region. But the possibility remains: Early humans might have established themselves on the island and spent hundreds of thousands of years adjusting, adapting, perhaps even speciating—long before the eventual arrival of today’s Homo sapiens.
Détroit and Mijares say it’s too early to speculate on the lineage of their fossils. Another group of globetrotters could have settled Luzon—perhaps purposefully—many years after those early tools were crafted, and still have had plenty of time to evolve. If that’s the case, Krueger says, it’s of “great consequence...it represents the ingenuity of our ancestors.”
In many ways, the unfurling tale of these fossils parallels the mysterious trajectory of Homo floresiensis, presumably stranded on Flores. This hints that there might be other isolated outcrops, holding the remnants of more populations, yet undiscovered. Mijares is already planning a return trip to Callao Cave, and several nearby sites in the Philippines. After all, he says, if they made it to Luzon, there was likely little keeping them off other islands.
“Understanding human evolution is like building a jigsaw puzzle: It’s one picture, even if you just have a few of the pieces,” Ward says. “Whether this is a new species or a population is an interesting question. But the important aspect of this is that we now have a new part of the puzzle started.”