Callao Cave on Luzon Island, Philippines.
Image: Callao Cave Archaeology Project

Scientists are reporting the discovery of a previously unknown species of ancient human that lived in the Philippines over 50,000 years ago. Evidence suggests the new species, named Homo luzonensis, was exceptionally tiny—and possibly even smaller than the famous Hobbit species uncovered on the island of Flores in 2004.

The story of human evolution just got a hell of a lot messier—and considerably more fascinating—owing to the discovery of a previously unknown human species. Bits of teeth and bone pulled from Callao Cave on the Philippine island of Luzon point to the existence of a distinctly human species, one deserving of the Homo designation in terms of its genus. At the same time, however, the fossils found in Callao Cave exhibit features unlike anything ever seen before, thus warranting the declaration of a completely new human species, Homo luzonensis. The details of this astonishing discovery were published today in Nature.

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Needless to say, this is a huge deal. New human species tend to not be discovered on a regular basis. The discovery of Homo luzonensis, with its curious set of physical characteristics, is telling us some surprising new things about human evolution and what happened to the pioneering hominins who left Africa so long ago.

This story begins in 2010 with the important discovery of a single human foot bone, dated at 67,000 years old, in Callao Cave. The exact species could not be determined, but it was the first direct evidence of a human presence in the Philippines. As an important aside, we’ve since learned that hominins—the sub-group of primates that are more closely related to us than to chimpanzees and bonobos—were living in the Philippines as long as 709,000 years ago.

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A butchered animal bone found a the Kalinga site in Luzon, Philippines, showed hominins were active on the island as far back as 709,000 years ago. The newly discovered species, called Homo luzonensis, lived in the Philippines around 67,000 years ago. It’s not known if the two groups were related.
Image: Thomas Ingicco, Mission Marche aux Philippines

Indeed, the story of human evolution is getting increasingly complicated. Hominins first appeared in Africa some 6 to 7 million years ago, with the first evidence of a hominid presence in Eurasia dating back about 1.8 million years ago, likely the archaic human known as Homo erectus. Incredibly, this dispersal happened long before our species, Homo sapiens, emerged; we finally entered onto the scene 300,000 years ago, spilling into Eurasia about 100,000 years later. There, we joined two other human species, the Neanderthals and Denisovans.

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Most extinct hominins are not our direct ancestors, but they’re our very close relatives. Each species went on its own evolutionary journey, adapting to their new circumstances and environments in different ways. By around 50,000 to 60,000 years ago, there were multiple human species living at the same time, doing their own thing at various locations in both Africa and Eurasia.

The scientists who found the human foot bone in the Philippines, a team led by Florent Détroit from the National Museum of Natural History in France and Armand Mijares from the University of the Philippines, kept working at Callao Cave in an effort to find more clues. These ongoing excavations resulted in the discovery of 12 new hominin elements, including teeth, a partial thigh bone, and several hand and foot bones. These fossils belonged to three different individuals, two adults and one child. No genetic evidence could be extracted from the specimens, and sadly, no skulls were found. These remains were pulled from the same stratigraphic layer as the foot bone found in 2010, dating these individuals to the same time period.

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Analysis of the bones and teeth suggested a previously unknown species of human. Also, these hominins appear to have been exceptionally small. The authors of the new study suspect H. luzonensis was subject to an evolutionary process known as insular dwarfism, in which a species’ body size becomes significantly reduced over time on account of limited access to resources. A similar thing likely happened to Homo floresiensis, an extinct human species popularly known as the Hobbit. The remains of these diminutive humans, who stood no taller than around 3 feet and 7 inches (109 centimeters), were discovered on the island of Flores in Southeast Asia back in 2004. The discovery of Homo luzonensis—who may have been shorter than the Hobbits—may signify the presence of yet another human species moulded by insular dwarfism—a rather astounding discovery, to say the least. It’s important to point out, however, that insular dwarfism in H. luzonensis remains a distinct, yet unproven, possibility. As the authors noted in the study, “further discoveries and more definitive evidence are needed.”

The Hobbits and H. luzonensis bear striking similarities, and they were around at roughly the same time, but their evolutionary relationship is not known. It’s conceivable that both human species are descendents of H. erectus, and that both ended up isolated on their respective islands.

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Five upper teeth of the newly described human species, Homo luzonensis.
Image: Callao Cave Archaeology Project

In terms of the distinct physical differences observed in H. luzonensis, the most notable were seen in their teeth and foot bones. Their molars, in particular, were unlike anything ever seen before in a human species. Writing in an accompanying News & Views article, anthropologist Matthew Tocheri from Lakehead University in Thunderbay, Ontario, described how their teeth exhibited both ancient and modern features.

When compared with the molars of other hominin species, H. luzonensis molars are astonishingly small, and the simplified surfaces of their crowns and their low number of cusps are features that look similar to the molar crowns and cusps of H. sapiens. Yet the shapes of H. luzonensis teeth share similarities with the teeth of H. erectus from Asia, and the size ratio of H. luzonensis premolars to molars is similar to that of Paranthropus, species of which are known for their massive jaws and teeth.

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This is unbelievably fascinating, but the story gets even more intriguing—and potentially even more confusing.

The third metatarsal—the long bone in the middle toe—in H. luzonensis is exceptionally strange, bearing an uncanny resemblance to those seen in Australopithecus—an ancient human ancestor that lived in Africa some 3 million years ago and never left Africa. Well, at least to the best of our knowledge. H. luzonensis also featured hand bones similar to those of Australopithecus.

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The toe bone in question: The third metatarsal of H. luzonensis.
Image: Callao Cave Archaeology Project

There are at least two possible explanations for this odd toe bone, neither of which seem wholly plausible.

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Crazy theory number one: Conventional thinking has it that Homo erectus was the only hominin species to have left Africa during the Early Pleistocene, the period between 2.58 to 0.78 million years ago. The evidence found in Callao Cave would seem to suggest that we’ve got it all wrong, and that other hominins, including groups more closely related to Australopithecus (or even Australopithecus itself!), made their way into Eurasia during this period. More evidence is needed to back this extraordinary possibility.

Crazy theory number two: An intriguing aspect about the H. Luzonensis third metatarsal is that it allows for more curved toes. This is significant because curved toes are great for tree climbing and hanging from branches. Australopithecus, a suspected tree climber, was equipped with this particular physical feature, and it would seem to suggest the same for H. luzonensis. But if H. Luzonensis is not closely related to Australopithecus, then why the curved toes and hands? It’s possible that some hominins retained their tree-climbing abilities for much longer than is conventionally appreciated. Another possibility is convergent evolution, in which similar physical characteristics appear in unrelated species. Should this be the case, if would mean that, after thousands of years of upright, bipedal locomotion, H. Luzonensis was returning to an arboreal existence, and evolving the requisite physical characteristics. It’s a mind-blowing theory, no doubt, but again, one in desperate need of further evidence.

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“The discovery of H. luzonensis underscores the complexity of the evolution, dispersal and diversity of the genus Homo outside of Africa, and particularly in the islands of Southeast Asia, during the Pleistocene,” the authors appropriately conclude in the new study.

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Paleontologist Adam Brunn, a researcher at Griffith University and an expert in the hominin colonization of southeast Asia, had been hearing rumours about the Luzon fossils for years, saying it’s “brilliant” to finally seeing the new findings published.

“It’s not everyday a new human species is discovered. To describe one, to add a new relative to our family tree, is an enormous privilege—and a huge responsibility,” explained Brunn in an email to Gizmodo. “The discovery team has done a very meticulous and commendable job describing these new fossils, and their naming of a new species, in my opinion, is valid. This is a truly sensational finding.”

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Gerrit van den Bergh, a senior lecturer at the University of Wollongong’s Centre for Archaeological Science, said the researchers were “justified” in their decision to name the new species. He said it’s fascinating that Luzon is now the second known island, after Flores, in which a distinct human species existed before the arrival of H. sapiens.

“These insular species may have evolved on each island in perfect isolation, and you could say that their evolutionary pathway has gone in a completely different direction as those of their counterparts on mainland Asia, for example Homo erectus or the Neanderthals,” wrote van den Bergh, who wasn’t involved with the new study, in an email to Gizmodo. “Most notably they became very small bodied, which is what also happened to other large mammals that became stranded on islands, such as various fossil pygmy elephants that we know once inhabited many islands in the Mediterranean and Island Southeast Asia.”

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Anthropologist Dean Falk from Florida State University had few quibbles with the new paper, but the one issue she did raise was beyond the control of the researchers: their inability to find a skull. Despite this major limitation, she “became convinced that the authors have identified hominins with unique dental features combined with at least some features of hands and feet that resemble Australopithecus,” adding that these “specimens may, indeed, represent a previously unidentified species,” wrote Falk in an email to Gizmodo.

The small teeth of H. luzonensis, she added, suggests they had small bodies, but we can’t know for sure. That said, “insular dwarfism would not be at all surprising on the island.”

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“You gotta love the curved hand and feet bones, and I don’t have a problem with speculating that their owners might have spent a good deal of time in trees,” Falk told Gizmodo. “Could it be that—at least some—hominins continued to sleep in tree nests until very recently, at least geologically speaking?”

Ultimately, however, the story of human evolution just got a lot more “bushier,” she said.

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No kidding. And a lot more interesting, if that’s even possible. As this new paper beautifully illustrates, the unfolding human origin story is turning out to be absolutely incredible.

[Nature]

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