How a new human species challenges textbook histories

Armand Mijares didn’t initially realize the significance of what he had found, but it would turn out to change the paleoanthropologist’s life – and rewrite human history.

At an archaeological site on the Philippine island of Luzon in 2007, his team unearthed a wide array of ancient animal bones, dated to be about 67,000 years old. The researchers couldn’t identify the fossils out in the field, so Dr. Mijares sent them to a zoologist colleague.

“He called me on my cellphone” one evening, recalls Dr. Mijares, an associate professor at the University of the Philippines. “He called me, ‘Hey, hi mate, you have human remains!’ I said, ‘What?’ ‘You have one human remain.’”

Among the ancient bones was a single human toe bone. But which human species did it belong to?

Now, after studying more hominin bones discovered at the site in 2011 and 2015, the researchers have come to a history-shaking conclusion: A human species previously unknown to science once lived on Luzon. The researchers introduced the new species, dubbed Homo luzonensis, in a paper published in the journal Nature on Wednesday.

This finding reverberates beyond the Philippines. It underscores a growing sense among human origins researchers that human evolution may not be as neat and linear as the story we often tell. H. luzonensis adds even more complexity to that picture.

“Obviously our idea of what was going on is much less nuanced than what was really going on,” says Susan Antón, a paleoanthropologist at New York University, who was not part of the study. “[This discovery] gives us a different perspective about our place in the world.”

Despite giving H. luzonensis a name, the hominin remains mysterious in many ways – and sorting out its story could illuminate the nuances of the whole genus Homo.

Currently, only a handful of bones represent the new species: some hand and foot bones, part of a thigh bone, and seven teeth. All the bones are particularly tiny, which suggests that this ancient human likely stood under 4 feet tall. But without the long bones, it’s difficult to be sure, says Dr. Mijares, co-lead author on the new paper.

That would make H. luzonensis the second known dwarf species in the genus Homo. The other short human, nicknamed “hobbit” for its stature, was discovered in 2004 on the Indonesian island of Flores. Both Homo floresiensis (the hobbit’s scientific name) and its newfound counterpart are thought to have lived up until around 50,000 years ago.

Both hominins likely underwent a process known as island dwarfism, when a species shrinks over generations via evolutionary selection due to limited resources. But how they got to the islands in the first place and who they were when they arrived remain open questions.

At the time, Homo sapiens, Neanderthals, and Denisovans were the only other human species known to be outside Africa. But becoming such distinct hominins as the two island dwellers takes generations of evolution, so they probably came from an earlier Homo lineage.

Researchers have previously found Homo erectus fossils, dated to show the species was in the region by 1 million years ago. So that species is an obvious contender. It’s also the only member of the genus Homo known to be in the region before H. sapiens arrived some 50,000 years ago, other than the Flores and Luzon dwarf humans.

The story told of H. erectus’ migration from Africa to Asia goes something like this: As the earliest hominin to have long legs conducive to long-distance walking, H. erectus was the first to stride out of Africa to conquer the world. A population in Southeast Asia somehow made it across wide, deep waters to these islands. Some scientists have suggested that the hominins might have been swept out to sea on a vegetation mat or some other accidental event. Once there, they established a population and, over generations, their features changed to make them better suited to the island environments.

“Remember Occam’s razor?” says Russell Ciochon, a paleoanthropologist at the University of Iowa, who was not part of the study. “What’s the most parsimonious hypothesis? We know that Homo erectus got there with time enough to disperse and evolve into these different species.”

But others aren’t so sure.

The Luzon fossils bear a strange mixture of features (that’s what convinced the research team this was an entirely new species). The tiny teeth look like both modern and archaic hominins in different ways. And the toe bone is curved, much more like the genus Australopithecus than any Homo.

“We could also explain it perhaps as something like early Homo leaving Africa either at or around the same time as early Homo erectus,” suggests Matt Tocheri, Canada research chair in human origins at Lakehead University and a Homo floresiensis researcher. H. erectus was the first member of the genus to have humanlike features on its whole body. Earlier members of the genus looked very human in the face and head, but from the neck down, they looked a lot more like Australopithecus.

Furthermore, Dr. Tocheri says, stone tools have been found in China dated to be about 2.1 million years old – a few hundreds of thousands of years before the oldest H. erectus fossil found outside Africa.

Such a scenario would be a major rewrite to conventional stories of early human dispersal, particularly as Australopithecus legs and feet were more suited toward a dual life as tree-climbers and local walkers than long-distance treks. And it still might be possible that H. erectus left Africa earlier than previously thought, and that the Luzon hominin evolved from H. erectus and that those seemingly archaic features were simply better adaptations for the island environment.

“It’s an interesting idea, but I think it would be foolish to make a commitment to one or the other in light of the fact that the data are simply not there to judge,” says William Kimbel, director of the Institute of Human Origins at Arizona State University, who was not part of the study. More data is needed, he says.

Ancient DNA samples would also help, but tropical islands are not environments conducive to preservation. Dr. Mijares and his colleagues attempted to extract some from the Luzon fossils, but couldn’t find any that had preserved.

Some paleoanthropologists are hesitant to call the Luzon hominin a new species just yet. “Because Homo is so much defined on cranial remains,” says Dr. Antón, “I would’ve wanted some heads.”

That doesn’t mean the team is wrong, she adds, but the significance of their find does not rest on whether you can call this hominin a distinct species. Instead, she says, the fact that a hominin evolved such wildly different features in the island context highlights just how flexible and variable humans can be.

This also shows that the human evolutionary story may not be as unique as we like to think, says Dr. Tocheri. “It’s really looking a lot more like it does for the evolutionary record of other mammals. At the end of the day, hominins are just another mammal.”

Dr. Kimbel agrees, “We are exceptional in many ways, but let’s not be fooled into thinking that our explanations [of how we came to be] have to be exceptional as well.”