In a cave in the Philippines, scientists have discovered a new branch of the human family tree.
At least 50,000 years ago, an extinct human species lived on what is now the island of Luzon, researchers reported Wednesday. It’s possible that Homo luzonensis, as they’re calling the species, stood less than 3 feet tall.
The discovery adds growing complexity to the story of human evolution. It was not a simple march forward, as it once seemed. Instead, our lineage assumed an exuberant burst of strange forms along the way.
Our species, Homo sapiens, now inhabits a comparatively lonely world.
“The more fossils that people pull out of the ground, the more we realize that the variation that was present in the past far exceeds what we see in us today,” said Matthew Tocheri, a paleoanthropologist at Lakehead University in Canada, who was not involved in the discovery.
In the early 2000s, Armand Salvador Mijares, a graduate student at the University of the Philippines, was digging at Callao Cave, on Luzon, for traces of the first farmers on the Philippines. Soon, he decided to dig a little deeper.
Researchers on the Indonesian island of Flores had discovered the bones of an extraordinary humanlike species about 60,000 years old. The scientists named it Homo floresiensis.
Some features were similar to ours, but in other ways Homo floresiensis more closely resembled other hominins (the term scientists use for modern humans and other species in our lineage).
Homo floresiensis was able to make stone tools, for example. But the adults stood only 3 feet high and had tiny brains. This strange combination led to debates about who, exactly, were their ancestors.
The oldest fossils of hominins, dating back over 6 million years, have all been found in Africa. For millions of years, hominins were short, small-brained, bipedal apes.
Starting about 2.5 million years ago, one lineage of African hominins began to evolve new traits — a flatter face, bigger brains and a taller body, among other features. These hominins were the first known members of our own genus, Homo.
Only later, about 1.8 million years ago, do the first fossils of Homo appear outside of Africa. One common species was Homo erectus, a species that spread to East and Southeast Asia. The youngest Homo erectus fossils, discovered in Indonesia, may be just 143,000 years old.
Our own lineage kept evolving in Africa. Homo sapiens emerged about 300,000 years ago, and only 100,000 years ago did we start leaving the continent. By 50,000 years ago, our species had reached Australia. (Some researchers believe that date should be pushed back to 65,000 years ago.)
One hypothesis, then, is that Homo floresiensis evolved from Homo erectus. So here was the question for Philippine archaeologists: Could hominins have reached Luzon as well as Flores?
“That inspired me to go back and go deep,” Mijares, now a professor at the University of the Philippines, said in an interview.
In 2007, he returned to Callao Cave. As his team dug into the cave floor, the researchers hit a layer of bones. At first, Mijares was disappointed by the fossils, which mostly belonged to deer and other mammals.
But when Philip Piper, an archaeologist at the University of the Philippines, later sorted through the finds, he noticed one that resembled a human foot bone. It was small, Mijares said, “and there was something weird about it.” But not much more could be learned from a single bone.
In 2011, on another dig, he and his colleagues found more humanlike fossils, including teeth, part of a femur and hand bones. In 2015, they found two molars, which they dated to at least 50,000 years ago.
All told, the fossils came from three individuals. And they were remarkable.
The teeth had a peculiar shape. Some of the front teeth had three roots, for example, whereas those of our species usually have just one. And the teeth were tiny.
“These adult teeth are smaller than any hominin known,” said Debbie Argue, a paleoanthropologist at Australian National University, who was not involved in the new study.
“Could it be that these teeth belonged to adults that were even smaller than Homo floresiensis?” she wondered.
The researchers did not find enough bones to estimate how tall Homo luzonensis stood. But they do display their own strange mix of traits. One toe bone, for example, looks nearly identical to those of early hominins living in Africa more than 3 million years ago.
“The combination of features is like nothing we have seen before,” said María Martinón-Torres, the director of Spain’s National Research Center on Human Evolution, who was not involved in the new study.
Taken together, Mijares and his colleagues concluded, the evidence pointed to a new species of Homo.
Drawing such a conclusion from a few bones is risky, acknowledged Huw Groucutt, a paleoanthropologist at the Max Planck Institute for Chemical Ecology.
Nevertheless, “I think the argument for a new species does look pretty convincing in this case,” he said.
Homo erectus may have been the ancestor of the tiny hominins on both Flores and Luzon — perhaps swept to the islands by storms, clinging to trees. It may even be possible that Homo luzonensis descended from hominins that came to Luzon hundreds of thousands of years earlier.
Last year, another team of scientists digging in a different cave on Luzon found the bones of a butchered rhinoceros. Near those remains, they also discovered stone tools dating back 700,000 years.
At the very least, the two studies indicate there were hominins on Luzon 700,000 years ago and 50,000 years ago. The question now is whether they belonged to the same population.
“I think it’s likely the same lineage,” said Gert van den Bergh, an archaeologist at the University of Wollongong in Australia and co-author of last year’s study. Over the next several hundred thousand years, he speculated, Homo erectus shrank as it adapted to life on Luzon.
Tocheri disagreed with that interpretation.
“I don’t really buy into the idea that it’s island-dwarfed Homo erectus,” he said. Instead, he suggests, the tiny island hominins had tiny ancestors — perhaps small hominins in Africa that expanded to Asia and wound up on Flores and Luzon, taking refuge from bigger hominins.
“But it begs the question,” Tocheri added. “If we’re finding these things way over there, there’s got to be a record of them all the way across the continent leading back to Africa.”
Sorting through these possibilities will demand more fossils of Homo luzonensis — and perhaps fossils from some of the many islands off the coast of Southeast Asia.
“I see it as an amazing opportunity to see several parallel experiments in human evolution on these islands,” van den Bergh said.