Prehistoric human fossils discovered in a cave called Liang Bua on the Indonesian island of Flores have long been shrouded in mystery.
When the first strange bones were unearthed well over a decade ago, scientists proposed that they represented an entirely new human species, dubbed Homo floresiensis and nicknamed "hobbit" for its 3.5-foot stature. But little was known about how such tiny humans came to be.
Perhaps they were actually sickly Homo sapiens that hadn't grown to their full size, not a different species, some scientists suggested at the time, citing the bones' relatively recent age (originally thought to be as young as 11,000 or 13,000 years old, but later turning out to be 60,000 to 100,000 years old). Or, as the more popular explanation went, maybe they had evolved from another archaic human species, like Homo erectus, that had undergone an evolutionary process called island dwarfing.
But there wasn't enough evidence in the fossil record to test these hypotheses, University of Wollongong archaeologist Gerrit van den Bergh said during a press conference – until now.
Archaeologists have been combing the island for more clues about these enigmatic people and their ancestors ever since. In 2014, they hit a jackpot: 700,000-year-old teeth and a jaw bone at a different Flores site, called Mata Menge.
The new fossils, described in two papers published Wednesday in the journal Nature, "end the argument" that Homo floresiensis could possibly be a sickly Homo sapien, Russell Ciochon, a paleoanthropologist at the University of Iowa in Iowa City, who was not part of the studies, tells The Christian Science Monitor in an email.
"Homo sapiens didn't exist 700,000 years ago. Our species evolved in Africa some time around 200,000 years ago," Daniel Lieberman, a paleoanthropologist at Harvard University who was not part of the studies, tells the Monitor in an interview. "So it's just simply impossible, unless the dates are wildly, wildly wrong."
So where did these tiny humans come from?
"One of the [new] fossil teeth … has characteristics that point to a Homo erectus ancestry," Dr. van den Bergh said. That molar has a fifth cusp, which Homo erectus also has, that wasn't on the previous fossil teeth found at the other site on Flores, Liang Bua.
But the tooth is still significantly smaller than any Homo erectus molar. "This adds crucial support to previous claims that Homo floresiensis is indeed a kind of dwarfed Homo erectus," he said.
Homo erectus likely reached 6 feet tall, while the hominins on Flores probably stood just over 3 feet tall. So how did they get so small?
The Flores humans probably underwent island dwarfism, the scientists suggest. In that scenario, a population of larger-bodied humans (like Homo erectus) shrunk over generations after they arrived on the island due to evolutionary selection that ensues in the restricted environment of an island. Other animals, like elephants, have been found to undergo this process too.
Homo erectus isn't the only candidate to be the hobbit's ancestor. Scientists are not ruling out the possibility that some other ancient hominin may have made its way to the Indonesian island.
It has been proposed that hominins that were already small, like Homo habilis or perhaps even Australopithecus, were Homo floresiensis ancestors. In fact, points out Dr. Lieberman, some parts of the original hobbit skeleton found over a decade ago, like the foot and wrist, display some features that are more primitive even than Homo erectus. This would hint support that one of those smaller, more primitive hominins was the ancestor.
But there is no evidence of either of those hominins ever being outside of Africa, and Homo erectus fossils have been found on nearby Java, dating to about 1.5 million years ago.
Hints in hobbit technology
In addition to the bones, archaeologists have also found stone tools on Flores. Tools have been found at Liang Bua and Mata Menge, and at a third site called Wolo Sege.
But not all these tools are the same. While the Liang Bua and Mata Menge tools are similar, the Wolo Sege tools, estimated to be about 1 million years old, are distinct.
"I think this difference in technology may reflect the presence of different hominin species at the two sites," Dr. Ciochon says. He describes a scenario in which a Homo erectus group colonized Flores by 1 million years ago, leaving evidence of their presence in the Wolo Sege tools. Then, as the ecological pressures simultaneously shrunk these humans' bodies, their technological needs changed too. So by the time that the Mata Menge humans lived, 700,000 years ago, they had both smaller bodies and differently styled tools. Then, the later populations, including those represented by the Liang Bua fossils, continued to use that same style tools.
But the team is still on the hunt for bones associated with those earliest tools.
"We want to know what the very first hominins to set foot on the island looked like," Adam Brumm of Griffith University, who is project co-leader with van den Bergh, said in the press conference. "And that requires finding hominin fossils that date to before 1 million years ago to go with these unusual tools that we find at around that period of time."
If this scenario is true, and towering Homo erectus was indeed on the island at 1 million years ago, the humans would have shrunk down over a very short time period.
"What is truly unexpected is that the size of the finds indicates that Homo floresiensis had already obtained its small size by at least 700,000 years ago," Yousuke Kaifu, from Tokyo's National Museum of Nature and Science, who helped examine the bones, said in a press release.
That would give the hominins just 300,000 years to shrink down to their iconic hobbit size.
"Now that might seem rather fast," van den Bergh said in the press conference. "The problem is that we have not much to compare this rapid rate of evolution with because there are no other islands where we have a sequence of primate fossils which you can follow through time."
So perhaps the process did happen very quickly.
Or, van den Bergh suggests, perhaps the ancestors to these hominins started shrinking on another island before ending up on Flores.
"It is possible that the ancestors of Homo floresiensis came from the North, from the island of Sulawesi," he says. Stone tools have been found on Sulawesi suggesting an archaic human presence, but they date to between 118,000 and 194,000 years ago.
Regardless, the shrunken hobbits "show that the human lineage, just like every other lineage, is subject to the forces of selection" on islands, Lieberman says.