About 1.1 million years ago, an ancient species of human may have stood atop an island mountain in tropical Southeast Asia and plotted a voyage that indelibly etched itself into human evolution in the region.
University of Iowa paleoanthropologist Russell Ciochon believes that those early humans spied other islands in the Indonesian archipelago and somehow managed to settle them—a diaspora that led to new human species, from the “Hobbit” hominins found on the island of Flores to still-unnamed fossils of another potential human species unearthed in the Philippines.
And, who knows? Maybe more species are still waiting to be found.
It’s an unfolding tale of the arrival of Homo erectus into the present-day Indonesian archipelago and how that parental lineage branched into new species as small groups likely colonized nearby islands, became isolated as the climate changed, then died out—for reasons we still don’t fully understand.
Ciochon outlined this theory of early human dispersal and speciation in an extensively researched paper published in the journal Evolutionary Anthropology. In their review, Ciochon and co-author Roy Larick argue that Homo erectus got happy feet relatively soon after arriving on the present Indonesian island of Java about 1.6 million years ago. The timing couldn’t have been much better: A global cooling had caused sea levels to fall, meaning more land was exposed between islands, in effect bringing them closer together. At some time periods, Flores was as few as 12 miles away from the nearest island that appears to have been part of Homo erectus's island-hopping migration.
As prey moved, the ancient humans followed, Ciochon thinks.
“The point is, if you’re up on a mountain, you could have seen those islands,” says Ciochon, professor in the Department of Anthropology and director of the Museum Studies certificate program at the UI. “You knew they were there.”
As Homo erectus hopscotched from island to island—likely floating on the currents or riding primitive rafts, the authors believe—they found themselves in relatively hospitable environments: Each place teemed with game, such as Pleistocene-era dwarf-elephant stegodons, and nearly zero competition. The colonizing groups flourished—some staying, others moving on, establishing more remote outposts with each stop.
“And then you get these new forms evolving,” Ciochon says.
Recent fossil finds seem to buttress the theory. In 2003, anthropologists found a nearly complete skeleton of a new species they called Homo floresiensis, named after the island of Flores where the specimen and fragments of other skeletons have been discovered. The species, nicknamed the “Hobbit” for its diminutive size, was much shorter than modern humans and had a skull less than half the size of Homo erectus remains found at sites on Java. Some argue the small stature came from “island dwarfing,” an accepted biological phenomenon in which large mammals physically shrink to better adapt to limited resources on an island.
Four years later, fossil fragments were unearthed in a cave on the Philippine island of Luzon, a good distance to the northeast of Java but plausibly reached by Homo erectus splinter groups as more land between islands was exposed and sea crossings shortened at various intervals.
Combined, the fossil record points to different species of human hunter-scavengers living and evolving separately in a relatively compact geographical range, yet always prone to succumbing as the climate, prey abundance, or other factors changed.
“No one thought that these same principles would apply to humans,” Ciochon says, “that they would fall prey to the same evolutionary pressures that [animals like] stegodons had.”
There may have been one other force that led to these hominin species’ extinction: modern humans. Fossil evidence shows Homo sapiens began arriving in the islands of Southeast Asia around 47,000 years ago, long after the other island species had taken root. Did Homo sapiens outcompete their ancient brethren? The question remains largely unanswered.
"Other hominins were in Island Southeast Asia prior to Homo sapiens arriving. If these species were still living, then it is possible that Homo sapiens outcompeted them and contributed to their extinction," Ciochon says. "The bottom line is that we do not have any definitive evidence for when these extinction events occurred, so we do not know if Homo sapiens were involved."
This summer, Ciochon will travel to two fossil sites on Java, the Bapang and Sangiran formations, to try to determine where individual Homo erectus specimens at each site came from and how they evolved once they arrived.
Ciochon has made annual anthropological forays to Indonesia since 1998. He's also examined remains in East Africa, Myanmar, China (where he still works), India, and Vietnam. His motivation was similar to ancient humans’ reason for migrating: less competition.
“The whole thing of discovery,” Ciochon, who has been at the UI since 1987, relates, “just to find something new, that’s what intrigued me. Finding new humans is just a trip because human fossils are the rarest of fossils, and everyone wants to know about them.”