Before this discovery, the earliest indicator that early humans, or hominins, were even on those islands had been a single foot bone from 67,000 years ago, uncovered in the Callao Cave on Luzon. That's quite a time jump.
Researchers came close to figuring out that Luzon may have been inhabited by early humans when stone tools and the fossils of large animals were discovered there in the 1950s. But they weren't able to securely date those findings to the Middle Pleistocene, which spans 126,000 to 781,000 years ago.
But recent excavations in the Kalinga province of northern Luzon uncovered 57 stone tools and more than 400 bones of animals like monitor lizard, Philippine brown deer, freshwater turtles and stegodons, a now-extinct animal in the same family as elephants and mammoths.
The biggest find was a 75% complete skeleton of a rhinoceros that was clearly butchered, with 13 of its bones displaying cut marks and areas where bone was struck to release marrow.
All of the remains were dated to 709,000 years ago using electron-spin resonance methods, which can date material in a way that radiocarbon dating can't. These methods can be applied to such things as tooth enamel and rocks that had been heated, like quartz found in sediment.
The Kalinga archaeological site on the Philippine island of Luzon.
The discovery is important for a multitude of reasons, said study author Thomas Ingicco, associate professor at the Muséum National d'Histoire Naturelle.
"First is the very old age of this site which multiplies by ten the formerly known early presence of Hominins in the Philippines," Ingicco wrote in an email.
"Second is the evidence for colonization of an ever-isolated island in The Philippines by the early Middle Pleistocene and therefore most likely by a hominin species other than Homo sapiens."
Although there is no direct fossil evidence to suggest who these early humans might have been, the "Kalinga toolmakers" represent a new area of interest and research.
"The butchery marks were a very good surprise," Ingicco said. "I only can think of two sites where you have evidences of butchery activities, one is the famous site of Choukoutien in China and the other is Ngebung in the Sangiran Dome of Java, Indonesia. So we actually know very little about these early Middle Pleistocene hominins' behaviour in Southeast Asia."
The finding adds to another intriguing area of continuing research that concerns the Callao Cave on Luzon. The previously discovered 67,000-year-old foot bone found in that cave appears to have come from an individual who had a form of dwarfism.
This is similar to the discovery of fossil evidence suggesting "hobbits" on the Indonesian island of Flores.
"The morphology of the fossil teeth also suggests that this human lineage represents a dwarfed descendant of early Homo erectus that somehow got marooned on the island of Flores," Yousuke Kaifu of Tokyo's National Museum of Nature and Science said of the 2016 studies.
The new Luzon evidence "might be mimicking what we know now on Flores Island, meaning an early colonization of an isolated island followed by the diminution in body size and speciation of this remote hominin population," Ingicco said. "Luzon Island might have been the place for similar endemic evolution of hominins into dwarfism just like what happened on Flores Island. There is a huge time gap between Kalinga and Callao archaeological evidences with a lots of questions in between."
These findings are like puzzle pieces that don't quite fit together yet.
"At Kalinga we have tools and butchery activities, and at Callao they have hominin remains and some butchery marks as well but no tools," Ingicco said. "Comparing the two sites is not easy at the moment."
How did the early humans reach these islands in the first place? Islands like Flores and Luzon are more than 1,700 miles apart, although the map would have looked much different 700,000 years ago.
It is not believed that early humans were capable of building watercraft. But it can't be ruled out completely, Ingicco said.
"Considering evidence of sea-crossing during the Middle Pleistocene is increasing in number, such a hypothesis cannot currently be rejected."
What about the animals?
Although fossils of large mammals were found, none of them belongs to carnivores -- which are not known for having good long-distance swimming skills, Ingicco said. But other large mammals are.
Small animals like rats, tortoises and lizards have been found to float on vegetation to reach islands. That scenario is likely here, according to the study.
Inclement weather, like typhoons, can also create natural rafts out of vegetation capable of carrying hominins and animals.
"Colonization of the islands could have been possible thanks to natural rafts such as floating mangroves that typhoons occasionally break off the coast," Ingicco said. "These floating islands would have come with animals and possibly hominins on them. Such natural rafts are quite well documented for historical periods and it is therefore a likely way of colonizing Luzon Island during the mid-Pleistocene by hominins."
The site is excavated for a month each year. The next excavation is planned for this summer.
The researchers hope that more excavations will help to answer the questions prompted by their latest findings.
"One is who made the stone tools and butchered the rhino," Ingicco said. "To answer this we have to continue excavating and hope to be lucky enough to find a Hominin fossil of some sort.
"The other question is the origin of the dispersion. Did they come from the North as our team suspect or did they come from the West as some other archaeologists suspect? Comparing the early Middle Pleistocene fauna of the Philippines with what is