SYDNEY — Humans originated in Africa, but when exactly our earliest ancestors left the continent and how they spread around the world has been intensely debated by archaeologists.

Two fossils unearthed in a cave in northern Laos suggest that Homo sapiens, our own species, was living in the region some 86,000 years ago, according to a new study. The finding challenges the prevailing idea that humans’ path across the globe was linear and took place in a single wave about 50,000 to 60,000 years ago.

“Chances are that this early migration was unsuccessful, but this does not distract from the fact that H. sapiens had arrived in this region by this time which is a remarkable achievement,” said study author Kira Westaway, an associate professor at Macquarie University in Australia, via email.

DNA analysis of present-day human populations has supported the hypothesis that early modern humans left Africa around 50,000 to 60,000 years ago, and archaeologists have thought our early ancestors likely followed coastlines and islands through southeast Asia toward Australia.

However, a growing number of older human remains discovered in China and the Levant show that this chapter in the human story is more complicated than first thought.

The migration 50,000 to 60,000 years ago “that contributes to our current gene pool may not have been the first,” Westaway said. “There may well have been earlier migrations that were not successful and therefore did not contribute their genetics to our modern populations.”

The two Laos fossils — a fragment of a leg bone and part of the front of a skull — were found in Tam Pa Ling cave. The archaeological site was discovered in 2009 when another partial skull was unearthed.

In addition to the latest finds, two jawbones, a rib and a phalanx have also been discovered at the site, and the physical features of the remains all suggested they belonged to early modern humans.

Dating fossils from the site proved tricky. They were too old for radiocarbon dating, which can only date remains from up to around 46,000 years ago. Also, the cave is part of a UNESCO World Heritage Site, which means the human fossils cannot be directly dated because of Laotian laws protecting the area, according to Westaway.

Instead, the team involved in the study, which published Tuesday in the journal Nature Communications, used two different techniques to estimate the fossils’ age.

The researchers measured the luminescence in quartz and feldspar minerals in the sediment layer, a method that reveals how long it’s been since a material with crystalline minerals was heated or exposed to sunlight.

As the excavation proceeded deeper, they also found two animal teeth in the same layer as the human remains and dated them by measuring the radioactive decay of uranium isotopes — chemical elements found in the tooth enamel — in a technique called electron-spin-resonance dating.

The two fossils were estimated to be 68,000 to 86,000 years old, with the leg bone fragment being the older find.

In addition to throwing the timeline of early human migration into question, the site also challenges conventional thinking that humans’ earliest journeys in the region would have involved skirting coastlines and island locations such as Sumatra, Philippines and Borneo.

The upland region in the heart of mainland southeast Asia was and is heavily forested, at an altitude of about 1,100 meters (3,609 feet) and a distance of at least 300 kilometers (186 miles) from the sea.

“The fascinating part of this research is the location of the cave. We know that hominins tended to move along river valleys inland, but this location confirms our suspicions that early Homo sapiens had the capacity to adapt and disperse through upland forested regions much earlier than anticipated,” Westaway explained.

Archaeologists believe the cave was occupied for almost 50,000 years, and the latest discoveries underscore the region’s exciting potential for paleoanthropology.

At a nearby site, known as Cobra Cave, a tooth believed to belong to a Denisovan, an elusive early human, has been found. However, that tooth is 70,000 years older than the earliest Tam Pa Ling remains, so there’s no evidence the two species of human mixed or coexisted, Westaway said.

The world’s oldest figurative rock art has been found in caves in Indonesia, and extinct human species, including the small Homo floresiensis and Homo luzonensis, have been found on islands in Indonesia and the Philippines.

The team expects to unearth more human fossils from the region.

“The site of Tam Pà Ling is one of five other sites (found) in that karst mountain that all contain human evidence,” Westaway said. — CNN