While we know that humans originated in Africa, when and how our ancestors first left the continent and how they spread across the globe remains a mystery that sparks lively debate among archaeologists.
The footsteps of our earliest ancestors have been a puzzle that has intrigued researchers. They are searching for clues as to when exactly humans left Africa and what routes they took as they sailed out into the unexplored world.
Two bone fragments recently discovered by archaeologists in a cave in northern Laos suggest that Homo sapiens were exploring Southeast Asia as early as 86,000 years ago. The discovery, published recently in Nature Communications, shows that humans migrated through the region earlier than previously thought. The discovery challenges the conventional wisdom that human travel around the world occurred in a linear fashion in a single wave some 50,000 to 60,000 years ago.
For more than ten years, the excavation team at Tam Pà Ling Cave found seven bone fragments buried between layers of clay. After digging to a depth of 7 meters, they finally reached the bedrock and were able to reconstruct the history of the cave. The sediments and bones found suggest that modern humans have lived in this mountainous region for at least 68,000 years, possibly longer. It appears that the bones were carried into the cave during a flood and the cave was uninhabited.
Challenges in Dating Ancient Human Fossils
Dating the fossils from this site is a complex challenge. The fossils are too old to be measured using radiocarbon dating methods, which are only effective up to 46,000 years ago. In addition, Lao laws protecting cultural heritage have prevented any damaging analysis of the human fossils found at the site.
However, the research team involved in this study was able to use two different techniques to estimate the age of the fossils. They used a luminescence technique to measure the luminescence of quartz and feldspar minerals in the sedimentary layers, a method that allowed them to determine how long the crystalline mineral material had been exposed to heat or sunlight.
In addition, as the excavation continued at greater depths, the research team found two animal teeth in the same layer as the human fossils. They used an electron spin resonance dating technique, which measures the decay of radioactive uranium isotopes - a chemical element found in tooth enamel - to determine the age of the teeth.
These results showed that the skull fragments and tibia were about 70,000 and 77,000 years old, but the tibia could be as old as 86,000 years. This is much older than the first fossil found at the site more than a decade ago, a piece of skull estimated to be 46,000 years old. It is also older than other bone fragments in the cave, such as two jaw fragments, a rib and a leg bone, which range in age from 46,000 to 70,000 years.
According to Laura Shackelford, an anthropologist at the University of Illinois Urbana who was part of the research team, the fossil record in Southeast Asia is limited because of the tropical climate, which decomposes most bones. There is still debate about when early humans first arrived in the region, their origins, and their migration routes. Laos, where the cave is located, is on a possible migration route of early humans to Australia, where the oldest archaeological sites date to about 65,000 years ago.
The cave, excavated in 2009, is not meant to be a permanent home for anyone. Each year, seasonal floods wash sediment and sometimes bones from the surface into the cave, creating layers of unique historical records. It's like a "trap for fossils," says Souliphane Boualaphane, one of the study's authors and an archaeologist with the Lao Ministry of Information, Culture and Tourism.
The Mystery that Remains
Human migration hypotheses based on DNA analysis suggest that Homo sapiens spread rapidly after a geological period known as Marine Isotope Stage 5, which lasted between 130,000 and 80,000 years ago. However, the discovery of fossils at Tam Pà Ling challenges these models by showing that human dispersal occurred before the end of Marine Isotope Stage 5.
According to Miriam Stark, an anthropological archaeologist at the University of Hawaii at Manoa who was not involved in the work, when researchers find different things, it doesn't mean the genetic models are wrong, but that the picture they reveal is still incomplete. Although the fossils are Homo sapiens, the 46,000-year-old skull fragments have a combination of ancient and modern human features, while the older fossils have more modern human-like features. For example, the older skull fragment lacks the prominent frontal bone usually associated with early humans and observed in younger fossils.
This is counterintuitive and suggests that the older fossils may not represent an evolution of the local population, but rather an early modern human group that migrated through the region. Still, the discovery is reassuring that there was an unsuccessful migration of modern humans into Asia that left no descendants, says Russell Ciochon, a biological anthropologist at the University of Iowa who wasn't involved in the work.
Despite providing data on a region that is still poorly understood, Tam Pà Ling provides additional insight into the timing of migration through the region. Shackelford and his team will continue to excavate the cave for more fossils. They are also trying to extract environmental DNA from the clay, which could provide clues about the flora and fauna that existed in the region tens of thousands of years ago. Discoveries outside the cave may also provide valuable insights into the early humans who inhabited the region.
