(CNN) - New evidence from Sri Lanka's oldest archaeological site suggests that early humans used sophisticated techniques to hunt monkeys and squirrels, according to a study published Tuesday in the journal Nature Communications.
Remains from a young adult female, a child between 5 and 6 years old and at least two infants were found in Fa-Hien Lena Cave. They were estimated to have lived between 5,000 and 6,000 years ago. Bone toolkits helped researchers determine that early humans lived in the cave as long as 30,000 years ago.
The other intriguing factor is that the cave opens out over the rainforest.
Researchers had known that Homo sapiens were able to adapt as they migrated across and out of Africa. In open savannas and coastal areas, they were able to hunt, butcher and eat game animals ranging from medium to large in size. But rainforests were considered to be a barrier to human migration, as they did not include the kind of animals they were hunting.
Rainforests sheltered smaller animals, and hunting them would imply more complex and sophisticated behavior than had been identified among these people, according to the researchers in the new study. They were also a smaller source of protein than larger game animals, so the complex effort required to hunt them would have yielded a smaller output and required more hunting.
But the discovery of bones belonging to monkeys and squirrels in the Fa-Hien Lena Cave, including cut marks and burns, is changing the perception that early humans avoided rain forests and small game. Sophisticated bone and stone tools were also found. The remains date to 45,000 years ago and show a sustained pattern lasting until about 4,000 years ago.
Although the bones of other animals were found, monkey and squirrel bones account for more than 70% of the remains. These species would have been vulnerable to exploitation and overhunting, the researchers said, and the early humans were sustainable in their methods to preserve the populations of their preferred game. This means the early humans were knowledgeable about the animals' life cycles and where they lived.
"Over the last two decades, research has highlighted human occupation of tropical rainforests in South Asia, Southeast Asia, and Melanesia at least as early as 45,000 years ago, so the potential for human reliance on small mammals in these settings prior to 20,000 years ago seems likely," said co-senior study author Patrick Roberts, leader of the stable isotope research group at the Max Planck Institute for the Science of Human History 's Department of Archaeology, in a statement.
Prime-age adult monkeys were deliberately targeted. The fact that both the squirrel and monkey species being hunted were primarily tree-dwelling, the early humans' hunting methods may have included projectiles, like darts and blowpipes. Bone points found at the site would have been ideal for this kind of hunting, and some of the broken points were used repeatedly.
Given the abundance of remains in the cave, researchers concluded that the monkeys and squirrels were brought back and processed. And then their own bones were put to use and honed into tools. Researchers found 36 bones that were turned into tools, indicating the hunters' preference for them. The monkey bones were presumably used to hunt other monkeys.
"This 'monkey menu' was not a one-off, and the use of these difficult-to-catch resources is one more example of the behavioural and technological flexibility of Homo sapiens," said Michael Petraglia, senior study author and professor in the Max Planck Institute for the Science of Human History's Department of Archaeology, in a statement.
All of this evidence shows that early humans migrating from Africa were able to adapt to environments and climate fluctuations, from deserts to rainforests. And the sustained hunting record of these primates is the oldest and longest, the researchers said. Before, small game hunting was thought to be limited to times of climate-caused crisis, especially about 20,000 years ago during the last Ice Age.
"We have evidence that people were living in rainforests from previous research, but exactly how they utilized forest resources, we didn't know," said Noel Amano, study author and postdoctoral student with the Max Planck Institute for the Science of Human History's Department of Archaeology. "For the first time, we have evidence of heavy reliance on forest resources by the people that were present in Sri Lanka as early as 45,000 years ago. For a long time, anthropologists and archaeologists saw tropical rainforests as a barrier to successful human dispersion."
Roberts noted, "This shows our species was able to diversify in varied settings all around the world. And it might be this that made our species more successful than other hominin species, like Neaderthals and Denisovans, and really allowed us to survive and be the last hominin standing on the face of the planet."