Using carefully selected stones weighing up to 33 pounds, some West African chimps have been pounding the nut of panda trees for generations, passing down the learned behavior and some of the hammer stones to their young, researchers report in the journal
"It is a very skillful behavior that takes up to seven years for them to learn," said Melissa Panger, a George Washington University researcher and co-author of the Science study. "It looks easy, but if you sit down and try it is a very difficult task."
The panda nuts fall to the ground inside an outer husk. When the husk is removed, there is a golf ball-sized nut protected by a tough covering that can require up to a ton of pressure to break open. Yet, if the animals pound too hard, the nut shatters and is inedible, Panger said.
"What is remarkable is that they are controlling the force precisely," she added.
Inside the tough covering are three kernels rich in nutrition. During nut-smashing season, some chimps spend two or three hours a day opening as many as 100 panda nuts.
The chimps have established nut-cracking stations, usually centered on a battered root of a hardwood tree used as an anvil. The animals gather nuts, put them on the anvil and then pound them with the heavy stones.
Panger said the chimps leave the hammer stones beside the anvils and some have apparently been used for generations. Some animals have been seen carrying pounding stones from one anvil site to another, just like a repairman might carry tools from place to place, she said.
The sites closely resemble sites used by the early human ancestor known as Homo habilis, which lived in what is now Kenya and Tanzania between 2.4 million and 1.5 million years ago. The tools this early "handy man" made are called Oldowan.
"Some of the stone by-products of chimpanzee nut-cracking are similar to what we see left behind by some of our early ancestors in East Africa during a period called the 'Oldowan,"' George Washington University archeologist Julio Mercader said in a statement.
Panger and Christophe Boesch, an expert in chimpanzee behavior of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said the implications of their findings are not yet clear.
Panger, who studies primate tool use, says it may be that the earliest humans used tools much sooner than some researchers believed.
"We know that flaked stone tools were used 2.5 million years ago, but stone tools may have been used by hominids as long as 5 million years ago," she said.
"If we look for assemblages of stone pieces like those we have found left behind by the chimpanzees, we can infer that those assemblages may relate to tool use, even if we don't have the tools."
The three researchers studied piles of stones and nutshells in the Ivory Coast's Tai National Park.
Nut cracking demonstrates a degree of sophisticated learning because it required the animals to select hammer stones at a distant rock outcropping and then carry them to the anvil sites for use. Panger said selection of the stones required some thought by the apes: The crude hammers had to have a flat surface on one side, a place to grasp the stone and had to be heavy enough to smash the tough nuts.
"The hammer stone has to be of a certain size and shape. They have to use it without crushing their fingers," said Panger. "Some of these hammers have been used so many times that they have deep pits, suggesting that they have been used for many generations, over and over again."
Mothers teach their children to bang on nuts and some young chimps have been seen hitting nuts with smaller stones, going through the motions learned from their parent.
The researchers said the nut smashing technique is known to only some bands of West African chimpanzees. It has not been seen among chimps in central Africa, although the apes there have nuts and stones available to them.
This suggests that nut smashing is a cultural, learned behavior that has not spread widely among the apes.
Chimps and humans shared a common ancestor somewhere between 5 million and 7 million years ago, and we share close to 99 percent of our DNA. The researchers say the sites may shed light on how human tool making evolved.
As the chimpanzees crack the stones against the anvils, chips fly off, the researchers noted. These sharp flakes resemble flakes that early humans used as knives and other tools, and the chimp behavior may illustrate just how the first humans discovered they could make tools from stones.
"For example, nut cracking during the Miocene and Plio-Pleistocene could have generated byproducts that eventually became the 'cutting' tools of Oldowan hominins," they wrote in their report.
The Miocene period ended 5.3 million years ago, while the Pliocene and Pleistocene lasted from then until 8,000 years ago, when the present epoch, the Holocene, began.
Robson Bonnichsen, a stone tool expert at Oregon State University in Corvallis, said humans first began the purposeful shaping of stone tools about 2.5 million years ago and that chips from that work have a distinctive shape that would not be confused by researchers with the pieces of stone randomly broken off of stone hammers by chimps.
Bonnichsen said the research by Panger, Mercader and Boesch gives important new insights into tool use by primates.
"This is really interesting," he said. "I applaud their work."