Are chimpanzees evolving in the wild?
March 6, 2007
If you were to put a sample of chimpanzee DNA next to a sample of human DNA -- and if you had any idea what you were looking at -- you would see that the samples are nearly identical. Chimps and humans share 96 percent of their DNA, and some new research suggests that chimps and humans may have split off from a common ancestor just 4 million years ago, which is a more recent estimate than the generally accepted timeframe of 5 to 7 million years. This would mean that it took about 4 million years for humans and chimpanzees to become completely separate species. The two are so close on the evolutionary ladder that observing chimps offers real glimpses into the way humans may have evolved. And a new, pretty major observation of chimp behavior may provide scientific evidence of a long-suspected theory about human evolution.
Chimpanzees are known to use tools. Many scientific studies, including Jane Goodall's famous work with chimpanzees in Gombe, Tanzania, have documented chimps using tools to complete or simplify tasks like cracking nuts open and getting termites out of logs. They've been observed using a stick to make an opening in a tree trunk big enough to where they get get an arm in to pull out some bugs or honey or other delicacy. But not many researchers have been able to effectively observe chimps outside of Gombe -- it's hard to get them accustomed enough to human presence to catch them acting naturally for long periods of time. And until now, no one has documented a definite case of chimpanzees using tools to hunt in the traditional sense.
Jill Pruetz of Iowa State University and Paco Bertolani from the University of Cambridge successfully observed chimpanzees in Fongoli, Senegal, from March 2005 to July 2006. Their work, published online in the journal Current Biology on February 22, 2007, reveals documentation of chimpanzees using tools to kill animals for food.
What they saw was a fairly methodical, step-by-step process of fashioning what most of us would recognize as a spear. The chimps would first break off a live tree branch, usually about 2 feet (0.6 meters) long; then pull off any leaves and twigs; then, in many cases, scrape off some of the bark at one end of the stick; and then, in many cases, create a point at that end of the stick by gnawing away at the tip with their incisors.
The sharp-point part is a big deal, because no one has seen a chimp sharpen a stick in order to use it to enlarge a hole in a log. This element supports the case that the chimps are creating a spear.
The chimps would then jab the stick into a hollow tree branch or a hole in a tree trunk, which are places where a lemur-related animal called a bushbaby (Galago senegalensis) sleeps during the day. The jabbing was in the motion humans typically think of as "spearing." Of 22 observed cases of this type of action, Pruetz and Bertolani saw only one in which the spear actually pulled a bushbaby out of a tree. But in some cases, the chimps would jab wildly with the stick and then pry open the hole from a bit of a distance in order to get to the bushbaby. Pruetz explains that this seems to indicate the chimps are in fact trying to kill or immobilize the animal with their jabbing, because bushbabies are quick -- if the animal were unharmed when the chimp pried open the hole, it would easily scurry away and evade capture. Also, the chimps would usually sniff or lick the spear after pulling it out of the hole, and the researchers found discarded spears with bushbaby fur stuck to them.
The study makes a case for this type of hunting as a localized adaptation -- in other words, an evolutionary survival trait. Typically, chimpanzees hunt red colobus monkeys for protein. In the overall chimpanzee population, that's the most common prey. But there are no red colobus monkeys in Fongoli. The Fongoli chimpanzee community had to adapt to a different protein source, and spear-based hunting seems to be one way in which the population has adapted. An interesting part of the revelation is Pruetz and Bertolani observed females and adolescent chimps doing most of the hunting with spears, not the adult males who are typically thought of as the hunters in chimpanzee populations. They surmise that the females and young ones may have come up with spear hunting as a way to compete with adult males for protein sources -- to catch a type of animal the males haven't been able to hunt successfully with their arms, legs and teeth. It is this latter observation that may provide insight not only into the behavior of chimpanzees, but also into the process of human evolution.
There is a common belief in the scientific community that females played a key role the evolution of human tool use. As observed in the chimpanzee community, the adult males are typically the last to pick up a new method of accomplishing a task. The adolescent males and females seem to be far more open to adaptation. And since adolescents spend most of their time with the females of the community, biologists have theorized that females are the primary innovators in tool use. The females use tools to adapt to changing conditions and pass along the adaptations to the young of the group, who easily pick up the new tool use. The adolescents eventually become the alpha males and the offspring-rearing females, solidifying the new tool in the daily life of the group. With 96 percent of human DNA matching chimpanzee DNA, the latest observations of female-dominant tool innovation in a chimpanzee community could provide fuel for the theory of female-driven tool innovation in human evolution.