Despite our ability to form a fist for defense, and our opposable thumbs that make dexterous tasks possible, the human hand may actally be more primitive than those of our closest living cousins the chimpanzees, researchers say.
While the specialized adaptations of our hands have long been assumed as a major evolutionary advantage, the human hand is less developed in terms of evolution than that of a chimp, having changed little from the hands of the last common ancestor shared with our simian cousins millions of years ago, scientists report.
In contrast, the hands of chimps and orangutans have changed much more since they split off from those ancestors to form new branches on the hominid family tree, they say in a study published in the journal Nature Communications.
It had long been believed that human hands evolved a longer thumb relative to the other fingers to allow precision grasping as humans began to learn to make stone tools.
However, the researchers say, their study shows that the human hand may have already had its current form as far back as the last common ancestor.
"These findings indicate that the structure of the modern human hand is largely primitive in nature, rather than the result of selective pressures in the context of stone tool-making," the researchers said in a release from the journal.
While human hands evolved little, chimpanzee and orangutan hands underwent considerable evolutionary change, with the fingers becoming elongated in comparison with the thumb to allow agile movement through forests by swinging on tree branches.
The study challenges a long-held assumption that the last common ancestor (or LCA) of humans and apes - the identity of which is still a matter of debate - would have resembled a chimpanzee with hands like modern-day chimps.
"Any evolutionary model of human hand evolution assuming a chimpanzee-like ancestor will likely be flawed from the beginning," says study leader Sergio Almecija of the Center for the Advanced Study of Human Paleobiology at George Washington University.
When hominins started producing flaked stone tools as early as 3.3 million years ago, in terms of overall shape and proportion their hands were pretty much like human hands today, he says.
The "primitive" nature of the human hand suggests that any changes that led to a widespread flowering of stone tool culture was likely neurological rather than structural -- in other words, the adaptation occurred in our brains, not in our hands, he says.