A team from the United States and France made the findings after traveling to an archaeological site along the northwest shoreline of Kenya's Lake Turkana. Two-faced blades and other large cutting tools had been previously excavated there along with primitive stone flakes.
Using a sophisticated technique to date the dirt, researchers calculated the age of the more advanced tools to be 1.76 million years old. That's older than similar stone-age artifacts in Ethiopia and Tanzania estimated to be between 1.4 and 1.6 million years old.
This suggests that prehistoric humans were involved in refined tool-making that required a higher level of thinking much earlier than thought. Unlike the simplest stone tools made from bashing rocks together, the early humans who shaped these more distinct objects planned the design and then created them.
This "required a good deal of forethought as well as dexterity to manufacture," said paleoanthropologist Eric Delson at Lehman College in New York, who was not involved in the research.
Results of the study, led by Christopher Lepre of Rutgers University and Columbia University's Lamont-Doherty Earth Observatory, appear in Thursday's issue of the journal Nature.
The stone tools, known collectively as Acheulian tools, are believed to be the handiwork of the human ancestor Homo erectus. The teardrop-shaped axes were "like a stone-age Leatherman or Swiss Army knife," said New York University anthropologist Christian Tryon.
The axes were suited for butchering animals or chopping wood while the thicker picks were used for digging holes.
Homo erectus walked upright like modern humans, but possessed a flat skull, sloping forehead and a smaller brain. It emerged about 2 million years ago in Africa. Most researchers think Homo erectus was the first to fan out widely from Africa.
There's archaeological evidence that the first to leave carried only a simple toolkit. The earliest sites recovered in Asia and Europe contain pebble tools and flakes, but no sign of Acheulian technology like hand axes.
Why that is "remains an open question," said anthropologist Sally McBrearty of the University of Connecticut, who had no role in the research.
Theories abound. Some surmise that the early humans could not find the raw materials in their new settlement and lost the technology along the way. Others suggest they later returned to Africa where they developed hand axes.
NYU's Tryon, who was not part of the study, has a different thought. Perhaps the early populations who expanded out of Africa didn't need advanced technology because there was less competition.
Early humans were "behaviorally flexible" and making hand axes "was something that they did as needed and abandoned when not needed," Tryon said.
The latest work does little to settle the issue, but scientists now have identified the earliest known site in the world containing Acheulian tools.
Geologists collected about 150 samples of sediment from the site in 2007. To come up with an age, they used a technique known as paleomagnetic dating, which takes advantage of the flip-flop of Earth's magnetic field every several hundred thousand years.
The tools were not too far from where the bones of Turkana Boy — the most complete skeleton of a prehistoric human — were unearthed in 1984.