New research suggests that ancient humans made sophisticated stone tools much earlier than believed.

A team of scientists from the United States and France say they used a technique to date mudstone from a site near Kenya's Lake Turkana. They determined the age of ancient tools found there to be 1.76 million years old.

Previous dates for such artifacts were estimated to range from 1.4 million to 1.6 million years old.

The tools the scientists dated included hand axes, cleavers and picks that displayed signs of symmetry and advanced planning. They say unlike the simplest stone tools that early humans made from hitting rocks together, the refined tools they dated required planning and design.

The scientists say one mystery that surrounds the tools is that the ancient humans did not take their stone creations along with them when they left Africa. Archaeologists are not sure why the early humans, called Homo Erectus, lost the technology.

The study appears in Thursday's issue of the journal Nature.