One hallmark of Homo erectus, a forerunner of modern humans, was his stone tools, an advanced technology reflecting a good deal of forethought and dexterity. Up to now, however, scientists have been unable to pin a firm date on the earliest known evidence of his stone tool-making.

A new geological study, being reported Thursday in the journal Nature, showed that tools from a site near Lake Turkana in Kenya were made about 1.76 million years ago, the earliest of their ilk found so far. Previous dates were estimates ranging from 1.4 million to 1.6 million years ago.

Although no erectus fossils were found with the Turkana tools, a skull of that species was excavated last year in the same sediment level across the lake. This suggests that Homo erectus was responsible for these particular tools, which were made with what scientists refer to as Acheulean technology. The term connotes the type of oval and pear-shaped hand axes and other implements that were a specialty of early humans.

American researchers at the Lamont-Doherty Earth Observatory, part of Columbia University, established the age of the Turkana tools by dating the surrounding mudstone with a paleomagnetic technique. When layers of silt and clay hardened into stone, this preserved the orientation of Earth’s magnetic field at the time, and an analysis of the periodic polarity reversals and other records yielded the age of the site known as Kokiselei.