A limestone hand axe found in a cave near Murcia, Spain, estimated to be 900,000 years old. (Michael Walker)

The first hand axes used by early humans in Europe are 400,000 years older than previously thought, geologists say, closing the gap with their first known appearance in Africa.

Scientists at the Berkeley Geochronology Center looked at two archaeological sites in southeastern Spain where hand axes were found. Their examination of the magnetic properties of the layers of rock at these sites suggests that the axes are 900,000 years old.

The oldest hand axes in Western Europe were previously thought to be only about half a million years old.

The appearance of large, shaped hand axes — most commonly made of flint, but also made of limestone and other rock — is a key development in the history of stone-age technology. In Africa, the first such tools have been found to be about 1.5 million years old.

This million-year gap between the first appearance of these tools in Africa and in Europe has puzzled scientists. This study closes that gap to 600,000 years.

The geologists found layers of rock at the sites near Murcia, Spain, where magnetic minerals were found to have a polarity opposite to what is now found. These polarity reversals are the result of natural changes in orientation of the Earth's magnetic field, which have occurred several times in the last few million years. The last reversal in the Earth's magnetic field took place 780,000 years ago.

Since these changes are global and well documented, geologists can date ancient artifacts using a method called paleomagnetism.

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