The Nature commentary by Rex Dalton says
Hand axes from southern Spain have been dated to nearly a million years old, suggesting that advanced Stone Age tools were present in Europe far earlier than was previously believed.
Acheulian axes, which date to at least 1.5 million years ago, have been found in Africa, and similar tools at least 700,000 years old have been found in Israel and China. But in Europe, sophisticated tool-making was thought to stretch back only around 500,000 years.
Cave sediment levels that included the two axes also held what some archaeologists believe may be small tools made using the so-called Levallois technique of shaping stone, known to have existed in Europe only about 300,000 years ago.
Homo neanderthalensis, Homo erectus and Homo heidelbergensis are all species known to be associated with Acheulian axes, which have two-sided cutting faces that were made of many types of stone for still-unconfirmed uses.
The Iberian axes were found at two sites dated to at least 760,000 and 900,000 years old, respectively. Gary Scott and Luis Gibert of the Berkeley Geochronology Center in California dated the sites using palaeomagnetic analysis, which uses known changes in the orientation of Earth's magnetic field over time.
The Solana del Zamborino and Estrecho del Quípar caves in the valley, where the axes were found, were first thought to be only about 200,000 years old.
But after dates of stone flakes at a nearby location indicated they were much older, Gibert, now a postdoctoral researcher at the Berkeley centre, and Scott homed in on the caves' rich sediments.
In addition to the palaeomagnetic technique, Gibert notes that a record in rock layers of the remains of micro-mammals such as rodents, developed by Walker's team at Estrecho del Quípar, was crucial in confirming the dates.
The type of archaoemagnetic dating that yours truly does relies on the phenomenon of secular variation, or continuous changes of up to several tens of degrees in the direction of the Earth's magnetic field. These changes can be recorded by heated features and artifacts, such as hearths. Scott and Gibert's work relies on magnetic reversals, which occur every few hundred thousand years, recorded by the sediments in which the tools are embedded.