A study of the magnetic properties of an archaeological site in North China reveals human occupation far earlier than previously thought.
A team of scientists say they have uncovered evidence of early humans in China dating back at least 1.6 million years, the oldest signs of early humans in North China.
In a paper published in the scholarly journal Scientific Reports, Chinese Academy of Sciences geologist Hong Ao and his team determined that tools and other artifacts found at the Shangshazui Paleolithic site in China's Nihewan Basin were deposited there between 1.6 and million 1.7 million years ago. Previously, the artifacts were thought to be 1 million years old.
"[The site] represents the oldest evidence of early human occupation in North China," writes Dr. Ao, in an email interview.
Determining the ages of stone artifacts in North China is tricky; unlike the famed Olduvai gorge in Tanzania, rocks there don't contain volcanic materials suitable for radiometric dating.
So instead, the team used a technique known as paleomagnetism.
Since the late 1920s, when Japanese geologist Motonori Matuyama examined basalt rocks from different layers of earth in Japan. He noticed that the magnetic polarity of some of the rocks was reversed, all on rocks dating to the early Pleistocene or older.
Over time, geologists came to accept that the magnetic North Pole and South Pole occasionally trades places, at what appear to be random time intervals. Each reversal takes between 1,000 to 10,000 years to complete. The last known reversal occurred about 780,000 years ago.
These magnetic reversals leave their imprints in some rocks. Just as a rod of iron can be magnetized by heating until it is red hot and then plunging in cold water, the magnetic particles within igneous rocks heated inside the Earth's crust will align with the earth's magnetic field, which ever way it happens to be pointing, and they will remain in that orientation after the rocks cool.
To determine the polarity of the rocks at Shangshazui, Ao and his team took 738 samples of earth from the site, carefully recording their orientations with a compass. They first heated the samples to remove the magnetic "overprint" in the rocks created by the North Pole's current orientation. They then heated the samples again, this time to higher temperatures, to tease out the earlier reversals.