Science Daily: Human Ancestor Older Than Previously Thought.
The team applied two different dating techniques to the sites. Like earlier work, they used the techniques -- U-series and Electron Spin Resonance, or ESR -- that are applied to fossilized teeth. They also used a technique called argon-argon dating that is applied to volcanic minerals in the sediments. All three methods use radioactive decay in different ways to assess age and all yielded robust and methodologically valid results, but the ages were inconsistent with one another.
The argon-argon results yielded highly precise ages of about 550,000 years old on pumices -- very light, porous volcanic products found at Ngandong and Jigar.
"Pumices are hard to rework without breaking them, and these ages are quite good, so this suggests that the hominins and fauna are this old as well," said project geochronologist Carl Swisher of Rutgers University.
By contrast, the oldest of the U-series and ESR ages, which were conducted at Australian National University by Rainer Grün, are just 143,000 years.
The difference in the ages means that one of the systems is providing an age for something other than the formation of the sites and fossils in them. One possibility is that the pumices are, in fact, reworked, or mixed in, from older rocks. The other possibility is that the ESR and U-series ages are dating an event that occurred after the sites were formed, perhaps a change in the way groundwater moved through the sites.
Either way, the ages provide a maximum and a minimum for the sites -- and both of these ages are older than the earliest Homo sapiens fossils in Indonesia. Thus, the authors concluded that the idea of a population of Homo erectus surviving until late in time in Indonesia and potentially interacting with Homo sapiens seems to have been disproven.