Homo erectus … clues and counter clues

At www.eurekalert.org/pub_releases/2011-06/nyu-fsh062811.php there is a report from PLoS One (online journal) on Homo erectus in Indonesia, widely regarded as a human ancestor as they resemble Homo sapiens in a variety of ways. It is standard theory that Homo erectus also migrated Out of Africa – but some 1.8 million years ago. Now, we all read these dates but they are pretty meaningless except in a geological context, as geochronology has a pattern. Now, it is how deposits are assigned on the geochronological framework that matters – what method was used to place it in one position  rather than another. This usually requires dating the deposit in which, for example, a hominid tooth was found, and in that way great faith is placed in the position of said hominid in the passage of time. So it is with Homo erectus – and this press release gives some surprising insights in the dating methodology that is deemed reliable in one paper and not so in another, which favours a different methodology. Now, it seems that Homo erectus became extinct, it is thought (a hypothesis rather than a fact) in Africa and most of Asia by 500,000 years ago – an awfully long time past. Paradoxically, consensus theory was that it happened to survive in Indonesia to somewhere around 35,000 years ago – roughly the same time Neanderthals became extinct from Europe and western Asia. However, in most models of the spread of Homo sapiens it is said the new super race had reached Indonesia, and Australia, somewhat before that date. It is problematical. Now, a deposit that included fossil animals and some bits and pieces of Homo erectus have been subject to redating as anthropologists were not happy. At first it was suggested the Homo erectus remains were older than the animal remains in the deposit, the assumption being that the deposit took thousands of years to accumulate. Research, apparently driven by the Out of Africa consensus, has just found the deposit was laid down over a short time period – code for very quickly. However, two dating methods came up with two new sets of dates. Electron spin resonance yielded a date of 143,000 years ago – meaning Homo sapiens can be accommodated at any time after that date. In contrast, the argo-argon methodology was used to date pumice in the deposit and this came up with a date of 500,000 years ago – the same date they are assumed to have become extinct further afield. What is going on? The researchers suggest the pumice is an old geological deposit that has become mixed into a later deposit, and that 143,000 years ago is the preferred date. That is of course possible – it is also convenient. What method was used in the first instance, prior to the research, to obtain a date between 50 and 35,000 years ago? and why is that not reliable?