Let us begin by noting that “Europe” is an arbitrarily defined geographical unit occupied for the last few hundred years or so by people who believe that “Europe” is the Center of the Universe. Therefore, this statement: “The earliest hominin occupation of Europe is one of the most debated topics in palaeoanthropology” is more about European Ego than it is about human prehistory.
Nonetheless, there is an interesting find … of a hominid mandible … reported in the current issue of Nature that relates to human prehistory in the region of Western Eurasia.
The mandible is associated with a “Mode 1” lithic industry. This means that the stone tools found in the same stratigraphic unit as the mandible are relatively undifferentiated and probably of the type produced by the earliest European hominids. Often, Mod1 one industries in Africa are pre-Acheulean, somtimes called Oldowan, and tend to be close to two million years old or older. Mode 1 industries occur in much of Eurasia over a much broader, and later, period of time.
The mandible is also found in association with the bones of animals that show evidence, according to the report, of modification by humans. The bones were not made into things (tools or ornaments) but rather, show evidence of butchery of the animals with the use of stone tools.
This find comes from the famous localityof Atapuerca, in particular, the site Sima del Elefante, in Spain. Atapuerca has produced some of the most important early human fossil remains in Europea. This particular mandible is from Level TE9.
There is not a direct radiometric date for this unit, however, several different dating techniques have been applied, including paleomagnetism, cosmogenic nuclides, and biostratigraphy. The sum of these methods (discussed in more detail below) indicates a date of about 1.1 to 1.2 million years ago. Assuming the date holds up, this would indeed be the earliest human bone in Europe.
The authors also claim that this find along with other material from other levels suggests a speciation event in this region. Although the authors are somewhat circumspect about this, they seem to be indicating that this is where (and when?) the human-Neanderthal split may have occurred.
The cave deposits in which this was found are extensive, about 18 by 15 meters in horizontal extent, and cohtaining 16 distinct strata.
Here is a photo of the mandible, part of Figure 2 in the paper:
It may seem difficult to associate such as small bit of bone with a particular type of hominid, let alone even identifying it as a human or human ancestor. But this bone (the anterior mandible) is chock full of diagnostic features, and quite a bit can be said about it. Many features pretty clearly link this fossil with “early Homo” and a smaller number of features provisionally link this bone with the particular hominid found previously at this site, known as Homo antecessor.
The animal bones provide clues to the date. The way this works is as follows: Over time, numerous studies of several different species show that a certain series of changes occurred at roughly the same time for each species (a tooth getting longer, for instance). Meanwhile, certain species are found to appear (because of migration or evolution) or disappear (because of climate change or extinction) at specific times. So, if a sedimentary deposit has a number of different animals represented in it, but no other way of dating it, it is possible to estimate a likely range of dates using the presence/absence and various metrics of these animals (usually mammals).
There is a little carnivore that links to a site dated in Italy at 1.4 mya. There is a rodent that shows up on sites dated to around 0.6 – 1.0 million years old. And so on. The fauna suggest a very broad date range, therfore, ranging from about a half million years ago to a million and a half years ago.
Paleomagnetic dating relies on the fact that tiny iron-bearing particles orient themselves to the Earth’s magnetic field under certain conditions. The Earth’s magnetic field changes orientation a little all the time, and totally reverses now and then. If you have a period of several hundred thousand years, you can then divide that into paleomag periods. The sediments where this mandible were found, as well as sediments above and below, have been studied in detail. This has been linked to studies of the mammal bones for various levels to produce a reasonable dated stratigraphy of these deposits. Paleomagnetic evidence strongly suggests that the upper part of layer TE16 dates to 0.78 million years ago.
Unfortunately, the paleomagnetics in combination with faunal dates are much more vague than one would ideally like, and it is possible that any of the layers above or below this mandible, and the layer in which it is found, could be of a fairly long time range. So, an additional technique was applied.
This technique, using the radioactive decay of a cosmogenic isotope of Aluminum (Aluminum-26). This isotope is formed with the interaction of cosmic rays and dust, slowly settles on the earth in minute quantities, and then proceeds to decay at a known rate. This technique may be affected by contamination of material moving through sediments, improper collecting, etc. but it can work. In the case of this site, two dates were obtained, one a few layers below the mandible, in layer TE7, dating to 1.13 Million years ago, and another in the layer the mandible was found it, dating to 1.22 Million years ago. Both have standard errors sufficient to not worry to much about this apparent reversal in age, but to suggest that all of the lower layers at this site are about 1.1 to 1.2-something million years old.
In my view, the chronology of this site needs further work, and there are probably many opportunities for this to happen. However, the overall age of this site and the specific ages of most of the layers is probably reasonably well understood, and I’m fully prepared to accept that this mandible and associated animal bones, and the tools, are somewhat over a million years old.
Carbonell, E. Et Al. The first hominin of Europe. Letter to Nature. Nature 452, 465-469 (27 March 2008) | doi:10.1038/nature06815; Received 15 October 2007; Accepted 4 February 2008