The final (?) word on those handaxes from Crete

While everybody was busy talking about unexpectedly old cutmarks and other Pleistocene goings-on last week, the paper by Strasser et al. (2010) describing the discovery of quartz handaxe assemblages on Crete quietly came out in Hesperia. This is a topic that was discussed at length on this blog, in several posts that generated a large amount of comments a few months back. The sticking point of all the arguments concerned the chronology of the handaxes, so without further ado, here's the money quote (Strasser et al. 2010: 185-186):

"The dating of the Palaeolithic in the Plakias region presents a considerable challenge, not least because of the long period of time that may have elapsed since the occupation of the earliest sites, during which postdepositional natural processes may have obscured the archaeological record. Additionally complicating the issue are the small number of sites, the lack of excavation, and the impact of modern development on the area, which has destroyed many sites.

Several approaches to dating were attempted, and our research on this topic continues. At Preveli 2, east of the Preveli Gorge, Palaeolithic artifacts are associated with a flight of marine terraces resulting from relatively high sea levels in the Pleistocene that were preserved by subsequent rock uplift. The lowest late Pleistocene marine terraces resulting from high stands of the sea at Preveli (14 ± 1 masl) and Schinaria (21 ± 1 masl) have 2-sigma calibrated radiocarbon ages of 45,400 ± 1,600 and 49,120 ± 2,890 years b.p., respectively, and are correlated with Marine Isotope Stages 3.3 and 3.4, both eustatic high stands. The higher terraces, at 59 and 96 masl, are unquestionably older. How much older? Assuming similar rates of rock uplift (1.4 ± 0.1 m/kyr) determined from the age-elevation relationships of the dated terraces at 14 and 21 masl, it is possible to estimate the approximate ages of the terraces associated with artifacts. This correlation provides an approximate age for the lithic artifacts. The higher terrace, at 96 masl, may belong to Marine Isotope Stage 5, possibly early 5e, ca. 110,000 b.p. Artifacts associated with the terrace at 59 masl could correlate with Marine Isotope Stage 5a, ca. 70,000 b.p. It should be stressed that these are rough approximations and these ages are probably minima that represent a terminus ante quem. If the uplift rate is changed, the terraces and the artifacts associated with them could be much older.

At Preveli 3, Preveli 7, Timeos Stavros 1, and Schinaria 5, Palaeolithic artifacts were found in outcrops of paleosols that exhibit the characteristics of the oldest maturity stage for such features, that is, Maturity Stage 6, or in geological terms, Marine Isotope Stage 6. Together these observations suggest an age of ca. 190,000–130,000 b.p. and serve as a terminus ante quem for the artifacts embedded within them. The stone tools were incorporated in the paleosols as part of a process described by Runnels and van Andel in Epirus: “the top of the Bt horizon itself would move gradually upward as a result of slow deposition, so engulfing any artifacts laid down on former land surfaces above it.” In other words, the Bt horizon, especially as much of the clay comes from eolian sources, will increase in thickness through time, slowly
engulfing clasts, such as stone tools, that were formerly in the A horizon.

In sum, the dating of the Palaeolithic sites is based on geological data derived from the study of marine terraces on the southwestern coast of Crete and our identification of paleosols, and these data place the Palaeolithic lithic artifacts firmly in the Pleistocene, ca. 130,000 b.p. or earlier. The chronology can be further refined, however, and a dating program
currently in progress may provide data for doing so." (references excised)

So, bottom line, the dating is largely indirect, but grounded by dated references points that provide a minimum age for the terraces where the handaxes were recovered. Interestingly, this indicates that these tools are, at most, 130,000 years old. Given that there do not appear to be more recent Paleolithic age (the rest of the implements reported in the paper are Mesolithic in age and techno-typology, which is an important discovery in and of itself), this implies that on current evidence, Crete was not occupied during the Late Pleistocene (ca. 130,000-10,000 BP). Why this was the case (and how an early colonization took place) is an interesting question that will need to be answered by future research.

In the meantime, here are a few take-away observations from the Strasser et al. (2010) report. First, on the basis of the drawing of the handaxes, these implements do appear to be human-made. Second, they are not isolated occurrences: the authors identified nine localities where these quartz tools were found, only three of which also yielded Mesolithic tools. This leaves open the possibility that the 'Paleolithic' sites represent task-specific components of the Mesolithic toolkit on Crete, but this is unlikely based on the association of handaxes with some of the terrace deposits described in the quote above. Third, as the authors indicate, this was not a case of a H. heidelbergensis (or a couple of them) washing onto Crete: the fact that nine sites (defined by the presence of a minimum of 20 stone tools) were found in a relatively small area indicates a somewhat sustained human presence on the southern coast of Crete. This does suggest that people got there purposefully (i.e., using some kind of watercraft), which leaves open the question of why Middle and Upper Paleolithic assemblages haven't (yet?) been found in the region or elsewhere on Crete (or any other large Mediterranean island, for that matter).

As a parting observation: one aspect of the discovery that definitely wasn't stressed in the media reports about these finds is how they were made. They were breathlessly reported as 'discovered' without much context. The authors actually set out to look for Mesolithic sites in southern Crete using a model developed for the Greek mainland that identified certain areas as having been most appealing for Mesolithic foragers. In other words, this wasn't a blind search for early stuff or simply a fortuitous discovery. Rather, it came about as the result of an explicit research design targeting some very specific questions. Good to see more of that in Paleolithic archaeology. 

References:

Strasser, T., Panagopoulou, E., Runnels, C., Murray, P., Thompson, N., Karkanas, P., McCoy, F., & Wegmann, K. (2010). Stone Age Seafaring in the Mediterranean: Evidence from the Plakias Region for Lower Palaeolithic and Mesolithic Habitation of Crete Hesperia, 79 (2), 145-190 DOI: 10.2972/hesp.79.2.145

Paleolithic radiocarbon legerdemain

Two fundamental but often underappreciated aspects of radiocarbon dating concern the ages it yields and the fact that these ages need to be calibrated in order to get an age that can be expressed in calendar years. I say these aspects are underappreciated because of the way radiocarbon age determinations are usually reported in news reports and, more rarely, in actual research papers.

First, age determinations. Radiocarbon dating is based on the observation that given 14C in previously living organisms decays at a constant, predictable rate. In this case, half of a sample's 14C decays in about 5730 years in exponential fashion. This means, that after 5730 years, 1/2 of the 14C of a previously living organism remain, 1/4 remains after 11,460 years, etc. The uncertainty or error range reported for all radiocarbon dates is due to imprecision in counting the radioactive decay of carbon atoms in a sample, and it is a critical component of the date. What the error range indicates is a 66% chance that the age of a sample falls within the interval it brackets. Double the error range, and the resulting interval is 95% likely to include it. What is important to note is that raw radiocarbon age ranges are centered on the date that is statistically most likely to be the correct one for a dated sample. In that sense, it is somewhat warranted to use the date as a shorthand to discuss how old a sample is. That is, for a bone point dated to 13,400 +/- 100BP (these dates are expressed as before present, with the present assumed to be 1950), it is technically OK to say that it is a 13,400 year-old point, since that age is the most likely to be correct within the interval defined by the error range.

The problem, however, is that radiocarbon years don't correspond to calendar years, and usually underestimate the true age range of any given sample. This is because the concentration of atmospheric radiocarbon has not been constant over time. However, this problem can be corrected through the use of calibration curves based on the radiocarbon dating of samples of known age and extrapolated from the discrepancy between the two ages. Samples whose calendar age can be determined include historical artifacts, as well as organic remains that grow or accumulate in yearly increments, such as trees (that accumulated a new grwoth ring yearly) or corals.

Until recently, reliable calibration curves only stretched back to ca. 24,000 years BP, but recent developments have extended the range of calibration curves past 40,000 years BP, although some debate remains about some of the finer details of these more extensive curves. Regardless, from the perspective of paleoanthropology and especially that research focused on the timing of the disappearance of the Neanderthals, this has been a real boon, since it allows researchers to finally discuss this process in the chronology of the rest of recent human prehistory. For the transition interval (i.e., the period 30-40ky BP), the discrepancy between radiocarbon and calendar ages has been argued to be on the order of 5000 years, meaning that a radiocarbon date of, say 35,000 BP translates into a calendar (or calibrated) age of about 40,000BP.

This precision is good, but in my view, it's been abused somewhat on two levels. First, people wanting to emphasize how old a given object or associated assemblage is now systematically use calibrated ages. The reverse, however, is not true and the age of unexpectedly recent finds such as the late-lasting Mousterian assemblages of Gibraltar routinely continue to be presented in radiocarbon ages (i.e., ca. 28,000 BP, as opposed to say 32-33,000 cal. BP). I supposed this practice makes sense from a PR perspective, but it certainly muddles arguments about prehistoric chronology, especially for that (large) segment of the public that doesn't understand the subtleties of radiocarbon dating. It also creates a gap between earlier research that published radiocarbon ages and current papers that use calibrated ages or, worse, a combination of both.

The second problem is that most researchers and science journalists continue yo present and discuss calibrated ages in terms of their central tendencies when this is absolutely unwarranted. This is due to the fact that calibration curves are based on irregularities in atmospheric radiocarbon concentrations at various times in the past. This means that the smooth curve centered on a given age that is obtained by radiocarbon dating turns into a curve often best described as a hair-raising rollercoaster. Here's an example from the OxCal web site to illustrate what I mean:

This plot shows how the radiocarbon measurement 3000+-30BP would be calibrated. The left-hand axis shows radiocarbon concentration expressed in years `before present' and the bottom axis shows calendar years (derived from the tree ring data). The pair of blue curves show the radiocarbon measurements on the tree rings (plus and minus one standard deviation) and the red curve on the left indicates the radiocarbon concentration in the sample. The grey histogram shows possible ages for the sample (the higher the histogram the more likely that age is).

This means that while the calibrated range corresponds to that of the original radiocarbon date, the mid-point of that range is not necessary the most likely one. This therefore means that, unlike for raw radiocarbon dates, it is often unwarranted to use the midpoint of a calibrated radiocarbon age range as shorthand for the most likely calibrated age of that sample.

I've simplified this discussion some for the sake of clarity (and kept it reference-free for the same reason), but it should now be clear why it really grinds my gears to see calibrated ages tossed around uncritically in the literature and, especially, in newsreports that discussed recent research that has an important chronometric dimension to it.

Two sides to every biface

It's been an interesting couple of weeks for people interested in handaxes. On one hand, there was the discovery of honking big handaxes in the Lake Makgadikgadi Basin (Botswana), which is currently dry. On the other, there was the report that the age for the oldest handaxes in Europe needs to be pushed back by as much as 300,000 years, based on new chronological and paleomagnetic information (Scott and Gibert 2009). In a way, these two discoveries are related more intimately than might seem to be the case.

First, the new ages for the Spanish handaxes have several implications, some of which are aptly discussed by John Hawks. The first is that how refined some handaxes are is not necessarily a good indicator of their overall age. That is, biface morphology doesn't simply gradually go from coarse to fine over time, and biface morphology is influenced by many factors and essentially reflect use considerations at the end of an individual handaxe's use-life (McPherron 2000). This was a well-established fact before this new study, but these dates underscore that lack of correlation especially well. A second implication is that the Acheulean (yup, that's how you spell it!) is therefore likely to be much older than previously assumed. The general consensus has been for some years that this industry first appeared in Europe around 600kya (cf. Monnier 2006). The age of 900kya for an Acheulean assemblage in Spain thus pushes back that date of first appearance by several hundred thousand years.

What is more, unless you accept that hominins using Acheulean tools came to Spain directly from Africa (across the Strait of Gibraltar?), this age implies that the Acheulean in more eastern parts of Europe must be even older, though hard evidence of this is currently lacking. The earliest Acheulean site outside of Africa is 'Ubediya, in the Jordan Valley, dating to ca. 1.4mya. Assuming a single origin for Acheulean technology, this would mean that the amount of time it took handaxes to diffuse across the European mainland is effectively cut almost in half and now stands at a maximum of about 500,000 years, a long time to be sure, but much less than the previously accepted almost million year interval. This has some important implications in constraining models of early hominin dispersion in Europe and how that relates to the subsequent development of Neanderthals (e.g., Hublin 2009).

And this would make sense, really, given the usefulness of handaxes as a technological innovation. The thing about handaxes is that they are generally described as unchanging over their 1.6my history, although this impression is based on morphology alone and doesn't really reflect the state of thinking among most scholars involved in Lower Paleolithic research. In a nutshell, handaxes were highly polyvalent from a functional perspective and not putting individual occurrences in proper context is what results in this mistaken impression of stasis (Machin 2009, Nowell and Chang 2009). A contextualized approach to handaxe variability is what allows archaeologists to seize on the richness and diversity of Acheulean behavior (Hosfield 2008). It is also what allows us to make sense of outliers like the Lake Makgadikgadi specimens.



By any standard, at 30+cm in length these things are frikkin' huge! Strikingly, the press release only mentions that these very large items were found, without any discussion of how their size is unusual and what this distinctiveness might mean. These specific artifacts are of uncertain age, and their function is also uncertain - at that size, it is unclear exactly what practical function they might have served, as they would have been rather unwieldy to use, unless they were somehow hafted, in which case their heft might be an indication of their ultimate function. Most people tend to assume handaxes were made and used as stand-alone hand-held tools. This lithic-centric view has led to some conjecture that the skill manifest in handaxe manufacture might have served as a form of 'advertisement' to potential mates by especially technically proficient knappers (e.g., Kohn and Mithen 2009). This has been challenged on both theoretical and practical grounds, most eloquently by Nowell and Chang (2008) who detail how such a model cannot, in fact, be argued to be founded on evolutionary theory as commonly defined.

Machin (2009:35-36) argues persuasively that handaxes morphology cannot be understood by reference to single-cause explanations since "variability is caused by the differing motivations and constraints – ecological, physiological, biological, cognitive and social – which act upon the individual agent at any given point in time." The sheer timespan and geographical distribution of handaxes certainly agrees with her - it's unlikely that handaxes served the same function in all contexts in which they are found. In a way, handaxes are perhaps best understood as an especially useful and versatile technological innovation that allowed them to be if not all things to all (pre-)people at least many things to many (pre-)people.

Getting back to the Spanish handaxes described by Scott and Gibert, this raises some interesting questions. The first among these is why, given their recognized usefulness, such implements would be so scarce when they are first documented in the record - at Estrecho del Quípar (the site dating to 900kya), there is only one handaxe in the assemblage, and based on its morphology (Fig. S4: flake scars are present on both sides of the piece, but is not very extensive at all toward the center of either face) some analysts might consider it a core or bifacially flaked cobble instead of a proper handaxe. To be fair, the authors refer to other studies that show that handaxes are not very frequent in most Acheulean assemblages (i.e., Monnier 2006), and they also describe a contemporary Spanish assemblage that lacks handaxes altogether to explain why and absence or low frequency of bifaces is not necessarily a problem to labeling the assemblage as Acheulean. However, this begs the question of what an Acheulean assemblage actually is if not one that contains handaxes, a question that Gilliane Monnier has addressed in great detail, concluding that

It is time for a comprehensive revision of the Lower/Middle Paleolithic periodization based upon a synthesis of multiple aspects of the archaeological record, including climate, subsistence, landscape use, mobility and exchange, symbol use, cognition, and biological evolution, in order to determine whether we should maintain a two-phase system [Lower vs. Middle Paleolithic] and, if so, how it should be defined. (Monnier 2006: 729)

If that's the case, what can we really say about the oldest appearance of the Acheulean without an in-depth consideration of these complementary - and necessary - lines of evidence instead of only focusing on the presence of large bifacial artifacts?

References:

Hosfield, R. Stability or Flexibility? Handaxes and Hominins in the Lower Paleolithic. In Time and Change: Archaeological and Anthropological Perspectives on the Long-Term in Hunter-Gatherer Societies (D. Papagianni, R. Layton and H. Maschner, eds.), pp. 15-36. Oxbow Books, Oxford.

Hublin, J.J. 2009. The Origins of Neandertals. PNAS 106:16022-16027.

Machin, A. 2008. Why Bifaces Just Aren't That Sexy: A Response to Kohn and Mithen (1999). Antiquity 82: 761-769.

Machin, A. 2009. The role of the individual agent in Acheulean biface variability: A multi-factorial model. Journal of Social Archaeology 9: 35-58.

McPherron, S.P. 2000. Handaxes as a Measure of the Mental Capabilities of Early Hominids. Journal of Archaeological Science 27:655-663.

Monnier, G. 2006. The Lower/Middle Paleolithic Periodization in Western Europe: An Evaluation. Current Anthropology 47:709-744.

Nowell, A., and M.L. Chang. 2009. The Case Against Sexual Selection as an Explanation of Handaxe Morphology. PaleoAnthropology 2009: 77-88.

Scott G.R., and S. Gibert. 2009. The oldest hand-axes in Europe. Nature 461:82-85.

Asking a "lady" her age

The 'Red Lady of Paviland' is a Gravettian burial originally found in 1823 Goat's Hole Cave at Paviland (UK), and which was recently returned to the National Museum of Wales from Oxford University where it had been kept since its discovery. The body is actually that of a young male and was found covered in red ochre as well as "with a number of artefacts including ivory wands, bracelets and periwinkle shells."

It was originally thought to date to about 18 kya, before more recent assays established it was 25-26 kya. Well, it turns out that Tom Higham and his team have determined it is, in fact, some 4000 years older, or "just over 29 kya".

Beyond the general 'older is better' paleoanthropological cachet of this new report, there are some interesting implications drawn from this new age:

"It would mean The Red Lady lived in an age when the climate was much warmer than it would have been 4,000 years later.

Dr Higham added: "The data that we have got now is making a lot more sense."

He said it was important for "our understanding of the presence and behaviour of humans in this part of the world at this time".

He also said it "might" suggest that the custom of burying people with artefacts originated in western Europe rather than eastern Europe as had previously been thought.

"This raises new questions about the way in which these people spread and lived on the continent," he added."

I don't know what else they're basing the claim for a Western European origin of burials, but a single burial is not much to go on for such an interpretation. I'm sure there'll be more about this in the write-up of the analysis, which should be published in the Journal of Human Evolution early in 2008 (the corrected proof wasn't available when I checked today).

An artist's rendition of the burial ceremony in the Gravettian
( http://museums.ncl.ac.uk/flint/archrit.html).