A revised chronology for Liang Bua places Homo floresiensis earlier than 60,000 years ago

Thomas Sutikna and colleagues report a significant revision to the stratigraphy of Liang Bua cave, which changes the geological age estimates attributed to the fossil and archaeological evidence of Homo floresiensis: “Revised stratigraphy and chronology for Homo floresiensis at Liang Bua in Indonesia”. Earlier work had placed many fossils attributed to H. floresiensis at geological ages younger than 20,000 years, with a last occurrence sometime between 11,000 and 13,000 years ago. Now, the new study shows that all fossil evidence of H. floresiensis is older than 60,000 years ago.

The paper effectively retracts a series of earlier dating results, including the chronologies in key papers by Morwood and colleagues (2004) and Roberts and colleagues (2009). Quoted in a Nature news story by Ewen Callaway, Richard Roberts shared some poignant thoughts about the initial work:

Roberts says that the peculiar geology of Liang Bua would have been hard to notice when the first hobbit bones were found on the final days of the 2003 field season. “Do I think we could have done a better job? Not with what we knew at the time,” he says. “We’re 10 years down the road, and we know a lot more and we’ve excavated a lot more.”

I totally accept that explanation. Cave sites are nearly always complex in some areas of their stratigraphy, and the chronology of a site will change with new information. We have seen redating and revision of stratigraphy in caves again and again in paleoanthropology. This is a normal aspect of the science and even large changes in the chronology have plenty of precedent when we look at the history of the field.

I decided to take a very close look at how the previous papers by Morwood and colleagues (2004) and Richards and colleagues (2009) went wrong, and to what extent other conclusions might be altered by the new chronology. Understanding these mistakes should help us to avoid making similar mistakes in the future. The current paper by Sutikna and colleagues (2016) goes a long way toward reducing inconsistencies in the current chronology of the site. But the new chronology in the paper now contradicts many of the conclusions of earlier papers describing the archaeological and faunal context of Homo floresiensis. Reading the new work carefully and comparing to earlier papers, I don’t understand how to resolve many of the earlier conclusions with the current chronology.

This post is a long one, with several main conclusions:

1. Although the earlier work by Morwood and colleagues (2004) and Roberts and colleagues (2009) did not recognize a large unconformity with a hiatus between the layer containing LB1 and overlying layers, they did recognize and plot the sloping stratigraphy. The unconformity itself would likely not have led to an incorrect chronology if the previous work had used a better radiocarbon sampling scheme.
2. Radiocarbon sampling errors were not obvious in these earlier papers because they depicted their sample locations incorrectly in the published site profiles. Morwood et al. (2004) shows that they obtained radiocarbon samples from immediately within the area of the LB1 skeleton, while Roberts et al. (2009) report the same samples in a different location, and new samples at a lower depth than LB1 and within the same stratigraphic layer. According to the new chronology and site profile from Sutikna and colleagues (2016), both those earlier reports incorrectly depicted the stratigraphic positions of their radiocarbon samples.
3. Both earlier papers reported other dating results that were inconsistent with the radiocarbon chronology, but nonetheless argued that these results were consistent because of weaknesses in the data. As a result, although on the surface the papers seemed to rely upon multiple methods of dating, the entire chronology was actually pinned by the radiocarbon sample locations. It appeared much more solid than it really was.
4. The new work does not adequately clarify several issues about the Liang Bua chronology, including the last occurrence date of Stegodon in the site, the status of the association of Homo floresiensis with heat-treated stone tools, and the composition of the post-H. floresiensis Pleistocene archaeological assemblage.
5. Anthropologists routinely overestimate the reliability of geological age estimates, particularly those provided with the initial descriptions of a fossil assemblage. This is a problem for understanding human evolution, and in communicating the science with the public.

The details

In the original pair of publications on these fossils in 2004, Morwood and colleagues reported on finds from two distinct areas of the cave: one near the wall and one near the center of the chamber. The LB1 skeleton and a number of other specimens come from the area near the east wall of the cave, and Morwood and colleagues established their position in the chronology by doing radiocarbon testing of charcoal samples, two of which they reported to have come from immediately adjacent to the fossil hominin skeleton. These radiocarbon samples led to the conclusion that the LB1 skeleton was approximately 18,000 years old, and other samples from elsewhere in the excavation showed that some Stegodon remains and archaeological material that Morwood and colleagues attributed to H. floresiensis was as recent as 12,000 years old. A later review and addition of new dates by Roberts and colleagues (2009) did not change the chronology of either area, and introduced additional dates that all confirmed the original findings.

There are several distinct issues of chronology, only some of which are addressed in the new paper:

1. The chronology of the Sector VII excavation unit that produced the LB1 skeleton and other hominin material, with the initial date for LB1 estimated at 18,000 years old. Later excavation reported by Morwood and colleagues (2009) expanded this excavation area by adding the neighboring Sector XI. The new paper rejects this chronology and provides a new one in which all H. floresiensis fossils are older than 60,000 years.
2. The oldest occurrence date of skeletal material attributable to Homo floresiensis, which Morwood and colleagues (2004) reported from Sector IV in the center of the front chamber at between 74,000 and 95,000 years old. The new paper changes the relevance of this material but is ultimately unclear about the chronology within Sector IV.
3. The oldest occurrence of archaeological material in the cave, which Westaway and colleagues (2007) reported at around 190,000 years old. The new paper repeats this and reports no new data to change it.
4. The latest occurrence of Stegodon and other extinct fauna, which Morwood and colleagues reported at around 12,000 years old. Morwood and colleagues suggested that Homo floresiensis persisted until this time. The new paper is unclear about how the the faunal and archaeological associations of H. floresiensis are changed by the new chronology.

Sector VII situation

Sutikna and colleagues focus most closely upon the excavation area near the east wall of the cave, including Sectors VII and IX reported by Morwood and colleagues (2004; 2009), and later Sectors XIII, XIV, XV and XVI. This area of the deposit includes three different sequences separated by complex surfaces within the excavation areas. The H. floresiensis fossil material comes from a lower sedimentary sequence, which has a sloping and eroded top surface, forming a “pedestal” of sedimentary layers in the excavation area. The new work establishes that the top of this lower sequence has an age older than around 46,000 years. The H. floresiensis fossil remains lie below this date and Sutikna and colleagues identify three datable tuffs and a series of TL and uranium-thorium age determinations that place the fossils older than around 60,000 years.

Above the pedestal surface, after a substantial period of time including erosion of the pedestal layers, a series of well-stratified, highly sloping layers was deposited. These include several datable layers. This sequence is the original source for the charcoal that was radiocarbon dated by Morwood and colleagues (2004). According to Sutikna and colleagues (2016), these layers have nothing to do with the Homo floresiensis fossils. Above these strata, after a hiatus marked by flowstone, are Holocene deposits from the last 5000 years, including modern human skeletal remains.

After reading this new description of the situation, I expected to go back to the original publication by Morwood and colleagues and find all this stratigraphic detail missing. Instead, I was surprised to discover that they recognized essentially the same things. here is the site profile published by Morwood et al. (2004):

These are the four walls, north, east, south and west, of the Sector VII excavation square. Morwood and colleagues clearly depicted the sloping layers that are identified by Sutikna and colleagues (2016). On the north wall of the sector, those overlying layers are clearly shown at the same depth as the skeleton on the south wall. The unconformity is not there, but the sloping stratigraphy is clearly depicted.

Looking at that site profile, any reader of that paper—including me—would reasonably think that the radiocarbon samples came from amid the skeletal remains of LB1, within the same layer. You can see the “6” and “7” radiocarbon samples pictured there directly within the “skeleton location”, indicated with a dotted line. It doesn’t get much clearer than that. The text refers to the two radiocarbon samples as “associated with the skeleton”. Further, they indicate that they took a luminescence sample (labeled “42”) from immediately adjacent to the skeleton, also within the same layer.

Sutikna and colleagues are saying that this was wrong. The radiocarbon samples were not even taken from within the same layer as the skeleton. If you look at the site profile from the paper from Sutikna and colleagues above, you can see that the sample locations are completely different from the Morwood et al. (2004) profile. The unconformity did not make them different, the original paper apparently just described the locations completely wrong.

The resulting chronological inconsistency might have been caught, if Morwood and colleagues had used the luminescence sample as an independent line of evidence. The luminescence sample from the location labeled “42” in the diagram came back with a TL date of 35±4 kyr, and an IRSL date of 6.8±0.8 kyr. The higher TL date seems inconsistent with it having been taken from the overlying layers that generated the much more recent radiocarbon ages, while the lower IRSL date seems completely out of reason. Neither of them accords well with the new chronology of Sutikna and colleagues, but either might have raised a red flag in the initial publication.

Morwood and colleagues (2004) ended their discussion of the luminescence dating as follows:

The TL and IRSL ages bracket the time of deposition of the hominin-bearing sediments to between 35±4 and 14±2 kyr, which is consistent with the ¹⁴C ages centred on 18 kyr.

It was hard for me to understand that conclusion when I read it, but I worked through the text carefully. Instead of seeing these two discrepant dates, with non-overlapping error bars, as inconsistent, they used the physics of the process to argue that they in fact could be used as maximum and minimum age estimates. Therefore, they argued that the TL and IRSL estimates, each different from the radiocarbon date by large amount, were actually an upper and lower bracket that confirmed the radiocarbon age.

Hence an inconsistency became a consistency, and a chronology pinned on only a single piece of information instead appeared to rely upon two independent sources of information. This is a basic error that reinforced the incorrect description of sampling locations.

The work by Morwood et al. (2004) was followed with additional excavation in an adjacent area, Sector XI, and additional dating by multiple methods, all reported by Roberts et al. (2009). Here is the profile diagram for Sectors VII and XI from that paper:

This figure differs from the earlier profile because of the addition of Sector XI, which doubles the length of the east and west walls of the excavation area. The position of the LB1 skeleton is given by the star.

You can see that the radiocarbon sample locations are reported differently in this profile. In this diagram, the radiocarbon samples are “+” signs labeled with the date in kyr. The original two dates from Morwood et al. (2004) are indicated there as the 18.1 and 18.5 kyr dates. These two samples now appear to lie within the sloping stratigraphy instead of within the area of the skeleton itself. They are at the same depth as LB1 but in an overlying layer. Now two newly-reported radiocarbon samples with calibrated date estimates of 19.1 and 19.2 kyr appear to be in the same level as LB1, but at a lower depth. The circle here is the same luminescence sample reported in Morwood et al. (2004) with its two mutually inconsistent dates.

Roberts and colleagues (2009) did not note this difference in the locations of the radiocarbon samples, now in different stratigraphic positions from those reported in Morwood et al. (2004). However, they did report the date of LB1 differently:

They claimed here that the radiocarbon samples actually bracket the LB1 skeleton, and that despite the sloping stratigraphy there is an association of radiocarbon age with depth. They even provide a figure demonstrating this association:

This is as clear a figure as you will ever see showing how easy it is to mislead yourself with statistics.

Roberts et al. (2009) applied this figure to show the regular relationship of calibrated radiocarbon age is with depth in all sectors of the Liang Bua excavation. The figure shows that there is no inversion of dates from the expected increase with depth. Such a figure does lend some confidence in the lack of mixture or turbation of the sediments.

However, here the problem was the sampling scheme. The stratigraphy was complex and sloping. Yet all the radiocarbon dates seem to have been selected within a single vertical transect through the layers. That strategy guaranteed that significant lateral variation in the stratigraphy, including possible unconformities, would be missed.

Roberts and colleagues’ diagram of the Sector VII/XI excavation area recognizes the sloping stratigraphy and underlying pedestal. Their notes on this diagram reflect the differences that are evident between the Sector IV and Sector VII/XI stratigraphic situations. Nonetheless they were still persuaded by the regular relationship of ¹⁴C ages and depth within their single vertical transect through the deposit. They claimed that this was strong evidence that LB1 is between 19.8 and 15.9 kyr.

Again, they failed to note the strong inconsistency between different dating methods. Again in this case, the TL dates continued to show much older dates than the radiocarbon, and Roberts and colleagues argued that the TL dates should be considered as “maximum” and the much younger IRSL estimates as “minimum” estimates for the same samples. These thereby defined a range of error from Holocene to 40,000 years ago for some samples, encompassing all the radiocarbon date estimates. Additionally, Roberts and colleagues (2009) had ESR dates on Stegodon enamel, which likewise were systematically older than the radiocarbon chronology, even under the early uptake model which resulted in lower date estimates than the linear uptake model. No Stegodon specimen subjected to direct ESR dating by Roberts and colleagues (2009) had a date anywhere near as young as the 12,000 years that Morwood and colleagues (2004) had proposed as the last occurrence date for Stegodon in the site. Morwood and colleagues (2005) had even claimed that “a well-defined occupation floor” produced H. floresiensis remains together with Stegodon material dating to 18,000 years ago, a claim repeated and supported by Van den Burgh et al. (2009). The ESR determinations should minimally have raised some question about this.

The point is not that the ESR dates were accurate, because from today’s perspective that is likewise questionable. The point is that two sources of evidence cannot be considered independent if one of them is never applied to question or challenge the other.

I don’t think any of these errors were malicious or intentional, but it is important to take account of how papers on geological dating and stratigraphy can go very wrong. These kinds of errors are not rare, as I’ll discuss below; they are common. Understanding them is much like being a crash investigator; you have to understand which parts of the system failed to do better in the future.

I’m satisfied at the description of the situation that Sutikna and colleagues have now provided for Sector VII and the adjacent newer excavation area. The new paper states that the earlier paper by Morwood and colleagues is in error, which for me ends the questions. They provide direct dates on the hominin fossils, and their more intensive sampling has allowed them to construct a much more complete chronology of the deposit and its contents, which has no major inconsistencies.

Sector IV

The new study reports minimal change to the chronology of material from the center of the cave, which Morwood and colleagues (2004) had put between 95,000 and 74,000 years ago. This is the source of the LB2 specimen, for which Sutikna and colleagues provide ²³⁴U/²³⁰Th age estimates of 71.4 ± 1.1 and 66.7 ± 0.8 kyr. This fossil is within the same range of ages as the Homo floresiensis fossil remains from the area near the east wall of the cave.

However, as I discuss below, there are some serious remaining questions about the Sector IV chronology that Sutikna and colleagues do not address. The sector is a black box in this paper, no profile or additional context is provided, although there are some new date results including the date on the LB2 ulna. From this sector came the majority of evidence of artifacts and Stegodonreported by Morwood and colleagues (2004). Those Stegodonremains generated ESR dates that seem to be inconsistent with the chronology reported in the new paper by Sutikna and colleagues. But I cannot tell from the information provided what exactly is going on with this part of the excavation area.

One hundred ninety thousand years

The oldest evidence of hominin activity in Liang Bua is much older than the H. floresiensis fossil remains. The paper concludes:

The new stratigraphic and chronological evidence for Liang Bua indicates that a pedestal of remnant deposits, dating to more than ~46 kyr cal. BP, has an erosional upper surface that slopes steeply downwards to the north and is unconformably overlain by sediments younger than ~20 kyr cal. BP. All skeletal remains assigned to H. floresiensisare from the pedestal deposits dated to approximately 100–60 kyr ago, while stone artefacts reasonably attributable to this species range from about 190 kyr to 50 kyr in age. Parts of southeast Asia may have been inhabited by Denisovans or other hominins during this period, and modern humans had reached Australia by 50 kyr ago. But whether H. floresiensis survived after this time, or encountered modern humans, Denisovans or other hominin species on Flores or elsewhere, remain open questions that future discoveries may help to answer.

The reference to the stone artifacts as early as 190,000 years ago does not emerge from any new analyses in this paper. This confused me at first, but I followed the authors’ citation to the work of Westaway et al. (2007). That paper reported on an examination of the sedimentary layers exposed in the rear chamber of Liang Bua, as part of the “conglomerate cliff”. They showed that a basal conglomerate layer containing stone artifacts likely was the earliest sedimentary deposit now present in the cave, with a date approximately 190,000 years ago. This is in an area that was not re-evaluated by Sutikna and colleagues.

Aspects that remain unresolved by the new work

The new paper focuses most of its attention on the geological age of the Homo floresiensis remains. While this is an important issue, it is far from the only important issue. The paper does not provide enough information to evaluate several issues that seem to depend closely on the chronology of the site. These include:

1. The last occurrence dates of Stegodon and other extinct fauna.
2. Whether any of the H. floresiensis-bearing levels contain evidence of fire or application of heat to artifacts.
3. How much of the archaeological material formerly “associated” chronologically with H. floresiensis is actually from much later deposits.

If Morwood and colleagues (2004) really were in serious error about the stratigraphy of Liang Bua, all of these issues must be examined, but the new paper is either silent or does not provide a clear answer on them.

Stegodon

Morwood and colleagues (2004) put the last occurrence of Stegodon in the deposit at approximately 13,000-11,000 years ago. The new paper clearly states that this date must be discarded because the associated radiocarbon samples are from the sloping layers that overlie the pedestal of sediment containing Homo floresiensis.

Further, the new paper by Sutikna and colleagues conducted direct U-series dating on seven specimens of Stegodon, finding all of them to exceed 40,000 years ago, and indeed, they suggest that the latest dates here are likely in error because they underlie sediments that have been dated to older dates by other methods:

The Stegodon bone samples (all from Sector XI) span a modelled age range of 80.6 ± 11.3 to 40.5 ± 2.0 kyr, with the youngest minimum age deriving from a bone (U-s-05/LB/XI/51/04) recovered from the same sediments and depth as LB6. Delayed diffusion of uranium into the dense bone matrix of Stegodon may account for the youngest minimum ages appearing more recent than those obtained for the bones of H. floresiensis, the speleothems and the sediments.

The paper later refers to the layers between T2 and T3, which contain Stegodon as well as extinct giant maribou stork fossils, and for which they provide a geological age between 66,000 and 49,000 years ago.

While the paper says all these things, it nowhere gives a clear last occurrence date for Stegodon. The paper provides direct dates only for Stegodon material taken from the new excavation area in Sector XI, and does not revisit material from the original excavation. The new paper seems to leave completely unresolved whether any Stegodon material was recovered at higher levels in the cave.

Morwood et al. (2004) clearly state that Stegodon material was recovered from higher in the deposit, “immediately below the ‘white’ tuffaceous silts derived from volcanic eruptions that coincide with the extinction of this species.” These correspond with the tuffs T7 and T8 in Sutikna et al. (2016), and they provide a date for charcoal just beneath T7 of 12.7 kyr.

Is this the last occurrence date of Stegodon? Or did Morwood and colleagues (2004) also err in their placement of Stegodon material just below T7 and T8? This is no small difference; there’s a vertical difference of more than a meter and up to two meters.

As I read the new paper, the latest date that they provide in association with any Stegodon material is 40,500 years from ²³⁴U/²³⁰Th, and the paper argues that date is too young, and the Stegodon samples should in fact be older than the speleothem and sediment ages that all exceed 58,000 years ago.

The last occurrence dates of Stegodon and other extinct fauna are pretty central to the question of when modern humans arrived on Flores and what their subsistence activity may have been like. Also, the Stegodon remains were central to Morwood and colleagues’ (2004) argument about a “big game” hunting kit that they associated with H. floresiensis.

On the other hand, the new paper is completely silent about the ESR/U-series dates on Stegodon enamel carried out by Roberts et al. (2009). Most of these come from Sector IV of the cave. Only two specimens are reported with coupled ESR/U-series dates, both of which are vastly older than either the early uptake (EU) or late uptake (LU) models for other specimens. The EU-LU dates from the other teeth each cover a very wide range of dates. Several of the teeth have date ranges between 20,000 and 40,000 years ago, which Roberts and colleagues (2009) saw as unremarkable, considering that the last occurrence was apparently 13,000-11,000 years ago. Now these dates are remarkable. Sutikna and colleagues (2016) refer only to one of these Stegodon ESR dates, with the coupled ESR/U-series date which is much older. They also reiterate that this tooth was found a meter below the LB2 specimen, which now has a direct U-series date of more than 66.7±0.8 kyr. So it looks like the Stegodon should be older than the ESR results from Roberts et al. (2009), although Sutikna and colleagues are silent on this question.

Is 58,000 the last occurrence date of Stegodon in the cave? Is it 40,500? Is it 12,700? Should we use the ESR/U-series dates from Roberts et al. (2009)? I don’t have a clue. I wish the paper had been clear about this important issue.

Fire

Morwood and colleagues (2004) put H. floresiensis in direct association with charcoal, which they used for radiocarbon dating. That obviously raised the prospect that H. floresiensis controlled and used fire. This is exactly what Morwood and colleagues (2004) wrote:

Concerning the behavioural context of H. floresiensis, associated small faunal remains include those of fish, frog, snake, tortoise, varanids, birds, rodents and bats. Many are likely to have accumulated through natural processes, but some bones are charred, which is unlikely to have occurred naturally on a bare cave floor.

In a later publication, Morwood and colleagues (2005) reported on burned stones and a “circular arrangement” of burnt stones.

Use of fire by hominins is indicated by charred bone and clusters of reddened and fire-cracked rocks. These include a cluster of three burnt, water-rolled, volcanic pebbles from Spit 84 (840 plusminus 5 cm depth) in Sector VII, and a circular arrangement of five similarly burnt pebbles from Spit 43 (435±5 cm depth) in Sector XI.

Van den Burgh and colleagues (2009) presented a thorough study of the faunal collection to that point, identifying some burned bone elements within the Holocene assemblage, but noting none within the Pleistocene deposits that contained H. floresiensis. That made it appear that Morwood and colleagues (2004) were incorrect to associate burned bone with the H. floresiensis layers.

Brumm and colleagues (2006) further emphasized that Homo floresiensis at Liang Bua co-occurs with heat-treated artifacts. Again, it looked like H. floresiensis must have been intentionally making fires, cooking, and heat-treating stone for tool manufacture.

Moore and colleagues (2009) investigated heat-treating along with other technological elements in the Liang Bua assemblage. They found that heat-treating was more significant in the Holocene levels, with burned and heat-fractured fragments making up 18% of the assemblage. By contrast, these are only 1% of the Pleistocene assemblage. Still, that is 29 fragments of burned or heat-fractured stone in apparent association with H. floresiensis, according to Moore et al. (2009).

Roberts and colleagues (2009) attempted TL dating on one of the burnt pebbles that Morwood et al. (2005) reported from Sector VII, which they suggest as evidence of fire use by H. floresiensis:

The TL-U and IRSL ages of >200 ka for sample LBS7-44, from close to the base of Sector VII, indicate that this burnt pebble was not heated to a sufficiently high temperature to empty the relevant electron traps in the near-surface quartz or K-feldspar grains. At the present time, therefore, evidence for the use of fire by ,em>Homo floresiensis</em> – preserved as charred bone and as clusters of fire-cracked and reddened rocks (Morwood et al., 2005) – is stratigraphically bracketed by TL-B and 14C ages of between ~41 ka and 20 ka.

Are these still in association with H. floresiensis now? Or were they part of the charcoal-bearing deposits that were sampled by Morwood et al. (2004). This is totally unclear from the new chronology. Likewise, it is unclear whether any burned faunal material derives from the same layers as H. floresiensis fossils.

Again, I wish the new paper had been clear about this. Much of the archaeological evidence uncovered by Morwood et al. (2004) came from Sector IV, and Sutikna and colleagues do not change the chronology of this sector substantially. But some of the burned artifacts apparently come from the Sector VII and XI area that has been massively revised. It’s totally unclear from the earlier papers, including Moore and colleagues (2009), who just lumped all early H. floresiensis-associated material into a single stratigraphic unit (Unit 4).

Even if the conclusion is that we cannot be sure about the association of burned remains from the original 2001-2004 excavations, that is important information to share.

Material culture

Sutikna and colleagues conclude with the following statement about the archaeological situation:

All skeletal remains assigned to H. floresiensis are from the pedestal deposits dated to approximately 100–60 kyr ago, while stone artefacts reasonably attributable to this species range from about 190 kyr to 50 kyr in age.

Here is my problem with that statement: Much of the archaeological material “associated” with H. floresiensis has been associated on the basis of a stratigraphic picture that is now clearly wrong. All earlier publications “associated” H. floresiensis with 18,000-year-old charcoal. It may not be so easy to work out from the excavation records which archaeological and faunal material actually belongs to the pedestal sediments, because earlier publications have mostly reported that material in terms of the 10-cm excavation spits instead of stratigraphic layers.

What this means it that some of the archaeological material formerly associated with H. floresiensis on the basis of the incorrect chronology was actually produced by modern humans within the last 20,000 years.

How much? We don’t know.

Sutikna and colleagues do report that the archaeological material from the top of the pedestal deposits, which predate 50,000 years, can be distinguished from the Holocene material on the basis of raw material selection—Holocene modern humans used a much higher proportion of chert. On this difference, they base an argument for the possible persistence of H. floresiensis into these layers between T2 and T3 despite the lack of fossil evidence. But they are silent on the issue of the later Pleistocene deposits—the ones that contain the 18,000-year-old to 11,000-year-old charcoal that were incorrectly associated with H. floresiensis.

The fallible science of geological age estimation

Dates aren’t set in stone, they are part of science, and subject to change as we discover more and better information. Especially with any new discoveries, we should be cautious about assuming anything about their age. The history of the field shows that a geological age estimate is the one thing most likely to turn out wrong in the future!

Ironically, geological age is the one thing that anthropologists seem to assume is the most solid. They often assume that a team’s interpretation of the anatomy of a fossil is just one interpretation, while the geological age stands external to the interpretive process.

When we look at the history of the field, that assumption is clearly wrong. Again and again, important specimens have been published with geological age assessments that were later shown to be off by huge amounts. Rarely have I seen a paper describing a geological age estimate that showed any kind of doubt or hesitancy at all. Most of them read like Morwood and colleagues’ (2004) paper, totally convincing in showing the reasonableness of their approach and the resulting age estimate.

Yet like any other area of science, geological age estimates are sometimes wrong.

Again and again, anthropological teams have relied upon incorrect geological age estimates to inform their interpretation of the anatomy. By relying too heavily on a date, they make mistakes of interpretation that could be avoided by separating the anatomical description from the geological age.

In many cases such over-reliance on geological age has caused them to overemphasize “derived” or “advanced” features of fossil remains, so that they can demonstrate the “first occurrence” of such features. In other cases, anthropologists have overemphasized the “primitive” traits of a fossil to argue for the impossibility of it being an ancestor of other forms at the same geological age or earlier.

The resulting errors of interpretation can be subtle and hard to root out. It is quite true that a later fossil cannot be literally the genealogical ancestor of an earlier one. But this puts rather a lot of stock in an accurate age for the earlier fossil specimen. But the later fossil may well represent a species that existed much earlier in time: first appearance dates are not equivalent to speciation times.

I don’t think we should ignore dates—they are very important for some purposes. But we have to be careful to distinguish the issues of phylogeny, function, and pathology that must be resolved by anatomy alone, and not allow our beliefs about the age of the fossils to bias those analyses.

What about Homo floresiensis?

There seems to be a widespread assumption that radiocarbon dates are the most reliable. The Liang Bua case is just one of many recent cases to show that radiocarbon dates are not infallible. In this case the error emerged from a poor sampling scheme in which samples were selected with insufficient attention to the complexity of the stratigraphic situation.

Look again at the figure from Roberts and colleagues (2009), showing the association of radiocarbon age with depth:

It seemed almost as regular as a natural law, and the LB1 skeleton was clearly bracketed between 15,000 and 20,000 years.

This is such a valuable example showing that anthropologists are very poor judges of which geological data are reliable. When people showed skepticism about the dating of Liang Bua, it was never the radiocarbon chronology they questioned, it was the oldest occurrences of hominin fossils in Sector IV, which seemed so poorly defined by the wide TL and U-series maximum and minimum ages. Yet these ages now appear to be reliable, while the highly accurate-seeming radiocarbon ages turned out to be wrong, because they were poorly sampled with respect to the stratigraphy.

Many other cases have faced similar issues, in which the radiocarbon dates may be correct but their association with fossil hominins or artifacts is questionable. In other recent cases, the problem has been with the radiocarbon methods, as improved ultrafiltration methods have revealed many errors in previous dates obtained with earlier methods. This is true even for some sites dated within the last ten years, as radiocarbon labs presently rely upon different protocols for preparing material.

Most critically, considering that even within the last ten years there have been huge innovations in radiocarbon sample protocols, we should anticipate that further innovation may continue to change methods and dating results in the future.

Science is not really self-correcting; we have to correct it. This means we need to rigorously challenge our own intrinsic biases. As long as we keep testing hypotheses by collecting new data and revisiting old data, we make progress toward identifying errors in previous results. The new understanding of the stratigraphy of Liang Bua is just one step in this process, and we should expect that the geological age of these fossils will continue to be refined. Indeed, the most current result may itself turn out to be wrong, and we’ll need to change ideas again. Stranger things have happened before. Much stranger.

We need to do better informing our colleagues and the public about the process of this science.

References

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