Early Pleistocene

I am pleased to announce a new open science initiative, focused on a discovery that is unique in paleoanthropology. Together we are going to find out if the Malapa site has preserved evidence of soft tissue from an ancient hominin species.

If you've arrived at this page from outside the site, here's a link to the main project headquarters.

In the August, 2011 issue, National Geographic reported on the Malapa fossils, including a teaser that the site may preserve skin from two hominin individuals. (I pointed to the article last month.)

The suggestion is obviously surprising. Many readers will remember how much controversy surrounded claims about soft tissue preservation from dinosaurs several years ago. Yet extraordinary preservation contexts do exist in the fossil record. Indeed, a few years ago Lee Berger's team, including several of the people now working on the Malapa hominins, identified hair preserved inside hyena coprolites from Gladysvale cave, more than 200,000 years old and only a short distance from Malapa [1].

Could Malapa present the first evidence of soft tissue from a fossil hominin? If so, what can it tell us about human evolution?

The day the National Geographic article was published online, I was standing with Lee in his lab looking at what might be australopithecine skin. I'm not talking about an imprint of skin, like a skin cast. These appear to be thinly layered, possibly mineralized tissue.

Suppose it's really skin, or some other soft tissue, I thought. How would you go about testing the hypothesis? Extraordinary claims require extraordinary evidence. Even if you could demonstrate it to your own satisfaction, what would it take to convince the doubters? How many distinct observations would be possible from these objects? What instruments would you use, and what comparative samples would you need?

Lee said this was his problem as well. He has access to some of the most sophisticated technology in the world. Some kinds of observations are obvious. He can micro-CT the apparent soft tissue evidence, look within the rock at its structure. He can sample the chemical content, and use scanning and confocal microscopes to examine it. He could sacrifice a small sample to be microscopically dissected. At the end, he would have an answer involving all these comparisons. But would it be convincing?

Lee then made an inspired proposal: What if the process itself were an experiment?

Much of the criticism of other surprising fossil discoveries has been fueled by their secrecy. Science done by a closed process means fewer eyes looking at data, and too many chances for errors to pass unnoticed. Unnoticed, that is, until publication. Then, a firestorm of controversy may erupt as the scientific community at last examines the methods and results closely. In anthropology, the most critical errors are often missed comparisons -- sometimes simple things that a research team could have looked at, if they had only thought of it.

An open process has the chance of improving research by broadening it. We want stronger, clearer results, and we want to anticipate every important criticism. If a significant comparison can be added by people who have the right tools, why not get those people involved? If we stand a chance of finding those people by making the process more open, why not do it?

Lee suggested that this soft tissue evidence could be the basis of a true experiment in whether paleoanthropology could be done as open science. I've been agitating about open science for years, and I volunteered right away to host the experiment and work to make it a success. We went immediately to Rachelle Keeling, the graduate student who will be coordinating the project, and described how we thought it could work. She was enthusiastic about the idea of a truly new kind of scientific project, one that had the potential to involve so many people in the process of discovery.

And so, after a month of putting things into order, here we are. How can you participate in the project, or at least follow its progress?

I have set up a home page for the project, here as a special category page on the blog. This page is the online headquarters of the work, and includes a feed that will have all project updates. As the project proceeds, it will generate suggestions, results, and press. I'll be tracking all of these and updating as we learn more.

The project has an official e-mail address hosted here: skin@johnhawks.net. We want to hear from anyone with the expertise or ideas to solve this problem. Rachelle and I will be reading through the e-mails, discussing them with other project members, and following up on them.

We don't know what to expect but I hope we get hundreds of responses. We can't promise replies to anyone, but everyone will receive an automatic acknowledgement that we've received their messages, and we will follow up personally with those that have suggestions or proposals we can take action on. We're going to ask people to participate in the project, perform research, and coauthor the scientific work: this is real open science.

Members of the Malapa team are biologists who know comparative skin and hair biology. I'll be posting quite a lot about these biological topics for people following the project.

We know that there are many researchers who have been working with methods that would be useful on these unique samples of possible soft tissue. People working with the trace chemistry of organic compounds in mineral samples, people working with the microscopic structure of other ancient soft tissue samples, people who study preservation of organic materials in forensic contexts. There are many others that I don't even know I should be listing.

If you know a person with the right expertise to help, please share this information and encourage her to write.

Most important to the success of the project is showing that we can produce top quality science by this open process. That means we need journals to acknowledge the value of open science instead of penalizing it for not being secret and embargoed. If you're a journal editor reading this, I'm calling you out. And if you're a reviewer or editorial board member, you can support this project and encourage more like it by encouraging the submission of open manuscripts.

And if you don't have a suggestion right now, keep watching. This project will develop and I expect it to become more interesting as it becomes broader. I can't predict how it will end, and that's pretty exciting!

References

I've been ranting on Twitter all day about the new paper on the "earliest Acheulean" by Christopher Lepre and colleagues [1], published in Nature today. The first time I read through the paper, I really thought they'd miffed it. I mean, really, they published a paper on the earliest Acheulean artifacts without putting a picture of them in the paper.

What actually bothered me more was the lack of any discussion at all about why the assemblage is Acheulean as opposed to, say, Developed Oldowan. The word Oldowan appears only in the context of saying that many localities within the same Kokiselei site complex have only Oldowan-typical assemblages. This started bothering me less as I ran through the citations to earlier work on the Kokilelei localities. But that raised another point of irritation: This Acheulean locality was briefly described already, a long time ago. Why is this news? And given that both descriptions are so superficial, where's the fuller account?

I had to stop and think about why I was finding this all so irritating. I mean, it's a paper about dating an archaeological locality. It's a perfectly good paper about dating an archaeological locality, full of details about the local geology, methods of sampling and analysis. My reactions weren't a criticism of the paper, really -- although if you're going to write a high-profile paper about your site, maybe you should actually feature the archaeology of the site?

I've been digging through references all afternoon, trying to get straight exactly why this paper doesn't mention the Developed Oldowan at all. I'm not saying I favor the Developed Oldowan -- just that we deserve some kind of thoughtful review of what constitutes an "earliest Acheulean" site. Is it a purely typological definition based on the presence of bifaces made on large flakes, or is there something more here? That's going to take me a bit longer to review, so I'll just report on some of what I found.

This isn't news. Hélène Roche and colleagues reported on this locality in 2003, in Comptes Rendus [2], including a date range between 1.79 and 1.65 million years ago. They describe it as "without doubt, one of the oldest Acheulean assemblages in Africa." That's right, if you can read French, you're eight years ahead of Nature.

This paper adds precision to the earlier estimate, and it's really important to do this well. But if you've been reading about the archaeology of Plio-Pleistocene Africa, finding a date of 1.76 million years for this locality with an Acheulean assemblage is totally expected.

Roche and colleagues [2] provided only a short description of the KS4 assemblage. Even so, it's more than provided in the current paper by Lepre and colleagues [1]. Here is what the current paper includes about the assemblage:

The KS4 assemblage (Supplementary Fig. 2) is characterized by the presence of pick-like tools with a trihedral or quadrangular section, unifacially or bifacially shaped crude hand-axes, and a few cores and flakes, all derived from the same mudstone bed. A single subsurface, in situ origin for KS4 is ensured by excavations at the main test trench that recovered several spectacular sets of refitted lithic artefacts (Supplementary Fig. 3). To the exception of a few cores made on basalt, the rest of the assemblage has been knapped from large cobbles or tabular clasts of locally available aphiric phonolite.

The supplementary information does include photos of three bifacial artifacts and two refits. But there is no technical analysis of the artifacts beyond the paragraph above. There's not even a summary of the number of artifacts found at the site.

Roche and colleagues added more details (my translation of the French):

Kokiselei 4 is a highly eroded site in which a series of more or less extensive trenches (total 19 m²) were dug. Among these only one (KS4A) yielded in situ artifacts in sufficient numbers to form an archaeological horizon, with a vertical dispersion limited to only fifteen centimeters, and no faunal remains. Some objects, distributed in a more diffuse fashion, were found in two other test pits (KS4B and KS4C); these are lower in elevation than the main horizon. In parallel to the test pits, a systematic surface collection across 104 m² (metric grid) was performed, which comprises the total sample of lithic material from KS4 (n = 167). It is characterized by robust, rough pieces of varying sizes, often very large, some scrapers and notches made on cobbles or flakes, by very large cores, by proto-bifaces or bifaces, and by picks with a trihedral section. Two thirds of the proto-bifaces or bifaces are manufactured on oblong pebbles, relatively flat, some quite large, whole or broken into two in the middle according to the major axis and very few retouched. Only a few are free of cortex and / or shaped enough to be called bifaces, the proto-bifaces in turn are made more coarsely, as if the concept of an elongated shape and sharp point was well integrated, but the operating scheme was inadequately implemented. All the tools characterizing a very early Acheulian are present, and it is to this cultural period that we attribute KS4.

Roche and colleagues also described the other localities, all Oldowan, at a similar superficial level of detail. The conclusion that Acheulean and Oldowan were two industries overlapping at the same time in this area was suggested in that paper.

That, obviously, leads to the real scientific story here. How could there be two different stone tool traditions overlapping across some fairly large area for more than 300,000 years? If we count Developed Oldowan, that makes three. Some people would count two Developed Oldowans A and B!

I'm inclined to think that the scenario is false. These really aren't distinct cultural traditions. Archaeologists have created definitions of archaeological assemblages, and the definitions have changed over time. Initially the definitions were entirely typological -- you have a handaxe, you've got Acheulean. Over time, the definitions have become less typological and more inclusive of technical elements -- you make bifacial artifacts on very large flakes, you've got Acheulean. But these technical categories are not unique or necessarily difficult to invent, and may have been repeatedly invented in different groups, just in the way that different groups of chimpanzees have invented nutcracking and termite fishing methods. For these early assemblages, we don't have any way of telling who made what -- the only hominin fossils from Kokilelei, for example, are teeth of A. boisei. We don't know how many different kinds of hominins there were. Maybe there was only one.

Early Homo is a bundle of mysteries, in other words, and the archaeology doesn't help. Can we make any sense of the development of early stone tool technology, from its initial beginnings to the handaxe-dominated assemblages? What does it mean that both Oldowan-like and Acheulean-like industries dispersed widely throughout the Old World? This is a really interesting scientific problem, involving information transfer, emergent sets of behaviors, invention and creativity, and their effects on survival.

The paper by Lepre and colleagues discusses the problem of Oldowan and Acheulean coexistence briefly, reviewing the idea that Homo erectus may be tied to Acheulean, leaving open the question of whether more than one toolmaking species existed before 1.5 million years ago. The paper is noncommittal, but I would frame the question very differently. It's self-evident that Acheulean cannot have been a culture, because no human or animal culture exhibits its spatial and temporal properties -- appearing episodically across three continents over a span of 1.5 million years. The real question is whether we can make sense of the many different Acheuleans, and whether other Oldowans (possibly Developed Oldowans) might have similar heterogeneity. Asking whether an Oldowan-bearing population in Africa first dispersed to Dmanisi is begging the question.

Finding these answers is surely a lot more interesting than what the press has done with this article.

That's probably what irritates the the most about this: how boring the article and reporting seem to make this topic. When I did the Google News search this afternoon, there are no fewer than 165 news articles worldwide. Nature made its cover image this week a photo of one of the bifaces. You can't get much more of a press push than that for an archaeology story. None of the stories go beyond the very simple "oldest Acheulean" story. Now, I'm used to seeing the "oldest X" storyline a lot in paleoanthropology, it's a perennial favorite of journalists who can't think of anything more interesting to write. But in this case, it's the worst angle -- because it's the part that isn't actually news!

References

Shanti Pappu and colleagues [1] report on date estimates resulting from new excavations at the old site of Attarampakkam, India. The news element is that they date an Acheulean occurrence to as old as 1.5-1.6 million years ago. At the oldest, these dates would make the Acheulean in India equal in age to the earliest occurrences in Africa.

The dates themselves depend on the decay of cosmogenic nuclides in the artifacts themselves. This is a kind of exposure dating -- as the artifacts are exposed to cosmic rays at the Earth's surface, they build up radioactive isotopes of beryllium and aluminum (¹⁰Be and ²⁶Al), which have half-lifes of 1.39 million and 717,000 years, respectively. When they are buried deep underground, their exposure to cosmic rays stops, and the radioactive isotopes can only decay. Then the ratio of the two isotopes in the sample reflects the time since deep burial. But like other exposure methods, in practice this depends on a model of exposure time, burial speed, and radioactivity within the soil, which lends substantial uncertainty to the dates. The lower 95% confidence interval of each of the date estimates reported in the paper is still over a million years, leading to the minimal conclusion that the site is that age or older.

Robin Dennell has written an accompanying short essay that gives a broader view of the Acheulian in South Asia [2]. The essay includes a great paragraph summarizing the now-obsolete idea that Acheulean reached India only a half million years ago:

How does this new evidence affect our understanding of the South Asian Acheulian? Previously, the general consensus was that the Indian Acheulian was less than 0.6 to 0.5 Ma (5) and was thus much younger than that in the Levant (eastern Mediterranean). There, the earliest dates of 1.4 Ma, from ‘Ubeidiya in Israel, probably indicate a dispersal of hominins from Africa (6). A second influx of African immigrants is indicated by the discovery of African types of cleavers and hand axes at Gesher Benot Ya'aqov (GBY), in Israel, dated to 0.78 Ma (7). This evidence implied that the Acheulian dispersed eastward toward South Asia only several hundred millennia after it first appeared in the Levant. It also implied that the spread of Acheulian bifacial technologies into South Asia was broadly contemporaneous with its first appearance in Europe, where the earliest sites date from ∼0.5 to 0.6 Ma (8). Some have attributed this expansion of the Acheulian into South Asia and Europe to Homo heidelbergensis. This Middle Pleistocene type of hominin is known mostly from Europe, where it was first defined, but is also recognized by some (but not all) researchers at African sites such as Bodo, Ethiopia, and Kabwe, Zambia, and even at some sites in China (9).

The "Homo heidelbergensis" model is in such utter disarray right now, I'm not sure many paleoanthropologists have realized the full extent of the problems. You should know that I don't believe in Homo heidelbergensis, never have. A couple of months ago, I was discussing some of the issues about mutation rate estimation with a very prominent geneticist, and the conversation turned to Homo heidelbergensis. What a shock the Denisova sequence should have been to those itching to see a H. heidelbergensis incursion into Asia!

Notice however, the intrinsic nuttiness of archaeological interpretation. Oh, we have the first evidence for Acheulean in India around 600,000 years ago? Well, that's around the same age as the Bodo fossil from Ethiopia! What a coincidence! Maybe this new kind of hominin expanded from Africa and carried the Acheulean to India! And Sima de los Huesos is around 600,000 years old, too -- and there's a handax in the pit! My gosh, we need a name for those hominins!

Well, the nice thing about a hypothesis built on mere coincidence, is that it only takes one observation to falsify it. Million-year-old handaxes in India ought to do it, and how. That's the message of Dennell's essay, and the subtext of the paper by Pappu and colleagues. What I find interesting is the extent to which the fact was hinted by earlier discoveries in South Asia but hampered by weaknesses in stratigraphic control and dating. From Pappu and colleagues:

Sparse radiometric ages from sites in India have situated the Acheulian within the Middle Pleistocene, with a few dates suggesting an early Middle to Early Pleistocene age. However, these ages often exceed the limits of confidence of the methods used (2). They include an electron spin resonance (ESR) mean age of 1.27 ± 0.17 Ma, assuming linear U uptake, on two herbivore teeth from Isampur (23); an ESR age of ~0.8 Ma (lacking uncertainty envelopes) on calcrete from the Amarpura formation, Rajasthan (24), which has been correlated with the Acheulian site of Singi Talav (4); dates ranging from ~1.4 to 0.67 Ma for the tephra at Bori (Kukdi river) (25); and paleomagnetic measurements with evidence of reversals at the sites of Bori, Morgaon, Gandhigram, Andora, and Nevasa (26). However, the reliability of these ages has, in each case, been questioned on various grounds (5, 27, 28). Likewise, the age and stratigraphic position of artifacts and faunal remains from the Early Pleistocene Dhansi formation along the river Narmada are yet to be firmly established (29). Based on data from controlled excavations and two independent dating methods, our ages from Attirampakkam show that the Acheulian in India is older than previously thought. Evidence from other sites in South Asia should be reconsidered and redated.

Much evidence already exists in the South Asian Acheulean that could be more accessible. The Acheulean in the region has been a long block of undifferentiated time, despite some very well-resolved sites. In addition to this much older dating for early Acheulean, India also has some of the youngest Acheulean assemblages anywhere -- for example, Haslam and colleagues [3] earlier this month reported on an Acheulean assemblage from around 130,000 years ago in northeastern India. That's long after the large biface tradition begins to give way to Middle Paleolithic and MSA toolkits in Europe and Africa.

On the topic of Denisova, Haslam and colleagues were writing before that genome was reported. But they did know about the Neandertal genetic results, including the evidence of Neandertal ancestry within India. Nevertheless, they assert a scenario in which the makers of earlier and later Acheulean in South Asia are the same biological population, without substantial gene flow from regions to the west, including the Neandertals.

Recent reports of the draft Neanderthal genome suggest that Neanderthals and H. sapiens likely did interbreed successfully soon after the latter had left Africa (Green et al., 2010), with the probable location of such contact to the west of India, in the Middle East. The southern limit of the Neanderthal range is unknown (Dennell and Roebroeks, 2005), but we emphasise that the continuity seen in the Middle Pleistocene South Asian technological record suggests that taxa derived from earlier hominin dispersals, and not Neanderthals, were the creators of the Indian Late Acheulean. Greater biological separation between dispersing humans and resident Indian hominins may have precluded viable genetic mixing (although see Liu et al., 2010 for an alternate view from East Asia), while similarities in certain technological strategies may have rendered cultural exchange a somewhat more likely occurrence.

Well, the Denisovans didn't have to live in India when the ancestors of Melanesians ran across them and intermarried. But Denisova and the Neandertal genomes now make it very likely that the inhabitants of South Asia were one or the other. And even if South Asians were yet a third group, as yet unattested from genomes, it is no longer credible to suppose that they were isolated from Europe or Africa for a million years previous. The tools just don't have that much to do with the populations.

References

Yesterday the Journal of Human Evolution released a new paper by Rhonda Graves and colleagues, titled “Just how strapping was KNM–WT 15000?” [1]. The paper challenges almost 25-year-old estimates for the body size of this important 1.5 million year old skeleton.

For all this time, the textbooks have reported that early Homo in Africa had the same tall and elongated physique as current East African people like the Maasai. The new paper says that the textbooks are wrong -- the skeleton doesn't represent an individual who would have grown to be 6'1" (185 cm), instead it was near the end of its growth trajectory, for an adult height of around 5'4" (163 cm).

That's a pretty massive change, and when the authors presented this work at the AAPA meetings last spring, it wasn't without controversy. So naturally we should look closely at the paper, understand its conclusions, and assess what this new estimate means for our understanding of early Homo. As you might guess from reading some of my earlier posts, I've been thinking that the body sizes of the rest of the Pleistocene record add up to a fairly simple picture. One of the few outliers from this picture was KNM-WT 15000. I'm inclined to think that the new estimate fits the bigger picture -- for example, I wrote this spring about "Shrinking erectus".

Which means, of course, that I should be even more skeptical.

KNM-WT 15000 was a juvenile at the time of death, and so any estimate of body size involves some assessment of the skeleton's state of development. This has presented a problem for assessing how much the individual had still to grow at the time of its death. The eruption and development of the teeth appear to be consistent with a fairly young age at death, by most estimates younger than 11 years, and by some as young as eight. That's using a human frame of reference. If we turn to a frame of reference drawn from chimpanzees or other apes, the estimated age at death from tooth development is even a bit younger. In contrast, the state of bone development seems to indicate a somewhat higher age at death: older than 11, and by some estimates as old as 15 years.

Graves and colleagues, looking at this apparent mismatch between dental and skeletal development in this specimen, suggests that we need to look at a broader range of possible developmental models for early Homo erectus. A modern human developmental model is not a good fit, and neither is an ape developmental model. So their study involves creating a range of possible developmental trajectories for early Homo. These trajectories are based on data from living apes and humans, but altered by accelerating some phases or changing the intensity of the adolescent growth spurt.

The growth spurt is very important to this issue, because it's one way that humans and most other primates differ greatly. Growth during that phase of development contributes disproportionately to the tall stature of modern humans. If Homo erectus didn't have the same kind of growth spurt as we do, then the stature of this specimen would have been a lot shorter than we would estimate for a human of the same age.

The section of Graves and colleagues' discussion that covers the adolescent growth spurt is, to my mind, the central issue in the paper. Their review begins with a survey of literature on why a growth spurt exists. Most assume that there is some kind of trade-off between early weaning in humans, brain growth, and a large adult body size–with the optimal solution being slow juvenile somatic growth, fast juvenile brain growth, and they “catch up” of somatic growth during adolescence. Graves and colleagues assert that this pattern was not present in early Homo erectus, and that a more chimpanzee like growth spurt may be a better model.

The velocity growth curves for human stature and chimpanzee total body length (summed length of crown-rump, femur, and tibia) highlight the difference between modern human and chimpanzee growth and development (Fig. 1). Both species exhibit growth spurts, but these spurts differ in rate, timing, and duration (Leigh, 1996). Pre-pubertal growth spurts in mass have been documented in many primates ([Tanner, 1962], [Laird, 1967], [Timiras and Valcana, 1972], [Leigh, 1996], [Leigh and Shea, 1996] and [Hamada et al., 1996]), but to date only slight increases in crown-rump length and total body length have been observed in chimpanzees (Hamada and Udono, 2002). Male chimpanzees (and possibly macaques) undergo a small growth spurt in length during the period between emergence of the first and third molars ([Watts and Gavan, 1982] and [Tanner et al., 1990]), but peak velocity is not as high and the growth spurt not as extended as in modern human adolescence. The velocity of chimpanzee growth decreases slightly between the ages of four and eight, and then begins to decline rapidly until adult total body length is reached at between 12 and 13 years of age. Chimpanzee growth spurts therefore differ in their onset, offset, and intensity compared to the modern human adolescent growth spurt (see Fig. 1; [Bogin, 1993] and [Bogin, 1996]). The growth spurts in the “ALH 12.3/25%” and “ALH 12.3/50%” curves approximate the juvenile pre-pubertal growth spurt exhibited by chimpanzees, which is of shorter duration and lesser magnitude than the full-blown modern human adolescent growth spurt. We contend that these curves most closely match what is currently known about growth and development in H. erectus but acknowledge that the data currently available limit our ability to choose a single curve. It is also possible that future studies documenting growth in wild chimpanzee length may provide evidence to support a different set of growth curves.

Their small stature estimate for KNM-WT 15000 doesn't entirely hang on this point, but this assumption about the growth spurt makes more difference than any other single factor.

We can reasonably ask: is there any other support for this assumption?

The apparent mismatch between dental and skeletal developmental patterns in the specimen is consistent with the lack of a humanlike growth spurt. But evidence from the skeleton itself is weakened by the fact that KNM-WT 15000 appears to have suffered from some kind of growth pathology, as argued by Latimer and Ohman [2]. The pathology argument has mostly come into play over the issue of vertebral canal size in the specimen, but anything that affected skeletal growth may well have affected the relation between epiphyseal closure and dental eruption. Naturally, if the developmental pathology was a significant influence on growth, then we shouldn't be using WT 15000 as a model for early Homo erectus stature anyway.

A more relevant argument is that KNM-WT 15000 is really an outlier when we assume that it would have grown to a very tall stature. On first appearance, this seems correct. We have quite a number of femora from Homo erectus, both inside and outside of Africa. Only two of them approach the length that had been estimated for the Nariokotome adult stature estimate. KNM-WT 15000's former adult estimate is the extreme.

But looking more closely, both those tall individuals come from generally the same time and place as KNM-WT 15000. KNM-ER 1808 and KNM-ER 736 both preserve partial femur shafts with estimated lengths above 480 mm. Both specimens are a bit older than Nariokotome, between 1.6 and 1.7 million years old. KNM-ER 1808 in particular contributed heavily to the argument that early Homo erectus had a very tall stature, because the partial skeleton includes a fragment of pelvis, argued to be female. A tall woman makes for a very tall species.

Still, these two specimens don't seem as significant in 2010 as they did twenty years ago. The Gona pelvis suggests that we don't really know the sex of KNM–ER 1808. Its pelvic fragment looks female in the context of living human dimorphism, but quite possibly male compared to the Gona individual. Henry McHenry [3] estimated adult statures for the KNM–ER 1808 and KNM–PR 736 femurs, both around 5'10" (180 cm). Those are the tall end of stature estimates for Homo erectus, both taller than average for living humans. But perhaps neither is surprising when taken as the largest and of the distribution that on the whole is relatively small bodied. An estimate of 163 cm for the adult height of KNM-WT 15000, as suggested by Graves and colleagues, would not be an outlier in this population, but neither would an estimate as large as 180 cm.

So I think the comparative evidence is equivocal. Revisiting the specimen with a smaller estimate is reasonable, but I think our ability to assess the accuracy of any estimate is very limited. In light of the pathology of KNM-WT 15000, it may not be very relevant to understanding body size evolution in early Homo, anyway.

The main problem facing us with understanding body size in early Homo is deciding which specimens should be included in which taxa. If we exclude everything except the relatively tall ones, like KNM–ER 1808 and KNM–ER 736, then we are going to end up with a tall stature estimate for a population, putatively H. erectus. But if we include some of the smaller specimens, like KNM–ER 993, or KNM–ER 803 – both contemporaries of the Nariokotome skeleton – than the average for this more inclusive population will be a lot lower. In East Africa 1.5 million years ago we can't assign an isolated femur to a species, and we won't have a good answer for this issue until we have many more associated specimens.

I tend to think that small stature is the null hypothesis now, given our knowledge of the small stature of the Dmanisi hominins, and the moderate body size of middle Pleistocene Homo everywhere else. There are a few specimens that represent individuals as tall as those indicated by KNM-ER 736 and KNM-ER 1808, but none taller, and many much shorter.

It's a much deeper topic than one skeleton, but the problems assessing stature in that skeleton help to highlight the difficulty of the problem in a global sense.

UPDATE (2010-09-18): A reader suggests that I give a link to a 2004 paper by Shelley Smith, which compared the dental and skeletal maturation of KNM-WT 15000 to a large growth series of modern Canadians [4]. She found cases in the sample with comparable mismatches of dental and epiphyseal age estimates, and argued that we can't exclude a humanlike growth spurt for early Homo. That's one reason why I think this issue can't be resolved -- the variation in humans is great enough to encompass the known fossil specimens.

A similar lack of resolution applies to enamel growth increments in KNM-WT 15000 ("Dental growth in early Homo"). The specimen can't be distinguished from Australopithecus, but the range in modern humans is very extensive.

At the moment, skeletal correlates of growth don't give us the resolution to answer these questions definitively about early Homo. If we had more specimens, we could at least reduce the component of error from sampling, which would help considerably. But we can't expect that anytime soon.

References