What have I been doing?
I spent much of the weekend digesting and writing notes on a couple of papers from last week, including the widely-reported Genographic Project paper on the mtDNA of Khoisan and other Africans.
After putting around 5000 words into it, I have decided this is becoming more of a research project than a blog post. It is now possible to build a much more interesting picture of the evolution of Africans continent-wide during the late Middle Stone Age.
This picture does not include many of the elements suggested in the press reports accompanying the paper. In particular, there is no evidence for the proposition that tens of thousands of years of droughts decimated humans down to a small number of tiny bands, which later reunited to conquer the earth. That account goes far outside the science presented in the AJHG paper by Behar and colleagues.
We are working in a wonderful time, when data from many different genetic projects are becoming available to examine the detailed pattern of evolution in parts of the world where archaeology and physical anthropology have been incomplete. But there has been a tendency by many geneticists to exaggerate the weaknesses of traditional artifactual and skeletal evidence. This is manifested in an unwillingness to approach multiple data sources with the idea that they must each be consistent with a single population history.
Since we have not only new genetic evidence, but also new fossil and archaeological evidence coming online, the time may have come for a new synthesis of the prehistory of Africa.
Omo Kibish redating
The news stories (nature.com) are focusing on the idea that the "earliest" modern humans are now 35,000 years earlier than they had been. This is the amount by which the Omo Kibish specimens are now believed (McDougall et al. 2005) to be older than the previous contenders for "earliest modern humans," the 160,000 year-old Herto hominids (White et al. 2003). A bit of a discussion has been underway on the Palanth forum as to whether the 195,000 year estimate is really warranted, or whether there is more properly considered a broader range of error.
It is important to have good stratigraphic placement and dates for the Omo Kibish specimens. Most of the Middle Pleistocene African fossils are associated only with poor dates or imperfectly known proveniences. Aside from the recent Herto sample, the dates for other important specimens are truly uncertain. So the knowledge that these hominids are broadly contemporaneous with Herto is immensely valuable.
Why are the Omo Kibish hominids considered to be modern?
This depends on one's definition of "modern humans." Many paleoanthropologists do not accept a distinction that sharply separates "modern" humans from "archaic" humans. For these scientists, the Omo Kibish specimens may simply be considered as representatives of their time and place, part of an evolutionary series leading to recent humans.
There is no question that features of recent aspect occur within the late Middle Pleistocene African sample. Especially Omo 1 has similarities in overall cranial shape with more recent people. Such similarities also may characterize its facial form, although these details are subject to the reconstruction. Taking the Omo skulls as a sample, together with the Herto sample, the full range of anatomy spans from relatively modern to substantially archaic. Here are some pocket descriptions (leaving out the child's skull BOU-VP-16/5):
- Omo 1: The occiput is rounded in profile, with only slight flattening of the parietals above lambda. In posterior view, the outline of the skull shows a maximum breadth relatively high on the parietal bones, narrowing significantly lower on the temporals toward the cranial base. The supraorbital torus is not greatly thickened even at the midline, and it thins toward the edges in a form that is continuous with the frontal squama. The interorbital space is relatively narrow. And as reconstructed, the skull appears to have both malar notches and a chin.
- Omo 2: Here, the skull is markedly more angled in the occiput than in Omo 1. The maximum breadth of the skull is at the base, across the temporals, and the sides of the skull are more or less vertical. There is a well-marked angulation of the parietal bones, meeting in a rounded keel. There is a slight angular torus on the parietal, and a well-marked nuchal torus transversely extensive across the occiput. But the frontal bone is continuous into the supraorbital region with no sulcus separating them, and the supraorbital torus itself is relatively thin laterally--perhaps as much or more so than Omo 1.
- BOU-VP-16/1 (Herto): The skull has a distinct angulation in the occiput, as great as Omo 2, with a very long nuchal plane. The skull appears to be slightly broader across the parietals than at the base, but the sides are essentially vertical: there is no distinct parietal boss. The nuchal torus is marked across the occiput, with a distinct downward-projecting inion. The browridge is moderately thick centrally, with a strong superciliary portion, and a clear sulcus dividing it from the curving frontal profile. There is a clear division between this superciliary arch and the lateral torus, which at its lateralmost extent is in the same range of thickness as the Omo 2 lateral torus. The zygomatic bone is very large, with a massive forward-facing cheek, hollowed into a canine fossa medially and malar notch inferiorly.
Several commentators have raised the issue of whether this sample contains multiple species (one going so far as to posit that the "species" immediately ancestral to our own might be preserved alongside the "modern humans" in the personage of Omo 2). A lateral comparison of the three skulls (where their comparable parts are most visible) shows that the differences are not that extensive. The Herto skull and Omo 2 are very similar in profile, with BOU-VP-16/1 being slightly higher in the forehead. Omo 1 contrasts with these in its rounded occiput, but the frontal profile of all three specimens are similar, as are their lateral torus thicknesses. Omo 1 and 2 diverge greatly in the position of their greatest cranial breadth and shape of their cranial walls; BOU-VP-16/1 is intermediate between them. All three are robust, with Omo 1 the least robust of the three. Presumably, all three are males. Their variation is extensive, but not surprising for three crania in a single region of the world.
Are they modern humans? As White and colleagues (2003) show, the Herto skull is outside the range of all recent humans in several cranial measurements. This is no doubt true for Omo 2 as well (although possibly not for Omo 1). But these are not recent, they are ancient. As a sample, they are certainly significantly different from any living sample. They are also certainly significantly different from Neandertals, and from earlier Africans.
So do we define "modern" humans in contrast with some earlier group? Or do we define them based on the variability within living people?
The answer here really is in the word "definition." If modern humans were really an evolutionary individual--a "thing" that could be discovered--then we shouldn't have to define them. We should be able to discover the boundaries of the group by examining discontinuities among fossil specimens. The fact that we have to find a definition (and that we have such trouble doing so) is in my mind sufficient to suggest that "modern" humans are not an evolutionary individual.
References:
McDougall I, Brown FH, Fleagle JG. 2005. Stratigraphic placement and age of modern humans from Kibish, Ethiopia. Nature 433:733Ð736. Nature
White TD, Asfaw B, DeGusta D, Gilbert H, Richards GD, Suwa G, Howell FC. 2003. Pleistocene Homo sapiens from Middle Awash, Ethiopia. Nature 425:742Ð747.
The hygienic dater
I've just been reading a useful paper by Andrew Millard, which reviews the chronometric dates of African and Near Eastern fossil hominids from the Middle and early Late Pleistocene. The overall theme is that we don't know the dates nearly as well as we would like -- or as well as many comparative analyses have assumed.
The highlight is the list of specimens with primary references to different date estimates. Anyone with a good training in paleoanthropology probably has a feel for which specimens have relatively good dates and which are real hands-up-in-the-air cases. Kabwe makes for a good example of the latter:
Kabwe (Broken Hill), Zambia. The remains of "Rhodesian Man," along with faunal remains, were discovered in 1921 by miners (Klein, 1973). The principal dating is based on Klein's (1973) assessment that the fauna is similar to that at Elandsfontein and broadly similar to those from Olduvai Gorge Upper Bed II through to Bed IV. There are no chronometric determinations. On the basis of the faunal correlation to Olduvai (Fig. 1), an age of younger than 1780 ka and, depending on the chronology for Olduvai, either older than 990 ka (on the long chronology) or, more likely, older than 490 ka (on the short chronology) may be assigned (see under Olduvai above). This is consistent with Elandsfontein being older than 330 ± 6 ka (Table 1).
Millard's discussion of "chronometric hygiene" takes up much of his discussion. This is nothing more than the simple idea that we should weed bad dates out of our analyses. For example, he singles out Florisbad as a specimen that has been handled poorly in the literature:
Use of the literature. In conducting this review of the chronometric evidence for African and Near Eastern hominids, the search for the detailed chronometric data was hampered by overreliance of many authors on the secondary literature. It is not uncommon to find a date cited from a publication, which upon checking simply cites another publication, which cites another, which cites the paper that first suggested the date. Frequently in such a chain of citations, the justification for the original date is lost, and in some cases, error limits disappear. For example, the ESR date of 259 ± 35 ka for the Florisbad hominid (Grün et al., 1996) can be applied to the Florisbad fauna, but somehow in the discussion of Stynder et al. (2001), this becomes simply "a maximum age of around 250 ka" (p. 372) for the Florisbad Faunal Span, and in McBrearty and Brooks (2000), it becomes a bald 260 ka age without any uncertainty for the Florisbad hominid itself. Sometimes, the primary proposal for a date is based solely on comparisons of morphology to the best-dated fossils at the time of publication, and for later papers to suggest evolutionary sequences based on this date is obviously problematic. Given the flux in dating methods, the fact that problems have often been identified some time after the introduction of these methods, and the changing understanding of the dates of faunal successions, every author should be beholden to check the basis of the dates cited and apply some basic chronometric hygiene (Millard 2008:19).
Of course, there is an irony here, since Millard's effort has generated a massive secondary source listing date estimates for all these hominids! I agree whole-heartedly with his sentiment, though -- everyone should do a better job of reading and citing papers.
But the effect of all this hygiene is to emphasize that most of the Middle Pleistocene remains a muddle, with very few well-resolved dates across the entire span. Millard describes faunal correlations as a relatively weak source of evidence in Africa. Above the time span effectively covered by ESR/TL, there is little to rely on.
References:
Millard AR. 2008. A critique of the chronometric evidence for hominid fossils: 1. Africa and the Near East 500-50 ka. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.11.002
The "dark matter" of modern human origins
I'm just looking through the January/February 2008 Evolutionary Anthropology, which is all about modern human origins in Africa. The special issue resulted from a conference at Stony Brook, along with a few additions to round out the topic.
I'll have some things to say about these articles, but one thing struck me. I'll describe the problem:
Dan Lieberman's paper, "Speculations about the selective basis for modern human cranial form," discusses five categories of functional requirements that might have been involved in the evolution of the "modern" human cranial anatomy. Each of these imposes distinctive requirements on the form of the head -- not all of which are fully understood -- but all of which changed in ways that parallel the basic changes in cranial form of the Late Pleistocene.
But Tim Weaver and Charles Roseman's paper, "New developments in the genetic evidence for modern human origins," claims that the modern human cranial anatomy originated by genetic drift, without any substantial selection:
Evolutionary quantitative genetic analyses, in fact, show that Neandertal and modern human cranial differences can be explained by genetic drift, making it unlikely, at least for the cranium, that modern human anatomical features were spread by natural selection rather than a range expansion out of Africa. An important point is that these analyses do not simply compare the magnitude of the morphological differences between Neandertals and modern humans; they are multivariate tests of how the patterns of covariation across different cranial measurements compare to those expected for divergence by genetic drift. Natural selective hypotheses designed to account for Neandertal and modern human cranial differences would also need to show multivariate consistency with the observed patterns of variation. While it may be possible to imagine natural selective scenarios that mimic genetic drift for a single measurement, such as fluctuating directional natural selection, the scenarios become much less plausible for multivariate patterns of variation (Weaver and Roseman 2008:78).
Both these papers cannot be correct. A full text search of Lieberman's paper does not find the words "drift" or "random," and "neutral" only appears as part of "neutral horizontal axis." Yet Weaver and Roseman cite the neutrality of cranial form as the main evidence against Eswaran's model of an adaptive dispersal of cranial form. According to them, all of Lieberman's "speculations" must be wrong.
I thought maybe I could get some insight into this dilemma by reading Günter Bräuer's paper, "The origin of modern anatomy: by speciation or intraspecific evolution." That title sounds fairly clear -- if we're talking about a speciation of modern humans to explain their anatomy, that sounds like the kind of rapid change that ought to indicate selection of some kind.
Bräuer shows some skepticism toward Lieberman's ideas about cranial evolution:
In my view, Lieberman, McBratney, and Krovitz's interpretation that anatomical modernization can be boiled down to just a few autapomorphies or genetic changes will be difficult to accommodate within the current fossil evidence (Bräuer 2008:27-28).
OK, but does this disagreement mean that Bräuer is likewise skeptical of adaptive hypotheses to explain modern cranial form? Again, a full text search fails to find the words, "drift," "neutral," or "random." But neither does it find the word "selection." Bräuer is concerned with describing the pattern of evolution of the modern human cranial form, but is entirely noncommittal on the question of why it evolved. That would seem to be problematic in itself: wouldn't we expect a different pattern of evolution if natural selection caused the changes, than if genetic drift caused them? Wouldn't the two causes make different predictions about the role of speciation in the process?
I'll have more to write about Bräuer's interesting paper, but on this issue, I think that is all I can extract from it. Osbjorn Pearson's paper, "Statistical and biological definitions of 'anatomically modern' humans," has more to say on the issue. Pearson cites the work that suggests modern human cranial form evolved under random genetic drift, saying:
Ideally, one would like to partition morphological distance into differences due to genetic drift, adaptation, and environmental interactions with ontogeny. Recently, several promising studies have shed light on these issues, including the amount of morphological diversity in recent humans that likely reflects genetic drift and the effects of the toughness of foods on the cranial morphology and occlusion of nonhuman primates, retrognathic mammals (for example, hyraxes), and humans from different parts of the world. Nevertheless, much remains to be done before these relationships become completely clear (Pearson 2008:40-41).
He later suggests (p. 44) that "rapid morphological change due to drift during population bottlenecks" may be involved in the evolution of modern cranial form. On the other hand, Pearson also suggests that "selection for new, advantageous traits or genes, or some combination of the two [selection and drift]" may have occurred. That would seem fairly noncommittal.
However, Pearson's description of the series of events -- a stepwise, sequential series of anatomical changes ultimately in a worldwide context up to and including the Holocene -- seems pretty unlikely to result from genetic drift alone. Indeed, Pearson writes,
In common with many other parts of the world, [African] crania that have dimensions or suites of morphological traits that make them statistically indistinguishable from the living populations appear only during the Holocene (Pearson 2008:45).
If the evolution of modern cranial form is a process that continued into the Holocene, it is quite impossible to have been caused by drift alone, since the effective population sizes of human populations were too large, and drift could hardly have caused a "nearly universal pattern of gracilization" (ibid.). So Pearson's paper certainly heightens the contrast between the adaptive and drift scenarios. If the events are as Pearson describes them, the "genetic drift alone" hypothesis must be false.
Philip Rightmire's paper is about earlier events, and Chris Stringer and Nick Barton's paper is a conference review. That leaves only Ian Tattersall and Jeff Schwartz's paper, "The morphological distinctiveness of Homo sapiens and its recognition in the fossil record: clarifying the problem," to clarify the problem.
Tattersall and Schwartz direct their attention to the kinds of features that are suitable for identifying a species from the fossil record -- uniquely derived features, or "autapomorphies." In their view, species must be accurately diagnosed from sets of specimens ("alpha taxonomy") before any kind of evolutionary hypotheses can be tested.
Because of this, they don't talk very much about the kinds of evolutionary forces that might cause the patterns they see. The paper includes only one reference to "random" and "adaptive," both in a single sentence:
However, there are some materials of this period [the late Middle Pleistocene] that fall outside, but not far outside, the strictest definition of Homo sapiens as based on the living species. Most of these (for example, Border Cave 5, Boskop, Fish Hoek, Klasies River Mouth except for AP 6222, and maybe Cave of Hearths) form a generally poorly dated South African group in which cranial structure largely conforms to the modern Homo sapiens morphology except that, most notably, the bipartite brow and/or the inverted-T-shaped chin are lacking. Do such fossils represent distinctive and now extinct populations of Homo sapiens that lacked two or more of the most striking autapomorphies of the living species merely as a result of random (or even adaptive) population variation? Or did they belong in life to one or more distinctive reproductive entities whose histories did not impinge, at least biologically, on that of today's Homo sapiens? (Tattersall and Schwartz 2008:52, emphasis added)
The bolded sentence is important. Tattersall and Schwartz view adaptive and random variations as equivalent: small changes between populations that may occur even without the kind of significant isolation that would invite a taxonomic interpretation. They contrast these in the next sentence with "distinctive reproductive entities whose histories did not impinge." And they are correct; modern human populations have morphological differences as a result of both selection and drift, and their histories certainly have impinged on each other.
But it makes a difference whether selection or drift was the cause of changes, because selection is more powerful than drift. Weak selection can cause a level of morphological differentiation that would require long isolation by random drift alone. If selection were involved in African regional differentiation, there may be no reason to posit "distinctive reproductive entities whose histories did not impinge" -- in fact, their histories almost certainly would have impinged.
In other words, the relation of the pattern of features to the taxonomic status of the populations depends on the evolutionary forces that generated the pattern.
As Weaver and Roseman note, their hypothesis that modern human cranial form evolved neutrally depends on the pattern of evolution of different features, not the amount of evolution of any single feature. But the amount of evolution must still be explained; under their hypothesis, it must have occurred in small populations over a substantial period of time. In their hypothesis, the cranial differentiation of African late Middle/early Late Pleistocene fossils would have emerged during relatively long periods of parital or complete isolation. Under that hypothesis, Tattersall and Schwartz would be correct to place these fossils into different taxa, only one of which was ancestral to living people -- or at least principally ancestral, allowing for some small amount of hybridization and introgression.
In contrast, Lieberman's adaptive hypotheses are consistent with the evolution of modern human cranial morphology within a broader, larger population. Patterns of selection may explain the variation among the fossils. Today's humans may have emerged from a population with substantial cranial polymorphism. That scenario would seem to be consistent with the patterns described by Pearson -- in which modern human cranial variation does not standardize until very late, perhaps even Holocene times. Only selection could cause this kind of evolution within the large populations of the last 10,000 years, or even within the large populations of the last 70,000 years.
I picked this problem first, because it was the first to stand out to me in the papers. It does seem a fairly glaring contradiction. I don't expect the authors to have noticed the contradiction in advance; I think that they approach the question of human origins from fundamentally different viewpoints.
As you can tell, two of the papers are not concerned with the causes of evolution at all -- their aim is to map the pattern of morphological variation onto putative speciation events. But it seems to me that if we approach the fossil record with the idea that speciation is the major cause of such patterns, then we have already assumed how the evolution happened. It may not have escaped your notice that this is the major reason for disagreement about modern human origins: One group of authors wants to assume the conclusion, foreclosing further discussion.
I don't have any complaints about the papers that were chosen for the issue -- in fact, I'm interested in reading the current opinions of all these authors. So far, I would say that each paper is a well-written expression of its authors' ideas, and I appreciate having all that in one place.
But it does seem a little strange that a special issue devoted to modern human origins in Africa doesn't have more, um, diversity of opinion. Several of the papers discuss multiregional evolution. They apparently believe that it is an important enough viewpoint to include their reasons for disbelieving it. One of the papers (Weaver and Roseman) includes a section about genetic introgression, kindly citing my work. Another (Bräuer) claims that it is reasonable to include all Middle Pleistocene humans in Africa and Europe as part of "one polytypic species, Homo sapiens" (Bräuer 2008:32).
So the work of those of us who write about evolutionary mechanisms seems to be making an impact. Still, it's kind of like "dark matter" -- you only know about the ideas because of their effects on what you can read! In this case, you can read a lot of peoples' opinions about these ideas -- you just can't read them from the people who thought of them.
What boring meetings these must be, with everybody agreeing with each other all the time, and nobody to point out all these contradictions!
References:
Bräuer G. 2008. The origin of modern anatomy: by speciation or intraspecific evolution? Evol Anthropol 17:22-37. doi:10.1002/evan.20157
Lieberman DE. 2008. Speculations about the selective basis for modern human cranial form. Evol Anthropol 17:55-68. doi:10.1002/evan.20154
Pearson OM. 2008. Statistical and biological definitions of "anatomically modern" humans: Suggestions for a unified approach to modern morphology. Evol Anthropol 17:38-48. doi:10.1002/evan.20155
Tattersall I, Schwartz JH. 2008. The morphological distinctiveness of Homo sapiens and its recognition in the fossil record: Clarifying the problem. Evol Anthropol 17:49-54. doi:10.1002/evan.20153
Weaver TD, Roseman CC. 2008. New developments in the genetic evidence for modern human origins. Evol Anthropol 17:69-80. doi:10.1002/evan.20161
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks