Friday, July 25, 2008

Cranio-morphological Variation

Cranio-facial variation is perhaps the most overrated phenotypic aspect of human body in societies, aside from skin tone. Not surprising, considering that it is the most regularly exposed part of the body that noticeably sports considerable variation, and goes without saying, forms a biological basis around which an individual's unique identity is established. Socio-ethnic identification is secondary to individual identity in society. Such is the obviousness of cranio-facial variation that images of certain archetypes or "idealized" types have been implanted or socially conditioned in the minds of folks, as socio-ethnic identifiers; this took a particularly notorious turn from ca. 19th century within European bio-anthropological circles, wherein human populations were grouped into narrow rigid "idealized" types, which were usually also identified with major geographical locations. These were presented as non-overlapping types, as though the so-grouped human populations didn't sport intra-group and intra-population variation, and by extension, variation manifestation that is shared with groups outside a reference point group. Howells types rely on such typological groups, and which Forensic science has embraced via Fordisc 2.0. Recent studies have shown that such approach is considerably limited in its ability to account for intra-group variation, and hence, fails the test of classifying individuals of a given population to the correct population of origin, because an 'unknown' or a 'known' specimen may well cluster with individuals in not one but multiple populations. A study on Spanish cranial series ended up having the said specimens cluster all over the map, with individuals in the series clustering with a multitude of groups spanning continents.

"Variation in racial classification represents the lack of a Spanish sample within the FORDISC 2.0 database as well as the human variation inherent within them. Individual crania were classified according to the best fit with the existing samples of the database, but the samples clearly were inadequate to elucidate the specific geographical origin of the overall Spanish sample…some crania were classified into groups with no clear geographic or ancestral relationship with the Spanish sample…

The authors also agree that additional and more complete samples from different geographical regions and groups are needed to augment the existing databases
."

"race classification of all individuals in this sample using the Forensic Data Bank option. Of the 95 individuals, 42 (44 percent) were classified as white, 35 percent as black, 9 percent as Hispanic, 4 percent as Japanese, 4 percent as American Indian, and the remaining three individuals as Chinese and Vietnamese" - Ubelaker et al., Application of Forensic Discriminant Functions to a Spanish Cranial Sample, 2002.

Williams et al. 2005 provided another example of this, when their examination of Meroitic cranial series showed that the series couldn't be classified into a single homogeneous entity, but rather, produced clusters with multiple series from distinct geographical regions. They say:

The Howells series. Fordisc 2.0 could not effectively classify ten of the crania, and of the remainder, eight were identified as Late Period Dynastic Egyptian, six as Zalavar, four as Easter Islander, three as Lake Alexandrina Tribes, and three as Norse (Medieval Norway). Eight were not significantly different from eight separate populations: Teita, Andaman Islands, Zulu, Arikara, Santa Cruz Island, Ainu, Hokkaido, and Atayal.

"The Howells series. Fordisc 2.0 could not effectively classify ten of the crania, and of the remainder, eight were identified as Late Period Dynastic Egyptian, six as Zalavar, four as Easter Islander, three as Lake Alexandrina Tribes, and three as Norse (Medieval Norway). Eight were not significantly different from eight separate populations: Teita, Andaman Islands, Zulu, Arikara, Santa Cruz Island, Ainu, Hokkaido, and Atayal."

“Fordisc 2.0 classified the Nubian crania with populations over an enormous geopraphic range, including North and Central Europe, Easter Island, the Andaman Islands, Japan, Taiwan, South Africa, Australia, and North America. “

“If Fordisc 2.0 is revealing genetic admixture of Late Period Dynastic Egypt and Meroitic Nubia, then one must also consider these ancient Meroitic Nubians to be part of Hungarian, part Easter Islander, part Norse, and part Australian Aborigine, with smaller contributions from the Ainu, Teita, Zulu, Santa Cruz, Andaman Islands, Arikara, Ayatal, and Hokkaido populations. In fact, all human groups are essentially heterogeneous, including samples within Fordisc 2.0. Using Fst heritability tests, Relethford (1994) demonstrated that Howells’s cranial samples exhibit far more variation within than between skeletal series. There is no reason to assume that the heterogeneity of the Late Period Dynastic Egyptian population exceeds that characterizing our Nubian sample. This heterogeneity may also characterize the populations in the Forensic Data Bank; Fordisc 2.0 classified the Meroitic Nubians not as either all black or all white but as black, white, Hispanic, Chinese, Japanese, and Native American.”

“We suggest that skeletal specimens or samples cannot be accurately classified by geography or by racial affinity because of (1) the wide variation in crania of the known series that crosscuts geographic populations (polymorphism), (2) the clinal pattern of human variation, and (3) cultural and environmental factors. Even a presumably homogeneous population such as the Meroitic Nubians shows extensive variation that preclude its classification as a geographic group.”

Apparently multiple variables go into shaping variations in human crano-facial development. Williams et al. put this simply, when they say:

“Finally, the assumption that cranial form is an immutable “racial” character is very likely to be false, given the diversity of studies of immigrants and the known effects of food preparation and masticatory stress upon cranial form. Cranial form, like other aspects of the body, is a phenotype partly determined by heredity but also strongly influenced by the conditions of life.”

As far as the "conditions of life" is concerned, "acclimatization" is one notable factor. Michael A. Little and Jere D. Haas summarize it, when they say:

“there is a recognition of the importance of the process of acclimatization. Acclimatization is defined as a biological response to repeated exposure to a climatic stressor (Prosser 1964). It is an expression of the genetic plasticity of the population, and with acclimatization, it is assumed that an adaptive response can occur without genetic change. Three forms of physiological acclimatization have been recognized: (1) simple, reversible acclimatization-a physiological response to a stressful environment that gradually disappears at the cessation of the stress; (2)irreversible acclimatization—acquired as a result of climatic stress but remaining after the stress is removed; and (3) developmental acclimatizationmuch like irreversible acclimatization except that the exposure must occur at a particular time during the growth process. Until the process of acclimatization was appreciated and its varieties identified much inter-operation variation was attributed to genetic difference. Attempting to define the limits of acclimatization has become as important as defining the underlying genetic basis of adaptation. Indeed it is only after acclimatization is accounted for that genetic adaptations can be identified.

A third manner by which contemporary studies of climatic adaptation differ from earlier efforts is through an appreciation of the role of culture (Baker 1960). Anthropologists since the time of Darwin have played lip service to cultural factors using such generalities as “sexual selection” and “cultural selection.” But it is only with recent empirical studies that cultural mechanisms are being identified and quantified. These have shown that culture is a buffer that modifies rather than eliminates climatic exposure (Wulson 1949, Planalp 1971, Little and Hanna 1978).” - Courtesy of Michael A. Little and Jere D. Haas—Human Population Biology: A Transdisciplinary Science, 1989.

From the genetic standpoint, it is generally known that when only a small segment of a larger population diverges and then locates elsewhere to assume the role as a founder group, the likelihood of loss of diversity is a strong possibility that goes along with it. Hence, pronounced reduction of diversity characterizing the newly-divergent offshoot group allows the distribution of certain traits to figure more prominently than the case would be in the parent population, and alternatively, other traits die out more dramatically. The result of such development, has been invoked in marked departure of offshoot founder groups from their ancestral population. As Little & Haas note, cultural behavior patterns chime in to "modify" these variations. Hence, putting acclimatization effects aside [not to leave out natural selection in the complex mix of factors], on one hand, certain variations brought upon by genetic mutation can be quite dramatic secondary to random genetic drift, wherein certain variations are magnified while others not so much; on another hand, cultural behavior patterns, which can be exemplified in "sexual selection" tendencies, factor in and contribute further to the sustenance and prevalence of certain elements of the overall variation over other elements, and hence, lending hand in certain intra-population morphological tendencies.

*Subject to modification upon new information without notice.
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*References

Ubelaker et al., Application of Forensic Discriminant Functions to a Spanish Cranial Sample, 2002

Williams et al. 2005, Forensic Misclassification of Ancient Nubian Crania: Implications for Assumptions about Human Variation.

—Michael A. Little and Jere D. Haas—Human Population Biology: A Transdisciplinary Science, 1989.

—Discussion link: Nile Valley discussion board.

4 comments:

Barklays said...

Another excellent analysis. I have heard about the weaknesses of Fordisc but no one has broken it down in such clear detail as you do on your page. Fordisc's weaknesses are also paralleled by weaknesses in CRANID as some others have written.

CRANID purports to match skulls with ethnicity and placeof origin. As regards Egyptians, CRANID places the them in a "Meditarranean" bloc. However there is one major problem. CRANID stacks the deck by using samples from a single cemetery at Giza, in (northern) Lower Egypt dating around the final dynastic periods of Egypt (Dyn 26-30), to plot dendrograms suggesting that the population of ancient Egypt lies within a "European/Mediterranean bloc."

In short the database is deliberately front-loaded towards a single cemetery close to the Mediterranean to serve as a "representative" standard in defining the ancient peoples. This skewed loading however, is not representative of the ancients as a whole, for it excluded samples from the same time period based on several important cemetery sites at Elephantine, in Upper Egypt, further south, bordering Nubia. CRANID's database thus excluded one of the historic areas of Egyptian civilization, the area from which Egypt was united, and which ushered in the flowering of its civilization.

This selective card stacking is pointed out not by wild-eyed men in kente cloth, but by respected mainstream Egyptologist Barry Kemp who points out in hos 2005 "Ancient Egypt Anatomy of a Civilisation" --

"If, on the other hand, CRANID had used one of the Elephantine populations of the same period, the geographic association would be much more with the African groups to the south. It is dangerous to take one set of skeketons and use them to characterize the population of the whole of Egypt."
(Source: Barry Kemp, Ancient Egypt Anatomy of a Civilisation, Routledge: 2005, p. 55; see also FW Rosing, 'Qubbet el Hawa und Elephantine; zur Bevolkerungsgeschichte von Agypten,' (trans: Qubbet el Hawa and Elephantine, Populations in the history of Egypt) Stuttgart and New York, 1990, p. 209, Abb 134)

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a good page in the US:
http://www.africanamericanculturalcenterpalmcoast.org/originofthenilevalleypeoples.htm

Mystery Solver said...

As you've correctly noted, CRANID and Fordisc share essentially the same problems. They fail to capture full diversity *within* populations, precisely because adjacent populations to those tested and beyond, not to mention *temporally diverse* specimens, are not given due consideration. As a result, only a *snapshot* of the actual *clinal* pattern and *gradual changes* in trends of diversity contained within larger populations spanning greater geographical stretch is available to the observer, who may or may not choose to interpret his/her observation to fit a certain preconceived [subjective, as opposed to objective] desired way. I mean, how does one expect to see the *full range* of clinal changes in diversity between two geographic reference points of say "A" and "B", and not expect to see more abrupt change from skimming through a small test sample size in "A" and "B" respectively, after having jumped over or ignored a series of reference points between the two? There is a great degree of overlap between populations, if not many a times the case, that a greater degree of variation occurs within designated populations than across or between them. This comes to the fore, when we revisit the examples provided by the aforementioned Spanish, Egyptian and "Nubian" or Meroitic sample sets. Their member test specimens have been found to sport cranio-morphometric affinities with patterns observed in not one, but in several cases, multiple geographical locations. From using the likes of Fordisc, and by association, relying on *stereotyping* vis-a-vis *deficiently-built* or incomplete database sets or index, crania from either of the said example test sets could very well be wrongly designated to a certain socially-designated segment of a population, if not geographic origin.

Proud Canadian said...

I only started getting into this topic area about a year ago, and the more I read pages like yours, the more I realize how necessary it is for balanced, scholarly information on the topic is.

I ran into someone the other day who quoted a study from Hammer 1997 (The geographic distribution of human Y chromosome variation) and Cavali-Sforza 2004 ("Levant versus the Horn: Corridor for migration" I think it was). But just looking at the study, Hammer et al drew their Egyptian samples from the far north, from the city of Cairo, and use this as a "representative" of all Egypt, referencing a 1994 Cavalli-Sforza article in relation to this selection.

As for the 2004 Cavalli-Sforza paper, I will have to look more at it again but just from its list of African populations used as a point of comparison, it excludes Somalians, Nubians, Sudanese, and all Ethiopians except the Falashas. And it's samples of Egyptians are mostly Arab and Berber types. Its comparison found few NRY? matches with various sub-Saharan populations but plenty with Middle Eastern populations. I am still scratching my head over this.. Could it tie in with what you say about adjacent populations?

Just to throw out a few other questions here:

1) You mention capturing the full diversity within populations. Would it be right to say then that there is more diversity *within* native African populations that between Africans and non-Africans? For example, a Bantu herdsman compared to a Swedish fisherman would seem to have more difference..

2) Would this diversity relate to a mixture of races- say white Mediteraneans or Caucasoid peoples from Palestine/Mesopotamia coming into East and North Africa to create this diversity?

3) How does the DNA pattern match up in general with the cranial/skeletal work doen by Fordisc and CRANID?

4) Brace 2005 (Brace, et al. The questionable contribution of the Neolithic and the Bronze Age to European craniofacial form, Proc Natl Acad Sci U S A. 2006 January 3; 103(1): p. 242-247.)seemed to have found greater clustering between adjacent populations such as the Egyptians, Somalians, Nubians, Ethiopians, and even Kenyans than between Europeans, Bantus, Arabs, and other Eurasian groups. Yet in his 1993 "Clines versus Clusters" article he found some of these populations clustering closer to the peoples of India. How could the clusters change in so short a time, between 1993 and 2005?


Scratch head...

Mystery Solver said...

proud canadian writes:

1) You mention capturing the full diversity within populations. Would it be right to say then that there is more diversity *within* native African populations that between Africans and non-Africans? For example, a Bantu herdsman compared to a Swedish fisherman would seem to have more difference..

While there is apparently greater diversity in Africa in general than anywhere else, it is generally the case that there is a great degree of intra-population variation, if not greater than that between populations; the Spanish and "Nubian" specimens serve as perfect examples of this fact; this would be the case, whether it is in Africa, Europe, Australasia or the Americas. Africa being generally more diverse doesn't render it more predisposed to intra-population variation than anywhere else. If however, you mean to say that there are likely more morphological patterns in Africa, than say Europe, well, that may well be the case [Recall Hiernaux about much of the world means being found in sub-Saharan Africa alone].


proud canadian writes:

2) Would this diversity relate to a mixture of races- say white Mediteraneans or Caucasoid peoples from Palestine/Mesopotamia coming into East and North Africa to create this diversity?

There is no such thing as "Mediterranean" or "Caucasoid" type, nor have I come across anything objective, that would suggest what your question is asking, unless of course, you can demonstrate so objectively...which would require providing the *scientific meanings* of the said terms, and how such a phenomenon as that you described, can objectively discern what is African and what isn't to begin with, so as to suggest "non-African" impact in "East and North African" diversity.

Ps - Analyzing two geographically distant populations at point "A" and point "B" has the effect of giving false impressions of local diversity, because it ignores the variations in populations in between the two reference populations; ignoring this, would also mean ignoring the gradual change in morphological and even genealogical trends between any such two given populations.