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Angkor — Medieval ‘Hydraulic City’ — Unwittingly Engineered Its Environmental Collapse

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The architects of Cambodia’s famed Angkor – the world’s most extensive medieval “hydraulic city” – unwittingly engineered its environmental collapse, says research by UNSW scientists and a team of international scholars.


Angkor at sunset. (Credit: Image courtesy of University Of New South Wales)

This revelation, published in the Proceedings of the National Academy of Science, supports a disputed hypothesis by French archaeologist Bernard-Philippe Groslier, who 50 years ago suggested that the vast medieval settlement of Angkor was defined, sustained, and ultimately overwhelmed by over-exploitation and the environmental impacts of a complex water-management network.

A succession of monarchs ruled the Angkor area from about 800 AD, producing the architectural masterpieces and sculpture now preserved as a World Heritage site. By the 13th century the civilisation was in decline, and most of Angkor was abandoned by the early 15th century, apart from Angkor Wat, the main temple, which remained a Buddhist shrine.

Groslier surmised that a network of roads, canals and irrigation ponds established between the 9th and 16th centuries proved too vast to manage. He argued that extensive land clearing for rice fields supporting up to a million people living beyond Angkor’s walled city produced serious ecological problems, including deforestation, topsoil degradation and erosion.

Latter-day archaeologists disputed Groslier’s view because he was unable to support his hypothesis with empirical data about the landscape beyond Angkor’s central temple complex.

Using modern day aerial photography and high-resolution ground-sensing radar, the international research team, including UNSW’s Professor Tony Milne, studied an area of nearly 3000 square kilometres, confirming Groslier’s hypothesis by correlating their images to existing maps, topographic data sets and supporting information from extensive ground-based archaeological investigations.

The team discovered more than 1000 man-made ponds and at least 74 more temple sites in the Angkor region, revealing ruins covering an area of 1000 square kilometres.

The study’s radar images were acquired from NASA via an airborne imaging radar (AIRSAR) data instrument capable of accurately reconstructing surface structures through cloud cover.

“The instrument can produce high-resolution images detecting surface structures as small as 20 cms in height and distinguish very subtle differences in surface vegetation and soil moisture,” says Professor Milne from the School of Biological, Earth and Environmental Sciences.

“This was of particular use in uncovering the archaeological landscape at Angkor. The distinctive spatial patterning of features manifests itself primarily in slight variations in topographic relief. This also influences the amplitude or ‘brightness’ of the radar signal returned to the sensor.”

“Both the topographic relief and the surface brightness can be helpful in identifying the possible location of former roads, canals and rice fields,” says Professor Milne. “When excavations were carried out, they prove to be the site of a canal or temple moat”.

PNAS | September 4, 2007 | vol. 104 | no. 36 | 14277-14282

A comprehensive archaeological map of the world’s largest preindustrial settlement complex at Angkor, Cambodia Damian Evans; Christophe Pottier; Roland Fletcher; Scott Hensley; Ian Tapley; Anthony Milne; Michael Barbetti

Edited by Michael D. Coe, Yale University, New Haven, CT, and approved June 29, 2007 (received for review March 17, 2007)

The great medieval settlement of Angkor in Cambodia [9th–16th centuries Common Era (CE)] has for many years been understood as a “hydraulic city,” an urban complex defined, sustained, and ultimately overwhelmed by a complex water management network. Since the 1980s that view has been disputed, but the debate has remained unresolved because of insufficient data on the landscape beyond the great temples: the broader context of the monumental remains was only partially understood and had not been adequately mapped. Since the 1990s, French, Australian, and Cambodian teams have sought to address this empirical deficit through archaeological mapping projects by using traditional methods such as ground survey in conjunction with advanced radar remote-sensing applications in partnership with the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL). Here we present a major outcome of that research: a comprehensive archaeological map of greater Angkor, covering nearly 3,000 km2, prepared by the Greater Angkor Project (GAP). The map reveals a vast, low-density settlement landscape integrated by an elaborate water management network covering >1,000 km2, the most extensive urban complex of the preindustrial world. It is now clear that anthropogenic changes to the landscape were both extensive and substantial enough to have created grave challenges to the long-term viability of the settlement.

Written by huehueteotl

September 13, 2007 at 9:03 am

Posted in Blogroll, World History

Physics Strips St. Francis Of His Tunic

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The tunic believed to have been worn by Saint Francis of Assisi preserved in the Church of Saint Francis in Cortona (Province of Arezzo) dates back to the period in which the saint lived, whereas the tunic preserved in the Church of Santa Croce in Florence was made after his death.


Carbon 14 measurements, which allow a relic to be dated, show that the tunic in Santa Croce dates back to some time between the late 13th century and late 14th century and thus could not have belonged to the “Poor Man of Assisi,” who died in 1226. (Credit: Copyright 2002 INFN)

Carbon 14 measurements, which allow a relic to be dated, show that the tunic in Santa Croce dates back to some time between the late 13th century and late 14th century and thus could not have belonged to the “Poor Man of Assisi”, who died in 1226. These and other discovers were made possible through the analysis of the relics with a tandem particle accelerator, which was performed by the Laboratory of Nuclear Techniques for Cultural Heritage (LABEC) of the INFN of Florence.

The results of the study were presented in Florence at the European Conference on Accelerators in Applied Research and Technology (ECAART) and will be published in the volume “L’eredità del Padre: le reliquie di San Francesco a Cortona” (which will be released in a few weeks by Edizioni Messaggero di Sant’Antonio). The volume will include the complete results of an interdisciplinary investigation which included both scientific and humanist research and which was promoted by the Tuscany Province Chapter of the Franciscan Order “Friars Minor Conventual”.

The analyses were conducted with a radiocarbon method, measuring the radiocarbon using Accelerator Mass Spectrometry (AMS). From each tunic, researchers took from 5 to 7 samples of fabric, each of which was smaller than one square centimetre and weighed around 10 milligrams. Multiple samples were taken to avoid doubts or ambiguities (due to, for example, the presence of patches that were added to the tunic at a later time), thus increasing the analysis’ validity.

Each sample of wool was then treated so as to extract only the carbon, obtaining a small graphite pellet weighing about 0.8 milligrams. The pellet was then placed in the accelerator’s chamber, where it was exposed to a beam of cesium ions, “scratching” the pellet’s surface and extracting carbon isotopes 12, 13, and 14. The accelerator used by the INFN separately measured the quantity of the three isotopes. Relics are dated by calculating the ratio of carbon 14 to carbon 12, the quantities of which are “counted” in the accelerator’s detectors. Both great delicacy and exceptional sensitivity are required for taking these measurements; in fact, the ratio of carbon 14 to carbon 12 is only around one to one trillion, or even lower.

The analysis of the tunic preserved in the Basilica of Santa Croce in Florence showed that it dates back to a period between the end of the 1200s and the end of the 1300s, revealing that it was made at least 80 years after Saint Francis’ death and thus could not have belonged to him.

By contrast, the dates of all of the fragments taken from the tunic in the church in Cortona coincide with the period of Saint Francis’ life (the average results show that the tunic was made between 1155 and 1225). The tunic is one of three Franciscan relics, which also include a finely embroidered cushion and a book of gospels believed to have been brought to Cortona by Friar Elia, Saint Francis’ first successor as leader of the order.

LABEC researchers also analysed the composition of the precious metal thread used to embroider the cover of the cushion on which the Saint’s head was placed upon his death, and they used the carbon 14 method to date the fabric of the cushion itself. Moreover, the book of gospels was subjected to in-depth codicological and paleographic investigations by researchers at the University of Siena. Based on both the scientific evidence and humanistic research, the cushion and the book of gospels were also found to date back to the period in which Saint Francis lived.

Note: This story has been adapted from a news release sciencedaily.com.

Written by huehueteotl

September 12, 2007 at 11:05 am

Nanomagnetic Sponges To Clean Precious Works Of Art

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Chemists in Italy are reporting “a real breakthrough” in technology for cleaning and conserving priceless oil paintings, marble sculptures and other works of art in a new article.


Nanomagnetic sponge. (Credit: Courtesy of Piero Baglioni, University of Florence, Italy)

In the report in ACS’ Langmuir, Piero Baglioni and colleagues describe development and successful testing on artworks of “nanomagnetic sponges” that could have a range of other applications in cosmetics, detergents, and biotechnology.

Highlighting potential uses in art conservation, the report explains that conservators often use solvents and other cleaning agents in a gel formulation, somewhat similar in consistency to gelatin desert. Compared to liquids, gels have less of a tendency to soak deep into the surface of artwork and cause damage. Gels, however, are difficult to remove from painted surfaces and may leave behind undesirable residues.

The new nanomagnetic sponges — made from nanoparticles so small that about 10,000 would fit across the diameter of a human hair — overcome that problem, the report states. The sponges can be loaded with solvents and other cleaning agents, and cut with a knife or scissors into desired shapes for application to specific, soiled areas of a painting. When the cleaning is done, conservators can remove the gel with a magnet.

“The nanomagnetic gel represents the most advanced and versatile system for cleaning and will have a dramatic impact on the conventional methods used in the conservation field and in several other fields where fine tuning of the release or uptake of confined material is required,” the report states.

Reference; “Nanomagnetic Sponges for the Cleaning of Works of Art” Langmuir, Aug. 14, 2007.

Written by huehueteotl

September 7, 2007 at 9:04 am

Posted in Blogroll, World History

First Beehives In Ancient Near East Discovered

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Archaeological proof of the Biblical description of Israel really as “the land of milk and honey” (or at least the latter) has been uncovered by researchers from the Hebrew University of Jerusalem Institute of Archaeology.+


Close-up of one of the ancient beehives found at Tel Rehov in Israel. (Credit: Hebrew University photo by Amihai Mazar)

Amihai Mazar, Eleazar L. Sukenik Professor of Archaeology at the Hebrew University, revealed that the first apiary (beehive colony) dating from the Biblical period has been found in excavations he directed this summer at Tel Rehov in Israel’s Beth Shean Valley. This is the earliest apiary to be revealed to date in an archaeological excavation anywhere in the ancient Near East, said Prof. Mazar. It dates from the 10th to early 9th centuries B.C.E.

Tel Rehov is believed to have been one of the most important cities of Israel during the Israelite monarchy. The beehives there were found in the center of a built-up area there that has been excavated since 1997 by Dr. Nava Panitz-Cohen of the Hebrew University. Three rows of beehives were found in the apiary, containing more than 30 hives. It is estimated, however, based on excavations to date, that in all the total area would have contained some 100 beehives.

Each row contained at least three tiers of hives, each of which is a cylinder composed of unbaked clay and dry straw, around 80 centimeters long and 40 centimeters in diameter.

One end of the cylinder was closed and had a small hole in it, which allowed for the entry and exit of the bees. The opposite end was covered with a clay lid that could be removed when the beekeeper extracted the honeycombs. Experienced beekeepers and scholars who visited the site estimated that as much as half a ton of honey could be culled each year from these hives.

Prof. Mazar emphasizes the uniqueness of this latest find by pointing out that actual beehives have never been discovered at any site in the ancient Near East. While fired ceramic vessels that served as beehives are known in the Hellenistic and Roman periods, none were found in situ, and beekeeping on an industrial level such as the apiary at Tel Rehov is hitherto unknown in the archaeological record. Pictorial depictions of apiaries are known from Pharaonic Egypt, showing extraction of honey from stacked cylinders which are very similar to those found at Tel Rehov.

Cylindrical clay beehives placed in horizontal rows, similar to those found at Tel Rehov, are well-known in numerous contemporary traditional cultures in Arab villages in Israel, as well as throughout the Mediterranean. The various products of beehives are put to diverse use: the honey is, of course, a delicacy, but is also known for its medicinal and cultic value. Beeswax was also utilized in the metal and leather industries, as well as for writing material when coated on wooden tablets.

The term “honey” appears 55 times in the Bible, 16 of which as part of the image of Israel as “the land of milk and honey”. It is commonly believed that the term refers to honey produced from fruits such as dates and figs. Bees’ honey, on the other hand, is mentioned explicitly only twice, both related to wild bees. The first instance is how Samson culled bees’ honey from inside the corpse of the lion in the Soreq Valley (Judges 14: 8-9). The second case is the story of Jonathan, King Saul’s son, who dipped his hand into a honeycomb during the battle of Mikhmash (Samuel I 14:27).

While the Bible tells us nothing about beekeeping in Israel at that time, the discovery of the apiary at Tel Rehov indicates that beekeeping and the extraction of bees’ honey and honeycomb was a highly developed industry as early as the First Temple period. Thus, it is possible that the term “honey” in the Bible indeed pertains to bees’ honey.

Cultic objects were also found in the apiary, including a four-horned altar adorned with figures of naked fertility goddesses, as well as an elaborately painted chalice. This could be evidence of deviant cultic practices by the ancient Israelites related to the production of honey and beeswax.

Study of the beehives found at Tel Rehov is being conducted with the participation of various researchers. Dr. Guy Bloch of the Silberman Institute of Life Sciences of the Hebrew University is studying the biological aspects of the finds; he already discovered parts of bees’ bodies in the remains of honeycomb extracted from inside the hives. Dr. Dvori Namdar of the Weizmann Institute of Science succeeded in identifying beeswax molecules from the walls of the beehives, and Prof. Mina Evron from Haifa University is analyzing the pollen remains in the hives.

Dating of the beehives was done by measuring the decaying of the 14C isotope in organic materials, using grains of wheat found next to the beehives. This grain was dated at the laboratory of Groningen University in Holland to the period between the mid-10th century B.C.E. until the early 9th century B.C.E. This is the time period attributed to the reign of King Solomon and the first kings of the northern Kingdom of Israel following the division of the monarchy. The city of Rehov is indeed mentioned in an Egyptian inscription dating to the time of the Pharaoh Shoshenq I (Biblical Shishak), whom the Bible notes as the contemporary of King Solomon and who invaded Israel following that monarch’s death.

A particularly fascinating find at the site is an inscription on a ceramic storage jar found near the beehives that reads “To nmsh”. This name was also found inscribed on another storage jar from a slightly later occupation level at Tel Rehov, dated to the time of the Omride Dynasty in the 9th century BCE. Moreover, this same name was found on a contemporary jar from nearby Tel Amal, situated in the Gan HaShelosha National Park (Sachne).

The name “Nimshi” is known in the Bible as the name of the father and in several verses the grandfather of Israelite King Jehu, the founder of the dynasty that usurped power from the Omrides (II Kings: 9-12). It is possible that the discovery of three inscriptions bearing this name in the same region and dating to the same period indicates that Jehu’s family originated from the Beth Shean Valley and possibly even from the large city located at Tel Rehov. The large apiary discovered at the site might have belonged to this illustrious local clan.

The excavations at Tel Rehov were supported by John Camp from Minneapolis in the U.S. with the participation of archaeological students from the Hebrew University of Jerusalem and numerous volunteers.

Note: This story has been adapted from a news release issued by The Hebrew University of Jerusalem.

Written by huehueteotl

September 6, 2007 at 10:40 am

Homosexual Civil Unions – A Tradition Dating From 16th Century

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Homophobia is more recent in history than its promoters are likely to enjoy. A compelling new study from the September issue of the Journal of Modern History reviews historical evidence, including documents and gravesites, suggesting that homosexual civil unions may have existed six centuries ago in France. The article is the latest from the ongoing “Contemporary Issues in Historical Perspective” series, which explores the intersection between historical knowledge and current affairs.

Commonly used rationales in support of gay marriage and gay civil unions avoid historical arguments. However, as Allan A. Tulchin (Shippensburg University) reveals in his forthcoming article, a strong historical precedent exists for homosexual civil unions.

Opponents of gay marriage in the United States today have tended to assume that nuclear families have always been the standard household form. However, as Tulchin writes, “Western family structures have been much more varied than many people today seem to realize, and Western legal systems have in the past made provisions for a variety of household structures.”

For example, in late medieval France, the term affrèrement — roughly translated as brotherment — was used to refer to a certain type of legal contract, which also existed elsewhere in Mediterranean Europe. These documents provided the foundation for non-nuclear households of many types and shared many characteristics with marriage contracts, as legal writers at the time were well aware, according to Tulchin.

The new “brothers” pledged to live together sharing ‘un pain, un vin, et une bourse’ — one bread, one wine, and one purse. As Tulchin notes, “The model for these household arrangements is that of two or more brothers who have inherited the family home on an equal basis from their parents and who will continue to live together, just as they did when they were children.” But at the same time, “the affrèrement was not only for brothers,” since many other people, including relatives and non-relatives, used it.

The effects of entering into an affrèrement were profound. As Tulchin explains: “All of their goods usually became the joint property of both parties, and each commonly became the other’s legal heir. They also frequently testified that they entered into the contract because of their affection for one another. As with all contracts, affrèrements had to be sworn before a notary and required witnesses, commonly the friends of the affrèrés.”

Tulchin argues that in cases where the affrèrés were single unrelated men, these contracts provide “considerable evidence that the affrèrés were using affrèrements to formalize same-sex loving relationships. . . . I suspect that some of these relationships were sexual, while others may not have been. It is impossible to prove either way and probably also somewhat irrelevant to understanding their way of thinking. They loved each other, and the community accepted that. What followed did not produce any documents.”

He concludes: “The very existence of affrèrements shows that there was a radical shift in attitudes between the sixteenth century and the rise of modern antihomosexual legislation in the twentieth.”

Reference: Allan Tulchin, “Same-Sex Couples Creating Households in Old Regime France: The Uses of the Affrèrement.” Journal of Modern History: September 2007.

Written by huehueteotl

August 27, 2007 at 12:34 pm

AIDS Interferes With Stem Cells In The Brain

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A prominent problem in AIDS is a form of dementia that robs one’s ability to concentrate and perform normal movements. Scientists at the Burnham Institute for Medical Research (Burnham) have discovered how HIV/AIDS disrupts the normal replication of stem cells in the adult brain, preventing new nerve cells from forming.

Drs. Stuart Lipton, Marcus Kaul, Shu-ichi Okamoto and their colleagues uncovered a novel molecular mechanism that inhibits stem cell proliferation and that could possibly be triggered in other neurodegenerative diseases as well. 

A normally functioning adult human brain has the ability to partially replenish or repair itself through neurogenesis, the proliferation and development of adult neural progenitor/stem cells (aNPCs) into new nerve cells. Neurogenesis can take place only within specific regions of the brain, such as the dentate gyrus of the hippocampus.

The hippocampus is the brain’s central processing unit, critical to learning and memory. aNPCs differentiate, adapt, and assimilate into existing neural circuits and mature with guidance from neurotransmitters, the chemical substances that nerve cells use to communicate with one another. The brain’s self-renewal through neurogenesis is impaired in AIDS dementia, Alzheimer’s, Huntington’s, and other neurodegenerative diseases, as evidenced by a greatly reduced number of aNPCs in brain tissue from individuals suffering from these diseases. The Burnham team focused on the determining the effect of a protein associated with AIDS, called HIV/gp120, which plays a key role in the pathogenesis of AIDS dementia.

In initial work with cell cultures in Petri dishes, the researchers methodically ruled out the possibility that HIV/gp120 would be inducing the death of stem cells and determined instead that HIV/gp120 was acting by inhibiting stem cell proliferation. Next, they confirmed these results in a special mouse strain bred to express HIV/gp120 in its brain. This mouse model for AIDS dementia mimics several features of the disease process found in humans. They observed a significant decrease in the number of proliferating stem cells in the brains of HIV/gp120-mice compared with similar tissue from normal, wild-type mice.

HIV/gp120 is known to interact with two receptors, called chemokine receptors, which are expressed on aNPCs. The researchers discovered that the same two receptors were targeted by HIV/gp120 sourced from either mouse or human brain tissue.

In search of a mechanism behind the finding that HIV/gp120 reduced proliferation of aNPCs, the scientists studied the effect of the protein on the cell cycle. Cells undergo seasons or cycles, known as G1, S, G2, and M (for mitosis, or cell division). They found that cells exposed to HIV/gp120 got stuck in the G1 or resting phase, and that the cell cycle was arrested.

Cell cycle is studied intensively by cancer researchers who have delineated certain “checkpoint” pathways that can jam cell proliferation, one of the key behaviors of cancer. Checkpoint pathways are overcome by cancers when they fool the body’s normal machinery into producing more cancerous cells. With dementia, it turns out that the opposite is true: the Burnham team discovered that HIV/AIDS could co-opt the checkpoint pathway to prevent stem cells in the brain from dividing and multiplying.

One such checkpoint pathway is modulated by an enzyme called p38 mitogen-activated protein kinase (MAPK), whose activity is known to disrupt the cell cycle. In mature nerve cells, the Burnham team had previously shown that HIV/gp120 activates the p38 MAPK pathway to contribute to cell death. Lipton and colleagues now report that the p38 MAPK pathway is also the mechanism underlying decreased stem cell proliferation in the brain associated with HIV/AIDS. Under experimental conditions, they were able to neutralize the p38 MAPK pathway and restore stem cell proliferation.

“We show for the first time how HIV/AIDS inhibits proliferation of neural stem cells and prevents the formation of new nerve cells in the adult brain,” said Dr. Stuart Lipton, Director of Burnham’s Del E. Webb Center for Neuroscience, Aging, and Stem Cell Research.

“The fact that the mechanism of action involves the p38 MAPK enzyme is fortuitous because drugs to combat that pathway are being tested for other diseases. If they prove effective, they might also work to protect the brain. Thus, this study offers real hope for combating the bad effects of HIV/AIDS on stem cells in the brain.” Lipton went on to state, “It will be important to see if HIV/AIDS acts similarly on stem cells for other organs in the human body, as this may impact on the disease process as a whole.”

Cell Stem Cell, Vol 1, 230-236, 16 August 2007

HIV/gp120 Decreases Adult Neural Progenitor Cell Proliferation via Checkpoint Kinase-Mediated Cell-Cycle Withdrawal and G1 Arrest

Shu-ichi Okamoto,1, Yeon-Joo Kang,1,4 Christopher W. Brechtel,1,2,4 Elisa Siviglia,1,4 Rossella Russo,1 Arjay Clemente,1 Anne Harrop,1 Scott McKercher,1 Marcus Kaul,1,3, and Stuart A. Lipton1,2,3,

1 Center for Neuroscience, Stem Cells, and Aging, Burnham Institute for Medical Research, La Jolla, CA 92037, USA
2 Department of Neurosciences, University of California, San Diego, La Jolla, CA 92093, USA
3 Department of Psychiatry University of California, San Diego, La Jolla, CA 92093, USA

Summary

Impaired adult neurogenesis has been observed in several neurodegenerative diseases, including human immunodeficiency virus (HIV-1)-associated dementia (HAD). Here we report that the HIV-envelope glycoprotein gp120, which is associated with HAD pathogenesis, inhibits proliferation of adult neural progenitor cells (aNPCs) in vitro and in vivo in the dentate gyrus of the hippocampus of HIV/gp120-transgenic mice. We demonstrate that HIV/gp120 arrests cell-cycle progression of aNPCs at the G1 phase via a cascade consisting of p38 mitogen-activated protein kinase (MAPK) → MAPK-activated protein kinase 2 (a cell-cycle checkpoint kinase) → Cdc25B/C. Our findings define a molecular mechanism that compromises adult neurogenesis in this neurodegenerative disorder.

Written by huehueteotl

August 17, 2007 at 10:58 am

Handsome By Chance: It Is Not About Makeup For Neanderthals

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 Chance, not natural selection, best explains why the modern human skull looks so different from that of its Neanderthal relative, according to a new study led by Tim Weaver, assistant professor of anthropology at UC Davis


Model of the Neanderthal man. Exhibited in the Dinosaur Park Münchehagen, Germany. (Credit: iStockphoto/Klaus Nilkens)

“For 150 years, scientists have tried to decipher why Neanderthal skulls are different from those of modern humans,” Weaver said. “Most accounts have emphasized natural selection and the possible adaptive value of either Neanderthal or modern human traits. We show that instead, random changes over the past 500,000 years or so – since Neanderthals and modern humans became isolated from each other – are the best explanation for these differences.”

Weaver and his colleagues compared cranial measurements of 2,524 modern human skulls and 20 Neanderthal specimens, then contrasted those results with genetic information from a separate sample of 1,056 modern humans.

The scientists concluded that Neanderthals did not develop their protruding mid-faces as an adaptation to icy Pleistocene weather or the demands of using teeth as tools, and the retracted faces of modern humans are not an adaptation for language, as some anthropologists have proposed.

Instead, random “genetic drift” is the likeliest reason for these skull differences.

Weaver conducted the research with Charles Roseman, an anthropologist at the University of Illinois at Urbana-Champaign, and Chris Stringer, a paleoanthropologist at the Natural History Museum in London.

Journal of Human Evolution Volume 53, Issue 2, August 2007, Pages 135-145 doi:10.1016/j.jhevol.2007.03.001 

Were neandertal and modern human cranial differences produced by natural selection or genetic drift?

Timothy D. Weaver a, b, Charles C. Roseman c and Chris B. Stringer d


aDepartment of Anthropology, University of California, One Shields Avenue, Davis, CA 95616, USA
bDepartment of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany
cDepartment of Anthropology, University of Illinois at Urbana-Champaign, 109 Davenport Hall, 607 South Matthews Avenue, Urbana, IL 61801, USA
dDepartment of Palaeontology, The Natural History Museum, London SW7 5BD, UK
Received 10 July 2006; accepted 1 March 2007. Available online 23 May 2007.

Abstract

Most evolutionary explanations for cranial differences between Neandertals and modern humans emphasize adaptation by natural selection. Features of the crania of Neandertals could be adaptations to the glacial climate of Pleistocene Europe or to the high mechanical strains produced by habitually using the front teeth as tools, while those of modern humans could be adaptations for articulate speech production. A few researchers have proposed non-adaptive explanations. These stress that isolation between Neandertal and modern human populations would have lead to cranial diversification by genetic drift (chance changes in the frequencies of alleles at genetic loci contributing to variation in cranial morphology). Here we use a variety of statistical tests founded on explicit predictions from quantitative- and population-genetic theory to show that genetic drift can explain cranial differences between Neandertals and modern humans. These tests are based on thirty-seven standard cranial measurements from a sample of 2524 modern humans from 30 populations and 20 Neandertal fossils. As a further test, we compare our results for modern human cranial measurements with those for a genetic dataset consisting of 377 microsatellites typed for a sample of 1056 modern humans from 52 populations. We conclude that rather than requiring special adaptive accounts, Neandertal and modern human crania may simply represent two outcomes from a vast space of random evolutionary possibilities.

Keywords: Neandertals; Hominin evolution; Evolutionary quantitative genetics; Population genetics

Written by huehueteotl

August 17, 2007 at 10:00 am

Posted in Blogroll, World History