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November 2005

Earliest European Farmers Left Little Genetic Mark On Modern Europe
Posted: Saturday, November 12, 2005
Source: American Association for the Advancement of Science

The farmers who brought agriculture to central Europe about 7,500 years ago did not contribute heavily to the genetic makeup of modern Europeans, according to the first detailed analysis of ancient DNA extracted from skeletons of early European farmers.

The passionate debate over the origins of modern Europeans has a long history, and this work strengthens the argument that people of central European ancestry are largely the descendants of "Old Stone Age," Paleolithic hunter-gatherers who arrived in Europe around 40,000 years ago rather than the first farmers who arrived tens of thousands of years later during the Neolithic Age.

This paper appears in the 11 November 2005 issue of the journal Science published by AAAS the nonprofit science society.

The researchers from Germany, the United Kingdom and Estonia extracted and analyzed DNA from the mitochondria of 24 skeletons of early farmers from 16 locations in Germany, Austria and Hungary. Six of these 24 skeletons contain genetic signatures that are extremely rare in modern European populations. Based on this discovery, the researchers conclude that early farmers did not leave much of a genetic mark on modern European populations.

"This was a surprise. I expected the distribution of mitochondrial DNA in these early farmers to be more similar to the distribution we have today in Europe," said Science author Joachim Burger from Johannes Gutenberg Universität Mainz in Mainz, Germany.

"Our paper suggests that there is a good possibility that the contribution of early farmers could be close to zero," said Science author Peter Forster from the University of Cambridge in Cambridge, UK.

To get at questions surrounding the ancestry of modern Europeans, the researchers studied mitochondrial DNA from early farmers in Central Europe. Mothers pass mitochondrial DNA to their offspring primarily "as is," without mixing or recombination with mitochondrial DNA from fathers. Mitochondrial DNA, therefore, provides a way for researchers to piece together how closely members of a species are related, using maternal lineages as a guide, explained Burger.

In the new study, the researchers attempted to extract mitochondrial DNA from the skeletons of 56 humans who lived in various parts of Central Europe about 7500 years ago. These ancient humans all belonged to well known cultures that can be identified by the decorations on their pottery -- the Linearbandkeramik (LBK) and the Alföldi Vonaldiszes Kerámia (AVK). The presence of these cultures in Central Europe marks the onset of farming in the region. These farming practices originated in the "Fertile Crescent" of the Near East about 12,000 years ago.

From bones and teeth of these 56 skeletons, the researchers extracted mitochondrial DNA sufficient for analysis from 24 of the skeletons. Six of the 24 early farmers belonged to the "N1a" human lineage, according to genetic signatures or "haplotypes" in their mitochondrial DNA that the researchers studied. These six skeletons are from archeological sites all across central Europe. Few modern Europeans belong to this N1a lineage, and those that do are spread across much of Europe.

The other 18 early farmers belonged to lineages not useful for investigating the genetic origins of modern Europeans because their genetic signatures from the scrutinized region of mitochondrial DNA are widespread in living humans, according to the authors.

Using the tools of population genetics and a worldwide database of 35,000 modern DNA samples, the researchers investigated the genetic legacy of early European farmers based on the fact that six of the 24 early European farmers are from a lineage that is now extremely rare in Europe and around the world.

At least 8 percent of the early farmers belonged to the N1a lineage, according to the researchers who estimate the range was between 8 and 42 percent.

Even this conservative estimate of 8 percent stands in stark contrast to the current percentage of central Europeans who belong to the N1a lineage -- 0.2 percent. This discrepancy suggests that these early farmers did not leave much of a genetic mark on modern Central Europeans, the authors say.

"It's interesting that a potentially minor migration of people into Central Europe had such a huge cultural impact," said Forster.

Small pioneer groups may have carried farming into new areas of Europe, the authors suggest. Once farming had taken hold, the surrounding hunter-gatherers could have adapted the new culture and then outnumbered the original farmers, diluting their N1a frequency to the low modern level. A range of archeological research supports different aspects of this hypothesis, the authors say.

Alternatively, a different population may have replaced the early farmers in Central Europe, eliminating most of the N1a types, but archaeological evidence for this scenario is scant, according to the authors.


Wolfgang Haak, Barbara Bramanti, Guido Brandt, Marc Tänzer, Kurt Werner Alt and Joachim Burger at Johannes Gutenberg Universität Mainz in Mainz, Germany; Peter Forster, Shuichi Matsumura and Colin Renfrew at University of Cambridge in Cambridge, UK; Richard Villems at Tartu University in Tartu, Estonia; Detlef Gronenborn at Römisch-Germanisches Zentralmuseum in Mainz, Germany. This study was supported by the Bundesministerium für Bildung und Forschung (BMBF)

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Neutron Star Discovered Where A Black Hole Was Expected
Posted: Thursday, November 3, 2005
Source: Chandra X-ray Center
Date: 2005-11-03

A very massive star collapsed to form a neutron star and not a black hole as expected, according to new results from NASA's Chandra X-ray Observatory. This discovery shows that nature has a harder time making black holes than previously thought.

Scientists found this neutron star -- a dense whirling ball of neutrons about 12 miles in diameter -- in an extremely young star cluster. Astronomers were able to use well-determined properties of other stars in the cluster to deduce that the progenitor of this neutron star was at least 40 times the mass of the Sun.

"Our discovery shows that some of the most massive stars do not collapse to form black holes as predicted, but instead form neutron stars," said Michael Muno, a UCLA postdoctoral Hubble Fellow and lead author of a paper to be published in The Astrophysical Journal Letters.

When very massive stars make neutron stars and not black holes, they will have a greater influence on the composition of future generations of stars. When the star collapses to form the neutron star, more than 95% of its mass, much of which is metal-rich material from its core, is returned to the space around it.

"This means that enormous amounts of heavy elements are put back into circulation and can form other stars and planets," said J. Simon Clark of the Open University in the United Kingdom.

Astronomers do not completely understand how massive a star must be to form a black hole rather than a neutron star. The most reliable method for estimating the mass of the progenitor star is to show that the neutron star or black hole is a member of a cluster of stars, all of which are close to the same age.

Because more massive stars evolve faster than less massive ones, the mass of a star can be estimated from if its evolutionary stage is known. Neutron stars and black holes are the end stages in the evolution of a star, so their progenitors must have been among the most massive stars in the cluster.

Muno and colleagues discovered a pulsing neutron star in a cluster of stars known as Westerlund 1. This cluster contains a hundred thousand or more stars in a region only 30 light years across, which suggests that all the stars were born in a single episode of star formation. Based on optical properties such as brightness and color some of the normal stars in the cluster are known to have masses of about 40 suns. Since the progenitor of the neutron star has already exploded as a supernova, its mass must have been more than 40 solar masses.

Introductory astronomy courses sometimes teach that stars with more than 25 solar masses become black holes -- a concept that until recently had no observational evidence to test it. However, some theories allow such massive stars to avoid becoming black holes. For example, theoretical calculations by Alexander Heger of the University of Chicago and colleagues indicate that extremely massive stars blow off mass so effectively during their lives that they leave neutron stars when they go supernovae. Assuming that the neutron star in Westerlund 1 is one of these, it raises the question of where the black holes observed in the Milky Way and other galaxies come from.

Other factors, such as the chemical composition of the star, how rapidly it is rotating, or the strength of its magnetic field might dictate whether a massive star leaves behind a neutron star or a black hole. The theory for stars of normal chemical composition leaves a small window of initial masses - between about 25 and somewhat less than 40 solar masses - for the formation of black holes from the evolution of single massive stars. The identification of additional neutron stars or the discovery of black holes in young star clusters should further constrain the masses and properties of neutron star and black hole progenitors.

The work described by Muno was based on two Chandra observations on May 22 and June 18, 2005. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

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