Volcanic rock offers clues to antiquity of life
Posted: Monday, April 19, 2004
Source: National Science Foundation
Shell Beads From South African Cave Show Modern Human Behavior 75,000 Years Ago
ARLINGTON, Va.- Perforated shells found at South Africa's Blombos Cave appear to have been strung as beads about 75,000 years ago-making them 30,000 years older than any previously identified personal ornaments. Archaeologists excavating the site on the on the coast of the Indian Ocean discovered 41 shells, all with holes and wear marks in similar positions, in a layer of sediment deposited during the Middle Stone Age (MSA).
"The Blombos Cave beads present absolute evidence for perhaps the earliest storage of information outside the human brain," says Christopher Henshilwood, program director of the Blombos Cave Project and professor at the Centre for Development Studies of the University of Bergen in Norway.
The shells, found in clusters of up to 17 beads, are from a tiny mollusk scavenger, Nassarius kraussianus, which lives in estuaries. They must have been brought to the cave site from the nearest rivers, 20 kilometers east or west on the coast. The shells appear to have been selected for size and deliberately perforated, suggesting they were made into beads at the site or before transport to the cave. Traces of red ochre indicate that either the shell beads themselves or the surfaces against which they were worn were coated with this widely used iron oxide pigment.
A few years ago, Blombos excavators found chunks of inscribed ochre and shaped bone tools that challenged the then-dominant theory of behavioral evolution, which held that humans were anatomically modern at least 160,000 years ago but didn't develop critical modern behaviors until some punctuating event 40,000 or 50,000 years ago. Henshilwood and his colleagues (including Francesco d'Errico and Marian Vanhaeren of the University of Bordeaux, France, and Karen van Niekerk of the University of Bergen) believe the Blombos bone tools and ochre show that modern behavior like the use of external symbols developed gradually throughout the Middle Stone Age, not suddenly when our ancestors spread from Africa to Eurasia. Similar finds at other recently excavated African sites support their belief.
Some critics have argued that those African artifacts did not represent true symbol use. The newly found beads strengthen Henshilwood's assertion considerably. According to Henshilwood, "Agreement is widespread that personal ornaments, such as beads, incontrovertibly represent symbolically mediated modern behaviour. Until now, the oldest beads in Africa date to about 45,000 years. The discovery of 41 shell beads in sand layers at Blombos Cave accurately dated as 75,000 year old provides important new evidence for early symbolically organized behaviour in Africa."
Blombos Cave contains artifacts from both the Middle and Later Stone Ages. The artifact-rich layers are clearly separated by a layer of dune sand deposited about 70,000 years ago. While LSA strata, which are less than 2000 years old, also contain Nassarius shells, they are a different color from those in the MSA strata. Also the LSA shell sizes and the placement of the piercing differ from, and are less uniform than, the MSA shells. Sand grains surrounding the MSA artifacts, dated by optically stimulated luminescence, show they were buried-removed from sunlight, which "resets" the dating clock-75,000 years ago. Burnt lithics, or stone, found nearby in the same strata, were independently dated by thermoluminescent techniques as 77,000 years old. Thousands of individual grains of sand were dated to search for signs of mixing between the Middle and Later Stone Age layers; none was detected.
Henshilwood and coworkers thus conclude that ancient Africans deliberately selected the shells and modified them for use as beads at least 75,000 years ago. To Henshilwood, this clearly indicates that the cave's early inhabitants used symbols in modern fashion. "Once symbolically mediated behaviour was adopted by our ancestors it meant communication strategies rapidly shifted," he says, "leading to the transmission of individual and widely shared cultural values - traits that typify our own behaviour."
Excavation of the Blombos site has been funded by the National Science Foundation (US), the South African National Research Foundation, the Center National del la Rechereche Scientifique, the European Science Foundation, The University of Bergen, the Anglo Americans Chairman's Fund and the British Council.
Send page by E-Mail Missing link found in black hole
Posted: Saturday, April 17, 2004
Astronomers have found support for the existence of medium-sized black holes, the journal Nature has reported.
Nature said on Thursday that such black holes may also provide the link between two other types of the phenomenon.
Likely to be formed in dense star clusters, medium-sized black holes could prove the existence of stellar-mass black holes and supermassive ones that reside in galaxy centres.
Stellar-mass black holes are between two and 10 times the Sun's mass, while supermassive ones are between a million and a billion times the Sun's mass.
Collisions between stars in the flattened disc of spiral galaxies are very unlikely because the spaces in between the stars are so great.
Stellar collisions are more likely to occur in clusters of new stars, where the more massive ones sink in towards the cluster's core, collide and merge.
"The result is tantalising - this could well be how the building blocks of supermassive black holes formed"
Simon Portegies Zwart, of the University of Amsterdam, Netherlands, and colleagues ran computer simulations using MGG 11 galaxy as their model.
They demonstrated that repeated collisions between stars could produce a so-called "runaway star" with increasing mass.
Once those stars exceed 260 solar masses, they collapsed to form black holes without a significant loss of their mass.
The researchers believe that this process may produce medium-sized black holes of between about 100 and 1,000 solar masses.
Some also believe it could provide a foundation for the formation of supermassive black holes.
"The result is tantalising - this could well be how the building blocks of supermassive black holes formed," says Nate McCrady, from the University of California at Berkeley, US.
Send page by E-Mail Star Making Peaked Five Billion Years Ago
Posted: Tuesday, April 13, 2004
Source: University Of Pennsylvania
Star Making Peaked Five Billion Years Ago; Expect Darkness
PHILADELPHIA -- The universe reached the climax of its star-building activity five billion years ago -- more recently than previously thought -- according to researchers at the University of Pennsylvania and the University of Edinburgh.
The astronomers sifted through the fossil record of 96,545 nearby galaxies to chronicle the complete history of star formation over time. Their findings, reported in the April 8 issue of the journal Nature, also determined that the more massive a galaxy the earlier its stars were formed, indicating that galaxies form stars differently depending on weight.
"If we want to understand how structure in the Universe formed and evolved, then we need to understand the history of the stars," said Raul Jimenez, an assistant professor in Penn Department of Physics and Astronomy. "Fortunately we can read the history of the stars. By analyzing all of the light coming out of a particular galaxy that is, the entire spectrum of visible light we can effectively see the entire 'fossil record' of that galaxy at one glance."
While paleontologists generally need to dig down to find their fossil record, Jimenez and his colleagues needed only to look up, in this case to look up data from the Sloan Digital Sky Survey. To sift through data from nearly 100,000 galaxies, Jimenez and Edinburgh astronomer Alan Heavens created a program called Multiple Optimized Parameter Estimation and Data Compression (MOPED) that analyzes spectrum data quickly by compressing information into more manageable blocks.
"Stars of different masses evolve with different luminosities, so by looking at the integrated spectrum of a galaxy we can track those different luminosities, their masses and, therefore, how long ago they were born," Jimenez said. "Our method takes into account all the stars that are present in the observed galaxies today and allows us to create the most complete history of star formation yet assembled."
According to the researchers' findings, star formation in the universe peaked, on average, about five billion years ago. By the time our own sun was born, about 4.7 billion years ago, almost half of the stellar mass in the universe since the big bang was already created. Star formation has drastically dropped off since then and, as new stars are not being created faster than old stars are dying, this will lead to the gradual dimming of the universe.
The findings also show a difference in star formation between low-mass and high-mass galaxies. Galaxies with a higher mass, our own Milky Way among them, formed most of their stars well before galaxies of a lower mass did.
"The mass-dependence of the star-formation history explains why previous surveys showed a much earlier date for star formation, since those studies were only able to examine more massive galaxies," Jimenez said.
In an accompanying paper submitted to the Monthly Notices of the Royal Astronomical Society, Jimenez and his colleagues have explored how these results shed light on the assembly of dark matter, the mysterious component of the universe that holds galaxies together.
Contributing astronomers also include Benjamin Panter and James Dunlop of the Institute for Astronomy at the University of Edinburgh.
Funding for this research was partly provided by the National Science Foundation with a grant to Jimenez.
Send page by E-Mail Black Hole Fits Inside Earth's Orbit
Posted: Monday, April 5, 2004
Radio Astronomers Lift 'Fog' On Milky Way's Dark Heart; Black Hole Fits Inside Earth's Orbit
Source: University Of California - Berkeley
Thirty years after astronomers discovered the mysterious object at the exact center of our Milky Way Galaxy, an international team of scientists has finally succeeded in directly measuring the size of that object, which surrounds a black hole nearly four million times more massive than the Sun. This is the closest telescopic approach to a black hole so far and puts a major frontier of astrophysics within reach of future observations. The scientists used the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to make the breakthrough.
"This is a big step forward," said Geoffrey Bower, of the University of California-Berkeley. "This is something that people have wanted to do for 30 years," since the Galactic center object, called Sagittarius A* (pronounced "A-star"), was discovered in 1974. The astronomers reported their research in the April 1 edition of Science Express.
"Now we have a size for the object, but the mystery about its exact nature still remains," Bower added. The next step, he explained, is to learn its shape, "so we can tell if it is jets, a thin disk, or a spherical cloud."
The Milky Way's center, 26,000 light-years from Earth, is obscured by dust, so visible-light telescopes cannot study the object. While radio waves from the Galaxy's central region can penetrate the dust, they are scattered by turbulent charged plasma in the space along the line of sight to Earth. This scattering had frustrated earlier attempts to measure the size of the central object, just as fog blurs the glare of distant lighthouses.
"After 30 years, radio telescopes finally have lifted the fog and we can see what is going on," said Heino Falcke, of the Westerbork Radio Observatory in the Netherlands, another member of the research team.
The bright, radio-emitting object would fit neatly just inside the path of the Earth's orbit around the Sun, the astronomers said. The black hole itself, they calculate, is about 14 million miles across, and would fit easily inside the orbit of Mercury. Black holes are concentrations of matter so dense that not even light can escape their powerful gravity.
The new VLBA observations provided astronomers their best look yet at a black hole system. "We are much closer to seeing the effects of a black hole on its environment here than anywhere else," Bower said.
The Milky Way's central black hole, like its more-massive cousins in more-active galactic nuclei, is believed to be drawing in material from its surroundings, and in the process powering the emission of the radio waves. While the new VLBA observations have not provided a final answer on the nature of this process, they have helped rule out some theories, Bower said. Based on the latest work, he explained, the top remaining theories for the nature of the radio- emitting object are jets of subatomic particles, similar to those seen in radio galaxies; and some theories involving matter being accelerated near the edge of the black hole.
As the astronomers studied Sagittarius A* at higher and higher radio frequencies, the apparent size of the object became smaller. This fact, too, Bower said, helped rule out some ideas of the object's nature. The decrease in observed size with increasing frequency, or shorter wavelength, also gives the astronomers a tantalizing target.
"We think we can eventually observe at short enough wavelengths that we will see a cutoff when we reach the size of the black hole itself," Bower said. In addition, he said, "in future observations, we hope to see a 'shadow' cast by a gravitational lensing effect of the very strong gravity of the black hole."
In 2000, Falcke and his colleagues proposed such an observation on theoretical grounds, and it now seems feasible. "Imaging the shadow of the black hole's event horizon is now within our reach, if we work hard enough in the coming years," Falcke added.
Another conclusion the scientists reached is that "the total mass of the black hole is very concentrated," according to Bower. By making the "most precise localization of the mass of a supermassive black hole ever," the astronomers said that a mass of at least 40,000 Suns has to reside in a space corresponding to the size of the Earth's orbit. Most likely, however, all the black hole's mass -- equal to four million Suns -- is concentrated well inside the area engulfed by the radio-emitting object.
To make their measurement, the astronomers had to go to painstaking lengths to circumvent the scattering effect of the plasma "fog" between Sagittarius A* and Earth. "We had to push our technique really hard," Bower said.
Bower likened the task to "trying to see your yellow rubber duckie through the frosted glass of the shower stall." By making many observations, only keeping the highest-quality data, and mathematically removing the scattering effect of the plasma, the scientists succeeded in making the first-ever measurement of Sagittarius A*'s size.
In addition to Bower and Falcke, the research team includes Robin Herrnstein of Columbia University, Jun-Hui Zhao of the Harvard-Smithsonian Center for Astrophysics, Miller Goss of the National Radio Astronomy Observatory, and Donald Backer of the University of California-Berkeley. Falcke also is an adjunct professor at the University of Nijmegen and a visiting scientist at the Max-Planck Institute for Radioastronomy in Bonn, Germany.
Sagittarius A* was discovered in February of 1974 by Bruce Balick, now at the University of Washington, and Robert Brown, now director of the National Astronomy and Ionospheric Center at Cornell University. It has been shown conclusively to be the center of the Milky Way, around which the rest of the Galaxy rotates. In 1999, Mark Reid of the Harvard-Smithsonian Center for Astrophysics and his colleagues used VLBA observations of Sagittarius A* to detect the Earth's motion in orbit around the Galaxy's center and determined that our Solar System takes 226 million years to make one circuit around the Galaxy.
In March 2004, 55 astronomers gathered at the National Radio Astronomy Observatory facility in Green Bank, West Virginia, for a scientific conference celebrating the discovery of Sagittarius A* at Green Bank 30 years ago. At this conference, the scientists unveiled a commemorative plaque on one of the discovery telescopes.
The Very Long Baseline Array, part of the National Radio Astronomy Observatory, is a continent-wide radio-telescope system, with 10, 240-ton dish antennas ranging from Hawaii to the Caribbean. It provides the greatest resolving power, or ability to see fine detail, of any telescope in astronomy, on Earth or in space.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
UCLA Astronomers Detect Plasma At Black Hole
'Supermassive' Black Hole Found In The Center Of Our Galaxy
Send page by E-Mail Fins to Limbs: New Fossil Gives Evolution Insight
Posted: Thursday, April 1, 2004
for National Geographic News
April 1, 2004
Today researchers announced their discovery of a 365-million-year-old fossil limb bone of an ancient tetrapod. Tetrapods, including humans, are four-limbed animals with backbones. The fossil was found during road construction that revealed an ancient streambed.
See an illustration of the animal and a fin-to-limb evolution chart.
Scientists say the find will help shed light on how early animals evolved limbs from fins. This crucial adaptation enabled Earth's animal life to crawl from water to land.
The bone-a humerus, or upper arm or forelimb-is one of the earliest tetrapod limb bones ever found. (Tetrapods today include amphibians, mammals, reptiles, and birds, among others.)
The ancient bone shares features with primitive fish fins, but also has characteristics of a true limb bone. It bridges the gap between fish and amphibian.
"The transition wasn't all or nothing," said Ted Daeschler, a vertebrate zoologist with the Academy of Natural Sciences in Philadelphia "It's not that some animals were thrown on land. There were certainly other functions intermediate."
Daeschler and colleagues Neil Shubin and Michael Coates, paleontologists at the University of Chicago, say the fossil bone offers a window onto this intermediate stage.
The National Geographic Society Committee for Research and Exploration and the National Science Foundation supported the scientists' research. The trio describe the fossil in tomorrow's issue of the journal Science.
From Stream to Shore
"This new humerus shows some primitive features that are lost in later tetrapods but in this specimen have already begun to change their orientation into tetrapod-like configuration," said Jennifer Clack, a paleontologist at the University Museum of Zoology Cambridge, United Kingdom.
Clack, an expert on the fish-tetrapod transition, also wrote an essay on the discovery for Science.
To understand how tetrapods evolved limbs from fins, Shubin said it helps to imagine the environment that these creatures lived in. "Think of a shallow stream choked with plants, not of an open sea," he said. "At some level, these shallow streams approach a more terrestrial environment in the ways that animals would move around."
According to Daeschler, such an environment would require fish to have limblike fins to propel themselves along the surface of shallow waters, hold their position in a current, or lift their head to the water's surface to gulp air.
The humerus-which the scientists say clearly belongs to a limbed amphibian and not a lobbed-fin fish-suggests that the tetrapod was an animal that had a powerful forelimb with a large area for the attachment of muscles at the shoulder. Such a muscle is associated with the ability to perform a push-up.
"The [newfound] humerus enables comparisons with fish that were not possible until now," Shubin said. "There is a large crest on the lower side of this humerus-it is where the pectoral muscle would attach. This same expanded crest is seen in fish."
The presence of a place to attach a pectoral muscle in both fish and tetrapods suggests that the ability to perform a push-up is ancient, evolving first in fish and not in terrestrial animals as was originally believed.
"The notion is that this movement is primitive," Shubin said. He believes the movement first arose in fish that required the appendages to move. That motion was akin to how creatures eventually used limbs to walk on land, Shubin said.
According to the researchers, however, fish with limblike fins had no intention of walking on land. Rather, they were adapting to their environment.
"When fish used their fins to prop themselves up on the bottom … it was a useful invention at that time" that helped fish "make a good living" feeding in their aquatic environment, Daeschler said.
Roadside Fossil Trove
The ancient tetrapod humerus is among several plant and animal fossils collected from an ancient streambed in north central Pennsylvania in 1993. Like many fossil sites in the region, the streambed was exposed as road crews cut through the bedrock in the course of constructing a highway.
"What we need are fresh exposures of bedrock. Pennsylvania has wonderful sequences of rock from this time period," Daeschler said. "But most of it is covered by forest or farmland."
So when road crews cut through bedrock, researchers make arrangements to study the fresh exposures. They often bring layers of rock back to their labs for painstaking analysis.
The research team has previously discovered fossil remains of two other tetrapods-Hynerpeton and Densignathus-from this same ancient streambed. But the significance of the new humerus went undetected for several years, since only a small portion of the bone was exposed.
In 2001 Fred Mullison, a staff scientist at the Academy of Natural Sciences in Philadelphia, excavated the bone from the rock. Only then did the fossil's import became apparent.
Daeschler and colleagues say they are unable to discern whether the humerus belongs to Hynerpeton, Densignathus, or an entirely new tetrapod species.
"We can't prove it's either or neither of them. So rather than throw another name on this thing … we're saying it is an early tetrapod humerus," Daeschler said.
The research team hopes further excavations will reveal more tetrapod fossils and provide more insights into a key evolutionary milestone-fins to limbs-of life on Earth.
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