Cell evolution puzzle
Posted: Tuesday, June 17, 2003
Source: Washington University In St. Louis
About Face: Washington University Anthropologist Sets The Record Straight Regarding Neandertal Facial Length
New scientific evidence challenges a common perception that Neandertals -- a close evolutionary relative to modern humans that lived 230,000 to 30,000 years ago -- possessed exceptionally long faces.
Instead, a report authored by Erik Trinkaus, Ph.D., the Mary Tileston Hemenway Professor of Anthropology in Arts & Sciences at Washington University in St. Louis, shows that modern humans are really the "odd man out" when it comes to facial lengths, which drop off dramatically compared with their ancestral predecessors.
Trinkaus' findings, which will appear in a summer 2003 issue of the journal Proceedings of the National Academy of Sciences (PNAS), are based on two critical skull measurements on fossilized specimens. His article, "Neandertal faces were not long; modern human faces are short," will be available online the week of June 16 on the PNAS Web site, www.pnas.org.
Trinkaus' main objective was to see how Neandertal faces stacked up against others in the evolutionary lineage: 179 "recent humans" (dating back to the 18th century); 26 Late Pleistocene early modern humans; 24 Neandertals; and 23 archaic human Neandertal predecessors.
His research has effectively established a baseline for future anthropologists to categorize evolutionary patterns as being ancestral (having traits similar to those present in a remote ancestor) or derived (traits that have undergone a recent change).
"Basically, the issue is whether the 'big' Neandertal face is simply something they inherited from their ancestors, or whether it is something that is uniquely derived for them -- something that makes them divergent in human evolution," Trinkaus said. "This was just a short paper to demonstrate that Neandertals, put in their proper evolutionary context, do not have big faces; as the title says, modern humans have short ones."
In order to assess facial length, Trinkaus first measured the prosthion radius, or the distance that extends from the ear-hole out to the roots of the incisors. The second measurement, the mandibular superior length, is a projection of the linear distance from the middle of the condyles (a point on the jaw-joint) to the midpoint between the incisors. Together, the two measurements take into account how far the incisors project out relative to the core of the skull.
Data for older specimens -- which are limited both in number and by degree of preservation -- were taken from previously documented discoveries from around the world. For the recent humans, however, Trinkaus worked in a more hands-on fashion, spending a day measuring the skulls of 18th- and 19th-century "Old World humans" at the American Museum of Natural History in New York, which boasts the most geographically diverse sample of skeletons in North America.
After compiling the data for different groups of hominids, Trinkaus concluded that the Neandertal's overall facial projection was, if anything, average for a Pleistocene epoch sample and was similar to or even modestly reduced from their non-Neandertal archaic predecessors.
He also noted that their size was only moderately greater than those of early modern humans, but principally contrasted with recent, late Holocene humans (humans of today). Thus, from an evolutionary standpoint, there was nothing uniquely derived about Neandertal face lengths.
Still, Trinkaus sees several possible explanations for the misconception. First, the majority of the more complete Neandertal skulls that anthropologists have thus far unearthed happen to be those of large males, each of whose facial length scales with the rest of their body.
Trinkaus says there is also the influence of "time's arrow" -- that is, for most of the last century, Neandertals have been compared primarily with recent humans and not to their own predecessors. And quite simply, despite major discoveries over the last 40 years, no one had previously published a comprehensive study comparing Neandertal facial lengths to both recent humans and earlier ancestors.
"It was acceptable in 1950 to say Neandertals had big faces," Trinkaus said. "It's not acceptable in 2003."
Yet another possible reason was illustrated by the intense media fervor surrounding the 1999 discovery of a juvenile skeleton from Portugal. The skeleton is basically early modern human but bears some distinctive Neandertal characteristics, raising the possibility of interbreeding. Trinkaus asserts that the erroneous characterization that Neandertals had long faces may, at least in part, have been a way to distance ourselves from a more "primitive" evolutionary relation.
"Neandertals are the archaic humans that are closest to us," Trinkaus noted. "Closest to us in time, closest to us in behavior and in many aspects of their anatomy. They're very human but they're not quite us, and many people have great difficulty accepting that we may be closely linked to the Neandertals. Their world-view does not allow them to accept it because it downplays the idea that modern humans are very unique, that modern humans are very special."
So if Neandertals didn't have long faces, what's the significance of humans having short ones?
"As you go further back in time, the periods encompassed by a sample get bigger and bigger," Trinkaus concluded. "Faces basically remain long or decreased very slightly, but with modern humans they get a whole lot shorter very quickly. So the question is, 'why did they get shorter?' I don't know.
"But the point of this paper was not to answer that question, it was to allow us to frame that question. Because previously the question was, 'why did Neandertals have long faces?' And that's the wrong question.
"The correct question is -- 'why do modern humans have really short ones?'"
This story has been adapted from a news release issued by Washington University In St. Louis.
Send page by E-Mail Birds Do It. Bugs Do It. But Why Don't We?
Posted: Tuesday, June 17, 2003
Source: University Of Michigan
ANN ARBOR, Mich. -- Many creatures including our fellow primates the New World Monkeys rely on highly specific scent molecules called pheromones to find a suitable mate. Even our humble mammal cousin, the mouse, was found to have 140 genes just for pheromone receptors when its genome was completely sequenced earlier this year.
But humans are clueless when it comes to pheromone signals, according to University of Michigan evolutionary biologist Jianzhi "George" Zhang. He believes color vision put our pheromones out of business.
Our closest relatives on the primate family tree rely on "sexual swelling" and gaudy, colorful patches of skin to signal their reproductive fitness and fertility, Zhang said. In fact, though humans and these apes still carry genes that should create pheromone receptors in our noses, these genes have mutated to the point that they are merely pseudogenes---they don't function any more.
Zhang has used the genes of people and primates to get at the answer to this intriguing puzzle. Zhang (pronounced Zong), is an assistant professor in Ecology and Evolutionary Biology in the College of Literature, Science and Arts. Zhang's paper on the topic appears this week in the online Proceedings of the National Academy of Sciences.
Zhang believes that a significant gene duplication made the difference and that it happened sometime between 23 million years ago and the split of the New World and Old World primates about 35 million years ago.
An ancestor of the Old World primates (humans, chimps, gorillas, orangutans, gibbons, baboons and guerezas) developed a second copy of the red/green color-vision gene, which resides on the X chromosome. Female New World monkeys have full color vision because females have two X chromosomes that harbor both red and green color vision genes. But males only have one X chromosome, so New World males have only one copy of either the red or green gene, and that leaves them color-blind. After the red/green gene duplication in the Old World family however, even the males got color vision too.
"Color vision made pheromones unnecessary," Zhang said. As a channel for sexual signaling, color vision works better at a distance than pheromones, Zhang believes. A pheromone attaches to a water molecule, drifts about in the air currents and finally lands on the proper receptor in someone else's nose. The receiver can't immediately be sure who sent it, where it came from or when. But with sexual swelling, everyone in the troop can see precisely when and where the signal is, even at a significant distance.
Sexual swelling occurs in about 10 percent of all primate species, but only in the Old World species of Africa and Asia, which is where humans probably originated, as well.
To test their idea, Zhang's team zeroed in on a human gene called TRP2, which makes an ion channel that is unique to the pheromone signaling pathway. They found that in humans and Old World primates, this gene suffered a mutation just over 23 million years ago that rendered it dysfunctional. But because we could use color vision for mating, it didn't hurt us. In turn, the pheromone receptor genes that rely on this ion channel fell into disuse, and in a random fashion, mutated to a dysfunctional state because they haven't experienced any pressure from natural selection. Zhang calls this process "evolutionary deterioration."
Zhang's paper, "Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrrhine primates" appears in the Proceedings of the National Academy of Sciences online (http://www.pnas.org/cgi/doi/10.1073/pnas.1331721100)
For more information about Zhang, http://www.eeb.lsa.umich.edu/eebfacultydetails.asp?ID=96
For more information on the topic, visit "Pheromones and Mammals" (Howard Hughes Medical Institute)http://www.hhmi.org/senses/d230.html
The original news release can be found here.
Send page by E-Mail Stars Discovered In Centaurus A
Posted: Wednesday, June 11, 2003
Source: European Southern Observatory
One Thousand 'Wonderful' Stars Discovered In Centaurus A; First-ever Census Of Variable Mira-type Stars In Galaxy Outside The Local Group
An international team led by ESO astronomer Marina Rejkuba  has discovered more than 1000 luminous red variable stars in the nearby elliptical galaxy Centaurus A (NGC 5128). Brightness changes and periods of these stars were measured accurately and reveal that they are mostly cool long-period variable stars of the so-called "Mira-type". The observed variability is caused by stellar pulsation.
This is the first time a detailed census of variable stars has been accomplished for a galaxy outside the Local Group of Galaxies (of which the Milky Way galaxy in which we live is a member).
It also opens an entirely new window towards the detailed study of stellar content and evolution of giant elliptical galaxies. These massive objects are presumed to play a major role in the gravitational assembly of galaxy clusters in the Universe (especially during the early phases).
This unprecedented research project is based on near-infrared observations obtained over more than three years with the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope at the ESO Paranal Observatory.
The full text of this Press Release, with nine photos (ESO PR Photos 14a-i/03) and all related links, is available at: http://www.eso.org/outreach/press-rel/pr-2003/pr-13-03.html.
Mira-type variable stars
Centaurus A (NGC 5128) is the nearest giant galaxy, at a distance of about 13 million light-years. It is located outside the Local Group of Galaxies to which our own galaxy, the Milky Way, and its satellite galaxies, the Magellanic Clouds, belong.
Centaurus A is seen in the direction of the southern constellation Centaurus. It is of elliptical shape and is currently merging with a companion galaxy, making it one of the most spectacular objects in the sky, cf. PR Photo 14a/03. It possesses a very heavy black hole at its centre (see ESO PR 04/01) and is a source of strong radio and X-ray emission.
During the present research programme, two regions in Centaurus A were searched for stars of variable brightness; they are located in the periphery of this peculiar galaxy, cf. PR Photos 14b-d/03. An outer field ("Field 1") coincides with a stellar shell with many blue and luminous stars produced by the on-going galaxy merger; it lies at a distance of 57,000 light-years from the centre. The inner field ("Field 2") is more crowded and is situated at a projected distance of about 30,000 light-years from the centre.
Three years of VLT observations
Under normal circumstances, any team of professional astronomers will have access to the largest telescopes in the world for only a very limited number of consecutive nights each year. However, extensive searches for variable stars like the present require repeated observations lasting minutes-to-hours over periods of months-to-years. It is thus not feasible to perform such observations in the classical way in which the astronomers travel to the telescope each time.
Fortunately, the operational system of the VLT at the ESO Paranal Observatory (Chile) is also geared to encompass this kind of long-term programme. Between April 1999 and July 2002, the 8.2-m VLT ANTU telescope on Cerro Paranal in Chile) was operated in service mode on many occasions to obtain K-band images of the two fields in Centaurus A by means of the near-infrared ISAAC multi-mode instrument. Each field was observed over 20 times in the course of this three-year period; some of the images were obtained during exceptional seeing conditions of 0.30 arcsec. One set of complementary optical images was obtained with the FORS1 multi-mode instrument (also on VLT ANTU) in July 1999.
Each image from the ISAAC instrument covers a sky field measuring 2.5 x 2.5 arcmin^2. The combined images, encompassing a total exposure of 20 hours are indeed the deepest infrared images ever made of the halo of any galaxy as distant as Centaurus A, about 13 million light-years.
Discovering one thousand Mira variables
Once the lengthy observations were completed, two further steps were needed to identify the variable stars in Centaurus A.
First, each ISAAC frame was individually processed to identify the thousands and thousands of faint point-like images (stars) visible in these fields. Next, all images were compared using a special software package ("DAOPHOT") to measure the brightness of all these stars in the different frames, i.e., as a function of time.
While most stars in these fields as expected were found to have constant brightness, more than 1000 stars displayed variations in brightness with time; this is by far the largest number of variable stars ever discovered in a galaxy outside the Local Group of Galaxies.
The detailed analysis of this enormous dataset took more than a year. Most of the variable stars were found to be of the Mira-type and their light curves (brightness over the pulsation period) were measured, cf. PR Photo 14i/03. For each of them, values of the characterising parameters, the period (days) and brightness amplitude (magnitudes) were determined. A catalogue of the newly discovered variable stars in Centaurus A has now been made available to the astronomical community via the European research journal Astronomy & Astrophysics.
Marina Rejkuba is pleased and thankful: "We are really very fortunate to have carried out this ambitious project so successfully. It all depended critically on different factors: the repeated granting of crucial observing time by the ESO Observing Programmes Committee over different observing periods in the face of rigorous international competition, the stability and reliability of the telescope and the ISAAC instrument over a period of more than three years and, not least, the excellent quality of the service mode observations, so efficiently performed by the staff at the Paranal Observatory."
What have we learned about Centaurus A?
The present study of variable stars in this giant elliptical galaxy is the first-ever of its kind. Although the evaluation of the very large observational data material is still not finished, it has already led to a number of very useful scientific results.
Confirmation of the presence of an intermediate-age population
Based on earlier research (optical and near-IR colour-magnitude diagrams of the stars in the fields), the present team of astronomers had previously detected the presence of intermediate-age and young stellar populations in the halo of this galaxy. The youngest stars appear to be aligned with the powerful jet produced by the massive black hole at the centre.
Some of the very luminous red variable stars now discovered confirm the presence of a population of intermediate-age stars in the halo of this galaxy. It also contributes to our understanding of how giant elliptical galaxies form.
New measurement of the distance to Centaurus A
The pulsation of Mira-type variable stars obeys a period-luminosity relation. The longer its period, the more luminous is a Mira-type star.
This fact makes it possible to use Mira-type stars as "standard candles" (objects of known intrinsic luminosity) for distance determinations. They have in fact often been used in this way to measure accurate distances to more nearby objects, e.g., to individual clusters of stars and to the center in our Milky Way galaxy, and also to galaxies in the Local Group, in particular the Magellanic Clouds.
This method works particularly well with infrared measurements and the astronomers were now able to measure the distance to Centaurus A in this new way. They found 13.7 +- 1.9 million light-years, in general agreement with and thus confirming other methods.
Study of stellar population gradients in the halo of a giant elliptical galaxy
The two fields here studied contain different populations of stars. A clear dependence on the location (a "gradient") within the galaxy is observed, which can be due to differences in chemical composition or age, or to a combination of both.
Understanding the cause of this gradient will provide additional clues to how Centaurus A - and indeed all giant elliptical galaxies - was formed and has since evolved.
Comparison with other well-known nearby galaxies
Past searches have discovered Mira-type variable stars thoughout the Milky Way, our home galaxy, and in other nearby galaxies in the Local Group. However, there are no giant elliptical galaxies like Centaurus A in the Local Group and this is the first time it has been possible to identify this kind of stars in that type of galaxy.
The present investigation now opens a new window towards studies of the stellar constituents of such galaxies.
The research described in this press release is presented in a research article, soon to appear in the European research journal Astronomy & Astrophysics ("Long Period Variables in NGC 5128: I. Catalogue" by Marina Rejkuba et al.). It is available on the web as astro-ph/0305432.
 The team consists of Marina Rejkuba and David Silva (ESO-Garching, Germany), Tim Bedding (School of Physics and Astronomy, Sydney University, Australia) and Dante Minniti (Departamento de Astronomia and FONDAP Center for Astrophysics, Pontifica Universidad Catolica de Chile, Santiago, Chile).
 Mira (Omicron Ceti), a cool red giant star, is one of the most famous variable stars in the sky. David Fabricius (East Friesland, Germany) is recognized to have discovered it in 1596 while searching for planet Mercury. In 1638, Johann Holwarda from the same area determined its period to be 11 months and so established it as the first long-period variable discovered. In 1642, Johannes Hevelius (Danzig, now Poland) named the star Mira, "The Wonderful". It has later been found to be a binary star system. An entire class of variable stars has been named after this star. By astronomical definition, "Mira-type" stars are variables with a period of around 100 - 1000 days and with visual light variations of more than 2.5 magnitudes.
The original news release can be found here
Send page by E-Mail Oldest 'modern' human skulls found in Africa
Posted: Wednesday, June 11, 2003
Source: University Of California - Berkeley
160,000-year-old Fossilized Skulls From Ethiopia Are Oldest Modern Humans
Berkeley -- The fossilized skulls of two adults and one child discovered in the Afar region of eastern Ethiopia have been dated at 160,000 years, making them the oldest known fossils of modern humans, or Homo sapiens.
The skulls, dug up near a village called Herto, fill a major gap in the human fossil record, an era at the dawn of modern humans when the facial features and brain cases we recognize today as human first appeared.
The fossils date precisely from the time when biologists using genes to chart human evolution predicted that a genetic "Eve" lived somewhere in Africa and gave rise to all modern humans.
"We've lacked intermediate fossils between pre-humans and modern humans, between 100,000 and 300,000 years ago, and that's where the Herto fossils fit," said paleoanthropologist Tim White, professor of integrative biology at the University of California, Berkeley, and a co-leader of the team that excavated and analyzed the discovery site. "Now, the fossil record meshes with the molecular evidence."
"With these new crania," he added, "we can now see what our direct ancestors looked like."
"This set of fossils is stupendous," said team member F. Clark Howell, UC Berkeley professor emeritus of integrative biology and co-director with White of UC Berkeley's Laboratory for Human Evolutionary Studies. "This is a truly revolutionary scientific discovery."
Howell added that these anatomically modern humans pre-date most neanderthals, and therefore could not have descended from them, as some scientists have proposed.
The international team is led by White and his Ethiopian colleagues, Berhane Asfaw of the Rift Valley Research Service in Addis Ababa, Ethiopia, and Giday WoldeGabriel of Los Alamos National Laboratory in New Mexico. The results of the find will be reported in two papers in the June 12 issue of the journal Nature.
The research team also unearthed skull pieces and teeth from seven other hominid individuals, hippopotamus bones bearing cut marks from stone tools, and more than 600 stone tools, including hand axes. All are from the same sediments and, thus, the same era.
"These were people using a sophisticated stone technology," White said. "Using chipped hand axes and other stone tools, they were butchering carcasses of large mammals like hippos and buffalo and undoubtedly knew how to exploit plants."
They lived long before most examples of another early hominid, the neanderthal, or Homo neanderthalensis, proving beyond a reasonable doubt, White said, that Homo sapiens did not descend from these short, stocky creatures. More like cousins, neanderthals split off from the human tree more than 300,000 years ago and died out about 30,000 years ago, perhaps driven to extinction by modern humans.
"These well-dated and anatomically diagnostic Herto fossils are unmistakably non-neanderthal," said Howell, a co-author of the Nature paper that details the hominids and an expert on early modern humans. "These fossils show that near-humans had evolved in Africa long before the European neanderthals disappeared. They thereby demonstrate conclusively that there was never a neanderthal stage in human evolution."
Because the Herto fossils represent a transition between more primitive hominids from Africa and modern humans, they provide strong support for the hypothesis that modern humans evolved in Africa and subsequently spread into Eurasia. This hypothesis goes against the theory that modern humans arose in many areas of Europe, Asia and Africa from other hominids who had migrated out of Africa at a much earlier time.
The fossil evidence, said Asfaw, "clearly shows what molecular anthropologists have been saying for a long time - that modern Homo sapiens evolved out of Africa. These fossilized skulls from Herto show that modern humans were living at around 160,000 years ago with full-fledged Homo sapiens features. The 'Out of Africa' hypothesis is now tested, ... (and) we can conclusively say that neanderthals had nothing to do with modern humans. They went extinct."
The fossil skulls
The three fossil skulls remain in Ethiopia, but replicas made from them were compared by the research team with many examples of neanderthal and earlier hominid skulls, as well as those of modern humans. Many of the modern human comparison skulls came from a worldwide sample of skeletal remains in the collection of UC Berkeley's Hearst Museum of Anthropology.
The most complete of the three new fossil skulls, probably that of a male, is slightly larger than the extremes seen in modern Homo sapiens, yet it bears other characteristics within the range of modern humans - in particular, less prominent brow ridges than pre-Homo sapiens and a higher cranial vault. Because of these similarities, the researchers placed the fossils in the same genus and species as modern humans but appended a subspecies name - Homo sapiens idaltu -to differentiate them from contemporary humans, Homo sapiens sapiens.
Idāltu, which means "elder" in the Afar language, refers to the adult male's antiquity and individual age. The man, though probably in his late 20s to mid-30s, had heavily worn upper teeth and a brain size slightly larger than average for living people.
Scientists tracking evolution through changes in mitochondrial DNA, which is passed from mother to daughter, have estimated that humans derive their mitochrondrial genes from an ancestral mother nicknamed "Eve" who lived in Africa about 150,000 years ago. Other scientists studying the male Y chromosome have reached similar conclusions. The new Herto fossils are from a population living at exactly this time.
"In a sense, these genetic findings were impossible to seriously test without a good fossil record from Africa," said White. "Back in 1982, when Becky Cann and Allan Wilson of UC Berkeley were using molecules to study evolution, they concluded that the common ancestors of all modern humans lived in Africa 100,000 to 200,000 years ago. For the last 20 years we've been looking for good, well-dated fossil evidence of that antiquity."
Previously found fossils were younger, from sites scattered around Africa, often poorly dated and incomplete. These include fossil skull fragments from Klasies River Mouth in South Africa, dating from about 100,000 years ago, and Middle Eastern fossils from Qafzeh and Skhul dating from 90,000 to 130,000 years ago. Ethiopia has yielded some modern human fossils, including those from Omo, which are approximately 100,000 years old, and the Aduma fossil finds of the Middle Awash, which date from about 80,000 years.
While these previous discoveries appear also to be Homo sapiens, the new finds from Herto are older, well-dated and more complete without sharing characteristics of more primitive human ancestors such as Homo erectus or the neanderthals.
The fossil-rich site was discovered on Nov. 16, 1997, in a dry and dusty valley bordering the Middle Awash River near Herto, a seasonally occupied village. During a reconnaissance, White first spotted stone tools and the fossil skull of a butchered hippo emerging from the ground. When the team returned to intensively survey the area 11 days later, they discovered the most complete of the adult skulls protruding from the ancient sediment. It had been exposed by heavy rains and partially trampled by herds of cows.
A portion of the large adult's left front cranium (the braincase) had been crushed and scattered, but the team was able to excavate the rest of the skull, minus the lower jaw, and reconstruct it.
The child's skull, found nearby, was fragmented and scattered from having been exposed for many years. The team recovered most pieces of the cranium, more than 200 in all, from a 400 square-foot area, and Asfaw painstakingly pieced them together over a period of three years.
Based on the presence of unerupted teeth, the skull is that of a child of six or seven. Interestingly, this skull and a second adult's, too fragmentary to reconstruct, showed cut marks pointing to ancient mortuary practices, White said. The child's skull bore marks indicating that, after death, the muscles had been cut from the base of the skull. The rear of the cranial base was broken away and the edges polished, and the entire cranium was worn smooth as if by repeated handling. The second adult skull showed parallel scratches around the perimeter of the skull apparently made by a stone tool repeatedly drawn across the skull's surface in a pattern different from that created during defleshing, as for food. Even the nearly complete adult skull had a few cut marks.
The mortuary rituals of the Herto people differ from those of earlier hominids, some of whom cut flesh from skulls but apparently did not polish or decorate them with scratch marks. Modifications like those seen in the Herto skulls have been recorded by anthropologists from societies, including some in New Guinea, in which the skulls of ancestors are preserved and worshipped.
The Herto skulls were not found with other bones from the rest of the bodies, which is unusual, White said, leading the researchers to infer that the people "were moving the heads around on the landscape. They probably cut the muscles and broke the skull bases of some skulls to extract the brain, but why, whether as part of a cannibalistic ritual, we have no way of knowing."
The team also recovered more than 640 stone artifacts, though they estimate that the entire Herto area contains millions of such artifacts: hand axes, flake tools, cores, flakes and rare blades. Renowned African prehistorian J. Desmond Clark of UC Berkeley analyzed many of them before his death in February of last year. Clark and colleagues Dr. Yonas Beyene of Ethiopia's Authority for Research and Conservation of the Cultural Heritages and Dr. Alban Defleur of Marseilles, France, concluded that the stone tools were transitional between the Acheulean period, characterized by a predominance of hand axes, and the later flake-dominated Middle Stone Age.
"The associated fossil bones show clearly that the Herto people had a taste for hippos, but we can't tell whether they were killing them or scavenging them," said Beyene. "These artifacts are clues about the ancestors who made them."
Ancient lake shore
The early humans at Herto lived along the shores of a shallow lake created when the Awash River temporarily dammed about 260,000 years ago. The lake contained abundant hippos, crocodiles and catfish, while buffalo roamed the land.
The sediments and volcanic rock in which the fossils were found were dated at between 160,000 and 154,000 years by a combination of two methods. The argon/argon method was used by colleagues in the Berkeley Geochronology Center, led by Paul R. Renne, a UC Berkeley adjunct associate professor of geology. WoldeGabriel of Los Alamos National Laboratory and Bill Hart of Miami University in Ohio used the chemistry of the volcanic layers to correlate the dated layers.
The Middle Awash team consists of more than 45 scientists from 14 different countries who specialize in geology, archaeology and paleontology. In this single study area, the team has found fossils dating from the present to more than 6 million years ago, painting a clear picture of human evolution from ape-like ancestors to present-day humans.
"The human fossils from Herto are near the top of a well-calibrated succession of African fossils," White said. "This is clear fossil evidence that our species arose through evolution."
The work was funded by the National Science Foundation and the Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory, in combination with the Hampton Fund for International Initiatives of Miami University and the Japan Society for the Promotion of Science.
The original news release can be found here.
Send page by E-Mail When humans faced extinction
Posted: Monday, June 9, 2003
By Dr David Whitehouse
BBC News Online science editor
Humans may have come close to extinction about 70,000 years ago, according to the latest genetic research.
The study suggests that at one point there may have been only 2,000 individuals alive as our species teetered on the brink.
This means that, for a while, humanity was in a perilous state, vulnerable to disease, environmental disasters and conflict. If any of these factors had turned against us, we would not be here.
The research also suggests that humans (Homo sapiens sapiens) made their first journey out of Africa as recently as 70,000 years ago.
Unlike our close genetic relatives - chimps - all humans have virtually identical DNA. In fact, one group of chimps can have more genetic diversity than all of the six billion humans alive today.
It is thought we spilt from a common ancestor with chimps 5-6 million years ago, more than enough time for substantial genetic differences to develop.
The absence of those differences suggests to some researchers that the human gene pool was reduced to a small size in the recent past, thereby wiping out genetic variation between current populations.
Evidence for that view is published in the American Journal of Human Genetics.
Because all humans have virtually identical DNA, geneticists look for subtle differences between populations.
One method involves looking at so-called microsatellites - short, repetitive segments of DNA that differ between populations.
These microsatellites have a high mutation, or error, rate as they are passed from generation to generation, making them a useful tool to study when two populations diverged.
Researchers from Stanford University, US, and the Russian Academy of Sciences compared 377 microsatellite markers in DNA collected from 52 regions around the world.
Analysis revealed a close genetic kinship between two hunter-gatherer populations in sub-Saharan Africa - the Mbuti pygmies of the Congo Basin and the Khosian bushmen of Botswana.
The researchers believe that they are "the oldest branch of modern humans studied here".
The data also reveals that the separation between the hunter-gatherer populations and farmers in Africa occurred between 70,000 and 140,000 years ago. Modern man's migration out of Africa would have occurred after this.
An earlier genetic study - involving the Y chromosomes of more than 1,000 men from 21 populations - concluded that the first human migration from Africa may have occurred about 66,000 years ago.
The small genetic diversity of modern humans indicates that at some stage during the last 100,000 years, the human population dwindled to a very low level.
It was out of this small population, with its consequent limited genetic diversity, that today's humans descended.
Estimates of how small the human population became vary but 2,000 is the figure suggested in the latest research.
"This estimate does not preclude the presence of other populations of Homo sapiens sapiens (modern man) in Africa, although it suggests that they were probably isolated from each other genetically," they say.
The authors of the study believe that contemporary worldwide populations descended from one or very few of these populations.
If this is the case, humanity came very close to extinction.
Send page by E-Mail Monkey's Memory Cells Caught In The Act Of Learning
Posted: Monday, June 9, 2003
Source: NIH/National Institute Of Mental Health
NIH-funded scientists have detected direct evidence of individual brain cells signaling the formation of new memories. Neurons they call "changing cells" in the hippocampus, the brain's memory hub, emit telltale signals as a monkey learns an associative memory task, the researchers have discovered. While past studies established that new associative memories such as learning the name of a new acquaintance -- can't be learned without the hippocampus, none had pinpointed such smoking guns of memory acquisition at the neural level. Wendy Suzuki, Ph.D., New York University (NYU), and colleagues, report their findings in the June 6, 2003 Science. "When hippocampal cells undergo these striking changes in neuronal activity, its like watching a new memory being born," explained Suzuki, whose research was funded by the National Institute of Mental Health (NIMH) and the National Institute on Drug Abuse (NIDA). "The key to detecting 'changing cells' was to keep the task simple so that the monkeys could easily learn multiple new associations within a single recording session."
The researchers measured activity of individual neurons in the hippocampus as two monkeys formed new associative memories while playing a computer game. The monkey had to gaze at one of four identical targets superimposed on a complex visual scene to win a juice reward. For example, white dots appeared at four locations over a picture of a gorilla in a forest. The animals learned, through trial and error, which location was associated with the reward for each new scene. Eye-tracking technology detected if the animal was fixated on the correct dot and the reward was dispensed. Monkeys learned 2-4 new associations during each recording session. It took about a dozen trials to establish the memory for each scene.
Among 89 neurons that responded to the scenes, the researchers discovered a subset of 25 "changing cells" whose increase or decrease in activity paralleled learning, signaling that they were involved in the learning process. The changes in neural activity took place before, during or immediately after learning, suggesting that there is a gradual recruitment of a network of hippocampal neurons associated with the formation of new associative memories. By relating the timing of a neuron's changes to the animal's behavioral performance, the researchers were able to precisely pinpoint its role in learning. Some neurons changed only in response to one of several scenes, indicating a high level of specialization for particular stimuli.
Some of the changing cells showed sustained activity after the trials were over, while others returned to baseline. The researchers hypothesize that these "sustained changing cells" may not only participate in memory formation, but also in storage of the associations in long-term memory.
"Changing cells" were not detected until now because studies of long-term memory have typically not begun monitoring neurons in animals until after they had already been trained on experimental tasks for months, according to Suzuki. Also, the signals are hard to detect if the learning task is too difficult or too easy, resulting in only shallow increments of learning.
"This study provides a direct demonstration of learning-related neural plasticity in the hippocampus," notes Suzuki.
In addition to Dr. Suzuki, Dr. Sylvia Wirth and Marianna Yanike, NYU, conducted the recording experiments. Drs. Loren Frank, Anne Smith and Emery Brown, Harvard Medical School, developed algorithms used to analyze the neural and behavioral data.
The research was also supported by grants from the McKnight Foundation and the John Merck Fund.
NIMH and NIDA are part of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research. NIH is a component of the U.S. Department of Health and Human Services.
The original news release can be found here
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