星空女人频道

2013年8月2日星期五

Monster galaxies lose their appetite with age

Aug. 1, 2013 — Our universe is filled with gobs of galaxies, bound together by gravity into larger families called clusters. Lying at the heart of most clusters is a monster galaxy thought to grow in size by merging with neighboring galaxies, a process astronomers call galactic cannibalism.

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New research from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) is showing that, contrary to previous theories, these gargantuan galaxies appear to slow their growth over time, feeding less and less off neighboring galaxies.

"We've found that these massive galaxies may have started a diet in the last 5 billion years, and therefore have not gained much weight lately," said Yen-Ting Lin of the Academia Sinica in Taipei, Taiwan, lead author of a study published in the Astrophysical Journal.

Peter Eisenhardt, a co-author from NASA's Jet Propulsion Laboratory in Pasadena, Calif., said, "WISE and Spitzer are letting us see that there is a lot we do understand -- but also a lot we don't understand -- about the mass of the most massive galaxies." Eisenhardt identified the sample of galaxy clusters studied by Spitzer, and is the project scientist for WISE.

The new findings will help researchers understand how galaxy clusters -- among the most massive structures in our universe -- form and evolve.

Galaxy clusters are made up of thousands of galaxies, gathered around their biggest member, what astronomers call the brightest cluster galaxy, or BCG. BCGs can be up to dozens of times the mass of galaxies like our own Milky Way. They plump up in size by cannibalizing other galaxies, as well as assimilating stars that are funneled into the middle of a growing cluster.

To monitor how this process works, the astronomers surveyed nearly 300 galaxy clusters spanning 9 billion years of cosmic time. The farthest cluster dates back to a time when the universe was 4.3 billion years old, and the closest, when the universe was much older, 13 billion years old (our universe is presently 13.8 billion years old).

"You can't watch a galaxy grow, so we took a population census," said Lin. "Our new approach allows us to connect the average properties of clusters we observe in the relatively recent past with ones we observe further back in the history of the universe."

Spitzer and WISE are both infrared telescopes, but they have unique characteristics that complement each other in studies like these. For instance, Spitzer can see more detail than WISE, which enables it to capture the farthest clusters best. On the other hand, WISE, an infrared all-sky survey, is better at capturing images of nearby clusters, thanks to its larger field of view. Spitzer is still up and observing; WISE went into hibernation in 2011 after successfully scanning the sky twice.

The findings showed that BCG growth proceeded along rates predicted by theories until 5 billion years ago, or a time when the universe was about 8 billion years old. After that time, it appears the galaxies, for the most part, stopped munching on other galaxies around them.

The scientists are uncertain about the cause of BCGs' diminished appetites, but the results suggest current models need tinkering.

"BCGs are a bit like blue whales -- both are gigantic and very rare in number. Our census of the population of BCGs is in a way similar to measuring how the whales gain their weight as they age. In our case, the whales aren't gaining as much weight as we thought. Our theories aren't matching what we observed, leading us to new questions," said Lin.

Another possible explanation is that the surveys are missing large numbers of stars in the more mature clusters. Clusters can be violent environments, where stars are stripped from colliding galaxies and flung into space. If the recent observations are not detecting those stars, it's possible that the enormous galaxies are, in fact, continuing to bulk up.

Future studies from Lin and others should reveal more about the feeding habits of one of nature's largest galactic species.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer .

JPL managed and operated WISE for NASA's Science Mission Directorate. Edward Wright is the principal investigator and is at UCLA. The mission was selected competitively under NASA's Explorers Program managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah. The spacecraft was built by Ball Aerospace & Technologies Corp. in Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise .

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Arctic sea-ice loss has widespread effects on wildlife

Aug. 1, 2013 — With sea ice at its lowest point in 1,500 years, how might ecological communities in the Arctic be affected by its continued accelerating melting over the next decades? Penn State University Professor of Biology Eric Post and an international team of scientists tackle this question by examining relationships among algae, plankton, whales, and terrestrial animals such as caribou, arctic foxes and walrus; as well as the effects of human exploration of previously inaccessible parts of the region.

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"Arctic sea ice has declined by more than 86,000 square kilometers -- a space slightly larger than the state of South Carolina -- per year," Post said. "That's an area of critical habitat for many species and the rate of loss is increasing." Post added that an acceleration of this rate likely will be due, in part, to the loss the white surface provided by ice that reflects sunlight -- thereby causing a cooling effect. The highly reflective ice, Post added, will be replaced by a much-less-reflective, darker surface of open water -- and the effect will be accelerated warming and accelerated melting.

A domino effect of sea-ice melting on terrestrial animals, Post explained, could happen through a disruption in the food chain. Sea-ice algae and sub-ice plankton, which together account for 57 percent of the total annual biological production in the Arctic Ocean, already are being immediately affected by sea-ice melting because ice loss triggers a significant change in the blooming times of these organisms. Likewise, land adjacent to areas of sea-ice loss will experience significant surface warming inland from the coastline, affecting soil conditions and plant growth. Post and his colleagues hypothesize that, while invertebrate ocean-dwelling animals -- such as zooplankton that feed on algae and phytoplankton in the seas -- already are being affected, larger terrestrial animals such as caribou could find their land-dwelling food sources disrupted, as well, due to temperature changes affecting plant communities inland.

"A change in population mixing could be another, indirect effect of sea-ice melting," Post said. He explained that populations of wolves and arctic foxes that currently are isolated only during the summer could become even more isolated. A longer period of the year without ice, which promotes travel between populations, could lead to a decline in crossbreeding.

However, for other species, the effect of sea-ice loss could be just the opposite: "We know that, for some species, sea ice acts as a barrier to intermixing," Post explained. "So for these species, ice loss and a lengthening of the ice-free season likely will increase population mixing, reducing genetic differentiation." Post explained that, for example, polar and grizzly bears already have been observed to have hybridized because polar bears now are spending more time on land, where they have contact with grizzlies.

While such mixing of populations is not necessarily cause for concern, Post explained, it could lead to drastic changes in disease dynamics. For example, a population that currently is a host to a certain pathogen could carry that pathogen to another, previously unexposed population. "In addition, a decrease in sea ice in arctic Canada likely will increase contact between eastern and western arctic species, promoting mixing of pathogen communities that previously were isolated," Post said. "For example, phocine distemper virus (PDV) currently affects eastern Arctic seals. But if these seals begin to mix with western arctic seals, the virus may reach other, naive populations."

Post added that greater accessibility of previously remote parts of the Arctic to human exploration could be yet another unexpected consequence of sea-ice loss. "Retreating sea ice, longer ice-free seasons, and loss of sea ice are expected to promote development of shipping lanes and increased shipping traffic in areas that formerly were rather inaccessible," Post said. "This increased marine access likely will accelerate the pace of mineral and petroleum exploration in the Arctic, which in turn could affect both terrestrial and marine animals; for example, bowhead whales and Pacific walrus."

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'Soft' approach leads to revolutionary energy storage: Graphene-based supercapacitors

Published today in Science, a research team led by Professor Dan Li of the Department of Materials Engineering has developed a completely new strategy to engineer graphene-based supercapacitors (SC), making them viable for widespread use in renewable energy storage, portable electronics and electric vehicles.

SCs are generally made of highly porous carbon impregnated with a liquid electrolyte to transport the electrical charge. Known for their almost indefinite lifespan and the ability to re-charge in seconds, the drawback of existing SCs is their low energy-storage-to-volume ratio -- known as energy density. Low energy density of five to eight Watt-hours per litre, means SCs are unfeasibly large or must be re-charged frequently.

Professor Li's team has created an SC with energy density of 60 Watt-hours per litre -- comparable to lead-acid batteries and around 12 times higher than commercially available SCs.

"It has long been a challenge to make SCs smaller, lighter and compact to meet the increasingly demanding needs of many commercial uses," Professor Li said.

Graphene, which is formed when graphite is broken down into layers one atom thick, is very strong, chemically stable and an excellent conductor of electricity.

To make their uniquely compact electrode, Professor Li's team exploited an adaptive graphene gel film they had developed previously. They used liquid electrolytes -- generally the conductor in traditional SCs -- to control the spacing between graphene sheets on the sub-nanometre scale. In this way the liquid electrolyte played a dual role: maintaining the minute space between the graphene sheets and conducting electricity.

Unlike in traditional 'hard' porous carbon, where space is wasted with unnecessarily large 'pores', density is maximised without compromising porosity in Professor Li's electrode.

To create their material, the research team used a method similar to that used in traditional paper making, meaning the process could be easily and cost-effectively scaled up for industrial use.

"We have created a macroscopic graphene material that is a step beyond what has been achieved previously. It is almost at the stage of moving from the lab to commercial development," Professor Li said.

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Stimulating brain cells can make false memories

Aug. 1, 2013 — Any crime scene investigator can tell you that memories are unreliable; the way people remember a place or event changes over time and varies between individuals. But for the mice in one lab at MIT, the accuracy of memories is even more suspect. Howard Hughes Medical Institute researchers in that lab have discovered how to alter the animals' memories by turning on neurons in the brain that are associated with the memories and updating them with new information.

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The new findings, which appear in the journal Science, illustrate that a mouse can be made to fear a cage by giving it a foot shock while at the same time reactivating a memory of the cage to associate the two.

"The kinds of things that once existed only in the realm of science fiction movies like Inception and Eternal Sunshine of the Spotless Mind are now experimentally possible," says Steve Ramirez, a graduate student in the lab of HHMI investigator Susumu Tonegawa and first author of the new work.

Researchers knew that memories are stored by the brain in a small set of neurons. Understanding how this information is encoded could be key to understanding how human memory works as well as memory disorders. But identifying exactly which neurons are linked to specific memories has been technically challenging.

Ramirez and Tonegawa, along with Xu Liu, a postdoctoral fellow in Tonegawa's lab at the Massachusetts Institute of Technology, had previously developed a way to pinpoint the specific handful of neurons that are activated in the brains of mice in any particular situation. The technique relies on optogenetics, a method of controlling brain cells through bursts of light developed by HHMI early career scientist Karl Deisseroth at Stanford University. The researchers engineered brain cells to produce a light-sensitive protein whenever the neurons were activated in a new setting or situation. Then, by shining a light onto the brain through a fiber optic cable connected to the mouse's skull, they could reactivate only that subset of neurons. Even without reactivating the cells, they could determine which cells had been activated by measuring which contained the light-sensitive protein. The approach was described in a 2012 Nature paper.

More recently, the scientists wondered if they could alter the way a mouse remembered a setting by activating neurons associated with it. They chose to test this idea by attempting to change whether or not a mouse was afraid of a particular cage.

"In mice, fear can be seen as a binary behavioral output," says Ramirez. "Either the animal is exploring a box that it's interested in, and it's curious and sniffing around. Or, if it's displaying fear behavior, it's huddled in a corner not moving. So it's a very easy, very powerful readout of memory."

To see whether they could make an animal associate fear with a previously neutral setting, Tonegawa's lab group first exposed mice to one of four unique cages. Each cage had distinct flooring materials, artificial smells, and different lighting. As the mice scouted out the new room, whichever neurons were activated produced the special light-sensitive protein.

Next, the mice were moved to a second cage. This time, as the mice explored, the scientists used light to turn on the neurons that had been activated in the first cage and simultaneously shocked the feet of the mice. Then the mice were put back in the first area -- where they'd never received a shock. The mice were clearly fearful of the setting, Ramirez says, spending more than a quarter of their time frozen in place.

"We were astonished that this worked on the very first mouse we ever tried," he says. "We got the animal to be scared of an environment where technically, nothing bad had ever happened to it."

By contrast, when the mice were put in a third cage that they'd never been in before, they exhibited no fear. And in a control group of mice that had received shocks in the second cage but no neuron reactivation, the first cage never induced fear.

After the successful experiment, Tonegawa, Ramirez, and Liu looked at the details of which neurons in the brain had been responsible for inducing the memory of the first cage. The neurons, they found, were located in the dentate gyrus, part of the hippocampus. The dentate gyrus has previously been implicated in the formation of memories, and is one of the areas of the brain with the most new neuron generation during adulthood. But most evidence about its importance came from instances in which the area had been damaged and memories lost.

"This study gives us information on the basic mechanism that could be happening in the brain when memories or false memories are formed," says Ramirez. "Next, we want to see if we can do the same with not only fear memories but pleasure memories or memories of objects or memories of other mice."

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New type of protein modification may play a role in cancer and diabetes

"It appears to be an intrinsic feedback mechanism in glucose metabolism, but I suspect that its other functions throughout the cell will prove at least as interesting when they are more fully elucidated," said Benjamin F. Cravatt, chair of the Department of Chemical Physiology and member of the Skaggs Institute for Chemical Physiology at TSRI.

Cravatt and his postdoctoral fellow Raymond E. Moellering reported the finding in the August 2, 2013 issue of the journal Science.

In Search of New Protein Modifiers

The Cravatt laboratory has long studied the natural chemical modifications that can change the functions of proteins "on the fly," switching their biological activities on or off or otherwise altering them. The better known of these modifications include phosphorylation, the addition of a small molecule known as a phosphate group, and acetylation, the addition of an acetyl group.

In search of new protein modifiers, Cravatt and Moellering, whose postdoctoral fellowship is sponsored in part by the Howard Hughes Medical Institute and the Damon Runyon Cancer Research Foundation, decided to investigate a small molecule known as 1,3-bisphosphoglycerate (1,3-BPG). The molecule's chemical makeup suggested that it might readily react with some proteins to form semipermanent, function-altering modifications. 1,3-BPG is one of the main "intermediate" molecules produced during glycolysis, which is a core metabolic pathway that converts glucose to cellular fuel.

"1,3-BPG's intrinsic reactivity seemed odd to us, considering that it is such a central metabolite," remembered Moellering.

Moellering's initial test-tube experiments showed that 1,3-BPG does indeed react with certain lysine amino acids to modify GAPDH, the enzyme that mediates the production of 1,3-BPG. "That gave us the first indication that this reaction does happen, and that we should therefore start looking for it in cells," he said.

A Role in Glucose Metabolism

After devising new methods to detect this unique lysine modification in human cell cultures, Moellering soon found it -- on other glucose-metabolizing enzymes, as well as on proteins seemingly unrelated to glucose metabolism.

"With every step we took, the project became more interesting, because we were finding signs that this reaction occurs frequently in cells and in animal tissues, and in unexpected cellular locations, too," Moellering said.

He detected the signature of the new lysine modification not only on proteins in the main volume of the cell (the cytosol), but also in the DNA-containing cell nucleus and even on the cell's membrane compartments.

"It appears that wherever GAPDH goes within cells, it is capable of catalyzing the localized production of 1,3-BPG, which in turn reacts with nearby proteins to modify their structure and function," said Cravatt.

Moellering found that when 1,3-BPG's lysine modification occurs on glucose-metabolizing enzymes, it tends to inhibit their activities, causing a slowdown of central glucose processing and a consequent buildup of certain glucose metabolites in the processing pathway. Moellering and Cravatt suspect that these overabundant metabolites may end up being shunted into other cellular processes besides basic fuel-making -- processes that contribute to the synthesis of new molecules and even cell proliferation.

Moellering also discovered that 1,3-BPG and the modification it makes on proteins become more prevalent as glucose levels rise. Within the context of glucose metabolism, 1,3-BPG's modification thus seems to act as a "very old, maybe ancient feedback mechanism for regulating that central metabolic pathway," Moellering said.

Looking Ahead

The abnormal processing of glucose within cells features in a number of major diseases including cancer and diabetes. "Cancer cells, for example, bring in as much as 20 times more glucose than non-cancerous cells of the same type," Moellering noted. He now wants to find out whether 1,3-BPG is part of the problem in such cells. At abnormally high levels, it conceivably could help force glucose metabolism toward the runaway cell proliferation that is a hallmark of cancer.

Cravatt and Moellering also want to learn more about what 1,3-BPG's lysine modification does in the nuclei and membrane compartments of cells, where they found evidence of it. "We suspect that it works to connect glucose metabolism to other pathways, perhaps as a kind of signaling mechanism," said Moellering.

Already Moellering has uncovered evidence that there are enzymes that work to reverse 1,3-BPG's modification of lysines -- which underscores the likelihood that this modification represents a fundamental, dynamic mechanism in cells. "We'd like to discover which enzymes catalyze the removal of the modification," said Cravatt, "because then, in principle, we could use inhibitors of these enzymes to control the levels of the modification and get a better understanding of its biological functions as well as the conditions under which it occurs."

Funding for the study, "Functional Lysine Modification by an Intrinsically Reactive Primary Glycolytic Metabolite," was provided by the National Institutes of Health (CA087660), the Skaggs Institute for Chemical Biology at TSRI and the Damon Runyon Cancer Research Foundation.

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New water splitting technique efficiently produces hydrogen fuel

The CU-Boulder team has devised a solar-thermal system in which sunlight could be concentrated by a vast array of mirrors onto a single point atop a central tower up to several hundred feet tall. The tower would gather heat generated by the mirror system to roughly 2,500 degrees Fahrenheit (1,350 Celsius), then deliver it into a reactor containing chemical compounds known as metal oxides, said CU-Boulder Professor Alan Weimer, research group leader.

As a metal oxide compound heats up, it releases oxygen atoms, changing its material composition and causing the newly formed compound to seek out new oxygen atoms, said Weimer. The team showed that the addition of steam to the system -- which could be produced by boiling water in the reactor with the concentrated sunlight beamed to the tower -- would cause oxygen from the water molecules to adhere to the surface of the metal oxide, freeing up hydrogen molecules for collection as hydrogen gas.

"We have designed something here that is very different from other methods and frankly something that nobody thought was possible before," said Weimer of the chemical and biological engineering department. "Splitting water with sunlight is the Holy Grail of a sustainable hydrogen economy."

A paper on the subject was published in the Aug. 2 issue of Science. The team included co-lead authors Weimer and Associate Professor Charles Musgrave, first author and doctoral student Christopher Muhich, postdoctoral researcher Janna Martinek, undergraduate Kayla Weston, former CU graduate student Paul Lichty, former CU postdoctoral researcher Xinhua Liang and former CU researcher Brian Evanko.

One of the key differences between the CU method and other methods developed to split water is the ability to conduct two chemical reactions at the same temperature, said Musgrave, also of the chemical and biological engineering department. While there are no working models, conventional theory holds that producing hydrogen through the metal oxide process requires heating the reactor to a high temperature to remove oxygen, then cooling it to a low temperature before injecting steam to re-oxidize the compound in order to release hydrogen gas for collection.

"The more conventional approaches require the control of both the switching of the temperature in the reactor from a hot to a cool state and the introduction of steam into the system," said Musgrave. "One of the big innovations in our system is that there is no swing in the temperature. The whole process is driven by either turning a steam valve on or off."

"Just like you would use a magnifying glass to start a fire, we can concentrate sunlight until it is really hot and use it to drive these chemical reactions," said Muhich. "While we can easily heat it up to more than 1,350 degrees Celsius, we want to heat it to the lowest temperature possible for these chemical reactions to still occur. Hotter temperatures can cause rapid thermal expansion and contraction, potentially causing damage to both the chemical materials and to the reactors themselves."

In addition, the two-step conventional idea for water splitting also wastes both time and heat, said Weimer, also a faculty member at CU-Boulder's BioFrontiers Institute. "There are only so many hours of sunlight in a day," he said.

The research was supported by the National Science Foundation and by the U.S. Department of Energy.

With the new CU-Boulder method, the amount of hydrogen produced for fuel cells or for storage is entirely dependent on the amount of metal oxide -- which is made up of a combination of iron, cobalt, aluminum and oxygen -- and how much steam is introduced into the system. One of the designs proposed by the team is to build reactor tubes roughly a foot in diameter and several feet long, fill them with the metal oxide material and stack them on top of each other. A working system to produce a significant amount of hydrogen gas would require a number of the tall towers to gather concentrated sunlight from several acres of mirrors surrounding each tower.

Weimer said the new design began percolating within the team about two years ago. "When we saw that we could use this simpler, more effective method, it required a change in our thinking," said Weimer. "We had to develop a theory to explain it and make it believable and understandable to other scientists and engineers."

Despite the discovery, the commercialization of such a solar-thermal reactor is likely years away. "With the price of natural gas so low, there is no incentive to burn clean energy," said Weimer, also the executive director of the Colorado Center for Biorefining and Biofuels, or C2B2. "There would have to be a substantial monetary penalty for putting carbon into the atmosphere, or the price of fossil fuels would have to go way up."

C2B2 is an arm of the Colorado Energy Research Collaboratory involving CU-Boulder, the Colorado School of Mines, Colorado State University and the National Renewable Energy Laboratory in Golden. The collaboratory works with industry partners, public agencies and other institutions to commercialize renewable energy technologies, support economic growth in the state and nation and educate the future workforce.

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Extreme wildfires in Western U.S. likely fueled by climate change

These erratic fires are harder to contain and often result in catastrophic damage and loss of property and life. Although not analyzed in the study, the recent Arizona wildfire that began with a lightning strike and killed 19 firefighters appeared to be such an unpredictable, fast-spreading blaze, according to a state report.

The MSU-led study, which appears in the Journal of Applied Meteorology and Climatology, predicts the trend will continue in the western United States.

"Our findings suggest that future lower atmospheric conditions may favor larger and more extreme wildfires, posing an additional challenge to fire and forest management," said Lifeng Luo, MSU assistant professor of geography and lead author on the study.

The researchers analyzed current and future climate patterns projected by multiple regional climate models and their effect on the spread of fire in a mountainous region that includes Arizona, Idaho, Nevada, New Mexico, Utah and Wyoming. The study focused on August, the most active month for wildfires in the western United States.

August 2012 saw 3.6 million acres burn in the region, the most of any August since 2000. However, there were only 6,948 fires in August 2012 -- the second fewest in that 12-year timeframe -- meaning the fires were much larger.

Large wildfires are mainly driven by natural factors including the availability of fuel (vegetation), precipitation, wind and the location of lightning strikes. In particular, the researchers found that exceptionally dry and unstable conditions in the earth's lower atmosphere will continue contributing to "erratic and extreme fire behavior."

"Global climate change may have a significant impact on these factors, thus affecting potential wildfire activity across many parts of the world," the study says.

Co-authors include Ying Tang and Shiyuan Zhong from MSU, and Xindu Bian and Warren Heilman from the USDA Forest Service.

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Existing cropland could feed four billion more by dropping biofuels and animal feed

Aug. 1, 2013 — The world's croplands could feed 4 billion more people than they do now just by shifting from producing animal feed and biofuels to producing exclusively food for human consumption, according to new research from the Institute on the Environment at the University of Minnesota.

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Even a smaller, partial shift from crop-intensive livestock such as feedlot beef to food animals such as chicken or pork could increase agricultural efficiency and provide food for millions, the study says.

"We essentially have uncovered an astoundingly abundant supply of food for a hungry world, hidden in plain sight in the farmlands we already cultivate," says graduate research assistant Emily Cassidy, lead author of the paper published in Environmental Research Letters. "Depending on the extent to which farmers and consumers are willing to change current practices, existing croplands could feed millions or even billions more people."

Demand for crops is expected to double by 2050 as population grows and increasing affluence boosts meat consumption. Meat takes a particularly big toll on food security because it takes up to 30 crop calories to produce a single calorie of meat. In addition, crops are increasingly being used for biofuels rather than food production. This study sought to quantify the benefit to food security that would accrue if some or all of the lands used to produce animal feed and fuel were reallocated to directly produce food for people.

To get at that question, Cassidy and colleagues first mapped the extent and productivity of 41 major crops between 1997 and 2003, adjusting numbers for imports and exports and calculating conversion efficiencies of animal feed using U.S. Department of Agriculture data. The researchers assumed humans need an average of 2,700 calories per day, and grazing lands and animals were not included in the study. Among the team's findings:

Only 12 percent of crop calories used for animal feed end up as calories consumed by humans.
Only 55 percent of crop calories worldwide directly nourish people.
Growing food exclusively for direct human consumption could boost available food calories up to 70 percent
U.S. agriculture alone could feed an additional 1 billion people by shifting crop calories to direct human consumption.
When calculated on the basis of protein rather than calories, results were similar. For instance, of all plant protein produced, 49 percent ends up in human diets.

In addition to the global findings, the research team looked at allocation of crop calories in four key countries: India, China, Brazil and the U.S. They found that while India allocates 90 percent of calories to feeding people, the other three allocate 58 percent, 45 percent, and 27 percent, respectively.

Noting the major cultural and economic dimensions involved, the researchers acknowledged that while a complete shift from animal to plant-based diets may not be feasible, even a partial shift would benefit food security. Quantifying the impact of various strategies, they found that a shift from crop-intensive beef to pork and chicken could feed an additional 357 million people, and a shift to nonmeat diets that include eggs and milk could feed an additional 815 million people.

The researchers emphasized that they are not making diet prescriptions or recommendations, just pointing out opportunities for gains in food production. They noted that humans can completely meet protein needs with plant-based diets, but that crop systems would need to shift (e.g., toward more production of protein-rich legumes) to meet human dietary needs.

"The good news is that we already produce enough calories to feed a few billion more people," Cassidy says. "As our planet gets more crowded or we experience disasters like droughts and pests, we can find ways of using existing croplands more efficiently."

In addition to her role as Global Landscapes Initiative graduate research assistant with the Institute on the Environment, Cassidy is a graduate student in the Natural Resources Science and Management program in the University of Minnesota's College of Food, Agriculture and Natural Resource Sciences.

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The when and where of the Y: Research on Y chromosomes uncovers new clues about human ancestry

But even though we all trace our family lineage to a few common ancestors, scientists still don't know exactly when and how those few ancestors started to give rise to the incredible diversity of today's population.

A brand-new finding, made using advanced analysis of DNA from all over the world, sheds new light on this mystery. By studying the DNA sequence of Y chromosomes of men from many different populations, scientists have determined that their male most recent common ancestor (MRCA) lived sometime between 120,000 and 156,000 years ago.

It's the first time the human ancestry has been traced back through the male line by sequencing the DNA of many entire Y chromosomes.

And, it agrees reasonably well with previous findings about our female most recent common ancestor, made by studying DNA carried down through the human race's female line. Such studies used DNA from mitochrondria -- structures inside cells -- and placed that time of the most recent common ancestor between 99,000 and 148,000 years ago. That agreement makes the new finding especially significant:

The research was done by a team of scientists from Stanford University, the University of Michigan Medical School, Stony Brook University, and their colleagues, and is published in the journal Science.

The team hopes their work will lead to further research on Y chromosomes as vehicles for studying human history -- and tracing male lineages back to the common "Adam" ancestors.

Jeffrey Kidd, Ph.D., an Assistant Professor of Human Genetics and Computational Medicine & Bioinformatics who worked on the new study, notes that only recently has it become possible to sequence Y chromosomes, because of technical limitations of previous approaches.

The new paper details how the team was able to make reliable measurements of the sequence variation along the Y chromosome -- which is handed down only from father to son without exchanging, or recombining, genetic material with other chromosomes.

Kidd notes that this initial paper on Y chromosome sequence diversity provides important first evidence that the male most recent common ancestor did not live more recently than the female most recent common ancestor.

"We're interested in understanding the historical relationships between many different human populations, and the migration patterns that have led to the peopling of the world," he says. "We hope that others will make use of this approach and sequence additional chromosomes of interest that are related to the peopling of specific places."

The study involved Y chromosomes obtained through the Human Genome Diversity Project, and from other sources. It included chromosomes from 69 men in several populations in sub-Saharan Africa, and from Siberia, Cambodia, Pakistan, Algeria and Mexico.

The great migrations of our ancestors out of Africa, across Asian and Europe and into the Americas all helped shape today's populations -- as did more recent forces related to colonialism and ever-growing global mobility.

Genetic studies such as this one may help anthropologists understand those migrations -- and their timing -- even better by giving them a genetic "clock" to use when studying today's humans, or potentially DNA extracted from ancient bones. It may also help scientists understand the great genetic diversity seen across Africa, and the evolution process that led to modern humans.

The reconciliation of the timing of "Adam" and "Eve," however, may be this study's most important immediate implication.

"This has been a conundrum in human genetics for a long time," said Carlos D. Bustamante, PhD, a professor of genetics at Stanford and senior author of the study. "Previous research has indicated that the male MRCA lived much more recently than the female MRCA. But now our research shows that there's no discrepancy. In fact, if anything, the Y chromosome may be a bit older."

In addition to Kidd and Bustamante, the research team includes U-M's Elzbieta Sliwerska, Stanford's G. David Poznik, Brenna M. Henn, Muh-Ching Yee, Ghia M. Euskirchen, Alice A. Lin, Michael Snyder, and Peter A. Underhill, and Lluis Quintana-Murci from Institut Pasteur in Paris.

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