Human Evolution Driven By Changing Environment

Human Evolution Driven By Changing Environment

  9:46:00 AM    Science Lover

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A series of rapid environmental changes in East Africa roughly 2 million years ago may be responsible for driving human evolution, according to researchers at Penn State and Rutgers University.

The researchers examined lake sediments from Olduvai Gorge in northern Tanzania, looking for biomarkers -- fossil molecules -- from ancient trees and grasses. (Credit: Gail Ashley)

"The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland and an open grassland about five to six times during a period of 200,000 years," said Clayton Magill, graduate student in geosciences at Penn State. "These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years."

According to Katherine Freeman, professor of geosciences, Penn State, the current leading hypothesis suggests that evolutionary changes among humans during the period the team investigated were related to a long, steady environmental change or even one big change in climate.

"There is a view this time in Africa was the 'Great Drying,' when the environment slowly dried out over 3 million years," she said. "But our data show that it was not a grand progression towards dry; the environment was highly variable."

According to Magill, many anthropologists believe that variability of experience can trigger cognitive development.

"Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response," he said. "Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes -- how you interact with others in a group. Our data are consistent with these hypotheses. We show that the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use."

The researchers -- including Gail Ashley, professor of earth and planetary sciences, Rutgers University -- examined lake sediments from Olduvai Gorge in northern Tanzania. They removed the organic matter that had either washed or was blown into the lake from the surrounding vegetation, microbes and other organisms 2 million years ago from the sediments. In particular, they looked at biomarkers -- fossil molecules from ancient organisms -- from the waxy coating on plant leaves.

"We looked at leaf waxes because they're tough, they survive well in the sediment," said Freeman.

The team used gas chromatography and mass spectrometry to determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for different leaf waxes. The data enabled them to reconstruct the types of vegetation present in the Olduvai Gorge area at very specific time intervals.

The results showed that the environment transitioned rapidly back and forth between a closed woodland and an open grassland.

To find out what caused this rapid transitioning, the researchers used statistical and mathematical models to correlate the changes they saw in the environment with other things that may have been happening at the time, including changes in the Earth's movement and changes in sea-surface temperatures.

"The orbit of the Earth around the sun slowly changes with time," said Freeman. "These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa. Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water. The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement."

The team also found a correlation between changes in the environment and sea-surface temperature in the tropics.

"We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa," Freeman said. The researchers recently published their results in the Proceedings of the National Academy of Sciences along with another paper in the same issue that builds on these findings. The second paper shows that rainfall was greater when there were trees around and less when there was a grassland.

"The research points to the importance of water in an arid landscape like Africa," said Magill. "The plants are so intimately tied to the water that if you have water shortages, they usually lead to food insecurity.

"Together, these two papers shine light on human evolution because we now have an adaptive perspective. We understand, at least to a first approximation, what kinds of conditions were prevalent in that area and we show that changes in food and water were linked to major evolutionary changes."

The National Science Foundation funded this research.

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Engineers Reverse E. Coli Metabolism for Quick Production of Fuels, Chemicals

Engineers Reverse E. Coli Metabolism for Quick Production of Fuels, Chemicals

  10:54:00 PM    Science Lover

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In a biotechnological tour de force, Rice University engineering researchers this week unveiled a new method for rapidly converting simple glucose into biofuels and petrochemical substitutes. In a paper published online in Nature, Rice's team described how it reversed one of the most efficient of all metabolic pathways -- the beta oxidation cycle -- to engineer bacteria that produce biofuel at a breakneck pace.

Rice University engineering researchers Ramon Gonzalez (left) and
Clementina Dellomonaco reversed one of the most efficient of all
metabolic pathways -- the beta oxidation cycle -- to engineer bacteria
that make biofuels at a breakneck pace. (Credit: Jeff Fitlow/Rice
University)

Just how fast are Rice's single-celled chemical factories? On a cell-per-cell basis, the bacteria produced the butanol, a biofuel that can be substituted for gasoline in most engines, about 10 times faster than any previously reported organism.


"That's really not even a fair comparison because the other organisms used an expensive, enriched feedstock, and we used the cheapest thing you can imagine, just glucose and mineral salts," said Ramon Gonzalez, associate professor of chemical and biomolecular engineering at Rice and lead co-author of the Nature study.


Gonzalez's laboratory is in a race with hundreds of labs around the world to find green methods for producing chemicals like butanol that have historically come from petroleum.


"We call these 'drop-in' fuels and chemicals, because their structure and properties are very similar, sometimes identical, to petroleum-based products," he said. "That means they can be 'dropped in,' or substituted, for products that are produced today by the petrochemical industry."




Butanol is a relatively short molecule, with a backbone of just four carbon atoms. Molecules with longer carbon chains have been even more troublesome for biotech producers to make, particularly molecules with chains of 10 or more carbon atoms. Gonzalez said that's partly because researchers have focused on ramping up the natural metabolic processes that cells use to build long-chain fatty acids. Gonzalez and students Clementina Dellomonaco, James Clomburg and Elliot Miller took a completely different approach.


"Rather than going with the process nature uses to build fatty acids, we reversed the process that it uses to break them apart," Gonzalez said. "It's definitely unconventional, but it makes sense because the routes nature has selected to build fatty acids are very inefficient compared with the reversal of the route it uses to break them apart."


The beta oxidation process is one of biology's most fundamental, Gonzalez said. Species ranging from single-celled bacteria to human beings use beta oxidation to break down fatty acids and generate energy.


In the Nature study, Gonzalez's team reversed the beta oxidation cycle by selectively manipulating about a dozen genes in the bacteria Escherichia coli. They also showed that selective manipulations of particular genes could be used to produce fatty acids of particular lengths, including long-chain molecules like stearic acid and palmitic acid, which have chains of more than a dozen carbon atoms.


"This is not a one-trick pony," Gonzalez said. "We can make many kinds of specialized molecules for many different markets. We can also do this in any organism. Some producers prefer to use industrial organisms other than E. coli, like algae or yeast. That's another advantage of using reverse-beta oxidation, because the pathway is present in almost every organism."


The research was funded by Rice University.

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Sex (As We Know It) Works Thanks to Ever-Evolving Host-Parasite Relationships, Biologists Find

Sex (As We Know It) Works Thanks to Ever-Evolving Host-Parasite Relationships, Biologists Find

  1:55:00 PM    Science Lover

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It seems we may have parasites to thank for the existence of sex as we know it. Indiana University biologists have found that, although sexual reproduction between two individuals is costly from an evolutionary perspective, it is favored over self-fertilization in the presence of coevolving parasites. Sex allows parents to produce offspring that are more resistant to the parasites, while self-fertilization dooms populations to extinction at the hands of their biological enemies.

The relationship between the roundworm Caenorhabditis 
elegans and the pathogenic bacteria Serratia marcescens, 
pictured here together in a Petri dish, is helping scientists 
understand why sexual reproduction occurs as it does. 
(Credit: Image courtesy of Indiana University)

The July 8 report in Science, "Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex," affirms the Red Queen hypothesis, an evolutionary theory who's name comes from Lewis Carroll's Alice in Wonderland text: "It takes all the running you can do, to keep in the same place." The idea is that sexual reproduction via cross-fertilization keeps host populations one evolutionary step ahead of the parasites, which are coevolving to infect them. It is within this coevolutionary context that both hosts and parasites are running (evolving) as fast as they can just to stay in the same place.

"The widespread existence of sex has been a major problem for evolutionary biology since the time of Charles Darwin," said lead author Levi T. Morran. Sex does not make evolutionary sense, because it often involves the production of males. This is very inefficient, because males don't directly produce any offspring. Self-fertilization is a far more efficient means of reproduction, and as such, evolutionary theory predicts that self-fertilization should be widespread in nature and sex should be rare. However, as we all know, this is not the case.

The Red Queen Hypothesis provides one possible explanation for the existence of sex.

"The Red Queen Hypothesis predicts that sex should allow hosts to evade infection from their parasites, whereas self-fertilization may increase the risk of infection," said co-author Curtis M. Lively.

By combining the DNA of two parents, sex allows parents to produce offspring that are genetically diverse and different from their parents. Parasites that have adapted to infect one generation may have difficulty infecting the next generation. However, offspring produced through self-fertilization inherit the DNA of their single parent, thus any parasites adapted to infect the parent should also be capable of infecting the offspring.

Morran, a post-doctoral researcher, and Lively, a distinguished professor of biology, both in the IU Bloomington College of Arts and Science's Department of Biology, authored the report with biology undergraduates Olivia G. Schmidt, Ian A. Gelarden and Raymond C. Parrish II.



The team used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis. They genetically manipulated the mating system of C. elegans, causing populations to mate either sexually, by self-fertilization, or a mixture of both within the same population. Then they exposed those populations to the S. marcescens parasite. The parasites were either allowed to coevolve with C. elegans or were prevented from evolving. The researchers then determined which mating system gave populations an evolutionary advantage.

"We found that the self-fertilizing populations of C. elegans were rapidly driven extinct by the coevolving parasites, a result consistent with the Red Queen Hypothesis," Morran said. On the other hand, sex allowed populations to keep pace with their parasites. "Sex helped populations adapt to their coevolving parasites, allowing parents to produce offspring that were resistant to infection and ultimately avoid extinction," he noted.

In host populations where either sex or self-fertilization were possible, the evolutionary state of the parasite determined the most effective reproductive strategy. When the parasite did not coevolve, self-fertilization evolved as the dominant form of host reproduction. However, when the parasite was allowed to coevolve with the hosts, then sex became the favored reproductive strategy.

"Coevolution with the pathogen not only favored sex over self-fertilization, but also allowed sex to be maintained throughout the experiment," Morran said.

These results are consistent with the Red Queen Hypothesis and may go a long way toward explaining the widespread existence of sex.

"Coevolving parasites seem to be very common in nature," said Lively. "The experiment shows that coevolution with parasites, but not the presence of parasites per se, selects for higher levels of outcrossing. Thus the coevolutionary struggle between hosts and their parasites could explain the existence of males."

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Quantum Knowledge Cools Computers: New Understanding of Entropy

Quantum Knowledge Cools Computers: New Understanding of Entropy

  9:20:00 PM    Science Lover

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From a laptop warming a knee to a supercomputer heating a room, the idea that computers generate heat is familiar to everyone. But theoretical physicists have discovered something astonishing: not only do computational processes sometimes generate no heat, under certain conditions they can even have a cooling effect. Behind this finding are fundamental considerations relating to knowledge and a lack of knowledge. The researchers publish their findings in the journal Nature.

According to the latest theoretical studies, the ever-
increasing energy costs caused by supercomputers of 
the kind operated at the Swiss National Scientific 
Computing Centre in Manno (canton of Ticino) 
could be reduced. However, this would need 
a quantum computer. (Credit: Michele 
De Lorenzi / CSCS)

When computers compute, the energy they consume eventually ends up as heat. This isn't all due to the engineering of the computer -- physics has something to say about the fundamental energy cost of processing information.

Recent research by a team of physicists reveals a surprise at this fundamental level. ETH-Professor Renato Renner, and Vlatko Vedral of the Centre for Quantum Technologies at the National University of Singapore and the University of Oxford, UK, and their colleagues describe in the scientific journal Nature how the deletion of data, under certain conditions, can create a cooling effect instead of generating heat. The cooling effect appears when the strange quantum phenomenon of entanglement is invoked. Ultimately, it may be possible to harness this effect to cool supercomputers that have their performance held back by heat generation. "Achieving the control at the quantum level that would be required to implement this in supercomputers is a huge technological challenge, but it may not be impossible. We have seen enormous progress is quantum technologies over the past 20 years," says Vedral. With the technology in quantum physics labs today, it should be possible to do a proof of principle experiment on a few bits of data.

Landauer's principle is given a quantum twist

The physicist Rolf Landauer calculated back in 1961 that during the deletion of data, some release of energy in the form of heat is unavoidable. Landauer's principle implies that when a certain number of arithmetical operations per second have been exceeded, the computer will produce so much heat that the heat is impossible to dissipate. In supercomputers today other sources of heat are more significant, but Renner thinks that the critical threshold where Landauer's erasure heat becomes important may be reached within the next 10 to 20 years. The heat emission from the deletion of a ten terabyte hard-drive amounts in principle to less than a millionth of a joule. However, if such a deletion process were repeated many times per second then the heat would accumulate correspondingly.

The new study revisits Landauer's principle for cases when the values of the bits to be deleted may be known. When the memory content is known, it should be possible to delete the bits in such a manner that it is theoretically possible to re-create them. It has previously been shown that such reversible deletion would generate no heat. In the new paper, the researchers go a step further. They show that when the bits to be deleted are quantum-mechanically entangled with the state of an observer, then the observer could even withdraw heat from the system while deleting the bits. Entanglement links the observer's state to that of the computer in such a way that they know more about the memory than is possible in classical physics.

Similar formulas -- two disciplines

In order to reach this result, the scientists combined ideas from information theory and thermodynamics about a concept known as entropy. Entropy appears differently in these two disciplines, which are, to a large extent, independent of each other. In information theory, entropy is a measurement of the information density. It describes, for instance, how much memory capacity a given set of data would take up when compressed optimally. In thermodynamics, on the other hand, entropy relates to the disorder in systems, for example to the arrangement of molecules in a gas. In thermodynamics, adding entropy to a system is usually equivalent to adding energy as heat.

The ETH physicist Renner says "We have now shown that in both cases, the term entropy is actually describing the same thing even in the quantum mechanical regime." As the formulas for the two entropies look the same, it had already been assumed that there was a connection between them. "Our study shows that in both cases, entropy is considered to be a type of lack of knowledge," says Renner. The new paper in Nature builds on work published earlier in the New Journal of Physics.

In measuring entropy, one should bear in mind that an object does not have a certain amount of entropy per se, instead an object's entropy is always dependent on the observer. Applied to the example of deleting data, this means that if two individuals delete data in a memory and one has more knowledge of this data, she perceives the memory to have lower entropy and can then delete the memory using less energy. Entropy in quantum physics has the unusual property of sometimes being negative when calculated from the information theory point of view. Perfect classical knowledge of a system means the observer perceives it to have zero entropy. This corresponds to the memory of the observer and that of the system being perfectly correlated, as much as allowed in classical physics. Entanglement gives the observer „more than complete knowledge" because quantum correlations are stronger than classical correlations. This leads to an entropy less than zero. Until now, theoretical physicists had used this negative entropy in calculations without understanding what it might mean in thermodynamic terms or experimentally.

No heat, even a cooling effect

In the case of perfect classical knowledge of a computer memory (zero entropy), deletion of the data requires in theory no energy at all. The researchers prove that "more than complete knowledge" from quantum entanglement with the memory (negative entropy) leads to deletion of the data being accompanied by removal of heat from the computer and its release as usable energy. This is the physical meaning of negative entropy.

Renner emphasizes, however, "This doesn't mean that we can develop a perpetual motion machine." The data can only be deleted once, so there is no possibility to continue to generate energy. The process also destroys the entanglement, and it would take an input of energy to reset the system to its starting state. The equations are consistent with what's known as the second law of thermodynamics: the idea that the entropy of the universe can never decrease. Vedral says "We're working on the edge of the second law. If you go any further, you will break it."

Fundamental findings

The scientists' new findings relating to entropy in thermodynamics and information theory may have usefulness beyond calculating the heat production of computers. For example, methods developed within information theory to handle entropy could lead to innovations in thermodynamics. The connection made between the two concepts of entropy is fundamental.

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How Spiders Create Silk Threads

How Spiders Create Silk Threads

  5:55:00 PM    Science Lover

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How can a tiny spider body contain material for several decimeters of gossamer silk, and what governs the conversion to thread? Researchers at the Swedish University of Agricultural Sciences (SLU) in Sweden can now explain this process.

Artificial spider silk. (Credit: Image courtesy of 
Swedish University of Agricultural Sciences)

The new research findings are presented in an article in the scientific journal Nature.

"We have seen how the first part of the spider silk protein has a very special and important function. It quite simply controls when the protein is to be converted into gossamer," says My Hedhammar, one of the researchers at SLU.

By rapidly lowering the pH, a spider can initiate the conversion to silk. Before this, the protein needed to form the silk is stored in a gland in the spider's body.

When it is time to spin a thread, the protein passes through a canal where it is converted to gossamer. Along the canal, the conditions change: among other things, the pH is lowered from a neutral (pH 7) to a somewhat more acidic level, pH 6.

"The spider gossamer protein consists of three parts. At SLU, this time we have primarily studied the first part, named NT, and have seen that it has very special properties that are important to the spider. At neutral pH, NT helps the protein to remain in liquid form. When the pH goes down, NT sees to it that threads are formed rapidly and also in an orderly manner," says My Hedhammar.

It has long been a dream of researchers to be able to produce artificial spider silk, since it is one of the strongest materials known. There are therefore great hopes about what spider gossamer could be used for in the future, everything from surgical sutures to bullet-proof vests. Spider silk is a strong and elastic material, and it is moreover biodegradable. It could be of great importance in medical technology, for example.

To be able to produce artificial gossamer, basic research about how spiders go about it is a key piece of the puzzle. Numerous researchers around the world are trying to map this process.

At SLU several scientists are involved in this work, which is largely done with classical biochemical methods. These researchers have primarily conducted their studies using the spider Euprosthenops australis, a species that makes one of the strongest threads and that is moreover large enough to be dissected in a simple way. But the new findings about gossamer protein seems to apply to all spider silk, regardless of species.

The SLU researchers behind the research now presented in the new issue of the journal Nature are Glareh Askarieh, My Hedhammar, Kerstin Nordling, Anna Rising, Jan Johansson, and Stefan D. Knight.

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Why Does a Human Baby Need a Full Year Before Starting to Walk?

Why Does a Human Baby Need a Full Year Before Starting to Walk?

  1:09:00 AM    Science Lover

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Why does a human baby need a full year before it can start walking, while a newborn foal gets up on its legs almost directly after birth? Scientist have assumed that human motor development is unique because our brain is unusually complex and because it is particularly challenging to walk on two legs. But now a research group at Lund University in Sweden has shown that human babies in fact start walking at the same stage in brain development as most other walking mammals, from small rodents to elephants.

Why does a human baby need a full year before it can start walking, while a newborn foal gets up on its legs almost directly after birth? (Credit: iStockphoto/Beth Jeppson)

The findings are published in the journal PNAS.

The Lund group consists of neurophysiologists Martin Garwicz and Maria Christensson and developmental psychologist Elia Psouni. Contrary to convention, they used conception and not birth as the starting point of motor development in their comparison between different mammals. This revealed astonishing similarities among species that diverged in evolution as much as 100 million years ago. -- Humans certainly have more brain cells and bigger brains than most other terrestrial mammalian species, but with respect to walking, brain development appears to be similar for us and other mammals. Our study demonstrates that the difference is quantitative, not qualitative, says Martin Garwicz.

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Life from Space

Life from Space

  8:58:00 PM    Science Lover

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Introduction

Growing evidence suggests that life on earth may have been seeded from outer space.

Podcast
Life from Space

Transcript

1. Nucleobases from Space

DNA components from space. I'm Bob Hirshon and this is Science Update.

Life on earth may have been jump-started by a special delivery from outer space. This according to a new report from an international team of scientists.

The researchers identified key chemical components of DNA and RNA—components called nucleobases—in a meteorite that landed in Australia in 1969. Lead author Zita Martins of University College London says nucleobases have been found in other meteorites too, but it wasn't clear how they got there.

Martins:
Our study really proved that in fact they are extraterrestrial.

That's because they contain a heavy isotope of carbon that forms only in outer space. The findings add to a growing body of evidence that the first building blocks of life came from meteorites and comets: an attractive theory, because the primitive earth appears unlikely to have formed them spontaneously. I'm Bob Hirshon for AAAS, the Science Society.

2. Molecular Space Cloud

Seeds of life in space. I'm Bob Hirshon and this is Science Update.

There's growing evidence that life on earth may have started with organic chemicals from outer space. Now, scientists are using the massive Green Bank Telescope to find these molecules in our Milky Way today.

Anthony Remijan of the National Radio Astronomy Observatory says they're studying a rich cloud of gas and dust, like the one that formed our solar system long ago.

Remijan:
There must have been complex organic molecules in that cloud in order to seed the molecular complexity we see in the solar system today. We just haven't had the instruments yet to detect these large molecules until now.

They're surveying the cloud across a wide range of radio frequencies. Then they'll look through the data for distinct radio signatures that are unique to each organic molecule. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

How did life arise on earth? It's a question that scientists have asked since the dawn of science. These studies address the very earliest stages of evolution, before actual life even existed.

According to current models, the earth formed about 4.6 billion years ago, and the oldest known fossils are about 3.5 billion years old. Sometime in between—no one knows exactly when—microscopic life began. But before you even get to the origin of life, you have to explain the origin of complex organic chemicals: chemicals that are the building blocks of all living things.

Those chemicals wouldn't have been abundant on a brand-new earth, which had a very different and much simpler chemical composition than today's planet. No one knows for sure what the early earth was like, but evidence suggests that it began as a ball of hot molten rock, surrounded by a thin atmosphere of hydrogen and helium. Later, as the surface cooled, volcanic eruptions would have released heavier gases into the atmosphere, like ammonia, methane, and water vapor.

Some scientists believe that the first building blocks of life, perhaps early forms of the genetic molecules DNA and RNA, were formed by chemical reactions in this early earth environment. Others believe that these molecules came directly from space, by hitching a ride on comets or meteorites that slammed into the planet's surface. There is evidence supporting both sides, but these two studies focus on the second hypothesis, which was once considered far-fetched but has been gaining favor over the past decade.

One reason that many scientists have doubted the life-from-space hypothesis (sometimes called panspermia or exogenesis) is that comets and meteorites burn as they enter the earth's atmosphere. Some scientists have argued that the extreme heat would destroy any organic molecules on the comets or meteorites before they hit the ground. Martins' study, however, claims to have found important organic molecules inside meteorites that could only have come from space, based on the chemical signature of the carbon in those molecules.

While the meteorites Martins studied fell to earth just forty years ago, the findings suggest that the same thing could easily have happened four billion years ago. Furthermore, evidence suggests that the early earth got smacked with meteorites a lot more than today. More meteorites means more chances for those nucleobases to collect on earth in large enough quantities to get life going.

Remijan's work is just getting started, but instead of looking on earth, he'll look in outer space for signs of organic compounds. His team will use a new, powerful radiotelescope to look at a cloud of space dust that resembles the cloud that formed our own solar system. They will base their search on the fact that all molecules emit radio waves as they rotate and vibrate, and that each molecule gives off a distinctive radio signature. By pointing the telescope at the cloud and sifting through these radio frequencies, they should be able to identify the types of chemicals in the cloud. Finding organic molecules in that cloud would lend further support to the idea that life may have been seeded from space.

Now try and answer these questions:

1. What are two basic explanations for the origin of organic compounds on earth?
   
2. Why is it important to explain the origin of these chemicals?
   
3. What are nucleobases? How did Martins establish that the nucleobases in the Australian meteorites came from space?
   
4. How will Remijan's work add to our understanding of the origin of organic molecules?
   
5. What limits our scientific understanding of the origins of life on earth?

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Rare river dolphin 'now extinct'

Rare river dolphin 'now extinct'

  9:38:00 PM    Science Lover

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An extensive survey of its habitat failed to find any sign of the baiji

A freshwater dolphin found only in China is now "likely to be extinct", a team of scientists has concluded.

The researchers failed to spot any Yangtze river dolphins, also known as baijis, during an extensive six-week survey of the mammals' habitat.

The team, writing in Biology Letters journal, blamed unregulated fishing as the main reason behind their demise.

If confirmed, it would be the first extinction of a large vertebrate for over 50 years.

The World Conservation Union's Red List of Threaten Species currently classifies the creature as "critically endangered".

We have yet to take full responsibility in our role as guardians of the planet Dr Sam Turvey, Zoological Society of London

Sam Turvey of the Zoological Society of London (ZSL), one of the paper's co-authors, described the findings as a "shocking tragedy".

"The Yangtze river dolphin was a remarkable mammal that separated from all other species over 20 million years ago," Dr Turvey explained.

"This extinction represents the disappearance of a complete branch of the evolutionary tree of life and emphasises that we have yet to take full responsibility in our role as guardians of the planet."

If confirmed, it would be the first extinction of a large vertebrate for over 50 years.

'Incidental impact'

The species (Lipotes vexillifer) was the only remaining member of the Lipotidae, an ancient mammal family that is understood to have separated from other marine mammals, including whales, dolphins and porpoises, about 40-20 million years ago.

The white, freshwater dolphin had a long, narrow beak and low dorsal fin; lived in groups of three or four and fed on fish.

The team carried out six-week visual and acoustic survey, using two research vessels, in November and December 2006.

"While it is conceivable that a couple of surviving individuals were missed by the survey teams," the team wrote, "our inability to detect any baiji despite this intensive search effort indicates that the prospect of finding and translocating them to a [reserve] has all but vanished."

The scientists added that there were a number of human activities that caused baiji numbers to decline, including construction of dams and boat collisions.

"However, the primary factor was probably unsustainable by-catch in local fisheries, which used rolling hooks, nets and electrofishing," they suggested.

"Unlike most historical-era extinctions of large bodied animals, the baiji was the victim not of active persecution but incidental mortality resulting from massive-scale human environmental impacts - primarily uncontrolled and unselective fishing," the researchers concluded.

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