Historia de la tierra from guest532621

Why is East Africa so Dry?

Peter deMenocal, a marine geologist at Columbia University, has been investigating about the natural history of eastern Africa for more than a decade. There is evidence of desiccation and gradual change to open savannahs, grass-eating fauna and the appearance of modern humans. Now he and his team have an explanation for that climate change: the tropical oceans.

They analysed planktonic foraminifera (small single-celled organisms) in an old sediment in the Indian Ocean off the west coast of Australia. These calcifying organisms take more and more magnesium when the water temperature goes up. About two million years ago, the level of magnesium grew up, indicating hot climate on the eastern side of the Indian Ocean. But on the other side of the ocean, off the coast of Africa, just the opposite happened: the ocean got colder.

This change in sea temperatures in the Indian Ocean came at an important time, coinciding with a rapid drying of east Africa, and the beginning of modern circulation in the Pacific Ocean. They tested the theory with a series of computer models and there it is: rain over east Africa depends on the conditions in the Indian Ocean and the Pacific Ocean.

Two to four million years ago is a significant time: it is then that the ice ages began, and a lot about humans is related to that period of the Earth. Can this climate change be connected to the evolutionary changes that happened at the same time, including the appearance of Homo Erectus and of animals adapted to the grasslands? Well, there is a lot of changes happening at the same time but it is difficult to prove that they are all related.

By Jeff Tollefson at www.nature.com. Picture by Top-Pics TBK / Alamy.

Lefthanders are Ambidextrous

If you're left-handed, there's good news. Well, most utensils and tools are still almost impossible to use. But at least your handedness is no longer humiliating. Lefties used to be strange creatures, as society considered them criminals. Left-handed children were forced to write with their right hand, although it was very difficult! Now times have changed. No longer shamed or mocked, the left-handed coexist with us peacefully. 

However, doctors continue to study handedness. For example, scientists don't know why the prevalence of lefties has remained consistently at 10% of humanity over time. Or exactly how being left-handed, which relates to brain symmetry, translates into other functions of the brain.

Left-handedness has been seen as a key to the complex anatomy of the brain, so investigators continue to look for links to many other conditions, including immune disorders, learning disabilities and dyslexia, reduced life expectancy and schizophrenia. But it's not so simple.

Brain lateralization, the distribution of function into right and left hemispheres, is crucial for understanding language, thought memory and perhaps even creativity. For many years, handedness has been seen as an external clue to the balance in the brain between left and right.

For right-handed people, language activity is predominantly on the left side. Many left-handers also have left-side language dominance, but a significant number have language predominantly on the right side of the brain.

In general, left-handers have less asymmetric brains, with more perfect distribution over the two hemispheres. Perhaps a more exact way to think about them is as non-right-handers, because many of them are ambidextrous.

By Megan Gibson at www.time.com and Perri Klass at www.nytimes.com. Picture by United Press International; Gary Cameron/Reuters; Jonathan Ernst/Reuters

A Close Look at Mercury

It took six years but on Thursday, March 17, NASA's Messenger spacecraft finally settled into orbit around the small planet Mercury. Now, for the first time, a spacecraft will observe Mercury for at least a year and may answer some questions.

Why, for example, is Mercury so much denser than Earth, Venus and Mars? Scientists know the basic reason: Mercury has less rock and a higher percentage of iron than the other planets, and iron is denser than rock. But how did Mercury end up that way?

Another question is whether there could be ice hiding in craters at Mercury's poles. Radar soundings taken from Earth show that there's something highly reflective inside, and because the crater floors are in perpetual shadow, it's cold enough for ice to exist. The spacecraft's neutron detectors will look for signs of hydrogen — the H in H2O — which would suggest it might be ice.

However, the most mysterious question is why Mercury has a magnetic field. In theory, you need molten iron in the core to generate one, but Mercury is so small, its core should have solidified.

And there's plenty more: Messenger carries no fewer than seven different instruments, including high-resolution cameras that will map just about every inch of the planet in extraordinary detail, picking out objects as small as 18 m across.

But anyone who follows astronomy knows that there's an important rule in space exploration: When you try a new, more powerful instrument on a celestial object you're sure to raise more questions than you answer. That is when the fun really begins.

By Michael Lemonick at www.time.com. Picture by NASA

Origin of Life

Stanley Miller and Harold Urey carried out experiments in the 1950s that were the first serious try to find out how life on earth began. They passed sparks through a combination of methane, ammonia, water vapor and hydrogen — a mixture similar to earth's original atmosphere. At the University of Chicago, they created amino acids, the building blocks of proteins. Their reconstructed atmosphere was probably wrong, and the experiment had never been repeated with a more exact mixture

But everyone was wrong. After Miller left Chicago, he worked with Jeffrey Bada, a chemist from the USA, at the Scripps Institution. When Miller died in 2007, Bada received boxes full of his paraphernalia. He did not worry about them much for some time, but one day...

"It was just a stunning experience," Bada says. "All these little boxes full of equipment and vials from all Stanley's original experiments. It was an unbelievable collection of organic compounds and amino acids."

The new studies are potentially important because they are based on a more realistic picture of conditions on early earth. In particular, they suggest that life might have appeared not in a "warm little pool" but in the violent environment of volcanic eruptions. "We know that the early earth had few continents, but it had a lot of volcanoes, and they were hotter and more powerful than today. They would have been throwing hydrogen sulfide and other gases."

Volcanic plumes would also have initiated powerful discharges of lightning from the skies. And that is essentially what Miller's unpublished experiments showed. And when the scientists analyzed Miller's original samples they found a wide assortment of amino acids, including some that had never been made before. 

The big action now is how go from these simple molecules to self-replicating molecules. It's challenging science. It's just a matter of time before we get to know it.

By Michael Lemonick at www.time.com. Picture by UPI Photo / Landov

Heart Surgery without Surgery

Cardiologists are talking about a major advance: A study suggests that many people with a bad aortic valve, the heart's most important gate, can avoid open-heart surgery and have a new one placed through a tube in an artery. There may be a problem — a higher risk of stroke — and nobody is sure about how long these valves will last.

The aortic valve can harden and narrow with age, making it difficult for the heart to push blood through it. Severe cases are treated with surgery to replace the valve, but that's risky for many older people who have this problem. Without an operation, half die within two years.

Through an artery in the chest a new heart valve is attached to a balloon and advanced across the narrowed, older, diseased heart valve. The balloon is inflated and the new valve left in place.

A test of this gentler treatment in people too sick for surgery improved survival, doctors reported last fall. The new study involved nearly 700 people eligible but at high risk for surgery. Their median age was 84, and they were randomly assigned to get valves replaced through surgery or the new way. Twenty-eight people refused treatment when they learned they had been placed in the surgery group.

In the end, both groups did very well. After one year, about 24 percent of the artery patients and 27 percent of the surgery patients had died — considerably fewer than doctors had predicted.

However, strokes were twice as common in the artery-treated group — 8 percent versus 4 percent.
AP / Marilyn Marchione at www.time.com. Picture by AP / Edwards Lifesciences.

Frogs and Teeth

Dollo’s Law, a theory proposed by the scientist Louis Dollo in the 1800s, says that when a particular property is lost in a species, it never comes back. It explains why humans have no tails, birds and turtles have no teeth and snakes have not regained legs.

But a new analysis about frogs, done by a scientist at Stony Brook University, found that they lost teeth in the lower jaw at least 200 million years ago, but a particular type of marsupial tree frog regained those lower teeth about 20 million years ago.

“It’s a very clear-cut case of re-evolution because of the large time span,” said John Wiens, the Stony Brook biologist who wrote the paper in the journal Evolution.

Dr. Wiens analyzed DNA samples of 170 modern and fossilized frogs to approximate the dates of loss and re-evolution of the teeth. Most frogs have teeth on their upper jaws, which may have made the re-evolution in the tree frog, known as Gastrotheca guentheri, easier, Dr. Wiens said. “They already had teeth in the upper jaw, so they had the enamel, dentine and other necessities,” he said. “There was a way to facilitate new teeth after 200 million years.”

The species is the only known modern frog species with lower teeth, though certain other species with upper teeth have structures similar to teeth on the lower jaw. “That’s a big question now: What’s preventing the other frogs from developing real teeth on the lower jaw?” Dr. Wiens said.

By Sindya N. Bhanoo at www.nytimes.com. Picture by Chris Gash.