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This is SCIENCE IN THE NEWS, in VOA Special English. I'm Steve Ember.

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And I'm Faith Lapidus. On our program this week, twenty-five years into the AIDS crisis, new findings about its history ...

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Also, a new theory about the history of humans and chimps ...

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And a report on the dangers of indoor air pollution from solid cooking fuels.

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On June fifth, nineteen eighty-one, a medical report described an unusual infection in five young men in Los Angeles. The Centers for Disease Control reported that all five were homosexuals, and two had died. Soon doctors identified more and more cases. In time, the infection became known as Acquired Immunodeficiency Syndrome.

A United Nations report last week said the spread of AIDS worldwide appears to be slowing. But rates of new infections continue to increase in some areas of the world. The newest estimate is that around forty million people are currently infected with the virus that causes AIDS.

The report says about three million people died of AIDS in two thousand five. Four million others became infected.

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After twenty-five years, scientists continue to work on a vaccine to protect against H.I.V., the virus that causes AIDS. There is no cure for AIDS. But antiretroviral medicines can add years to survival.

Scientists say H.I.V., human immunodeficiency virus, is related most closely to an animal virus. This virus is known as S.I.V., simian immunodeficiency virus. Simians include apes and monkeys.

Scientists have known for years that S.I.V. has been found in chimpanzees living in captivity. But they were not sure if the virus existed in chimps in the wild. Now experts say they have direct evidence to confirm that the human virus came from chimps.

In west-central Africa, they have found what they call an important missing link in the history of H.I.V. An international team gathered chimpanzee waste from the forest floor in areas of southern Cameroon.

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Beatrice Hahn of the University of Alabama at Birmingham led the study. Doctor Hahn says tests on the droppings showed antibodies to S.I.V. Antibodies are white blood cells that fight invading organisms.

The scientists also did genetic tests on the waste to identify individual chimps as well as communities of chimps. Doctor Hahn says these tests showed which ones had the kind of S.I.V. that led to what is now the most common form of the human virus.

The researchers found infection rates as high as thirty-five percent in some chimp communities in southern Cameroon.

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The scientists believe that the virus spread from chimps to humans most likely through bushmeat hunters.

Developing the tests to do this study without further endangering chimp populations took seven years. Science magazine published the findings. But there are questions that remain, such as how the virus developed in chimps before it jumped to humans.

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The first known case of H.I.V. was found in a man from what is now the Democratic Republic of Congo. His name is not known. He had given blood in nineteen fifty-nine for research on genetic resistance to malaria.

Researchers at the University of Washington in Seattle stored the blood for future study. In nineteen eighty-six, five years after the first report of AIDS, the blood tested positive for H.I.V.

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You are listening to SCIENCE IN THE NEWS in VOA Special English.

Genetic experts say humans and chimps are ninety-eight percent related. A new study suggests that the development of humans from chimps may have been more recent than scientists have thought. And more complex.

The scientists who did the study say they found evidence of a period when humans reproduced with chimps. The result when different species are combined is a hybrid.

Scientists traditionally believe that hybrid populations die out. But the new theory suggests that this one continued to evolve into the humans of today.

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The theory comes from scientists at the Broad [pronounced brode] Institute. This is a joint program of the Massachusetts Institute of Technology and Harvard University. The findings appeared in the publication Nature.

David Reich led the team that did the research. In his words: "The study gave unexpected results about how we separated from our closest relatives."

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The scientists studied the genetic history of humans and of chimps and other apes. They believe that more than one million years after the ancestors of humans and chimps split, the two species produced hybrids. The theory is that modern humans developed from these hybrids, not from the earlier forms of humans.

The scientists say human females might have mated with chimps because males of hybrid species often cannot reproduce. They say the final split between humans and chimps took place perhaps less than five million four hundred thousand years ago.

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This estimate differs from findings by other researchers.

In two thousand two, scientists reported the discovery in Chad of a skull from a creature known as Toumai. Tests suggested that this nearly complete head bone was between six million and seven million years old.

The scientists who found Toumai described it as a new kind of hominid, a member of the family that includes modern humans. It apparently had human-like teeth and walked on two legs.

The researchers who did the new study say the dating of Toumai could be wrong. But if the fossil is that old, they say, then it may have come from before the final split into humans and chimps. Possible evidence for their theory that the separation took longer and was not as clean a break as people might think.

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The World Health Organization says half of the world's population burns wood, coal, animal waste or other solid fuels. More than three thousand million people use solid fuels to cook and to heat and light their homes.

But people who burn these fuels often breathe in large amounts of smoke. This can lead to pneumonia and other diseases.

The W.H.O. recently published a report about the dangers of solid fuels. The report says these fuels are the cause of one and one-half million deaths each year, mostly in Southeast Asia and southern Africa. Most victims of indoor air pollution are children and women.

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The World Health Organization says there has been little progress since nineteen ninety in supplying more people with modern cooking fuels. The report discusses what it would take to cut the use of solid fuels in half by two thousand fifteen.

To do that, almost five hundred thousand people every day would need to gain modern energy services. But experts say gains in health and productivity would more than pay for the costs required.

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The W.H.O. estimates a yearly cost of thirteen thousand million dollars to supply liquefied petroleum gas to half the people now using solid fuels. It says that investment would result in yearly economic gains of ninety-one thousand million dollars. Other kinds of fuel would cost more.

The report says there are simple solutions that could help people in the short term. These include cleaner-burning stoves and better systems to clear the air in homes. Longer-term solutions include a change to cleaner cooking fuels such as liquefied petroleum gas, biogas, ethanol or plant oils.

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The report is called "Fuel for Life: Household Energy and Health." It can be found on the World Health Organization Web site at w-h-o dot i-n-t (who.int). Enter the words "Fuel for Life" in the search box at the top.

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SCIENCE IN THE NEWS was written by Caty Weaver and Lawan Davis. Brianna Blake was our producer. I'm Faith Lapidus.

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And I'm Steve Ember. Read and listen to our programs at WWW.51VOA.COM. To send us e-mail, write to special@voanews.com. And join us again next week for more news about science in Special English on the Voice of America.