The study found that if HIV-infected heterosexual individuals begin taking anti-retroviral medicines when their immune systems are relatively healthy as opposed to delaying therapy until the disease has advanced, they are 96 percent less likely to transmit the virus to their uninfected partners. Findings from the trial, first announced in May, were published in the New England Journal of Medicine in August.

　　The study was funded by the U.S. National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.

　　"The HPTN 052 study convincingly demonstrated that anti- retroviral medications can not only treat but also prevent the transmission of HIV infection among heterosexual individuals," said NIAID Director Anthony Fauci in a statement. "We are pleased that Science recognized the extraordinary public health significance of these study results."

　　Science's list of nine other ground-breaking scientific achievements from 2011 include:

　　The Hayabusa Mission: After some near-disastrous technical difficulties and a stunningly successful recovery, Japan's Hayabusa spacecraft returned to Earth with dust from the surface of a large, S-type asteroid. This asteroid dust represented the first direct sampling of a planetary body in 35 years, and analysis of the grains confirmed that the most common meteorites found on Earth, known as ordinary chondrules, are born from these much larger, S-type asteroids.

　　Unraveling Human Origins: Studying the genetic code of both ancient and modern human beings, researchers discovered that many humans still carry DNA variants inherited from archaic humans, such as the mysterious Denisovans in Asia and still-unidentified ancestors in Africa. One study this year revealed how archaic humans likely shaped our modern immune systems, and an analysis of Australopithecus sediba fossils in South Africa showed that the ancient hominin possessed both primitive and Homo-like traits.

　　Capturing a Photosynthetic Protein: In vivid detail, researchers in Japan have mapped the structure of the Photosystem II, or PSII, protein that plants use to split water into hydrogen and oxygen atoms. The crystal-clear image shows off the protein's catalytic core and reveals the specific orientation of atoms within. Now, scientists have access to this catalytic structure that is essential for life on Earth -- one that may also hold the key to a powerful source of clean energy.

　　Pristine Gas in Space: Astronomers using the Keck telescope in Hawaii to probe the faraway universe wound up discovering two clouds of hydrogen gas that seem to have maintained their original chemistry for two billion years after the big bang. Other researchers identified a star that is almost completely devoid of metals, just as the universe's earliest stars must have been, but that formed much later. The discoveries show that pockets of matter persisted unscathed amid eons of cosmic violence.