By Victoria Gill
Science reporter, BBC News
The team slowly reconstructed what "Ardi" would have looked like
The discovery of a fossilised skeleton that has become a "central character in the story of human evolution" has been named the science breakthrough of 2009.
The 4.4 million year old creature, that may be a human ancestor, was first described in a series of papers in the journal Science in October.
It has now been recognised by the journal's editors as the most important scientific accomplishment of this year.
It is part of a scientific top 10 that ranges from space science to genetics.
The first fossils of the species, Ardipithecus ramidus, were unearthed in 1994. Scientists recognised their importance immediately.
But the very poor condition of the ancient bones meant that it took researchers 15 years to excavate and analyse them.
The most important thing to emerge from that excavation was the partial skeleton of a female creature, which has now been nicknamed "Ardi".
An international team of scientists unveiled the skeleton in a series of scientific papers published in Science in October.
Their careful examination of its skull, teeth, pelvis, hands and feet revealed that Ardi shared a mixture of "primitive" traits shared with its predecessors, and "derived" features, which it shared with later hominids, or human-like creatures.
It shared some of these derived features with humans.
Professor Tim White from the University of California, Berkeley in the US, was one of the lead scientists working on the project.
"This is not an ordinary fossil. It's not a chimp. It's not a human. It shows us what we used to be," he told Science Magazine at the time the research was published.
One of his team's key conclusions was that Ardi walked upright. This was based on the painstaking reassembly of its very badly crushed pelvis, which the scientists said had a shape that would have allowed Ardi to balance on one leg at a time.
Professor White said that some researchers had been sceptical about these conclusions.
"Some people have looked at the pelvis and said, 'my gosh, that's fairly squashed. Are you sure you knew how to put it together correctly?' So we're responding to that," he told Science magazine.
Ardipithecus was even more primitive than the famous "Lucy" fossil - a 3.2 million year old Australopithecus skeleton that was discovered in 1974.
Professor Chris Stringer, a palaeontologist from the Natural History Museum in London said that Ardi was likely "a remnant of a more ancient stage of human evolution" than Lucy.
"[It was] closer in many ways to the ancestor we shared with our closest living relatives, the chimpanzees, more than six million years ago," he said.
Nasa's discovery of water on the Moon was one of the runners up
The editor-in-chief of Science said that the Ardipithecus research represented a "culmination of 15 years of painstaking, highly collaborative research by 47 scientists of diverse expertise from nine nations."
The nine runners up in Science's list of this year's most important breakthroughs were published in a number of scientific journals, including Science, Nature and Proceedings of the National Academy of Sciences (PNAS).
The first runner up was Nasa's discovery of magnetised, rapidly rotating neutron stars called pulsars.
Others included the discovery that a compound called rapamycin boosted longevity in mice - the first time any drug has stretched a mammal's life span - and advances in gene therapy that could help treat a fatal brain disease.
The nine runners up were:
- Pulsar mystery: Nasa's Fermi gamma-Ray Space Telescope helped identify previously unknown pulsars - highly magnetised and rapidly rotating neutron stars.
- Extending life: Researchers found the compound rapamycin extends the life span of mice. The discovery was particularly remarkable because the treatment did not start until the mice were middle-aged.
- Supreme conduction: Materials scientists probed the properties of graphene - highly conductive single-layer sheets of carbon atoms - and started fashioning the material into experimental electronic devices.
- Plant survival: Scientists discovered the structure of a critical molecule that helps plants survive during droughts. This could help in the design of new ways to protect crops against prolonged dry periods.
- Laser tool: The SLAC National Accelerator Laboratory in California unveiled the world's first X-ray laser, a powerful research tool capable of taking snapshots of chemical reactions as they happen and studying materials in unprecedented detail.
- Gene Therapy: European and US researchers made progress in treating a fatal brain disease, inherited blindness, and a severe immune disorder by developing new strategies involving gene therapy.
- Magnetic monopoly: Physicists working with strange crystalline materials called spin ices created magnetic ripples that behaved like "magnetic monopoles" - fundamental particles with only one magnetic pole.
- Watery Moon: Nasa discovered water vapour in the debris when it deliberately crashed a rocket near the south pole of the Moon. The experiment was part of the space agency's Lunar Crater Observation and Sensing Satellite (LCROSS) mission.
- Hubble Repair: A final repair mission by space shuttle astronauts gave the Hubble telescope sharper vision, enabling it to produce some of its most spectacular images yet.