By Paul Rincon
BBC News science reporter
The discovery by Nasa's robotic rovers of a watery past on Mars has topped an eagerly awaited list of the 10 key scientific advances of 2004.
Science published the data from the rovers
Compiled each year by Science magazine, the list has always divided opinion, and this year's has proved no exception.
The rovers triumphed in a strong field, including the discovery of a dwarf human species in Indonesia.
But Donald Kennedy, editor of Science magazine, said selecting first place in the list "wasn't a headache".
Not everyone shared his assessment. For some, the announcement in February that South Korean scientists had cloned human embryos had far-reaching significance.
"It has a whole range of implications; it's a very important development," said Professor Christopher Higgins, director of the Medical Research Council's Clinical Sciences Centre in London, UK.
"I wouldn't put the rovers at the top. It's a great technological achievement, but they haven't found life. If they had, that would have been extraordinarily exciting."
The South Korean work was an important step along the road to therapeutic cloning. But Professor Higgins also sees philosophical implications in the work.
"The fact it can be done begins to move us away from some of the mysteries surrounding human beings; things like the existence of a soul, which frankly is pure imagination," he told the BBC News website.
"It begins to get us to that point at which we realise we are just a different form of animal. Science is about trying to understand where we come from, what our purpose is.
"Cloning a human embryo starts to address those questions. It may not be in the way that people like - as it may suggest there is no purpose - but I think it's very important."
Runner-up status went to the mind-boggling discovery that a dwarf species of human - dubbed "the Hobbit" by some - had survived on the Indonesian island of Flores until 13,000 years ago.
The study, published in the rival journal Nature, had seemed a strong contender for the top spot.
In his editorial, Mr Kennedy said the find - known to science as Liang Bua 1 or LB1 - had "gripped the imagination of many". But, he added, it also raised questions and controversy.
This direct image of an exoplanet (below left) did not make Science's list
"The lone skull and related postcranial remains are now under re-examination. We'll see how the story unfolds," Mr Kennedy wrote.
Also featured in Science's picks of the year was the discovery that junk DNA is not as useless as previously thought, and disturbing declines reported in plant and animal diversity.
But for writer and broadcaster Simon Singh, the stand-out discovery of the year was the first direct image taken of a planet circling another star. The picture, taken by astronomers in Chile, did not even make Science's top 10.
"We have found literally dozens of planets outside our Solar System and that in itself has changed our view of the Universe," Dr Singh told the BBC News website.
"We now know there are other planets because we can see their effects on other stars. But to actually see another planet - even though it's probably not at all like our own - is extraordinary.
"Extraordinary not only in that we have the technology to see this object, but because it suggests one day we might see a planet like Earth and perhaps see evidence of life.
"To me, it's an historic image and I can't believe it wasn't splashed on front pages around the world."
Science magazine's breakthroughs of 2004
- Winner: Water on Mars. Nasa's Mars rovers Spirit and Opportunity discovered compelling evidence for the prolonged existence of salty, acidic water on the surface of the Red Planet.
- Runner up: Indonesian "hobbit". A team of archaeologists made the mind-blowing discovery of a new species of human that stood only one metre tall and lived on the Indonesian island of Flores.
- Human cloning. South Korean researchers made headlines across the world after announcing they had cloned human embryos, the first published and "peer reviewed" evidence this technique could work with human cells.
- Understanding condensates. In 2004, scientists made giant leaps in understanding ultra-cold gases called condensates, shedding light on some key problems in physics.
- Hidden DNA treasures. Stretches of "junk DNA" proved to be far more important than previously thought. They turned out to be essential for helping genes turn on at the right time and in the right place.
- Pulsar pair. Astrophysicists discovered the first known pair of pulsars, spinning neutron stars that shoot out jets of radiation.
- Declining plant and animal diversity. There was disturbing news this year about the decline of species diversity from large studies that surveyed amphibians, butterflies, plants and birds.
- Water on tap. New results on the structure and chemical behaviour of water could reshape fields from chemistry to atmospheric science.
- Medicines for the World's Poor. "Public-private partnerships" emerged as a force in 2004, according to Science magazine, affecting the way medicines are developed and delivered to emerging nations.
- Genes in a Drop of Water. This year, researchers hit on a new way to identify lifeforms too small and too remote to see. They collected water from diverse environments and sequenced the genes floating in it.