By BBC News Online Science Editor Dr David Whitehouse

Research into human stem cells has been the most important advance of 1999, according to the influential journal Science.

In its annual top ten list of the biggest developments of the past twelve months, Science says, "in just one short year, stem cells have shown promise for treating a dizzying variety of human diseases".

The journal says that 1999 has been a pivotal time in which scientists and the public have grappled with the ethical and scientific implications of stem cell research.

Right at the end of last year, two research groups announced that they had for the first time isolated and cultured human embryonic stem cells - the parent cells of all tissues in the body.

This opened the way for a wave of new papers on stem cell research in 1999. Scientists believe that if they can now learn the complex chemical signals that make stem cells specialise, they will, in theory, be able to grow-up any tissue they desire in the lab.

This will herald a new era in transplant medicine when replacement cells - and perhaps one day even whole organs - will come "off the shelf" rather than from a human donor.

Allied to cloning technology, stem cells could even provide us with perfect-match tissue, greatly reducing the risks of rejection.

Range of disciplines

Four of the top ten stories were concerned with aspects of molecular biology; three with astronomy; two with the properties of matter; and one with the history of life on Earth. This is how the editors of Science magazine saw 1999.

• Genomic study: The reading and interpretation of the DNA genetic code of a living creature. With technology that allows for the rapid sequencing of an organism's DNA, scientists have been able to compare in detail the genetic make up of creatures that are closely and distantly related. The announcement of the near-complete sequence for human chromosome 22 was a major highlight of the year.
  
    Historic first: Chromosome 22 is sequenced
  
  
• Ribosome blueprint: Researchers produced the first complete molecular map of the ribosome, the machinery used by cells to produce proteins.
• New planets: The pace at which new planets circling nearby stars were discovered quickened in 1999. There are currently about 30 planets known to be orbiting stars in the solar neighbourhood.
  
   The first planetary system outside our own is discovered
  
  
• Early life: By finding telltale signs of cells that contained a nucleus in 2.7-billion-year-old Australian rocks, scientists said the world was populated by eukaryotes (cells that have a nucleus) much earlier than previously thought.
• Quantum world: During 1999, scientists created a bizarre new kind of low-temperature gas called a Fermion condensate that may form the basis for a new generation of clocks and lasers.
• Mystery explosions: Astronomers finally made some progress in understanding so-called gamma-ray bursters, which are enormous explosions in deep space. They emit more power in a few seconds than our Sun could in 10 billion years.
• Molecular memories: A series of discoveries made it somewhat easier to comprehend how our memories are formed by molecules in our brains.
• Flat universe: Astronomers found new evidence that the Universe is flat. This supported the Big Bang theory by establishing a precise balance between matter and energy.
• Photonic crystals: This year, scientists made significant steps in their quest to harness the power of photons in the same way that electrons are used in electronic circuits.

Science also voted the loss of the Mars Climate Orbiter (MCO) spacecraft as the blunder of the year.

The MCO was lost when engineers and scientists got their English and metric measurements mixed up, just as the spacecraft was about to enter Martian orbit. The craft burnt up, much to Nasa's embarrassment.

 The moment MCO mission control knew they had a problem

The decision by the Kansas State Board of Education to drop evolution from its science teaching has been voted the breakdown of the year, while the debate over genetically-modified (GM) foods has been voted the controversy of the year.

  Paul Reynolds reports from Kansas

  The BBC's Tom Carver: Pressure mounts of US farmers to reject GM crops