First runner-up: Ribosome revelations. Researchers this year got their closest look yet at one of the cell's most important players, the protein factory called the ribosome. Each cell must precisely fit together two protein-RNA subunits so that the resulting macromolecule--the ribosome--can produce protein. Biologists have been eagerly awaiting a close-up of this complex for clues about how it makes protein with such stunning accuracy.

Over the last 12 months, three groups have sharpened our blurry view, presenting structures in atomic detail. The new close-up of the larger subunit's structure revealed that RNA itself acts as an enzyme, helping to link one amino acid building block to the next. This realization of RNA's active role helped bolster the theory that RNA molecules were among life's first, with proteins coming along later.

Other runners-up, in random order:

Fossil find. Anthropologists have long agreed that the first humans--that is, members of the genus Homo--arose in Africa. Yet just when these early humans began to migrate out of Africa and inhabit other continents has been a matter of fierce debate. In May, a team of Georgian and German prehistorians reported the first undisputed proof that humans had indeed left Africa at least 1.7 million years ago. The spectacular find of two well-preserved skulls at Dmanisi, 85 kilometers south of the Georgian capital Tbilisi, sent waves of excitement through the anthropological community and united it in a rare consensus about the great importance of the find.

One word--organics. Earlier this month, the chemistry Nobel Prize was presented for the discovery in the 1970s that plastics can be made electrically conductive. That discovery led to the finding that plastics could act like semiconductors--the workhorse materials of the information revolution--and it sparked research efforts to make everything from lasers to computer circuits out of plastic and other organic molecules. This year, these efforts surged ahead on several fronts, raising hopes for applications such as wall-sized electronic displays and electronic price tags.

New cells for old. Scientists performed some amazing tricks this year with stem cells and cloning, further undermining the old idea that a cell's developmental path is irreversible. In one surprising result, researchers showed that brain cells from adult mice can, when injected into early mouse embryos, become cells in the heart, stomach, liver, and other organs of a developing fetus. Other scientists found that cells from adult human bone marrow had become liver cells in patients who received transplants. In mice, transplanted bone marrow cells can travel to the brain and become neuronlike cells. If these multitalented cells can be tamed, researchers might be able to repair tissues damaged in spinal cord injuries, heart disease, and other maladies.

Water, water, everywhere. The solar system looked wetter and wetter this year. And where there's water, there's talk, at least, of alien life.

The long-proposed ocean on Jupiter's moon Europa began to look very real after the Galileo spacecraft sped past the ice-covered moon early this year. Galileo picked up a weak magnetic field near Europa whose orientation depended on the changing orientation of Jupiter's much stronger field. Europa watchers could imagine only one material--salty ocean water--that could produce such a varying field. Researchers continued to debate whether a past ocean washed over Mars, but excitement grew with reports of how water seems to have meandered along or beneath the planet's surface for eons.

Cosmic BOOMERANG. As 1999 drew to a close, cosmologists were almost shivering with anticipation. A preliminary map of the sky from a balloon-borne telescope named BOOMERANG had served up tantalizing evidence that we live in a flat universe that will expand forever instead of falling back on itself eons from now. By the end of 2000, a thorough analysis, which was soon strikingly confirmed by a second independent balloon experiment called MAXIMA, removed almost all doubt that the universe is flat, even as it called into question some fundamental assumptions about the state of the early universe.

Good reception. Nuclear hormone receptors--proteins that sit on the membrane surrounding the chromosomes and turn genes on and off in response to certain hormones--are implicated in ailments such as cancer, heart disease, and diabetes. This year brought new insights into their involvement in the metabolism of cholesterol and fatty acids. Scientists identified new target genes and potential drugs that could block or enhance the receptors' signaling, perhaps leading to effective treatments for seemingly intractable diseases.

So NEAR ... After circling the 34-kilometer asteroid Eros for less than half a year, the NEAR Shoemaker spacecraft revealed the rock's deepest secret--under a mask of rouge hides a heart containing some of the most primitive matter in the solar system. The discovery solves the decades-old mystery of what kind of asteroid can supply the most common meteorites falling to Earth.

Quantum curiosities. Textbook wisdom once held that quantum mechanics ruled the small world, classical mechanics the large. Superposition, for instance, a quantummechanical phenomenon made famous by Schrödinger's half-alive, half-dead cat, is usually associated with atoms. But in March, physicists announced that they had an electric current to flow around a superconducting loop of wire clockwise and counterclockwise at the same time--a form of superposition. Although not yet cat-sized, these current loops were a few micrometers across, much bigger than atoms.

Until early this year, scientists were also convinced that the power of quantum computers rests on a quantum property called "entanglement" where the fates of two quantum objects are linked. But in January, one scientist showed that you could still get quantum-computer power without using entanglement, leaving researchers scratching their heads.

--THE NEWS AND EDITORIAL STAFFS

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An expanded version of the Breakthrough of the Year section, with references and links, is posted at ScienceOnline