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For 140 years, scientists argued over the evolutionary link between modern birds and a group of ancient reptiles dubbed dinosaurs — or "fearfully great lizards" — by Sir Richard Owen, the first director of London's Natural History Museum. Were the animals related, as their similar bone structures appeared to indicate? Perhaps. But skeptics still wanted to see more convincing evidence — and that meant feathers.

The final, feathery piece of the puzzle fell into place two years ago when farmers in Liaoning Province, in northeastern China, unearthed the missing link. Termed the "fuzzy raptor," the remarkable 124-million-year-old juvenile dinosaur fossil that they discovered had the bony, duck-sized skeleton of a meat-eating Dromaeosaurus and the feathers of a bird.

The fuzzy raptor and 12 other unique "dino-bird" fossils from the Geological Museum of China in Beijing are now making their first visit to Europe. From this week until May 5, 2003, visitors to the Natural History Museum can view the first exhibition of the Early Cretaceous fossils presented in a way that traces the evolutionary origins of birds. Beginning with a Late Jurassic treasure from the museum's own collection — the Archaeopteryx lithographica, the most ancient bird known to science — the exhibition takes visitors through the arguments and the evidence confirming that today's birds are indeed descendants of a group of small predatory dinosaurs that, rather than dying out (like the much bigger and mightier Tyrannosaurus rex), evolved feathers and then the ability to fly.

Is there still room for doubt? "Not now," says Angela Milner, associate keeper of paleontology and dinosaur researcher at the museum. "There's too much evidence. Fuzzy raptor is the missing link that clinches the relationship." Still, she adds, "there's no one modern bird that we can say is closest to the dinosaurs."

In 1861, a century and a quarter before the dino-birds of Liaoning were discovered, a small avian skeleton surrounded by delicate feather impressions was found in Germany, in a limestone formation at Solnhofer, northwest of Munich. Dating from the Late Jurassic period, the primitive bird — with clawed fingers, teeth and a bony tail — was preserved in stone 147 million years ago. Gazing upon its outstretched wings and slender body, the Bavarian quarryman who found it reportedly believed that he had found the remains of an angel. Described more scientifically as an ancient, long-tailed bird, the chicken-sized creature was labeled Archaeopteryx, or ancient wing. Only seven specimens of the rare fossil are known to exist, all from the lithographic (compact and fine-grained) limestone quarries near Solnhofer.

Fast-forward to Liaoning, 1995. Fossilized birds, captured in ancient sediment, are uncovered by local farmers. Slightly more advanced than Archaeopteryx — and named Confuciusornis by Chinese scientists — the fossils lacked long bony tails and toothed jaws, but still had wings with distinct fingers ending in claws. Large numbers of such fossils indicated that Confuciusornis, the oldest beaked bird found in China, lived in flocks.

A year later, a Liaoning quarry yielded a new small theropod dinosaur, a voracious, bipedal hunter. Excited scientists noted that the Sinosauropteryx had a fringe of "unbranched integumentary structures" — otherwise known as fluff — along its backbone. Was the fluff a primitive type of feathery covering that the warm-blooded little dinosaur — which could not fly — used for insulation?

In 1997, remains of the Caudipteryx and the Protarcheopteryx dinosaurs were found in the same quarry. Both fossils had feathers similar to those of modern birds, though they still lacked the distinctive, asymmetrical feathers needed for them to become airborne. (Flight feathers have narrow leading edges and are sculpted on the wing to provide lift.) Only one Protarcheopteryx specimen exists in the world. The sweeping action of its long arms, scientists say, later developed into an avian flight stroke.

When the fuzzy raptor was found in 2000, its perfectly preserved downy body feathers conclusively resolved the long-standing mystery of the evolutionary link between dinosaurs and birds. While that unique specimen — a two-legged, fast-running dromaeosaur, much like the Velociraptors of the film Jurassic Park — has yielded one secret, another big mystery remains: Did the first birds first fly from the ground up, or from the trees down? Indeed, it was wings, not feathers, that were most crucial to flight.

While scientists puzzle and argue over that question, the dino-birds themselves will be flying around Europe beginning next summer. Staff at the Natural History Museum say they're talking with museums in Germany, Italy, the Netherlands and others about hosting the exhibition over the next four years — a mere blink of the eye in paleontological time.

Natural History Museum: www.nhm.ac.uk

• MARYANN BIRD/London
• Feathered dinosaurs land in London for new exhibition