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Tiny creature may be ancestor of all mammals

Artist's impression of what Hadrocodium wui might have looked
Artist's impression of what Hadrocodium wui might have looked like  

PITTSBURGH, Pennsylvania (CNN) -- Researchers said Thursday that the discovery of the fossilized remains of what they believe is one of the tiniest mammals that ever lived has advanced their understanding of how mammals -- including humans -- evolved.

The discovery of a fossilized skeleton of an animal the size of a paper clip was made in China's Yunnan Province in 1985. The significance of the find was determined by an international team led by Dr. Zhe-Xi Luo of Pittsburgh's Carnegie Museum of Natural History.

"The mammalian family tree is part of our past," Dr. Luo told CNN. "A better understanding of our mammalian heritage gives a great meaning to our own existence."

The tiny animal --- dubbed Hadrocodium wui --- had a very large brain and advanced ear structure, characteristics that scientists say distinguish mammals from reptiles.

Harvard professor Alfred Crompton, co-author of the report published in the journal Science, said as some ancient reptiles made the transition from reptile to mammal, their lower jaws became a single bone while the three-bone structure that previously comprised the jaw lost its attachment to the jaw and became the middle ear. Scientists say a three-bone middle ear, along with a powerful jaw hinge, upper and lower teeth that match, and a large brain case are physical characteristics that distinguish mammals from reptiles.

Crompton called the evolutionary process that led to Hadrocodium wui a "crossing of the Rubicon" possible only in a very tiny animal.

"When you get that small," he said, "it is possible to probably move those bones off the lower jaw on to the skull. And that is the kind of step we were looking for."

Luo told CNN his team's discovery may contribute to understanding of the recently mapped human genome.

"The mammalian family tree is the road map for scientists to decipher the genetic make-ups of all mammals including human, and for understanding the history behind our biology," he said.

"The mammalian brain is the most important organ of our body. Its larger size and more elaborated structure is what distinguish us (the mammals) from all non-mammals, such as crocodiles, turtles, frogs, and fish," Luo continued. "The sensitive mammalian hearing allowed mammals to survive and to thrive to this day."

Cromption acknowledged that the discovery of Hadrocodium wui will have little impact on the man on the street. "It won't affect daily life," he said in a CNN interview. "But it does take us back closer to origins of mammalian life forms."

• Nature
Report: 3 new species of tiny primates discovered
November 14, 2000
• Undersea mountains yield living 'fossils'
June 26, 2000

• News at Carnegie Museum of Natural History

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