CAMBRIDGE, Massachusetts -- Moving a step closer to understanding how life began, scientists say they have produced a complex molecule similar to those that existed on earth 3 billion years ago.


Researchers at the Whitehead Institute for Biomedical Research and the Massachusetts General Hospital found that complex ribozymes -- catalytic molecules made of ribonucleic acid, or RNA, which promote chemical reactions -- could have originated in one step rather than evolving over time.


"We're trying to understand how life arose on earth. The experiments provide a little more evidence that helps us understand the origin of life," said Jack Szostak, a molecular biologist at the Massachusetts General Hospital and a collaborator on the study.


The study suggests complex ribozymes might have been important in the evolution of early life.


"We can get fairly complex enzymes more easily than we thought," Szostak said.


The findings were to be published in last Friday's issue of the journal Science.


Scientists have no direct way to study life forms that existed billions of years ago.


They study that world by trying to reconstruct molecules and to search for catalysts, like ribozymes, that might have been important.


The discovery of ribozymes in the early 1980s suggested there might be an "RNA world," a time before protein-based catalysts became the workhouses of biochemical reactions in modern cells and before deoxyribonucleic acid, or DNA, became the genetic code of life.


Ribozymes appear to fulfil two basic requirements as life's progenitor molecules: They contain the information to copy themselves and they can catalyze some reactions.


But little is known about the structure of ribozymes and the ancient world in which they existed. And few ribozymes have been identified.


"We hope to explore the range of reactions and the intrinsic properties of RNA," said David Bartel, principal author of the Science paper and a fellow at the Whitehead Institute.


The ability to generate such an efficient ribozyme from random sequences may hold promise for new ribozymes that could be used for diagnosing and treating diseases, the Whitehead Institute said.