It takes more than old bones to trace the evolution of life on Earth. Some scientists are going after molecules. They seek the ancient ancestors of proteins and their genes that have evolved into the machinery of today's living cells. This quest has just paid off big. Instead of trying to glean clues from very old fossils, one research team has resurrected the ancient living entity itself.
It has put that old protein through its evolutionary stages in the laboratory. This has demonstrated in a concrete way how tightly integrated biological systems in living cells today can arise through the slow step-by-step process of accumulating small favorable mutations. In presenting their work, Jamie Bridgham and colleagues at the University of Oregon in Eugene suggest that the process they demonstrated "will be a predominant theme in evolution, one that may provide a general explanation for how molecular interactions critical for life's complexity emerged in Darwinian fashion."
This has been a month for spectacular announcements in paleontology. On April 5, Nature and the National Geographic Society reported finding a fossil of the first "fish" that walked on land. This filled a major gap in the history of how aquatic vertebrates evolved into land-based creatures. But the publicity surrounding this discovery overshadowed the report of Dr. Bridgham's team in the April 7 issue of Science. That work also fills a major gap in evolutionary history.
Evolutionary biologists call it the lock-and-key problem. If you found an intricately constructed lock that only an equally intricate key would fit, it would be hard to explain how these could arise through accumulation of small changes to both key and lock through time. It would appear that the key had to be constructed with the lock already in mind and vice versa. It would look as though someone had designed them. Many biological molecular systems present this appearance. A protein will have an intricate structure that physically and chemically fits neatly into an equally intricate structure of a so-called receptor molecule. The combination then forms a biologically active unit.
Bridgham and his colleagues studied the lock-and-key relationship between a common hormone (aldosterone) and its molecular partner that binds with it in a tightly integrated unit. They reconstructed the ancestral genes of the molecules' ancestors. They also resurrected the ancestor of the hormone's partner. They then showed how the protein evolved an affinity for molecules similar to aldosterone long before aldosterone appeared on the scene. When it did appear, it just co-opted a partner that had a preexisting aptitude for the relationship.
In a commentary accompanying the Science paper, Christoph Adami at the Keck Graduate Institute of Life Sciences in Claremont, Calif. observes: that "it is now possible to reconstruct the ancestral genes of an existing species so that, as Darwin urged us to do, we can 'look exclusively to its lineal ancestors' to understand a gene's evolution."
Darwin had said that discovery of a biological structure that could not be explained by step-by-step evolution would "absolutely break down" his theory. Critics cite tightly integrated biological molecular systems as just such non-Darwinian structures that must have been "intelligently" designed. Dr. Adami notes that studies such as those of the Bridgham team "solidly refute all parts of the intelligent-design argument."