For such a structure to evolve at random would be a lengthy process. Instead, Dr. Gilbert suggests, the first genetic elements were simple modules, the forerunners of today's exons, and the exons were then mixed and matched to build up the lengthy chains that make longer and longer genes. By analyzing the structure of contemporary genes, it should be possible to discern the ancient modules within. For the modules to mix and match, Dr. Gilbert believes, they would have needed strips of extra DNA, like the leader on a reel of film, so that two modules could be spliced together without risk of cutting into their genetic message in the process . The introns, the elements interspersed between the coding regions of genes, are in this view related to the ancient leaders that enabled the modules to be assembled.
Critics say the theory leaves some awkward issues unexplained.
Introns are found in the DNA of plants and animals but not in that of bacteria. Dr. Jeffrey D. Palmer at Indiana University and a few other experts in molecular evolution suggest that Dr. Gilbert's theory is wrong. They say that the introns and the shuffling of genetic modules were mechanisms that did not arise until two billion years after life first began. And, they regard the introns not as clever Tinker Toy construction elements, but as just what they appear to be, pieces of junk DNA left over from billions of years of evolution. Noting that bacteria, presumably more primitive than plants and animals, do not have introns in their genes, they say it is more reasonable to assume bacteria never had introns than that they had and later lost them.