LOS ANGELES, May 22 (Xinhua) -- Ancient whales, four-footed land animals like large modern dogs, evolved into graceful swimmers through a series of small genetic changes during their embryonic development, scientists said on Monday.

    This finding, published in the May 22 issue of the Proceedings of the National Academy of Sciences, revealed the genetic basis behind one of the best-documented examples of evolutionary change in the fossil record.

    The gradual shrinkage of the whales' hind limbs over 15 million years was the result of slowly accumulated genetic changes, which influenced the size of the limbs.

    However, the actual loss of the hind limb occurred much further along in the evolutionary process, when a drastic change occurred to inactivate a gene essential for limb development, there searchers said.

    This gene, called Sonic Hedgehog, functions during the first quarter of gestation in the embryonic period of the animals' development, before the fetal period. In limbed vertebrates, the gene is required for normal limbs to develop beyond the knee and elbow joints.

    The researchers, led by Hans Thewissen at the Northeastern Ohio University, began by exploring the embryonic development of whales' cousins, the dolphins.

    The dolphins are intriguing because for a brief time during development they do sprout hind limbs, which quickly vanish again as the embryos reach the second month in a gestation period that lasts about 12 months.

    In most mammals, a series of genes is at work at different times, delicately interacting to form a limb with muscles, bones, and skin, the researchers explained.

    "The genes are similar to the runners in a complex relay race, where a new runner cannot start without receiving a sign from a previous runner," Thewissen said.

    In dolphins, the Sonic Hedgehog gene drops out early in the race, disrupting the genes that were about to follow it. That causes the entire relay to collapse, ultimately leading to the regression of the animals' hind limbs.

    The whales' story is more complex, according to the researchers.

    Between 41 million and 50 million years ago, whales' hind limbs did shrink greatly as the former land animals began a return to the sea.

    But their legs showed no change in the basic arrangement and number of bones, which proved that Sonic Hedgehog gene was still functioning. Its loss must have come later.

    The dramatic loss of Sonic Hedgehog expression was not the genetic change that drove hind limb loss in whales.

    Instead, the researchers concluded, whales' hind limbs regressed over 15 million years via "Darwinian microevolution": a step-by-step process occurring through small changes in a number of genes relatively late in development. Enditem