National Geographic : 2009 Feb
and lower arm bones and even the wrists of land animals: a missing link, if ever there was one. It may even have been able to live in the shallows or crawl in the mud when escaping predators. Equally intriguing, however, is what Tiktaalik has taught Shubin and his colleagues in the lab- oratory. e fossil s genes are lost in the mists of time. But, inspired by the discovery, the researchers studied a living proxy---a primitive bony sh called a paddle sh---and found that the pattern of gene expression that builds the bones in its ns is much the same as the one that assembles the limb in the embryo of a bird, a mammal, or any other land-living animal. e di erence is only that it is switched on for a shorter time in sh. e discovery overturned a long-held notion that the acquisition of limbs required a radical evolutionary event. "It turns out that the genetic machinery needed to make limbs was already present in ns," says Shubin. "It did not involve the origin of new genes and developmental processes. It involved the redeployment of old genetic reci- pes in new ways." ough modern genetics vindicates Darwin in all sorts of ways, it also turns the spotlight on his biggest mistake. Darwin s own ideas on the mechanism of inheritance were a mess---and wrong. He thought that an organism blended together a mixture of its parents traits, and later in his life he began to believe it also passed on traits acquired during its lifetime. He never under- stood, as the humble Moravian monk Gregor Mendel did, that an organism isn t a blend of its two parents at all, but the composite result of lots and lots of individual traits passed down by its father and mother from their own parents, and their grandparents before them. Mendel s paper describing the particulate nature of inheritance was published in an obscure Moravian journal in 1866, just seven years a er e Origin of Species. He sent it hope- fully to some leading scientists of the day, but it was largely ignored. e monk s fate was to die years before the signi cance of his discovery was appreciated. But his legacy, like Darwin s, has never been more alive. j What better evidence for Darwin's belief in the commonality of all species than to find the same gene doing the same job in birds and fish, continents apart?