Genome sequencing of “living fossil” fish sheds light on the evolution of land animals

In a newly published article in the scientific journal Nature (496, 311–316), scientists report a significant step in the understanding of the evolutionary process that led certain ancient fish to develop limbs and emerge onto dry land, becoming the ancestors of all amphibians, reptiles, birds, and mammals. This transition is one of the most important in the development of life on earth. The new research is based on the decoding of the genome of a so-called “living fossil” fish, the “coelacanth” (pronounced SEE-luh-canth). This group of fishes, known from fossils dating as far back as 400 million years ago, was long thought to have gone extinct 70 million years ago, in the Late Cretaceous Era. However, the discovery of a living specimen in the West Indian Ocean off of South Africa in 1938 demonstrated that a reclusive population still survived. More recently, a second species of living coelacanth, both belonging to the genus Latimeria, was discovered in Indonesia. Coelacanths have been colloquially known as living fossils because of their close resemblance, at least outwardly, to fossil specimens dating back tens and even hundreds of millions of years. But perhaps the most interesting aspect of these fish is their apparent close evolutionary relationship to the first land vertebrates. It has long been thought, based on paleontological and anatomical evidence, that all tetrapods (four-limbed, vertebrate animals—amphibians, reptiles, birds, and mammals) evolved, somewhere in the neighborhood of 400 million years ago, from fish closely resembling the now mostly extinct group of lobe-finned fish belonging to the class Sarcopterygia. Lobe-finned fish have a well-developed bony skeleton inside their fins, in contrast to the common ray-finned (teleost) fish. Living representatives of this lineage include lungfish, which are actually able to breathe air with lungs that are homologous (have the same genetic origin) as those of tetrapods, and the coelacanth. Basic anatomical similarities between the skeletal structures in the fins of lobe-finned fish and the limbs of tetrapods strongly suggested an evolutionary link. However, the details of the mechanism for the transformation of fins into limbs had yet to be understood. The newly reported research begins to fill in that gap. The technique of DNA sequencing, which in recent years has permitted the decoding of the human genome and those of a growing number of other species, has now been used to document the coelacanth’s DNA sequence. Among the resulting discoveries is a genetic mechanism that controls the growth of both the lobe-shaped fins in the coelacanth and of limbs in tetrapods. Scientists have long wondered whether lungfish or coelacanths were more closely related to tetrapods. In other words, which group is closer to the common ancestor of the earliest vertebrate animals to walk on land? Genetic sequencing holds the potential to answer such questions. Unfortunately, lungfish have an incredibly large genome, which cannot be effectively sequenced by existing methods. It should be noted that the size of a species’ genome has no necessary correlation with the complexity of the organism, due to duplications of DNA sequences and other “baggage” accumulated during the course of evolution, which is not a particularly neat process. Despite this, the scientific team whose research was published in Nature was able to decode segments of the lungfish genome and compare it to their newly