Molecular Clocks and Shark Evolution
The fossil record is vexingly incomplete, frustrating most attempts to reconstruct evolutionary history. Paleontologists can only hope that the creatures they collect as fossils left descendants, but geneticists can be absolutely certain that the genes they collect had ancestors. Recent evidence from studies in molecular genetics allow us to test the origin times of lamnid sharks.
Mitochondria are cellular organelles that have their own, slightly unusual kind of DNA. Mitochondrial DNA uses the same nitrogenous base pairs — adenine, thymine, cytosine, and guanine — as regular DNA, but its sugar-phosphate backbone zigzags somewhat, giving the molecule a 'kinked' appearance. Mitochondrial DNA (or mtDNA, for short) is preferentially used in genetic analyses for profound biological as well as practical reasons. Because mitochondria are responsible for packaging adenosine triphosphate (often known by the acronym ATP), the common energy currency that powers all multicellular life, mtDNA is replicated very conservatively. In addition, because it is shorter than DNA from most cells, mtDNA is somewhat less unwieldy to analyze. Just like regular DNA — although at a much slower rate — mtDNA is prone to replication errors and therefore changes slowly over time.
The average rate of change in mtDNA can be used as a kind of 'molecular clock'. In mammals, a 2 to 4 percent change in mitochondrial gene sequences corresponds to a million years' of evolutionary divergence. Anthropologist Rebecca Cann of the University of California and her co-workers have used this technique to estimate how long ago the common ancestor of modern humans appeared. Since mtDNA is inherited only through the female line (matriline), she was able to trace human origins back to a 'Mitochondrial Eve' who lived in Africa somewhere between 280,000 and 140,000 years ago. Mounting evidence from similar studies suggested that the molecular clock ticks at the same rate for all mammals, and perhaps all vertebrates. Could the same technique be used for sharks?
Geneticist Andrew P. Martin and his co-workers have measured mtDNA differences in several species of sharks. In order to calibrate his molecular clock for sharks, Martin needed to relate a genetic change in one or more populations of these animals to a reliably-dated geological event. He compared genes of two populations of a small species of hammerhead (Sphyrna tiburo) that were separated by the rise of the Isthmus of Panama, which occurred some 7 to 3 million years ago. To their surprise, Martin and his colleagues found that the rate of genetic change in sharks is positively glacial compared with that of mammals — some seven to eight times slower.
Based on his research to date, Martin estimates that in sharks a 1 percent difference in gene sequence corresponds to approximately 6 million years' divergence time. In a recent study, Martin measured the percent difference in the mtDNA of representatives of all three genera within the family Lamnidae. He found that Lamna was the most divergent genus, being roughly 7.6% different from Carcharodon. This genetic difference suggests that the separation of Lamna and Carcharodon occurred some 65 to 35 million years ago. Martin also found that Isurus differed from Carcharodon by about 7.1%. This difference suggests that these two genera diverged about 60 to 35 million years ago. Therefore, according to Martin's genetic studies, the White Shark can be traced back no more than about 60 million years ago.
Evidence from molecular genetics thus supports the paleontologist's proposed origin time of Carcharodon (if Isurolamna is included in its lineage) and Isurus (if Macrorhizodus praecursor is considered the earliest representative of that lineage). However, the genetic evidence suggests a far more ancient origination time for Lamna than is presently supported by the fossil record (65 million years ago versus 42-38 million years ago).
Using molecular clocks to calculate origin times of biological lineages is still in its infancy, and — like any newfangled technique — remains controversial. But both paleontologists and geneticists agree that, compared with other modern sharks, Carcharodon is a relatively ancient genus.