act-dog

This passage is from the article “The Rise and Fall of the Living Fossil” by Ferris Jabr (©2015 by Nautilus). The term “living fossil” refers to creatures that had emerged long ago and seemed to have stopped evolving. Like all living fossils, crocodiles were thought to have emerged in the distant past and then stayed largely unchanged. The standard theory held that the crocodilian species we know today originated in Africa during 5 the Cretaceous (145 to 66 million years ago), when the seven continents were much closer together. As the continents drifted apart, the crocodilians went with them, explaining how they ended up in a band of tropics encircling the globe. If that were true, then modern 10 crocodilian species should be very different from one another at the level of genes and molecules, because there would have been more than enough time for substantial mutations to accumulate. By the 1990s, however, molecular analysis revealed that immune system 15 molecules conserved across living crocodilian species were remarkably similar in structure and behavior. Intrigued by this puzzle, a post-doctoral research fellow at the University of Washington named Jamie Oaks began collecting DNA samples from all 23 living 20 crocodilian species, comparing sections of the genome where mutations were most likely to have appeared. Oaks did not find nearly as many differences between the modern crocodilian genomes as one would expect had those species diverged all the way back in the Cre25 taceous. He concluded that modern crocodilian species split from their last common ancestor between 8 and 13 million years ago, not long before ancient hominins split from their last common ancestor with chimpanzees. The living fossil theory of crocodiles had overes30 timated their evolutionary age by about a factor of 10. Oaks also noticed something odd about the DNA samples he had acquired from the iconic Nile crocodiles (Crocodylus niloticus): they did not match up with each other. In fact, the variation between them was 35 great enough to suggest that he was looking at two distinct species. If so, then not only were modern crocodiles much too young to be living fossils, but they had also continued to speciate after diverging from their basal ancestor—something living fossils are not sup40 posed to do. On its own, Oaks’ study was intriguing, but not enough to convince the larger scientific community to cleave the Nile crocodile into two species. Unbeknownst to him, however, a separate team of scientists was preparing to corroborate his results. In 45 the early 2000s, on an excursion to Chad, the wildlife conservationist Michael Klemens encountered some odd little crocodiles in a desert oasis. They were so docile that he and his companions could swim beside them without concern. He took a tissue sample from 50 one that had recently perished and sent it to the American Museum of Natural History in New York City, where Evon Hekkala, an assistant professor at Fordham University studying crocodilian diversity, sequenced its genome. When she compared the docile croc’s DNA to 55 other Nile crocodiles, she noticed some rather striking differences. Could these tame crocs be an entirely distinct species? DNA analysis of 123 African crocodiles—as well as 57 separate samples from museum specimens, 60 including crocodiles mummified in ancient Egypt— confirmed her suspicion. In a few sections of their respective genomes, all the mild-mannered crocs would have one DNA sequence, and all the typical Nile crocs another. They even had different numbers of chromo65 somes. “That made us very confident that there were actually two different populations and they were not mixing their DNA,” Hekkala says. The two different species had diverged between 3 and 6 million years ago: Crocodylus niloticus in the East and the smaller, 70 less aggressive Crocodylus suchus in the West. The vast majority of mummified crocodiles were C. suchus, suggesting that ancient Egyptians had recognized the difference. Together, Hekkala, Oaks, and other scientists 75 helped redraw the map of how crocodilians evolved in space and time, and conclusively removed them from the category of living fossils.