Credit: H. McDonald & Science
The evolutionary history of corals is a fascinating subject, recounting one of the oldest lineages of multicellular animal life. This story is full of mistaken identities, uncertain affinities, and exciting, new discoveries. It’s also a tale of extinctions and near-extinctions, and of adaptations and diversifications through the ever changing history of our planet. To begin our journey through coral evolution, let’s go all the way back to the origins of multicellular animals to gain a better perspective of the place of stony corals in the history of life on earth. By studying the rate of mutations in the genetic code of extant (=living) groups, scientists can extrapolate backwards to determine when any two organisms last shared a common genetic code (i.e. their last common ancestor). This method, termed the “molecular clock”, gives a rough estimate ranging from 700-1000 million years ago for the origins of the first metazoan (=multicellular) animals. There is a long gap in the fossil record from this presumed molecular origin and the appearance of the first definitive fossil evidence; this is to be expected given the absurd odds of anything from this period becoming fossilized, surviving a billion years of geologic destruction, and ultimately being discovered and identified. The recently described sponge, Eocyathispongia, known from 585 million year old rocks of the Ediacaran period, is the oldest known fossil that can be definitively identified as a metazoan. There are older fossils, but they are far more ambiguous in their interpretation. The next 40 million years offers a scant record of disputed fossils, many of which have been suggested as primitive jelly-like organisms. Finally, with the “Cambrian explosion” around 540 million years ago, we see the true blossoming of life on earth as we know it, with abundant fossil evidence of early corals. With such a sparse fossil record, it should come as no surprise that the relationships amongst the earliest branches of metazoan life have been contentious. For centuries, the proposed phylogenies have been based solely on morphology, principally concerning the cellular organization and tissue histology of these groups. But in the last few years, under the scrutiny of new molecular methods, we have gained new insights into the earliest animal interrelationships. Comb Jellies: Phylum Ctenophora
The common Pleurobranchia. Credit: AP Photo/University of Florida
One of the most significant recent breakthroughs in zoology has been the revelation that the comb jellies, which appear to be morphologically “advanced”, are in fact the earliest diverging lineage of metazoan life.… More:
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