Three Common Ancestors

Amphibians, especially frogs are being increasingly considered the "canary in the coalmine" of global warming. They seem to be very sensitive to minor variations in temperature and many populations are succumbing to fungal and parasite infections brought about by increased temperatures. Many species may be on the brink of extinction. A new study has shown that although amphibians may be among the first organisms prone to collapse in times of an environmental crises, they are also champions of recovery, rapidly diversifying and filling ecological niches post environmental trauma. An analysis which reconstructed amphibian family trees using a comparison of genes from 171 species show that amphibians diversified very rapidly soon after two of the biggest mass extinction of the Phanerozoic, the late Permian and the late Cretaceous. This is where the problems in the press report crept in.

In summing up the late Permian crises the press report says

"The work revealed that today’s amphibians have three common ancestors, which arose around 350 million years ago. That trio suddenly branched out between 250 million and 225 million years ago – around the time of a global mass extinction at the end of the Permian Period, when some 95 percent of all life forms had disappeared from the face of the planet".

Three common ancestors. I can almost hear biologists grinding their teeth and sharpening their knives at this careless use of the term common ancestor. Amphibians are a diverse group but they also share deep similarities, which have convinced biologists that all amphibians arose from just one common ancestor, i.e. just one ancestral species. Organisms with properties of amphibians originated just once in the history of life. What could the term 3 common ancestors mean? It certainly could not mean that amphibians arose independently on three separate occasions. The probability that three different ancestral species evolving identical amphibian features is vanishingly small. I think what the reporter meant was that about 350 million years ago an ancestral species split into two species, one of which founded the lineage that lead to amphibians, the other founded the amniotes (reptiles, birds, mammals). By 250 million years ago the amphibians had diversified into three main groups ( three orders of class amphibia). After the Permian extinction, the surviving species of these three orders diversified rapidly and eventually gave rise to frogs and toads, salamanders, and caecilians respectively.

Later speaking of the late Cretaceous extinction the report says

"In total, approximately 86 percent of frog species alive today, and more than 81 percent of salamander species descend from just five amphibian species that survived this mass extinction 65 million years ago".

Again, a very careless use of terminology which renders the sentence meaningless. There is no way biologists can acertain using a family tree reconstructed using genetic data that only 5 species survived the Cretaceous extinction. I searched out the original paper and this is what it said

"Approximately 86% of modern frog species and >81% of salamander species descended from only five ancestral lineages that produced major radiations in the late Cretaceous and early Tertiary".

5 ancestral Lineages not 5 surviving species. What this means is that several amphibian lineages (group composed of species related by descent from a common ancestor) survived the late Cretaceous mass extinction. Out of the surviving lineages, 5 were particularly successful.

From original paper to press release, this mutation in term lead to a very different reading of the evolutionary story.