We chose our title, The Mermaid’s Tale, because we like that it can be thought of as having two meanings. First, of course, it refers to the bodily arrangement of mermaids, but second, it allows us to tell the story of how biological traits, and the creatures that carry them, develop in the short term and what this means on the long term evolutionary time scale. Even religious fundamentalists, who would say they don’t accept the basic principles of evolution, know that there is no such thing as a mermaid. Ironically enough, the reason we, and they, can be so sure is because of the principles of evolution. (Image is an oil painting by John Waterhouse, 1909, now in the public domain.)
The mermaid’s body is said to be made of sections (torso and tail) that are stapled together from different parts of life’s phylogeny, often called the Tree of Life. The ‘Tree of Life’ is a representation of the relationships among organisms: species, like different kinds of cats or goldfish, are placed close together on this diagrammatic representation, because they share a more recent ancestor than other species. Fish and oak trees are very far apart, because they haven’t shared a common ancestor since far into the distant past. But members of similar species often have very similar biology, such as their basic body plan and structures.
The mermaid seems on the surface to be a perfectly plausible species. But it isn’t so. Her parts can’t be found on the same creature because they arose and evolved on lineages, branches on the Tree of Life, that separated from a common ancestor hundreds of millions of years ago. The genetic mechanisms that assemble organisms as they develop have a history that is characterized by sequestration, or isolation – a general principle of life in all its dimensions. In this case, the isolation in different species led to differences in the details of the embryological development of fish with scaly tails but no legs, and of mammals with legs, breasts, and fur, that accumulated over those hundreds of millions of years since they diverged from their common ancestor. The fish tail and the mammal torso now are each built by developmental processes specific to each lineage, each step in development contingent upon what came before – another general principle of life. The processes that build similar traits among closely related species within each lineage are similar as well. Because of contingency, and ‘inheritance with memory’, another basic principle referring to the processes that make offspring resemble their parents and species within lineages resemble each other, although both fish and mammals share many important traits – like having a backbone – they don’t, and can’t have the two basically different body halves that a mermaid does.
Interestingly, however, it was discovered not so many years ago to great surprise, fish and mammals, torsos and scaly finned tails, despite their deep separation in evolutionary time, do share many aspects of the basic processes of development, even while differing in the specifics. The way many of the same genes and genetic mechanisms are used are quite similar in both lineages. Indeed, we share some of this with insects and worms, and we share many of the same basic types of mechanism even with plants – though the vast majority of the specific genes used in those mechanisms are totally different. This is an important aspect of the discussions we hope to hold on this site.
Darwin’s basic insight about the unity of life, descended from a common ancestor, has been confirmed many times over by the lessons of genetics and developmental biology. There’s something deeply satisfying about that.