During his voyage across the world on the HMS Beagle, Darwin became convinced of the evolution of living things. The evidence initially came from biogeography and paleontology, but Darwin observed that comparative anatomy and embryology also convincingly demonstrated the existence of evolutionary relationships between diverse species.
Darwin proposed that those individuals with beneficial variations in their living conditions would be more likely to survive and pass these characteristics on to their offspring. These variations would accumulate generation after generation until, given enough time, the new individuals would have little or nothing to do with their ancestral individuals. Therefore, more similar species should have diverged from each other less time ago than very different species. Anatomical kinship therefore became an especially useful tool not only to organize and group living beings as taxonomy had been doing since time immemorial, but a whole new field was opened for the study of evolutionary relationships between species. that today we call phylogeny.
Similar structures with different functions
The skeletons of a dog, a man, a bird and a whale are strikingly similar , despite the different life forms of these animals and the diversity of their environments. The bone-to-bone correspondence can easily be seen in the extremities, for example. The configuration of the bones of the arm, forearm and hand is identical, even though the limb has different functions: to write in the human, to run in the dog, to swim in the whale and to fly in the bird.
If they are different structures but with the same organizational plan, it could be considered that all of them derive from the same ancestral organism. The basic structure of the forelimb already existed in the ancestors of these species and has been modified over time in a differential way in each group, so that today it is specialized in the different functions of swimming, running, writing and flying . They are, therefore, homologous organs, with different functions but which reveal the same anatomical structure and, consequently, the same evolutionary origin.
Comparative anatomy investigates the origins of the structures of organisms and differentiates those that tell us about kinship relationships (homologous) from those that, although similar in form or function, present us with different solutions to a common problem (analogous) . For example, bats , birds, and insects have developed wings that allow flight. However, while the wing structure in the first two responds to the same structural plan, and therefore shows homology, the wing of insects is completely different. It is an organ analogous to that of bats and birds and it does not help us to establish a kinship relationship between these groups .
Darwin has already realized that the degree of correspondence between anatomical structures reflects the level of kinship in evolution and allows us to reconstruct the evolutionary history of organisms. He was not the first to raise the immutability of species or to put the idea of evolution on the table. He did something much more important, propose a natural mechanism by which all the diversity of species on our planet could be explained. He never learned about the advances in the field of inheritance of characters and molecular genetics that would come in later decades, but he laid the foundations of what today is the cornerstone of the life sciences: the theory of evolution. .
Arturo Ariño is director of the Science Museum of the University of Navarra and Javier Novo is professor of Evolution at said university.
