The process of respiration is absolutely essential for life and for the most part we think of oxidative processes. Physiological respiration concerns the bulk flow of O₂ from the air to the cells of an organism and that of CO₂ in the opposite direction. Such gas exchange can occur through simple diffusion if the organism is small enough, but larger organisms need a respiratory system, involving structures such as gills or lungs. Here, the circulatory system is often employed for carrying gases to and from tissues. Inside the cells, a suite of metabolic processes then converts biochemical energy into a usable form, which is referred to as cellular respiration. And respiration is not only indispensable but also convergent...

When animals invaded the land (a process that happened many times independently), they needed to evolve air-breathing structures. An interesting example is provided by marine crabs, which have repeatedly moved onto land, mainly in tropical and subtropical regions. All these crabs have retained their gills, but have additionally developed simple vascularised lungs that show certain parallels to the lungs of pulmonate molluscs. Remarkably, however, the paired invaginated lungs of Trinidad mountain crabs are so complex that they are more analogous to the lungs of higher vertebrates, particularly birds, than to the lungs of other land crabs.

Remarkably, some amphibians have lost their lungs and rely entirely on their skin for gas exchange. Such lunglessness has evolved independently in two families of salamanders as well as in a frog and a caecilian, but the selective pressures involved remain rather elusive.

Sand-diving lizards have converged on similar solutions to obtain sufficient oxygen for respiration when buried underground. They employ forelegs and ridges on the body trunk to 'shield' breathing movements from surrounding sand, thus stopping sediment from blocking the space required for the ribcage to expand and deflate during in- and exhalation.

On a cellular level, there is also evidence for convergence, for example in animal haemoglobins, the oxygen-binding iron-based proteins contained in the red blood cells of almost all vertebrates. Even though the protein itself is ancestral to all animals, during its evolution various episodes of gene duplication have led to a number of different varieties, notably the β-globins. Some invertebrates employ haemocyanin, a copper-based respiratory protein, which is an excellent example of molecular convergence. Haemocyanin occurs in both the arthropods and molluscs, and is almost certainly of independent origin, as the protein structure is radically different in the two groups. While in arthropods the basic unit is a hexamer, mollusc haemocyanin occurs as an enormous cylinder based on a decamer arrangement. The active sites, however, are strikingly similar.

It has recently (and controversially) been suggested that chloroplasts, the organelles concerned with the transduction of energy in the cells of plants and other eukaryotes, might have evolved convergently by repeated "capture" of cyanobacteria. What is clear is that ongoing examples of primary endosymbiosis with cyanobacteria are happening before our eyes, most dramatically in the amoeboid Paulinella. We should not be surprised, therefore, if chloroplasts, which are, of course, central for photosynthesis, evolved several times. However, not only do they show some very interesting convergences with the mitochondria in terms of redox processes, but essential as mitochondria are to respiration, here too do not be too surprised if their acquisition also turns out to be convergent.