“Why haven’t marine mammals and reptiles developed the ability to breathe underwater?”

Marine mammals and reptiles have not evolved the ability to breathe underwater primarily because they have successfully adapted to their respective niches in their ecosystems without this specific adaptation. Evolutionary changes of such magnitude are complex and require multiple genetic mutations over extended periods of time.

Evolution does not involve deliberate design but builds upon existing anatomical structures. It operates through gradual modifications accumulated over thousands or millions of generations. If a particular change enhances an individual’s likelihood of successful reproduction, it is more likely to persist in the population. Environmental pressures and competition further drive the necessity for these changes to propagate.

The existing respiratory systems of marine mammals and reptiles have proven highly efficient in their respective environments, allowing them to reproduce successfully without the need for underwater breathing. For instance, whales have evolved the ability to store large amounts of oxygen in their muscles, which enables them to undertake deep dives. Considering other metabolic factors for their warm-blooded bodies, this adaptation often proves more advantageous than the development of gills.

Given the right combination of environmental pressures and a sufficiently long time span, it is conceivable that a respiratory structure capable of breathing both air and water, or solely water, could emerge. However, such a development would necessitate numerous mutations, each providing individual benefits or at least not resulting in significant drawbacks, for the changes to persist in the population.