Could Whales and Dolphins Develop Gills if Evolution Continued for 600 Million Years?

The evolution of complex organisms, such as whales and dolphins, into forms with gills is a fascinating hypothesis. While it might be highly unlikely, it’s an intriguing concept worth exploring.

Unlikely in the Extreme, but How Might it Happen?

Let's consider a hypothetical scenario where cetaceans eventually develop gills after another 600 million years. This would require a complex series of evolutionary steps. Some observational structure within a cetacean species would initially arise, drawing a very small amount of dissolved oxygen from seawater. It would likely be a byproduct of an existing structure with another function.

Oxygen Levels Decrease, Selective Pressures Increase

The oxygen levels in the ocean begin to decline. Members of the cetacean population with this functional structure would tire slightly later, swim a bit farther, or eat a bit better, allowing them to have just a few more offspring. To put this in perspective, one in a hundred thousand is considered “a few more.”

As the ocean's oxygen continues to deteriorate, the new cetaceans thrive marginally better in almost every generation, leading to a higher number of these organisms. Over time, natural variation causes some of these new structures to become slightly more efficient, and individuals with these structures are ever so slightly favored, leading to an overall increase in the population with these functional structures.

Evolutionary Advantages and Complex Adaptations

This dynamic continues for a thousand or ten thousand generations. In this process, various environmental factors contribute to selection pressures, leading to the development of a more complex system. Eventually, all cetaceans without oxygen-apparatus disappear, and those with the most developed structures increase their portion of the population.

At some point, an intelligent species, perhaps a descendant of humans, might encounter these cetaceans and remark, "Look here, I think these are gills." This would be a significant milestone in the evolution of cetaceans.

Why It's Unlikely

While this scenario is theoretically plausible, it is highly unlikely for several reasons.

No Preadaptations Exist

Cetaceans lack preadaptations that could be readily modified into gills. Furthermore, even if gills were developed, they would not confer a significant advantage because air contains much more oxygen than water. Thus, cetaceans' current air-breathing lungs already provide them with more than sufficient oxygen.

Energy Inefficiency

The development of gills in mammals would necessitate a significant adaptation. For a mammal to extract enough oxygen from water, it would need to pump a large volume of water through its gills. Gills, being thin structures in direct contact with water, would be in thermal equilibrium with the water, causing the mammal to require an efficient heat exchanger to maintain its body temperature.

Additionally, the process of breathing underwater requires a highly complex and energy-intensive system that is already well-adapted in cetaceans with their respiratory apparatus. Evolving a separate gill system would likely not be energy-efficient compared to their current air-breathing mechanism.

Therefore, while the idea of cetaceans developing gills is intriguing, it is highly improbable due to the lack of preadaptations and the energy inefficiency involved.

Still, the exploration of such scenarios is a valuable exercise in understanding the complex processes of evolution and the intricate adaptations that drive species to survival.