Millions of years ago, a land mammal decided to return to the water. This change, which completely transformed its body and its genetics, gave rise to one of the most unique groups of animals on Earth: the cetaceanswhich include dolphins, whales and porpoises. Today, these giants of the ocean reveal an extraordinary evolutionary history, visible in fossils, anatomy and, more recently, in their genomes.
The ancestors of cetaceans were small land mammals that lived approximately 50 million years. One of the first fossils to shed light on this connection was that of an animal called Pacicetusdiscovered in India and Pakistan. This animal, which resembled a deer and lived both on land and in water, gave rise to a series of adaptations for marine life.
In an evolutionary period of 10 million yearsthese mammals underwent drastic transformations:
- Its hind legs disappeared and transformed into powerful caudal fins.
- Their noses moved from the end of the snout to the top of the skull, forming the breathing holes or “spiracles”.
- His body lost almost all of its hair, adapting to aquatic life.
These anatomical changes, visible in fossils such as those of Dorudontidaeare a testament to how cetaceans adapted to the ocean to survive and thrive.
The transition to water not only changed the external shape of cetaceans, but also their genetics. Recent studies have identified several genes that were lost or modified during this process:
- Gen SLC4A9: related to saliva production, it ceased to be functional because saliva was unnecessary in an aquatic environment.
- Melatonin genes: Four genes related to this hormone were deactivated, allowing the cetaceans to adopt sleep alternating between cerebral hemispheres. This allowed them to rest while remaining alert to breathe on the surface.
Furthermore, adaptations to deep diving involved genetic modifications. For example:
- Genes F12 y KLKB1: They stopped working, reducing the risk of clot formation during prolonged descents and rapid pressure changes.
- DNA repair enzymes: They were adjusted to correct damage caused by reactive oxygen molecules, which increase with repeated diving and resurfacing.
The ocean is a hostile environment, full of microorganisms that can be harmful. Cetaceans faced strong evolutionary pressure to protect their respiratory and skin systems. Genetic studies show changes in genes related to:
- The skin: which now acts as a robust barrier against marine bacteria and viruses.
- The lungs: modified to deal with contaminants and avoid infection while breathing air at the surface.
A notable example is the gene PON1which was inactivated in cetaceans and other aquatic mammals. Although its full function is not fully understood, this loss appears to have helped prevent inflammation caused by the build-up of toxins during prolonged diving.
In their transition to the ocean, cetaceans lost and adapted several senses:
- Smell: the number of functional genes related to smell was drastically reduced, in some cases by as much as 80%. In toothed whales, such as orcas, the sense of smell completely disappeared.
- Gusto: Cetaceans no longer have genes to detect flavors such as sweet, bitter or umami. This is probably because they swallow their food whole, without needing to taste it.
- Vision: The eyes of cetaceans evolved to be more sensitive to blue light, predominant in the deep sea. However, some species, such as the sperm whale, lost both types of color receptors, becoming monochromatic.
Thanks to the analysis of genomes, scientists have been able to identify the molecular adaptations that accompanied the evolution of cetaceans. These tools have allowed us to raise new questions:
- How did toothed whales develop echolocationwhich allows them to navigate in the darkness of the ocean?
- What drove the increase in brain size in dolphins, whose brains are proportionally similar to those of great apes?
- What genetic factors explain the astonishing diversity of dolphin species, which make up almost half of the cetaceans alive today?
The impact of genetic changes on cetacean physiology is astonishing:
- Highly elastic lungs: Shared with other aquatic mammals, this trait allows cetaceans to rapidly inhale and exhale to the 90 % of your lung capacity.
- Muscles rich in myoglobin: The positive charge of this protein allows it to store high concentrations of oxygen, essential for prolonged dives.
These adaptations have allowed cetaceans to become some of the most efficient divers in the animal kingdom.
