About 375 million years ago, Tiktaalik, an unusual fish, ventured onto land with unique adaptations like lobed fins for walking and air sacs in its throat for breathing. Despite having gills, Tiktaalik is the earliest common ancestor of tetrapods, including humans. Given the evolutionary journey from fish to humans, the question arises: Why don't humans possess gills?
From Gills to Lungs: Evolution's Design for Land Dwellers and the Journey of Tetrapods
The practical aspect of having gills is that they require to be constantly wet to function, which is not suitable for animals living on land. Gills possess a large surface area and numerous tiny blood vessels, enabling efficient oxygen transfer to the bloodstream as water flows over them.
However, this design becomes inefficient when dealing with terrestrial environments. In contrast, human lungs excel at extracting oxygen from the air through gas exchange, making them well-suited for land-dwelling creatures.
Evolutionary biologist Chris Organ from Montana State University explains that lungs, surprisingly, existed before the transition from sea to land. Tetrapods already had both lungs and gills when they lived underwater.
The emergence of lungs was not a deliberate choice made during the move to land; rather, fish with pre-existing lungs were more likely to survive the transition. Without lungs, a fish attempting to live on land would perish due to the inefficiency of gills in a non-aquatic environment.
Similarly, scientists believe that the arms of tetrapods evolved for movement on the ocean floor, proving advantageous for subsequent land-dwelling activities like foraging and mobility. Natural selection favored these traits, leading to the evolution of longer limbs and hands over millions of years.
Soft tissues' poor preservation obscures the lung evolution fossil record. Despite this, the transition likely involved simple to subdivided structures, diaphragm included. Conversely, structures losing purpose vanish over time.
Gills, present in juveniles, disappeared in full-time land animals about 315 million years ago, coinciding with reptilian and bird/mammal ancestors. Primitive fish with lungs could extract air oxygen, aiding a successful land transition.
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Why Mammals Avoid Gills
In aquatic environments, mammals such as whales, porpoises, walruses, and manatees have consistently evolved with lungs instead of gills. This pattern in evolution involves significant adaptations to accommodate lungs, like placing blowholes on the heads of whales.
The absence of mammals with gills is attributed to the impracticality of having gigantic gills. Unlike fish, mammals require substantially more oxygen, particularly warm-blooded species like humans, who might need 15 times more oxygen per pound of body weight.
The efficiency of gills in fish relies on constant water movement, achieved by using mouths and gill flaps. If mammals had gills, they would need a substantial portion of their bodies dedicated to this respiratory system, making it impractical and inefficient.
The anatomical space required for large gills, coupled with the need for a mechanism to force water over them, is why mammals, including humans, have never evolved with gills.
Although mammals don't possess gills, embryonic development in humans reveals remnants of early gill-like structures called pharyngeal arches. These arches, present during embryonic stages, don't develop into functional gills but contribute to the formation of the jaw, throat, and ears.
This evolutionary relic underscores the shared ancestry between mammals and aquatic, gill-breathing species, demonstrating the fascinating traces of our evolutionary journey from aquatic environments to terrestrial existence.
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