About 37.5 billion years ago, a comical-looking fish called tiktaalik ventured ashore in a novel way of adapting: split-wings propelled itself to "walk" on land, and air sacs in its throat breathed oxygen from the air. Tiktaalik also has gills and is the common ancestor of the earliest known tetrapods or tetrapods.
Over a period of hundreds of millions of years, tetrapods evolved into countless species, including Homo sapiens. So, if humans evolved from fish, why don't we have gills?
Part of the answer is practical: gills need to be kept moist to work, which is not ideal for animals that don't live underwater. The gills have a large surface area with thousands of tiny blood vessels that allow oxygen to easily enter the bloodstream. When water rushes through the gills, oxygen diffuses in and carbon dioxide diffuses out, says Chris Ogan, an evolutionary biologist at Montana State University. If land animals have gills, they dry up quickly, making them an inefficient way to breathe. On the other hand, our human lungs are adept at absorbing oxygen from the air and entering our bloodstream through gas exchange.
But the lungs were there long before the transition between land and sea. "The lungs are actually surprisingly primitive in evolution," Neil Shubin, an evolutionary biologist at the University of Chicago and part of the team that discovered the Tiktalik fossil in 2004, told Live Science. When our fishy ancestors still lived underwater, they already had lungs in addition to their gills.
Fish won't come ashore to say'I need lungs; I'm going to evolve lungs'Organ told Live Science. Only fish with lungs can encroach on land and survive. If a lungless fish tries to live on land, it will die. "It's about these traits that have evolved for other reasons and then allowing this animal to take advantage of this new environment," he added.
Similarly, scientists believe that our fish ancestors evolved arms to move on the ocean floor, which later came in handy when searching for food and moving around land, Organ said. That's where natural selection comes in: because these arm-like structures are beneficial on land, animals evolved longer limbs and hands over the next few million years. The same thing can happen to the lungs. Soft tissues like lungs don't fossilize well, so scientists aren't sure how human lungs evolved, Organ said. But the available evidence suggests that early lungs first evolved into simple lungs of lizards and then into subdivided lungs specific to mammals. It's important to note that mammals evolved to have diaphragms – the muscles that regulate our breathing – that may have existed as early as 300 million years ago.
Conversely, structures that are no longer beneficial tend to disappear. Over time, the gills shrink and are confined to juvenile age until they finally reach their mark at about 3The Carboniferous period, 1.5 billion years ago, completely disappeared from full-time land animals, Organ said. Around that time, the ancestors of the first reptiles and the first birds and mammals began to evolve.
It seems strange that primitive fish had lungs. While gills are good at extracting oxygen from water, they don't always provide large amounts of oxygen, especially for large animals that need more oxygen. Seasonal changes can also affect the amount of oxygen in the water, Shubin noted. For example, if there are a lot of dead leaves in the water, those dead leaves will absorb oxygen. Thus, the air sacs – the primitive lungs – allow the fish to swallow air above the surface of the water to supplement oxygen intake. Modern barramundi, which has been around for more than 400 million years, has the same ability, which helps explain why landward movement is possible.
But we haven't completely lost those early gills. Human embryos have fishy physical characteristics: tiny folds called pharyngeal arches resemble gills, but we don't use them for breathing. While they're not exactly gills, they're definitely remnants of early gills, a bit like an old recipe that is now being made something different than before, Shubin says. Throughout embryonic development, these arches become part of the jaw, throat, and ears. "Every creature with a head goes through the pharyngeal arch stage," Shubin said. In other words, without the pharyngeal arch, the animal's head cannot be formed.
Aquatic, breathing gill species also have these arches in embryonic development. The only difference is that the arcuate develops into a true gilll along with the surrounding bones, muscles, nerves and arteries, Shubin says.
We can be thankful for the fish in us because the lungs we now use to breathe don't have clumsy gill slits.