the lungs We cannot live without them, nor can most of the animals that wear their skeletons inside. However, some unusual amphibians no longer bother with these delicate, mucus-y organs.
Scientists have discovered that some lungless animals initially grow these respiratory organs and then change hearts later in their development process.
These animals, plethodontid salamanders, have evolved from being lungless by breathing through their thin skin and mouth tissues for at least 25 million years. To do this, they have to coat themselves with goo — like they’ve coated their lung mucus on their outsides — because they can only absorb oxygen through their skin if it stays moist.
The lungless plethodontids are the largest group of salamanders with about 478 species, found mainly in the Americas, with some in Europe and South Korea.
Little is known about how these amphibians misplace their lungs, so Harvard University evolutionary biologist Zachary Lewis and colleagues took a closer look at these cold-blooded water lovers.
“We confirm that early lungs form in embryos of many species,” the researchers write in their paper. “Its morphological features resemble those seen in axolotls, Ambystoma mexicanumA lung salamander.”
These embryonic lungs develop and branch for about three weeks in one species, P. cinereus, before something interrupts them. Before baby salamanders hatch, early lung cells undergo apoptosis, a form of cell death.
The researchers suspect that lung development is halted because the cells do not receive the regulatory signals they need to multiply, which in other vertebrates come from the tissue surrounding the developing lungs, called mesenchyme. So they decided to test this hypothesis.
“We put mesenchyme from a lung-bearing salamander into a lungless salamander embryo and allow it to develop,” says Lewis. “This results in the formation of lung-like structures, which provides some evidence that lungless salamanders are able to continue to develop lungs.”
While these mucus-covered animals have no traces of lungs as adults, salamanders still have the genes and many of the developmental processes they need to grow.
But some of the signals needed to complete the process—for example from mesenchyme tissues—appear to be missing.
So why have so many tools and materials been preserved for making lungs in animals that didn’t use them for millions of years?
“The origin of the lung, for example, may play an important role in the development of adjacent organs such as the heart,” the researchers speculated, adding that some genes involved in the early formation of the lung are also used to form other organs. For example, a gene called Sonic the Hedgehog (yes, really) is used to develop lungs and organs.
These connections show how studying organ remains (vestigial structures)—like these or toe knobs in snakes—can help us understand the twisting and turning mechanisms behind developmental and evolutionary processes and provide insight into the function of our organs.
The sparse fossil record of salamanders makes it difficult for researchers to narrow down when these animals lost their dedicated respiratory organs, but genetic timelines place it between 25 and 110 million years ago, depending on whether it happened in an ancestral species or lung loss. Within this salamander family evolved several times.
“Apparently lungless salamanders do better without lungs because they make up about two-thirds of all salamander species,” Lewis says. “Perhaps the loss of the lungs enabled rather than hindered this remarkable evolutionary success.”
This research was published in 2015 Science advances.