Here's a breathtaking finding: Scientists say they've discovered an ancient vestigial lung in the coelacanth, a lobe-finned fish that's related to the ancient swimmer that went on to evolve into all tetrapods, humans included.
The find, described in the journal Nature Communications, helps to fill in some gaps in this mysterious lineage in the fishy family tree, and sheds light on how such fish might have looked around 410 million years ago.
Humans, like all mammals, reptiles, amphibians and birds, are tetrapods: four-legged vertebrate animals that all descended from an ancestral fish. This ancestor would have been a lobe-finned fish – somewhat different from the ray-finned fishes, from salmon to sardines, that fill the oceans today. Lobe-finned fish had rounded, fleshy fins, and they also had lungs along with their gills, which allowed them to breathe air.
Some lobe-finned fish still survive in the world's waters today, including the appropriately named lungfish as well as the coelacanth. The coelacanth had been considered extinct, known only from the fossil record, until the species Latimeria chalumnae was discovered in 1938 and hailed as a 'living fossil' -- a primitive species that had apparently preserved the physical features from long-gone ancestors.
But that's a misnomer, said Paulo Brito, a paleontologist at Rio de Janeiro State University in Brazil who co-led the paper.
"In reality, living fossils, they don't exist," Brito said. "If they're living, they're not fossils."
After all, there's a roughly 70-million-year gap in the record between the most recent fossil coelacanths and the extant, or still-living ones today. So there are probably many adaptations these fish have made in that time that may or may not be apparent from looking at a fossil skeleton.
While researchers figured that the coelacanth, like its cousin the lungfish, must have a lung, it's been surprisingly tough to find. Some thought that a fatty organ in the fish's body, thought to be used to control buoyancy, might have been that original lung.
But according to the fossil record, ancient coelacanths' lungs were calcified – that is, they had a series of bony plates around them that might have helped to regulate expansion and contraction. Since organs and soft tissue don't survive in fossils, these bony plates could be the giveaway that scientists were looking for.
So for this new paper, the scientists looked for these bony plates in still-living animals – and found them around a different organ that was completely separate from the fatty swim bladder.
"When we found these plates, that was a really, really happy day," Brito said.
The researchers used X-ray tomography to scan coelacanth specimens at different ages, including embryos, juveniles and adults. In the early embryonic stages, the young animals seemed to have a relatively large and potentially functional lung – but that as the animal grew, the lung's growth slowed. By the time the modern coelacanth reaches adulthood, the lung is purely vestigial.
"The lung now has no function at all … just like our appendix," Brito said.
While the ancestor of these modern coelacanths probably had a functional (albeit primitive) lung, the lung may have lost its ability as the fish moved to deeper and deeper waters, farther from the surface, making an air-breathing organ ultimately unnecessary.
In fact it's possible that the reason these animals lost their lung function is the same reason they're still around today. Fish living in deeper waters would have been safer from the searing heat that ravaged the surface of the planet after the asteroid that killed the dinosaurs hit the Earth some 66 million years ago.
"This study showed us the true lung of the extant coelacanth," Yoshitaka Yabumoto, an ichthyologist at the Kitakyushu Museum of Natural History and Human History in Japan who was not involved in the paper, wrote in an email. "Now we know coelacanths, including extant species, have a calcified lung. This is unique in vertebrates and might give us a clue to know the origin and development of lungs in vertebrate evolution."