People traditionally think that the lungs and limbs are key innovations that came with the transition of water vertebrates to land. But in fact, the genetic basis of air breathing and limb movement was already established in our fish ancestor 50 million years earlier. This, according to a recent mapping of the primitive fish genome by the University of Copenhagen, among others. The new study changes our understanding of a key milestone in our own evolutionary history.
There is nothing new that humans and all other vertebrates have evolved from fish. The conventional understanding has been that certain fish shone toward the earth about 370 million years ago as lizard-like primitive animals known as tetrapods. According to this understanding, our fish ancestors came out of the water into the earth turning their fins into limbs and breathing underwater into air breathing.
However, the limbs and lungs are not innovations that appeared as recent as previously thought. Our common fish ancestor that lived 50 million years before the tetrapod landed already carried the genetic codes of limb-like shapes and the breath of air needed to land. These genetic codes are still present in humans and in a group of primitive fish. This has been demonstrated by recent genomic research conducted by the University of Copenhagen and its partners. New research reports that the evolution of these ancestral genetic codes may have contributed to the transition of water to land from vertebrates, which changes the traditional view of the sequence and timeline of this great evolutionary leap. The study was published in the scientific journal Cell.
“The transition between water and land is an important milestone in our evolutionary history. The key to understanding how this transition went is to reveal when and how the lungs and limbs evolved. We can now show that biological functions occurred long before the first animals reached the ground, “said Guojie Zhang, professor and lead author at the Villum Center for Biodiversity Genomics in the University of Copenhagen’s Department of Biology.
Our synovial joint evolved from the ancestors of fish
Using pectoral fins with locomotor function such as the limbs, the bichir can move on the ground in a manner similar to the tetrapod. For some years now, researchers have believed that the pectoral fins of the bichir represent the fins that our ancestors had.
The new mapping of the genome shows that the joint that connects the so-called metapterygeal bone with the radial bones of the pectoral fin of the bichir is homologous to the synovial joints in humans: the joints that connect the bones of the arm and the ‘forearm. The DNA sequence that controls the formation of our synovial joints already existed in the common ancestors of bony fish and is still present in these primitive fish and in terrestrial vertebrates. At some point, this DNA sequence and synovial joint were lost in all common bone fish, the so-called teleosts.
“This genetic code and joint allow our bones to move freely, which explains why the bichir can move on the ground,” says Guojie Zhang.
First lungs and then swimming bladder
In addition, the bichir and some other primitive fish have a pair of lungs that anatomically resemble ours. The new study reveals that both bichir and crocodile lungs also function similarly and express the same set of genes as human lungs.
At the same time, the study shows that the lung tissue and swim bladder of most existing fish are very similar in gene expression, confirming that they are homologous organs as predicted by Darwin. But while Darwin suggested that swim bladders become lungs, the study suggests that swim bladders are more likely to evolve from lungs. Research suggests that our bony fish ancestors had primitive functional lungs. Through evolution, a branch of fish preserved lung functions best suited to air respiration and eventually led to the evolution of tetrapods. The other branch of the fish modified the lung structure and evolved with swim bladders, which led to the evolution of teleosts. Swimming blades allow these fish to maintain buoyancy and perceive pressure and therefore survive better underwater.
“The study sheds light on where the organs of our body come from and how their functions in the genome are decoded. Therefore, some of the functions related to the lungs and limbs did not evolve at the time they were produced the water-land transition, but are coded by some ancient mechanisms of gene regulation that were already present in our fish ancestor long before landing.It is interesting that these genetic codes are present in these ‘living fossil’ fish , which offer us the opportunity to trace the root of these genes, ”concludes Guojie Zhang.
Not just the limbs and lungs, but also the heart
Primitive fish and humans also share a common and critical function in the cardio-respiratory system: the arterial cone, a structure of the right ventricle of our heart that can allow the heart to deliver oxygen efficiently throughout the body and that it is also found in the bichir. However, the vast majority of bony fish have lost this structure. The researchers discovered a genetic element that appears to control the development of the arterial cone. Transgenic experiments with mice showed that when the researchers removed this genetic element, the mutated mice died due to thinner, smaller right ventricles, leading to congenital heart defects and impaired heart function.