We are more primitive fish than was ever thought

Traditionally, people think that lungs and limbs are important innovations that have accompanied the transition of vertebrates from water to land. In fact, the genetic basis of air breathing and limb movement was established in our fish ancestors 50 million years earlier. This is evident from a recent genome mapping of primitive fish, carried out by, among others, the University of Copenhagen. The new study changes our understanding of an important milestone in our own evolutionary history.

There is nothing new about humans and all other vertebrates evolved from fish. The conventional view is that some fish shook landward about 370 million years ago as primitive, lizard-like animals known as tetrapods. According to this understanding, our ancestors of fish came to land from water by converting their fins into limbs and breathing underwater in air breathing.

However, limbs and lungs are not innovations that appeared as recently as was once believed. Our common fish ancestor who lived 50 million years before the tetrapod first landed, already had the genetic codes for limb-like shapes and air-breathing necessary to land. These genetic codes are still present in humans and a group of primitive fish.

This has been shown by recent genomic research conducted by the University of Copenhagen and their partners. The new research reports that the evolution of these ancestral genetic codes may have contributed to the transition from water to land of vertebrates, changing the traditional view of the sequence and timeline of this great evolutionary leap. The study has been published in the scientific journal Cell.

“The transition from water to land is an important milestone in our evolutionary history. The key to understanding how this transition took place is to reveal when and how the lungs and limbs evolved. We can now demonstrate that the genetic basis underpins these biological functions occurred much earlier before the first animals made landfall, ”explained Professor and lead author Guojie Zhang of the Villum Center for Biodiversity Genomics, in the Department of Biology at the University of Copenhagen.

A group of ancient living fish could be the key to explaining how the tetrapod eventually grew limbs and breathed air. The group of fish includes the bichir that lives in shallow freshwater habitats in Africa. These fish differ from most other extant bony fish in that they bear traits that our early ancestors of fish had more than 420 million years ago. And the same properties are also present in people, for example. Using genomic sequencing, the researchers discovered that the genes necessary for lung and limb development have already appeared in these primitive species.

Our synovial joint originated from the ancestor of fish

Using pectoral fins with locomotor function such as limbs, the bichir can move on land in the same way as the tetrapod. Researchers have believed for several years that pectoral fins in bichir represent the fins that our early fish ancestors had.

The new genome mapping shows that the joint that connects the so-called metapterygium bone to the radial bones in the pectoral fin in the bichir is homologous to synovial joints in humans – the joints that connect the upper arm and forearm bones. The DNA sequence that regulates the formation of our synovial joints already existed in the common ancestors of bonefish and is still present in these primitive fish and terrestrial vertebrates. At some point, this DNA sequence and the synovial joint were lost in all common bony fish – the so-called teleosts.

“This genetic code and joint allow our bones to move freely, which explains why the bichir can move on land,” says Guojie Zhang.

Lungs first, then swim bladder

Additionally, the bichir and a few other primitive fish have a pair of lungs that resemble ours anatomically. The new study shows that the lungs in both bichir and alligator gar also function in a similar way, expressing the same set of genes as human lungs.

At the same time, the study shows that the lung and swim bladder tissue of most extant fish are very similar in gene expression, confirming that they are homologous organs, as predicted by Darwin. But while Darwin suggested that swim bladders were converted into lungs, the study suggests that swim bladders are more likely to have evolved from the lungs. The research suggests that our early bony fish ancestors had primitive functional lungs. Through evolution, a branch of the fish has retained lung functions that are more adapted to air breathing and ultimately led to the evolution of tetrapods. The other branch of fish altered lung structure and evolved with swim bladders, leading the evolution of teleosts. The swim bladders allow these fish to maintain buoyancy and sense pressure, allowing them to better survive underwater.

“The study explains to us where our body organs came from and how their functions are decoded in the genome. Therefore, some functions related to lungs and limbs did not evolve when the transition from water to land took place. encoded by some ancient gene regulation mechanisms that were present in our fish ancestor long before landing, interestingly, these genetic codes are still present in these ‘living fossil’ fish, which allow us to trace back the root of these genes, ”concludes Guojie Zhang.