We like primitive fish more than we once believed

People traditionally think that lungs and limbs are the most important innovations due to the transition of vertebrates from water to land. But in fact, the genetic basis of air respiration and limb movement was already established 50 million years ago in our fish ancestors. This according to a recent genome mapping of primitive fish done by the University of Copenhagen, among others. The new study changes our understanding of an important milestone in our own evolutionary history.

There is nothing new to humans and all other vertebrates that evolved from fish. The conventional notion was that certain fish shrank inland about 370 million years ago as primitive, lizard-like animals known as tetrapods. According to this understanding, our fish ancestors came from land to land by transforming their fins into limbs and breathing underwater.

However, limbs and lungs are not innovations that have occurred as recently as ever believed. Our common fish ancestor, who lived 50 million years before the tetrapod first came ashore, needed all the genetic codes for limb-like shapes and air respiration for landing. These genetic codes are still present in humans and a group of primitive fish.

This is evidenced 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 contributed to the swirling water-to-land transition, changing the traditional view of the sequence and timeline of this great evolutionary leap. The study was 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 developed. We can now show that the genetic basis of these biological functions occurred much earlier before the first animals came ashore, “said Professor and lead author Guojie Zhang, of the Villum Center for Biodiversity Genomics, at the University of Copenhagen’s Department of Biology.

A group of old live fish may hold the key to explaining how the tetrapod can eventually grow limbs and breathe in the air. The group of fish contains the bichir that live in shallow freshwater habitats in Africa. These fish differ from most other existing bony fish by bearing characteristics that our early fish ancestors had more than 420 million years ago. And the same characteristics are also in humans. Using a genomic sequence, the researchers found that the genes necessary for the development of lungs and limbs had already appeared in these primitive species.

Our synovial joint developed from fish father

Using pectoral fins with a movement function such as limbs, the bichir can move on the ground in a similar way to the tetrapod. Researchers have been believing for years that breast fins in bichir represent the fins our early fish ancestors had.

The new genome mapping shows that the joint that connects the so-called metataphygeal bone with the radial bones in the pectoral fin in the bichir is homologous to synovial joints in humans – the joints that connect the upper and lower arm bones. The DNA sequence that controls the formation of our synovial joints already exists in the common ancestors of bony fish and is still present in these primitive fish and on land vertebrates. At one point, this DNA sequence and the synovial joint were lost in all the common bone fish – the so-called teleostes.

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

First lungs, then swim bladder

In addition, the bichir and some other primitive fish have some lungs that look anatomically similar to ours. The new study reveals that the lungs in both bichir and crocodile also function in a similar way, expressing the same set of genes as human lungs.

At the same time, the study shows that the tissue of the lung and swim bladder of most existing fish are very similar in gene expression, confirming that they are homologous organs, as predicted by Darwin. But although Darwin suggested that swimming pools be converted to lungs, the study suggests that swimming pools are more likely to have evolved from lungs. The research suggests that our early ancestors of bony fish had primitive functional lungs. Through evolution, one branch of fish preserved the lung functions that were more adapted to air respiration and eventually led to the evolution of quadrupeds. The other branch of fish changed the lung structure and developed with swimming pools, which led to the evolution of teleostes. The swimming pools allow these fish to maintain buoyancy and pressure monitoring in order to better survive underwater.

“The study enlightens us on where our body organs come from and how their functions are decoded in the genome. So some of the functions related to lung and limbs did not develop when the transition from water to land took place. coded by some ancient gene regulatory mechanisms that already existed before our landing in our fish ancestor.It is interesting that these genetic codes still occur in these ‘living fossil’ fish, which gives us the opportunity to trace the root of this genes, ”concludes Guojie Zhang.