The transition from aquatic to terrestrial vertebrates is often thought to have brought key innovations such as air-breathing and limb movement. However, according to a recent study of primitive fish, the genetic basis for air breathing and limb movement had already been established 50 million years ago. As a result of this study, we have a better understanding of a significant evolutionary step for ourselves.

Traditional thinking holds that lungs and limbs are key innovations that came with the vertebrate transition from water to land. However, the genetic basis of air-breathing and movement of limbs was already present half a billion years ago in our fish ancestor. This is the result of a recent genome mapping project of primitive fish conducted among others by the University of Copenhagen. As a result of this study, we have a better understanding of a significant evolutionary step for ourselves. As all vertebrates have evolved from fish, humans are not new to the evolutionary process. Species of fish are usually understood to have flown to land as primitive lizard-like creatures known as tetrapods roughly 370 million years ago. Using this understanding, our fish ancestors came from the water to the land by converting their fins into limbs and breathing beneath the water into the air. As it turns out, limbs and lungs are not innovations that were as recent as once thought. Fish living 50 million years before the tetrapod first appeared on land in the sea already had the genes for limb-like forms and air breathing that were needed to land. Genetic codes that are representative of these ancestral species are still present in humans and some

Recent genomic research by the University of Copenhagen and its partners provides powerful evidence of this claim. In new research, it is suggested that the evolution of ancestral DNA codes contributed to the transition of vertebrates from land to water, so changing typical views on the sequence and timescale of this evolutionary step. Earlier this year, Cell published a study related to breast cancer. The transition from the water to the land is a major event in humanity’s evolution. For an understanding of how this transition occurred, one must analyze when and how the lungs and the limbs developed. Professor and lead author Guojie Zhang, from Villum Centre for Biodiversity Genomics at the University of Copenhagen’s Department of Biology, stated “We are now able to demonstrate that the genetic basis underlying these biological functions occurred much earlier than the first animals appeared on the shore.”

It could be possible to explain how the tetrapod lastly grew limbs and breathed by studying a group of ancient alive fishes. A species of bichir lives in shallow freshwater habitats throughout Africa as part of this group of fishes. Most of these fish have traits that our early fish ancestors might have had over 420 million years ago, which set them apart from most other extant bony fishes. The same traits can also be found in humans, for example. In a study of genomic sequencing, researchers found that the genes involved in the growth of lungs and limbs have already been expressed by

Our synovial joint evolved from fish ancestor

A bichir uses its pectoral fins with locomotor function as limbs, allowing it to move about in the same way as a tetrapod on land. It has long been believed by researchers that bichir has pectoral fins, which might be the same fins our early It is now known that the joint that connects the radial bones with the metapterygium bone in the pectoral fin of the bichir is homologous to the synovial joints of humans, that is, the joints that connect the forearm to the upper arm. Our synovial joints are formed by DNA sequences present in these primitive fishes and in terrestrial vertebrates. The synovial loci have been located in the genome since before the evolution of the fossil record. All of the common bony fishes — the teleosts — have lost this DNA sequence and the synovial joint at some point. As a result of the genetic code and the joints, we have free mobility of our bones, which is why the bichir can move around on land.

First lungs, then swim bladder

Furthermore, the bichir and some other primitive fishes have lungs that share anatomical appearance with our own. The new study indicates that the lungs of the bichir and alligator gar perform the same function and express the same genetic information as those of humans. During the study, however, it was also shown that the tissues of the lung and swim bladder of most extant fishes share a very similar gene expression pattern, introducing the hypothesis that their organs are homologous as Darwin predicted. The study claims that swim bladders evolved from lungs rather than the opposite, contrary to Darwin’s proposal that swim bladders converted into According to research, the lungs of our early bony fish ancestors was primitive but functional. During evolution, one branch of fish maintained the lung function that was better suited to air breathing and eventually led to the development As the other branch of fishes evolved, they compromised the lung structure and developed swim bladders, causing the emergence of the teleost. These fishes utilize their swim bladders in order to maintain buoyancy and feel pressure, thereby enabling them to be more successful at surviving underwater. We are now better able to understand where our various body organs come from, as well as how their functions are associated with our genes. Our ancestor was already alive much before land was terraformed, so certain features like lung and fins did not evolve for the first time at the time when we moved from water to land, but rather were encoded by ancient gene regulation mechanisms that were first present in fish. Interestingly, these genetic codes are still present in these ‘living fossil’ fishes, so we are able to trace their origins backward,” Guojie Zhang concludes.

FACT BOX 1: Not just limbs and lungs, but also the heart

A critical component of the cardio-respiratory system is also shared by primitive fish and humans An arteriosus called the conus, which is also present in the bichir, is a structure found in the right ventricle of the heart that might efficiently deliver oxygen into the body. Bony fish, however, have lost this structure to a great extent. A genetic element appears to be responsible for controlling the development of the conus arteriosus of the heart. A study with transgenic mice showed that when researchers removed the genetic element, the mice then died from thicker, smaller right ventricles, which result in congenital heart defects and compromised heart functions.

FACT BOX 2:

There is an abundance of fish species today that belong to the ray-finned subclass of bony fishes. There are gills, fins and a swim bladder in these types of fish. Among vertebrates, the tetrapod group comprises the largest number of species. Among tetrapods, all mammals, birds, reptiles, and amphibians are included, as they are descendants of the first animals that developed four limbs and lungs to adapt to life on the ground. Researchers theorize that air-breathing fishes survived the second mass extinction roughly 375-360 million years ago because the ability to breathe allowed them to stay alive. The depletion of oxygen in Earth’s oceans at that time caused a majority of species to become extinct. The study has been published in the scientific journal Cell. Some fish are able to survive on land because of their lung structures. As well as writing the genome for the lungfish, colleagues also contributed to the publication of another study on the fish’s genome. As of this writing, the genome is the biggest vertebrate genome ever decoded. A similar paper was published in Cell at the same time. The research is supported, inter alia, by the Villum Foundation.