Evolution News - Our Common Ancestry in Fish

Polypterus bichir

We’re more like primitive fishes than once believed – University of Copenhagen

Humans, like all terrestrial tetrapods, have a couple of major differences with the bony fish we are all believed to be descended from via an intermediate species such as Tiktaalik roseae, which crawled out onto land about 370 million years ago; the major differences being our jointed limbs and our lungs, all thought to be adaptations to walking on land and breathing air.

However, researchers at the University of Copenhagen believe they have found evidence to show that these structures could have been present in our ancestral fish some 50 million years before that. In fact, lungs may be the forerunner of the swim-bladder found in bony fish, rather than the swim-bladder being the forerunner of lungs.

The conventional view is that bottom-feeding fish, in low-oxygen environments, used their swim-bladder as a fresh-air supply by gulping air from the surface. The swim-bladder is connected to the gut. This, over time, evolved into the lungs and enabled this bottom-feeders to use the limbs they were evolving to crawl around in the silt, to come out onto land.

This research challenges that view by suggesting that some early bony fish may have had lungs that later evolved into swim-bladders in fish that needed to vary their buoyancy while some retained them as functional lungs which server the purpose of a fresh air store for foraging in an anoxic environment.

The same early fish were probably also evolving the synovial joints that made walking possible, that are now a feature of the terrestrial vertebrate skeleton. The Copenhagen team arrived at this conclusion by studying the genes of the primitive bichir and the alligator gar fish.

The University of Copenhagen press release explains:

People traditionally think that lungs and limbs are key innovations that came with the vertebrate transition from water 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 genome mapping of primitive fish conducted by the University of Copenhagen, among others. The new study changes our understanding of a key milestone in our own evolutionary history.

Bichir, Polypterus bichir

There is nothing new about humans and all other vertebrates having evolved from fish. The conventional understanding has been that certain fish shimmied landwards roughly 370 million years ago as primitive, lizard-like animals known as tetrapods. According to this understanding, our fish ancestors came out from water to land by converting their fins to limbs and breathing under water to air-breathing.

However, limbs and lungs are not innovations that appeared as recent as once believed. Our common fish ancestor that lived 50 million years before the tetrapod first came ashore already carried the genetic codes for limb-like forms and air breathing needed for landing. These genetic codes are still present in humans and a group of primitive fishes. This has been demonstrated by recent genomic research conducted by University of Copenhagen and their partners. The new research reports that the evolution of these ancestral genetic codes might have contributed to the vertebrate water-to-land transition, which changes the traditional view of the sequence and timeline of this big evolutionary jump. The study has been published in the scientific journal Cell.

"The water-to-land transition is a major milestone in our evolutionary history. The key to understanding how this transition happened is to reveal when and how the lungs and limbs evolved. We are now able to demonstrate that biological functions occurred much earlier before the first animals came ashore," stated by professor and lead author Guojie Zhang, from Villum Centre for Biodiversity Genomics, at the University of Copenhagen’s Department of Biology.

Our synovial joint evolved from fish ancestor

Using pectoral fins with a locomotor function like limbs, the bichir can move about on land in a similar way to the tetrapod. Researchers have for some years believed that pectoral fins in bichir represent the fins that our early fish ancestors had.

Alligator gar, Atractosteus spatula

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

"This genetic code and the joint allows our bones move freely, which explains why the bichir can move around on land," says Guojie Zhang.

Evolution of the jawed vertebrates

First lungs, then swim bladder

Moreover, the bichir and a few other primitive fishes have a pair of lungs that anatomically resembles ours. The new study reveals that the lungs in both bichir and alligator gar also function in a similar manner and express same set of genes as human lungs.

At the same time, the study demonstrates that the tissue of the lung and swim bladder of most extant fishes are very similar in gene expression, confirming they are homologous organs as predicted by Darwin. But while Darwin suggested that swim bladders converted to lungs, the study suggests it is more likely that swim bladders evolved from lungs.

Not just limbs and lungs, but also the heart

Primitive fish and humans also share a common and critical function in the cardio-respiratory system: The conus arteriosus, a structure in the right ventricle of our heart which might allow the heart to efficiently deliver the oxygen to the whole body, and which 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 conus arteriosus. Transgenic experiments with mice showed that when researchers removed this genetic element, the mutated mice died due to thinner, smaller right ventricles, which lead to congenital heart defects and compromised heart function.

The research suggests that our early bony fish ancestors had primitive functional lungs. Through evolution, one branch of fish preserved the lung functions that are more adapted to air breathing and ultimately led to the evolution of tetrapods. The other branch of fishes modified the lung structure and evolved with swim bladders, leading the evolution of teleosts. The swim bladders allow these fishes to maintain buoyancy and perceive pressure, thus better survive under water.

"The study enlightens us with regards to where our body organs came from and how their functions are decoded in the genome. Thus, some of the functions related to lung and limbs did not evolve at the time when the water-to-land transition occurred, but are encoded by some ancient gene regulatory mechanisms that were already present in our fish ancestor far before landing. It is interesting that these genetic codes are still present in these ‘living-fossil’’ fishes, which offer us the opportunity to trace back the root of these genes," concludes Guojie Zhang.

Sadly, the paper in Cell is behind a paywall and the publishers, Elsevier, want money just to reprint the abstract.

The point this discovery shows is how something in evolution that seems intuitively right still needs validation and, if found wanting, must be revised. In this way, science moves ever closer to the truth that scientific theories model. It now looks, if this discovery is confirmed, as though both lungs and synovial joints may have preceded and facilitated the move onto land, not followed from it and been the consequence of it.

But whatever the eventual truth of the matter, and there is no reason to suppose that this isn't it, one thing this discovery confirms with very little doubt, is the common ancestry of the vertebrates, whether terrestrial or aquatic, with the same or homologous genes being responsible for the same basic structures in our remote ancestors from 420 million years ago as are present in our own genomes today.

As I said in my book, What Makes You So Special? From The Big Bang to You:

Almost all your genes have spent much longer being something else than they have being human. Your ancestors were there when Europe and Africa split off from the Americas. They were there as small mammal-like reptiles when dinosaurs ruled the earth. They saw pterodactyls flying overhead. They survived the mass-extinction which ended the dinosaurs’ reign and they saw the birds and the bats grow wings and take to the air.

Your ancestors swam in the Cambrian seas and crawled out onto the land as early air-gulping fish destined to become four–legged animals with lungs. Your ancestors lived through the Carboniferous era when dense forests of tree ferns grew in steaming jungles where dragonflies with meter-wide wings flew. They saw the trees fall and form the piles of vegetation destined to be coal as the climate changed and the Carboniferous forests collapsed. They saw the first flowering plants as plants and insects formed their mutual-benefit society.

Your ancestors lived through the first great toxic waste disaster when the cyanobacteria produced oxygen and triggered a mass extinction; and they learned to turn it to their advantage by evolving aerobic respiration.

Your ancestors were bacteria; maybe they were archaea; they may have been the strange Ediacarans which were the earliest known multi-cellular organisms. In almost every one of your cells, in your genes, you carry a record of your evolution, of the entire human evolution story, and of a great deal of the evolution story of every other living thing.

Rubicondior, R.; What Makes You So Special? From The Big Bang to You, pp 143-144

This paper reinforces the truth of that statement.

---

Tweet Link