8 septembre 2016

L'origine de la colonne vertébrale

Chaque partie de notre anatomie raconte un chapitre de l'histoire de l'évolution de notre espèce. La colonne vertébrale s'avère être l'un des chapitres les plus fascinants.

Extrait de l'article:

The spine is certainly a useful innovation. It provides support for the body, as well as valuable protection for the spinal cord (...) But seeing as over 90% of all animals get by just fine without backbones, it is not obvious why this novelty arose in the first place. How did the spine emerge from a spineless world?

The evolutionary leap that birthed the vertebrates took place during the Cambrian period around 500 million years ago. (...) In an event known as the Cambrian explosion, Earth's oceans were filled with a huge array of life forms. Within a few tens of millions of years something new joined the ranks of the six-foot predatory arthropods and multi-eyed oddities. Animals that looked something like today's lampreys and hagfish.

These creatures were some of the first vertebrates. Their bodies were supported by rudimentary vertebrae and a stiff rod called a notochord made from a cartilage-like material. The notochord was the precursor to the backbone, the structure found in vertebrates from mice to dinosaurs to humans.

We know where these first vertebrates came from. Their predecessors are represented by fossils from those Cambrian seas: worm-like animals like Pikaia and Haikouella. These animals are not vertebrates, but they do possess a notochord. They belong in a more inclusive category called the chordates, which incorporates vertebrates and a few vertebrate-like groups.

It is worth noting that this means that we ourselves are chordates. In a throwback to our deep ancestry we briefly possess notochords during our development in the womb.

But the question of exactly where those first chordates came from has long proved controversial. (...) Over the years, virtually every invertebrate group – from molluscs to arthropods – has been suggested as a starting point for the origin of the chordates. The fossil evidence to reliably answer the question is lacking.

(...) some of our closest invertebrate relatives are a group of marine animals that includes the familiar starfish, sea urchins and sea cucumbers, as well as some unusual worm-like animals called the hemichordates. Like all of these animals, we are deuterostomes – meaning that during embryo development, our anus forms before our mouth.

In other words, we are vertebrates, which are a form of chordate, which are themselves a form of deuterostome.

(...) Though we appear to look nothing like sea urchins and their ilk, we do share a common ancestor, which (...) could have looked sea urchin-like, chordate-like, hemichordate-like - or like none of these.

Then, one of those early deuterostomes became the very first chordate by gaining a notochord.

Nori Satoh, a researcher at the Okinawa Institute of Science and Technology in Japan, has identified what he sees as the moment the notochord, and hence the chordate lineage, was born. "I have proposed that the occurrence of tadpole-like larvae is a key event that caused the evolution of chordates," he explains.

Early in life, many deuterostomes pass through a larval stage. But while acorn worm or sea urchin larvae might swim about by rhythmically moving tiny hair-like structures – cilia – on their bodies, chordate larvae have a tail that they beat.
"The notochord is the supporting organ of the beating tail," says Satoh.

(...) In this hypothesis, the early chordates ended up forsaking an evolutionary future as bottom-dwelling invertebrates, and instead retained their larval notochords to adulthood – just as some species of amphibians retain larval characteristics into adulthood today.

(...) As for why it would be beneficial to evolve a larval tail in the first place, and then retain it into adulthood, Satoh thinks this is obvious. "Swimming with a beating tail is much more efficient than locomotion with ciliary beats," he says. "The notochord gave the earliest chordates a unique advantage."

(...) "Presumably, as animals increased in size, it became advantageous to have something stronger than a notochord for the paraxial muscles to work against in swimming," remarks Holland.

A few living chordates still retain the ancient backbone-free condition where the notochord offers support. They can provide us with some intriguing glimpses into our evolutionary past.

The lancelet, or amphioxus, is a translucent, fish-like creature that represents one of the only surviving lineages of non-vertebrate chordates in the modern age. It is a "living fossil" that gives researchers like Holland an insight into how vertebrates evolved in those prehistoric oceans.

The anatomy of the lancelet hints at the spine's early evolution, as it possesses segmented muscles and a sheath around its notochord and nerve cord. The vertebral column in humans and other animals with a backbone is, put very simply, an elaboration of this same notochordal sheath.

(...) When researchers at the European Molecular Biology Laboratory found a notochord-like structure in a marine worm back in 2014, it supported an old theory of how chordates originated from ancient worms.

Intriguingly, it also suggested just how wrong our march of progress image really is. The appearance of the notochord might not have been a seminal turning point in the history of life after all.

In fact, the notochord might have appeared earlier, in our shared ancestor with starfish and other spineless deuterostomes. These creatures might then have given up on the structure, adapting to life on the seabed where a notochord was simply not a particularly useful feature.

Other animals might also have once had, and then lost a notochord. While lancelets are commonly identified as the closest living relatives of vertebrates, back in 2006 a paper in Nature claimed that this is not the case – pointing instead to tunicates, also known as sea squirts, as a better candidate.

(...) "Rather than the steady acquisition of progressively more chordate-like (and, by implication, human-like) features from an ancestor with nothing much to recommend it, the story becomes one of persistent loss."

(...) Evolution has no end point. There is no ideal that it is striving towards. A starfish is as highly-evolved as a human, and the fact that an ancestral starfish may have shed some of the traits we associate with sophisticated body forms demonstrates the absence of innate vertebrate superiority.

2 commentaires:

Hans Georg Lundahl a dit...

Lampreys have no jaws.

How are animals with skulls and jaws supposed to have developed from lamprey like proto-vertebrates?

And how are lampreys supposed to have eveloped from non-vertebrate chordates?

Prof Solitaire a dit...

Je ne suis pas un spécialiste de la biologie évolutive, mais ces phénomènes me semblent éminemment explicables... les plus anciens poissons fossilisés connus ne possédaient pas de mâchoire, mais une ouverture par laquelle ils aspiraient leur nourriture. Éventuellement, cette ouverture s'est modifiée afin de pouvoir s'ouvrir et se fermer, créant ainsi la première mâchoire.

L'origine du crâne est plus ancienne. Et cet article donne une explication plausible et intéressante de l'origine de la colonne vertébrale...