Why we have a spine, when over 90pc of animals dont
Although the backbone is one of the most important innovations in the history of life, its origins have long been shrouded in mystery
Josh Gabbatiss
A popular trope of science fiction is a world in which creatures with unusual body plans rule. From the octopoid Martians of War of the Worlds to the giant bugs of Starship Troopers, there is a creepy appeal in the idea that a race of spineless creatures could pose a real threat to humanity.
Because as far as we are concerned, the backbone is king. Whether or not we are truly the dominant organisms on Earth is debatable. But we are vertebrates and so are most of the large, charismatic animals that walk, swim and fly. It is only natural that we find ourselves drawn to what Henry Gee, Senior Editor at Nature, calls our own corner of creation.
The spine is certainly a useful innovation. It provides support for the body, as well as valuable protection for the spinal cord. The fact that our bodies are supported by an internal skeleton as opposed to an external one also allows for a greater range of movement, and means vertebrates can grow to far greater sizes than any invertebrate ever has.
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. Prior to this, life had been a pretty simple affair for hundreds of millions of years; single cells drifted around, occasionally getting together to form a colony. That was pretty much it.
We briefly possess notochords during our development in the womb
Then, boom! In an event known as the Cambrian explosion, Earths 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 todays 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. In 1909, the golden jubilee of Darwins On the Origin of Species, the Linnean Society of London organised an event to explore the origin of the vertebrates.
These early creatures were too squishy to leave much lasting evidence
It was not a rousing success: The disputants agreed on one single point, quipped zoologist Thomas Stebbing: namely, that their opponents were all in the wrong.
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.
This is because unlike later vertebrates, which are full of hard, fossilisable components like bones and teeth, these early creatures were too squishy to leave much lasting evidence. Today, only faded imprints of their soft bodies remain.
The result is an early history documented by what palaeontologist Phillippe Janvier refers to as squashed slugs. These are difficult to interpret, leaving plenty of room for speculation.
This means that we are, ultimately, defined by our anus
That said, in the past few decades the fields of evolutionary developmental (evo-devo) biology and molecular phylogenetics have given scientists a much better understanding of the evolutionary relationships between organisms.
This has included the finding that 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.
This means that we are, ultimately, defined by our anus, which is a bit humbling. But this really is the best way scientists have come up with for organising the deepest divisions in the animal kingdom.
Though we appear to look nothing like sea urchins and their ilk, we do share a common ancestor, which according to Linda Holland, a chordate researcher at the Scripps Institution of Oceanography, USA, 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.
The notochord is the supporting organ of the beating tail
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.
Satohs idea builds on the work of British zoologist Walter Garstang. 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.
In Satohs lab, work is underway to demonstrate the genetic underpinnings that could result in such a transformation.
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.
Anyone who has experienced the discomfort of a slipped disc will appreciate how useful the notochord still is
In the dangerous Cambrian oceans, full of highly mobile predators, faster swimming would have been a valuable asset for a creature trying to outpace an attacker.
In most modern vertebrates including humans - the notochord no longer provides support, but it still has an important role to play. It has become part of the discs that separate our vertebrae and that act as shock absorbers. Anyone who has experienced the discomfort of a slipped disc will appreciate how useful the notochord still is. BBC
