2023-04-18 09:03:22
By Hannah Bonner* y Mar Gulis
It does not matter that the tuna and the shark are fish and the human being a mammal. Evolutionarily, and contrary to what common sense suggests, tunas are closer to our species than to sharks. The reason: Tunas and humans have a common ancestor that lived approximately 400 million years ago, when its lineage had already diverged from that of sharks.
Imagen de Danilo Cedrone (United Nations Food and Agriculture Organization)
grandfather fish
To explain our kinship, we need to dive into the depths of the sea and go back 500 million years agoin the Cambrian period, at which time the first fish appeared. Seen with today’s eyes, those organisms were a very simple and somewhat scrawny version of what we consider a fish. They did not have lateral fins and their small mouths lacked jaws, but they did have typical fish characteristics: a head with two eyes and two nostrils, a notochord (precursor structure of the spinal column), zigzag muscles, and supporting structures. called branchial arches.
Although they don’t look like us at all, proto-fish like the Haikouichthys of the illustration are the ancestors of sharks and tuna, yes, but also of all vertebrates, including frogs, snakes, lizards, birds, fish and mammals.
Haikouichthys, one of the first fish. / Hannah Bonner (Planet Tuna)
The family is divided: fish with and without jaw
From these first fish evolved three groups that have survived in the seas to this day. The oldest and least common are ciclostomousa small group composed solely of the lampreys and the mixines. Like the first fish, these strange long-bodied beings lack jaws.
A lamprey, a jawless fish. / Hannah Bonner (Planet Tuna)
Later they appeared fish that with jaws. Their success in evolutionary terms was so great that they gave rise to many different species and that finally began to resemble what we know as fish. The jawed fish, in turn, were divided into two groups: the cartilaginous partsWhat are the sharks and the stripesand bony fish, which include all other fish. About 96% of all the fish that inhabit the seas today are bony fish: since the seahorses until the heritage or the tunas.
A shark (a cartilaginous fish) and a salmon (a bony fish). / Hannah Bonner (Planet Tuna)
Such is (the tree of) life: division after division
For their part, bony fish were divided into two groups: lobe-finned fish and those of ray fins.
Illustration by Hannah Bonner (Planet Tuna)
Los lobe-finned fish (scientific name ‘sarcopterygians’) have a series of bones at the base of the fin that bear a certain resemblance to the bones of our arms and legs. It’s not by chance: 370 million years ago, those bones evolved to become the limb bones of the earliest amphibians.. From those early amphibians evolved all other tetrapods, or four-legged animals, including humans.
Instead, the fish ray fins (or ‘actinopterygians’) have a series of parallel fin rays. They make up the vast majority of fish that exist today, including tuna.
Specifically, tunas belong to a subgroup of ray-finned fish called teleosts, which are distinguished by having developed a series of improvements in the jaws and fins. In turn, within the teleosts we find a family called Scombridae, which is the tuna family and its closest relatives, such as bonito and mackerel. Scombridae are open water predators with bodies perfectly adapted to outswim their prey.
The search for the common ancestor
Now that we know that humans are descended from lobe-finned fish and tunas are descended from ray-finned fish, the question is: who was our last common ancestor? Evidently it had to be some fish that lived before these two groups separated. We cannot know exactly who this ancestor was, but from the fossil record it has been calculated that he probably lived about 400 million years ago, at the beginning of the Devonian period. That’s a long time ago, but it still means we’re related, albeit very distant, to the tuna. And that the tuna, in turn, is more related to a human being than to a shark, because cartilaginous fish diverged from bony fish at an even earlier time.
Illustration by Hannah Bonner (Planet Tuna)
To know moresee Hannah Bonner’s video Are We Related to Tuna?
* This post is adapted from “Our relatives the tunas”, text published by Hannah Bonner, author and illustrator, on the web Planet Tunaa dissemination project of the Spanish Institute of Oceanography of the CSIC that unravels the secrets of tuna.
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