Once We All Were Fish: A Conclusion of Mysteries

     In 2022, a team of paleontologists from the University of Chicago took a closer look at another ancient lobe-finned fish that strongly resembled and was closely related to Tiktaalik and therefore, to tetrapods as well. The fish is known as Qikiqtania and lived in the freshwater swamps, lakes and rivers of the late Devonian around 365 million years ago. Qikiqtania displays a number of traits that set it apart from Tiktaalik in a peculiar way. Tiktaalik had sturdy, wristed, and limb-like pectoral fins. Qikiqtania, however, possessed pectoral fins with distinct humerus bones that lacked the ridges that would indicate where muscles and joints would be on a limb. Instead, its pectoral fins were instead much more curved and paddle shaped. This finding suggested that Qikiqtania was not adapted for moving on land, unlike Tiktaalik, and was evolving back towards a strictly aquatic lifestyle at the same time Tiktaalik was moving towards a terrestrial lifestyle. In other words, after fish had just started to leave the water, Qikiqtania went right back in.
     One of the most mysterious of all discoveries revolving around tetrapod evolution made in recent years were a series of trace fossils (a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself) from Poland known as the Zachelmie Trackways. The Zachelmie Trackways are the tracks of a tetrapod that was walking on dry land across the coast. The tracks indicate that this particular tetrapod was walking proficiently on all four limbs, meaning that it could not have been Ichthyostega or Acanthostega, which possessed limbs that were not adapted for doing so. But the most baffling thing about the Zachelmie Trackways is the time in which scientists have determined they were made. While Tiktaalik, a fish with the beginnings of limbs, lived around 380 million years ago and Ichthyostega and Acanthostega, primitive four-limbed vertebrates with limbs that were not developed for walking on land both lived around 365 million years, these tracks of a walking tetrapod are dated back to 395 million years. This indicates that tetrapods may have evolved and took to the land around 18 million years earlier than previously assumed, which is earlier than Tiktaalik lived as well. These findings might mean that animals like Tiktaalik and Ichthyostega might not be the pivotal transitional species we often think of them as. However, a number of scientists are skeptical about the potential implications of the Zachelmie trackways. For instance, some believe that the traces may have been left by fish nests or feeding traces rather than an animal walking on four limbs (meaning that the area in which they were made was actually underwater at the time). But others have suggested that the tracks imply that animals like Ichthyostega were not the ancestors of tetrapods.
     The series Once We All Were Fish has provided a fairly detailed timeline of the evolution of fish and how they evolved from one form to another. But how our fish ancestors left behind their aquatic forms and colonized the land is a subject of debate, controversy, and mystery to this day. While it may be unsatisfying to end this series on an unfinished puzzle, let it serve as a testament to the highly complex nature of evolution, the limitations of modern science, and how fortunate we are to understand as much as we do from the fossil record alone.