Increase in Fish Vision Preceded the Evolution of Tetrapods
Massive Increase in Visual Range Preceded the Origin of Terrestrial Vertebrates
One of the most significant steps (no pun intended), in vertebrate evolution took place some 385 million-years-ago when some types of sarcopterygian fish evolved rudimentary limbs and started to clamber around on land. These fish were the ancestors of all the Tetrapods (four-legged vertebrates), amphibians, reptiles, birds, mammals and of course ourselves. It could be argued that our species, Homo sapiens, is nothing more than a highly modified fish! How and why the transition from living in a watery world to one of a terrestrial lifestyle took place is hotly debated by palaeontologists. One team of researchers from the Northwestern University (Illinois), in collaboration with colleagues from the Natural History Museum of Los Angeles County and optical imaging specialists, have come up with a remarkable theory that explains the move from the water to the land.
Note the Eyes Located on the Top of the Skull in this Early Tetrapod
Improvement in eyesight was the driver for the momentous leap from water onto land and not limb evolution linked to a better ability to clamber about on terra firma.
Looking at the Position of the Eyes
The scientists which included corresponding authors Malcolm MacIver and Lars Schmitz (Northwestern University), spent a year running simulations comparing many different types of fossil. The researchers noticed that there was a dramatic increase in fish eye size, the eyes tripled in size and they shifted from the sides of the head to the top of the skull. These adaptations took place long before the first modified fins looking like limbs evolved. The team speculate that before permanent life on land came about, certain types of fish became ambush predators, hunting terrestrial invertebrates using similar tactics to modern-day crocodiles. Eyes near the top of the head would have allowed these creatures to observe the land without giving themselves away. They could spot potential prey on the land. Being able to spot potential food provided a distinct advantage and as light travels better in air than it does in water, these fish could see a lot further. Natural selection led to the evolution of bigger eyes and better eyesight.
The “buena vista” (good view) hypothesis that the study proposes states that being able to see food opportunities far away provided a stimulus to drive the evolution of locomotion to reach the potential prey. This aided the natural selection for improved, stronger limbs, first for brief forays onto land to ambush prey and later for a life in a terrestrial environment.
The Evolution of Bigger, Better Eyes Capable of Seeing Further Preceded the Conquest of the Land
Video Credit: Northwestern State
Hunting on land was a mental game changer for the early Tetrapods and we applaud the team behind the research who have collaborated with animators to produce a very helpful explanatory video. The video written and produced by Kristin Pichaske with the collaboration of Malcolm MacIver is provided here and it is great to see research summarised in such a way so as to make it much more accessible. Our congratulations to everyone involved in the study and also to the team behind this most informative video.
For an article on the evolution of Tetrapod locomotion: Walking Fish Provides Clues to the First Tetrapods