Sensory Capacity

Seeing and hearing underwater is markedly different from seeing and hearing on land. Anybody who has swum underwater knows this. Humans' eyes and ears are not optimized for seeing or hearing underwater. The early tetrapods faced the opposite challenge. To survive effectively on land, tetrapods had to develop changes to the way they saw and heard the world outside of the aquatic domain of their fish ancestors.

In reptiles and amphibians, hearing is reliant on a middle--ear bone called the stapes. This tiny bone amplifies vibrations from the eardrum, or tympanic membrane, and transmits them to the inner ear, where the brain can detect them. Fossils of the earliest tetrapods and of closely related lobe-finned fishes such as Eusthenopteron and Panderichthys provide evidence of the transformation of this bone from fish ancestors. The most likely scenario for the change is that the stapes evolved as a bony modification of a jaw-hinging structure around the same time that fins were beginning to transform into limbs with digits. By the time of the eight-fingered Acanthostega, about 20 million years later, this tiny skull bone had acquired the functionality of a primitive terrestrial ear. It probably was sensitive only to lower-frequency sounds, however, because of its bulky design and lack of a tympanic membrane of appreciable size. The early mammals were the first to evolve a larger eardrum that was good for hearing high-frequency sounds carried by the air.

The evolution of terrestrial vision most likely occurred in the Late Devonian, at about the same time that hearing began to adjust to air and tetrapods improved their ability to walk on land. The earliest tetrapods most likely spent much of their time in shallow streams and ponds, resting partially underwater much of the time. Changes took place over many generations of these animals to improve their ability to see, hear, breathe, and protect themselves from desiccation. It can be seen in the fossil record that the eyes of early tetrapods not only became larger than the eyes of their fish ancestors, but also began to migrate to the dorsal surface of the head. This gave the tetrapod the ability to lie on the bottom in shallow water while surveying for prey, and to look up out of the water while only partially exposing itself. Over millions of years, the lenses of the eyes modified ever so gradually to become optimized for seeing in the air. This probably occurred by the Late Devonian and coincided with a suite of biological improvements to locomotion, the senses, respiration, and water retainment that made tetrapods better equipped to remain outside of the water for longer periods of time.

There is little fossil evidence related to the sense of smell of the early tetrapods. One difference between fish and tetrapods is that a fish uses the nose only to detect smells, while a tetrapod uses the nose for breathing as well as for smelling. Available fossils show no significant difference between the nostrils of lobe-finned fishes and the nostrils of the earliest tetrapods. This leads to an assumption that the nose was used only for smelling. As Jennifer Clack points out, however, sampling the air for smells was probably a first step toward inhaling air for breathing.

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