Evolution Favors Asymmetry under Some Conditions

A quick glance in the mirror and equally fleeting views of other vertebrates suggest that bilateral symmetry of the body is commonplace among animals. But a closer look reveals important exceptions. Among flatfishes—including such well-known edible fish as sole, flounder, and halibut—there are startling examples of body asymmetry that also include the brain (Rao and Finger, 1984).

In adult flatfishes (Figure 1A), both eyes are on the same side of the head (Picasso wasn’t the first to think of this arrangement). This structural oddity emerges during development. When the fish hatches from an egg, it has an ordinary fishlike symmetrical form, but as it develops, one eye migrates across the top of the head (see Figure 1B). Some flatfishes are right-eyed, with both eyes on the right side of the head; others are left-eyed. Most flatfishes spend a large part of their time lying on the bottom of the sea, often covered with a light layer of sand, with only their eyes and nostrils exposed. When prey come near, the flatfish darts out to capture it. Having both eyes on the same side allows the fish to remain inconspicuous while retaining full vision to look for prey.

All flatfishes belong to the order Pleuronectiformes, and all fish in this order are asymmetrical as adults. Although both eyes are on one side of the head in these animals, the visual regions within the brain are symmetrical. However, the olfactory system in some flatfishes reveals a striking brain anatomical asymmetry (see Figure 1C). The right nostril lies above the left, and the right olfactory receptor and pathways, including the olfactory regions of the brain, are distinctly larger than their counterparts on the left.

As we’ve seen, human language is heavily represented in the left hemisphere in most people and that structural asymmetry of the planum temporale may be related to this asymmetry. Structural asymmetry is evident in the brains of other nonhuman vertebrates, although it is not as pronounced as in flatfishes. The right hippocampus is larger than the left in male rats. Nonhuman primates show anatomical asymmetries in the region of the temporal lobe analogous to those found in humans (Gannon et al., 1998). Functional studies in nonhuman vertebrates reveal many examples of lateralization, including the effects of lesions of the brain. Earlier we saw that a surgical cut of the left twelfth cranial nerve in an adult male bird results in the loss of song, whereas damage to the right side has little impact on the bird’s vocal behavior (Nottebohm, 1981). These examples support the notion that lateral specialization confers an important advantage, but in most instances the advantages are not as easy to understand as in the flatfish.

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