Humans owe their relatively long life span to living in societies that reduce the risk of extrinsic mortality. Other organisms in organized societies are also expected to exhibit a similar lengthening of life span over evolutionary time. One hundred million years before the first human stood up and walked, social insects existed in societies with cities, roads, division of labor, farming, slave making, and organized group defenses (Holldobler and Wilson, 1990). Sociality has resulted in a 10- to 100-fold increase in the life span of queens in ants, bees, and termites, a trend that was rigorously demonstrated using phylogenetic methods to compare life span and social structure across the insects (Figure 24.1; Keller and Genoud, 1997). The evolution of sociality and its associated increase in life span show a general trend that has independently evolved several times.
Ants represent an ideal system for studying how evolution effects the changes necessary for long life. In addition to the differences in life span among species, there are also order-of-magnitude differences in life span between different castes in a single species. For example, queens of the ant Lasius niger have been known to live nearly 29 years in a lab, while workers live for only a few years and males a few weeks (Kutter and Stumper, 1969; Holldobler and Wilson, 1990). Given that the same egg can become either a queen or a worker, differential gene expression seems to be the key difference between those two castes.
The traditional model systems used to study aging (Drosophila melanogaster, Caenborabditis elegans, Mus musculus and Saccharomycies cerivisiae) all share a short generation time, making them ideal experimental systems in terms of laboratory rearing and experimental manipulations. Unfortunately, this has introduced a bias in the types of life histories that have been sampled in modern aging research as almost all the mechanisms proposed for aging have been limited to these systems. Some of these mechanisms are not thoroughly understood in an evolutionary context and may not truly represent adaptations necessary for long life. Hence, these mechanisms need verification in long-lived species like humans. In addition, studies of long-lived species such as ants may also reveal new and novel life span extending processes and mechanisms that have withstood the test of evolutionary time.
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