Last scene of all,
That ends this strange eventful history,
Is second childishness and mere oblivion;
Sans teeth, sans eyes, sans taste, sans every thing.
So ends Lord Jacques's soliloquy “All the World's a Stage,” summing up human life history from infancy and childhood to toothless old age and death. In Chapter 4, we saw that some australopith species may have had juvenile growth periods shorter than or as long as those of some modern apes, but none were quite equal to modern humans in this regard.
Anthropologist Barry Bogin defined human childhood as the period between weaning and puberty during which offspring continue to depend on others for survival (180). For fossil hominins, the existence of childhood as so defined would be hard to determine. Applying Bogin's definition would require information about ages at weaning, which are difficult to acquire in fossil hominins, though not impossible (see Chapter 10). It would also be necessary to ascertain the length of time over which growing juvenile hominins actually depended on others for survival, and that would be even more difficult to determine. Nevertheless, through the study of teeth, the extension of juvenile growth periods, if not childhood as Bogin defined it, can be traced over the evolutionary history of the genus Homo. So too can the evolution of “second childishness” – old age.
In this chapter, we will see that within the genus Homo, changes in juvenile growth periods and longevity parallel each other in direction, though it is not clear that these changes in growth periods and longevity occurred in lockstep with each other. A common trajectory toward lengthening both juvenile growth periods and lifespans, but with possible differences in their rates of change, is consistent with the concept of “modular” life history evolution (186). The concept of “modularity” as applied to life history recognizes that different aspects of a species life history can respond to selection independently to some degree. This would be the case if the genes and developmental pathways that affect length of juvenile growth periods, for example, are not the same as those affecting longevity – or at least if not all of those genes and developmental pathways are the same.