Compared with other animals, humans have a very unusual pattern to our lives. We take a very long time to grow up, we are long-lived, and most of us stop reproducing halfway through our life span. A few other species have some elements of this pattern, but only humans have distorted the course of their lives in such a dramatic way. Most of that distortion is caused by the evolution of middle age, which adds two decades that most other animals simply do not get.
An important clue that middle age isn’t just the start of a downward spiral is that it does not bear the hallmarks of general, passive decline. Most body systems deteriorate very little during this stage of life. Those that do, deteriorate in ways that are very distinctive, are rarely seen in other species and are often abrupt.
For example, our ability to focus on nearby objects declines in a predictable way: Farsightedness is rare at 35 but universal at 50. Skin elasticity also decreases reliably and often surprisingly abruptly in early middle age. Patterns of fat deposition change in predictable, stereotyped ways. Other systems, notably cognition, barely change.
Each of these changes can be explained in evolutionary terms. In general, it makes sense to invest in the repair and maintenance only of body systems that deliver an immediate fitness benefit — that is, those that help to propagate your genes. As people get older, they no longer need spectacular visual acuity or mate-attracting, unblemished skin. Yet they
do need their brains, and that is why we still invest heavily in them during middle age.
As for fat — that wonderfully efficient energy store that saved the lives of many of our hard-pressed ancestors — its role changes when we are no longer gearing up to produce offspring, especially in women. As the years pass, less fat is stored in depots ready to meet the demands of reproduction — the breasts, hips and thighs — or under the skin, where it gives a smooth, youthful appearance. Once our babymaking days are over, fat is stored in larger quantities and also stored more centrally, where it is easiest to carry about. That way, if times get tough we can use it for our own survival, thus freeing up food for our younger relatives.
These changes strongly suggest that middle age is a controlled and preprogrammed process not of decline but of development.