Why have humans not evolved longer life? Like the turtles, trees, etc.

Hi John, I'm afraid that no one knows the answer for sure. But, there are two ways of looking at possible explanations

First, it could be that a longer life was not necessary for survival. In fact, it could be a detriment as a longer life means more competition between parents and offspring. Also, a longer life is usually associated with slower breeding, which is a serious problem for future adaptation; if a population cannot breed fast enough to adapt to a new challenge, it will go extinct. Old animals dying out and allowing the next generation to take over is how evolution happens, so if you slow this process you slow evolution.

Second, it could be that it simply never happened by chance. Although evolution is not random, it can only work with what is available at the time, so it is possible that extended longevity was just never "discovered" in humans. For example, our mammalian physiology is very different from turtles or trees, meaning that the kind of longevity you are talking about may not even be possible.

Having said that, humans in industrialised nations have a life expectancy of about 2.5x the span of humans who lived 10,000 years ago! It could be said that the evolution of intelligence, which resulted in advances in science/medicine/engineering, has indirectly resulted in a huge gain in human life expectancy.

Well, I think the whole premise of this question os mistaken. Humans are very long lived -- that's why we say "dog years" are worth seven human years: because we live seven times as long as they do!

Part of the reason we're so long lived is because of medical science: as John pointed out, our intelligence has allowed us to change the game, and factors that were killers even a generation are no longer problems in most of the world. For example, polio was a major cause of infant mortality in our grandparents' generation, but hardly an issue at all now.

But I don't think our long lifespans are merely caused by elongation. We also breed very late in life compared with other animals: for example, dogs breed at one or two years of age, cows within a similar range. In effect, we've stretched out our whole lives.