Life expectancy is increasing in most countries and has exceeded 80 in several, as low-mortality nations continue to make progress in averting deaths. The bulk of the mortality reduction has occurred since 1900 and has been experienced by only about 4 of the roughly 8,000 human generations that have ever lived. Moreover, mortality improvement in humans is on par with or greater than the reductions in mortality in other species achieved by laboratory selection experiments and endocrine pathway mutations. Researchers say this observed plasticity in age-specific risk of death is at odds with conventional theories of ageing.
The study by researchers at the GermanMax Planck Institute for Demographic Research, published in the Proceedings of the National Academy of Sciences of the United States, investigated data at Swedish and Japanese men - - from two countries with the longest life expectancies today. It concluded that their counterparts in 1800 would have had lifespans that were closer to those of the earliest hunter-gatherer humans than they would to adult men in both countries today.
The primitive hunter gatherers, at age 30, had the same odds of dying as a modern Swedish or Japanese man would face at 72.
Why do we age?According to classical evolutionary theory, ageing evolved because the selective forces of nature diminish as age increases. Annette Baudisch and James W. Vaupel, institute scientists, suggest that we may have to find an alternative explanation to understand why we age, and they presented their argument in a November 2012 issue of 'Science,' a US magazine.
If decreasing selection pressure were the actual reason for the evolution of ageing, then all species should be equally affected by these declining evolutionary forces and mortality and fertility in all animals should display patterns over age that are typical for ageing, i.e. mortality should rise and fertility should fall. “The data paint a completely different picture,” said author Annette Baudisch. Although younger adult ages are generally subject to higher selection pressure than later ages for both humans and animals, there are species such as the desert tortoise that experience decreasing mortality over adult ages. For the freshwater polyp hydra, the risk of dying at any age is the same over the entire lifespan.
“We have to approach an explanation for the ageing process differently in order to close this gap between theory and data,” Baudisch said.
An example of an alternative explanation is the so-called “allocation theory,” typically applied in the field of life history biology as well as in economics. The theory assumes that, because resources are limited, it is important that they are used wisely. In economics these resources may include, for example, labour, capital, and raw materials. For a living organism, essential resources include water, light, and nutrients. The resources can be used to produce offspring that will preserve the animal’s genetic lineage, or they can be used to secure the organism’s own survival.
“Every organism has a difficult decision to make about how best to use these resources at each stage of life,” Baudisch said. In making these decisions, the organism faces conflicting goals. Where is it better to invest: in survival or in reproduction? Both factors are essential to “evolutionary fitness.”In most cases they cannot be maximized simultaneously. The consequences of these decisions can vary greatly from life form to life form. The freshwater polyp hydra, for example, can increase the number of offspring it produces without noticeably affecting its own chances of survival. But for other species, like for mammals, an increase in fertility is associated with substantial additional costs,which can affect not only the survival chances of each individual juvenile, but also the well-being of the adult animals.
“To better understand ageing, we first have to learn more about how conflicts between the goals of growth, survival, and reproduction play out among various animal and plant species,” Baudisch said. It is only by understanding these processes that we can one day determine which factors are responsible for the increase in mortality with age among humans, and which factors have contributed to the dramatic increase in human life expectancy over the past century.
Max Planck Institute for Demographic Research's summary of the research
Since the species evolved 200,000 years ago in Africa, there have been some 8,000 generations of Homo sapiens. Over the course of human evolution, life expectancy has risen considerably. The greatest "jump" in longevity in humans, however, has occurred in the last 100 years within only four generations: Since 1840, the life expectancy of a newborn baby in a Western industrialised nation has risen by approximately 3 months per year. This trend continues today and is surprisingly constant. There is currently no clear sign that this trend of increasing life expectancy has reached its upper limit. In the past, many studies on human mortality focused primarily on the economic and health-related consequences of an ageing society. Only rarely has the issue been discussed in a broad evolutionary context. "Yet it is precisely this which is so fascinating, especially for research into ageing," says Oskar Burger from the Max Planck Institute for Demographic Research in Rostock. In his study he was able to show how biologically unique the latest development in human mortality actually is.
"To carry out such a comparative study, the primary requirement is a ‘baseline’ with which you can compare the various stages of human mortality," explains Oskar Burger. "Ideally, this baseline should reflect the typical or average mortality pattern experienced by humans during their existence." By far the longest period of human existence has been spent as hunter-gatherers in small groups. "Thanks to anthropological fieldwork, we have a very good average mortality profile for hunter-gatherers living today, which we used as our standard," says Burger. The scientists compared this baseline with various human populations, including the populations of Sweden and Japan, both of which currently have particularly high life expectancy, as well as a group of slaves who lived in Trinidad in the 19th century, who had exceptionally low life expectancy.
As expected, the mortality of the slaves in all age groups was higher than that of the hunter-gatherers, who ranked somewhere in the middle. The mortality of Swedes in 1800 and 1900 was still very similar to that of the hunter-gatherers, while the modern populations of Sweden and Japan had by far the lowest mortality. "Yet the biggest surprise came when we included the data for wild chimpanzees in our analysis," says Oskar Burger. "The mortality profile of the hunter-gatherers was significantly closer to that of the chimpanzees than to that of the modern populations in Sweden and Japan." Thus, the risk of death for a Swede living today is over 100-times lower than for a hunter-gatherer, while the mortality of the same hunter-gatherer is only 10-times lower than that of a wild chimpanzee.
"These results indicate that just the last 100 years of modern development have caused the mortality rates of humans to drop more dramatically than they did during the evolution of a chimpanzee-like ancestor into Homo sapiens", states Oskar Burger.
The scientists also compared the profiles of humans with those of laboratory animals specifically bred to live longer using a whole variety of techniques such as dietary modification and genetic manipulation. What they discovered was that even when the evolution of these species was provoked artificially, very few of the attempts achieved a reduction in mortality per generation equal to or exceeding that seen in the most recent evolution of humans since 1900. "The reduction in human mortality over the past century really is biologically unique. No other species has experienced anything like it," says Oskar Burger. Genetic changes within only four generations can be virtually excluded as the cause of the drop, which is probably due almost solely to environmental factors such as improved nutrition and medical care. "We do believe, however, that other species could in principle achieve comparable reductions in mortality. In future we want to extend our research to other taxa, primarily primates and other mammals. This will enable us to better estimate the plasticity of human mortality and provide a biological perspective," says Burger.
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