October 16, 2012
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.