The Claim And Criticisms

In October 2016, molecular geneticist Jan Vijg published a paper claiming that the human lifespan was limited to 115 years. This kindled a vigorous controversy among scientists, and on June 28 of this year, five groups of scientists published formal rebuttals to the claim.

Vijg's work analyzed demographic data from the 20th century, taken all over the world, and demonstrated that peak age plateaued at about 115 years starting in the mid-1990s. Based on their results, the authors concluded that the natural human age limit is 115 years old and that there is the probability of less than 1 in 10,000 of living to be more than 125 years old.

You could probably guess, not everyone in the scientific community agrees. Most criticisms arise from the way the Vijg team handled their data, and their process for drawing conclusions. First, the Vijg team tested their data to prove whether or not the plateau they felt they observed after 1995 was in fact present. In other words, they generated a hypothesis and then tested it using the same dataset, which is typically unacceptable, as it causes inaccurate results due to severe overfitting, a fit based on error or noise, not a real relationship.

Second, the team's actual data set was very small because in each year they counted only the oldest person who died. They then subjected this inordinately small sample to standard linear regression techniques, which was not appropriate based both on the small sample size, and the additional fact that the individuals being counted were outliers who should have been subject to extreme event analysis. In fact, the decline suggested in the 2016 conclusions appears to be suggested by a single death in the data set.

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Moreover, other scientists reanalyzed the data and found it consistent with multiple lifespan trajectories, not just the one reported in 2016. Finally, several scientists in their rebuttals point to the overall body of work on the biology of aging over the past few decades which suggests that the human lifespan has been far more flexible than previously believed; which alone indicates that the proposed limit should be viewed with extreme caution.

Takeaways From The Controversy

The authors of the original study stand by their work and disagree with the criticisms of the statistical methods used. Vijg also believes that the real cause of the outcry is not the data, which is convincing, but the fact that aging can't be stopped and there is a limit to human life: “I guess the main message is that a lot of people have difficulty accepting that everything now points toward an end in the increase of maximum lifespan,” Vijg told The Scientist.

University of Illinois at Chicago professor of public health Jay Olshansky, who was involved in neither the original study nor the rebuttals, thinks the criticizing scientists are missing the real point of the 2016 study, which he clarified for The Scientist: “The most important message to get across, in my view, is that we should not be trying to make ourselves live longer, we should only be trying to extend the period of healthy life.”

However, there are many others pushing the limits of human longevity right now who disagree strongly enough to put their money where their philosophy is. Since research has demonstrated that transfusions of younger blood, or parabiosis, was able to rehabilitate cognitive abilities in mice, a startup called Ambrosia has started to offer a human clinical trial of parabiosis for paying clients. Peter Diamandis of the genotype research facility, Human Longevity, Inc., is searching for the key to using nanomachines or stem cells to regenerate our bodies. And Metformin, which has been shown to prevent cancer and extend life in animals, began clinical testing as an anti-aging drug in February.

There are so many possibilities in motion that it does seem hard to agree with a firm limit to the human lifespan. In the end, time will resolve the controversy once and for all. Ironically, we'll only be here to see it if the critics are right.

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