Birthdays are good

By: Katelynn Wilton

Posted: 10/26/09

Last Friday, Professor Sherratt of Carleton University, presented on "The Evolution of Ageing" (yes, that is aging, spelled correctly, he claimed, from the other side of the river.) His talk, held at 4:00 p.m. in Science Center 213 presented research into the cause of aging. The work was done in collaboration with Dr. Rob Laird, who now works at Lethbridge University.

Dr. Sherratt began with a description of aging. Not only is it the physiological breakdown of the body with age, but it also entails the increased chance of mortality with increased age, along with a decreased fitness for reproduction. He explained to the audience that the chance of each person in the room dying approximately doubles every eight years, despite the age.

Dr. Sherratt then discussed aging as seen in other species. He explained that other animals also age. In particular, he showed trends in the damselfly and antler fly indicate senescence, which is normally considered a sign of age at the cell level. He also discussed how data can even be collected to show that the tyrannosaur showed signs of aging. In addition, bacteria also showed decreased ability to reproduce as they age.

However, although it seems like a grim topic, there is an upside. Dr. Sherratt described the "mortality plateaus" which show that although aging gets bad, and an individual's chance of death becomes high, but once it reaches a certain point, it does not get any worse. Humans display this pattern, particularly in incidences of cancer.

The question then became "why do we age?" Dr. Sherratt shared several modern theories on this question with the Clarkson audience. The first was named the "shit happens" theory. This theory indicates that various wear and tear occurs on the body, and eventually it wears out. Aging is just a sign of this damage to the body. However, Dr. Sherratt showed that it was not that simple. If that were true, then what would explain the different aging rates among and within species? Also, there is a heritable basis for variation, and long life can be selected for in some organisms. In addition, as his twins highlighted, many twin studies have corroborated this.

Dr. Sherratt then showed us the ultimate truth: a birthday card for a ninety year old showed on the screen. It read, "Birthdays are good for you. Statistics show that those who have the most live longest." So, this explains everything.

However, Dr. Sherratt continued on another topic, Wallace's theory that organisms age and die to make room for the next generations. This "group selectionist" theory, however, is not backed by specific genes that create this programmed death.

Sir Peter Medawar proposed the mutation accumulation theory, which proposes that genes with late acting effects are not selected for because at that point, there are so few organisms left, that natural selection is less effective. This benign neglect results from natural selection ignoring the effects that genes may have on older individuals in order to focus on the more important genes needed in younger life in order for the organism to get to the point where the late in life genes will even matter. As an example, Dr. Sherratt described a scenario in which you could have two kids per year for two years, or one kid per year over five years. The most beneficial, was not as obvious as one would think. Given two offspring per year, each offspring could then mate the following year, and the original parent would have spread their genetic material further. However, there are limited genes available to support this theory and it does not explain the mortality plateau seen in different species.

Dr. Sherratt then began to describe the theory closest to his own. Reliability theory involves describing elements as vital to a process, and redundancy is built in order to extend life. Although this is originally an engineering problem, it applies well to the aging of organisms as well. However, Dr. Sherratt altered it to include evolutionary advantages, to create a new model of aging. In his model, you can assume that the amount of redundancy of any element is heritable. He explained then how the number of redundancies could be determined by natural selection. An organism will have no advantage if it has more redundancy that it will never use, so each organism will, on average, end up with an amount of redundancy that should not normally result in the vital function failing.

However, he did state the portions of the body did not all age at the same rate. Some organs and systems age more rapidly, and are much more likely to be the cause of death.