About 225 million years ago, at the very dawn of mammalian existence, the creatures that humans and all other extant mammals would descend from were diminutive creatures about the size of a shrew. This handicap on size placed them squarely on the dinner plate of dinosaurs, and many other larger creatures that lived on our planet at the time. It may also be the reason that their ancestors, including humans, don’t live longer than we do. “The hypothesis is that mammals spent 100 million years during the time of the dinosaurs at the bottom of the food chain,” says Joao Pedro de Magalhaes, a molecular biogerontologist at the University of Birmingham who recently published an essay on the reasons for mammalian aging. During this extended period of evolution, it’s possible that mammals evolved out of many of the anti-aging traits that some reptiles, birds, and fish retain today — there’s no point in ensuring a healthy genetic life if you are just going to get eaten before you reach a couple of years of age. Read More: A Complete Dinosaur Timeline to Extinction: How Long Did They Roam Earth?
De Magalhaes has always been interested in learning why some animals seem to live so much longer than others. He worked with a team to sequence the genome of bowhead whales— one of the mammals with the longest known lifespans — to find genetic clues related to aging. He also keeps a database of the known lifespans of more than 4,000 species— perhaps the largest database of its kind. Mostly what he researches is senescence — the process of deterioration with age — rather than calendar years in many of these cases. The chances of death for humans doubles every 6 to 7 years in adulthood, and for many mammals, this senescence rate is even higher. De Magalhaes was fascinated to read a 2022 study published in Science that compared the aging of 77 species of reptiles and amphibians. The research found that many turtles, crocodilians, and salamanders have very low aging rates. For turtles, the authors attributed some of this to the hard shells of the long-lived species, as well as their slow pace of life. Read More: Negligible Senescence: Why Do Some Animals Age Differently?
Conversely, most mammals have a much higher rate of senescence. Scientists haven’t learned all the answers to why most mammals haven’t evolved to resist aging the way that some reptiles and fish do. But one factor is likely related to teeth. Whereas humans only replace a full set of teeth once in their lives (when we lose our baby teeth), creatures like sharks replace teeth continuously. Before the advent of fillings and dentures, tooth infections were a common source of mortality. » …
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