Wish You Could Live As Long As A Tortoise? Blame The Dinosaurs

Thanks to science, we now have a reasonably good understanding of how we age. But why do we age? Why has evolution left us with a body that will start to fail after 80 years or so? After all, living longer would give us more opportunities to produce offspring, which is what evolution is all about. This evolutionary strategy seems to have worked for long-lived organisms like bowhead whales and giant tortoises, which may live upwards of 200 years. It’s not just a question of lifespan either. Research shows that in some animals, the biological processes responsible for ageing progress very slowly and may even be switched off entirely in some species. Mammals in particular seem to have gotten the short straw – we tend to age faster, have less capacity to regenerate our tissues, and may also get more cancer when compared to reptiles and amphibians. 

So why has evolution led us mammals down a different path? Since we can’t go back in time, we will probably never know exactly how we got here, but the dinosaurs may have had something to do with it – the scary ones, specifically. You see, when weighing up the evolutionary advantages of a long lifespan, it’s important to realise that such an evolutionary trait isn’t free. Maintaining and repairing DNA, recycling damaged cells and other processes needed to prevent ageing all cost a lot of energy. For most species, energy is a scarce and valuable resource that can only be obtained from food. This means that if an organism ages slowly, it’s going to have less energy to do other things, including reproduce. 

This trade-off is pretty readily observable – species with shorter lifespans tend to reproduce more (with some exceptions). The downside to this is that organisms from such a species must start reproducing as soon as they are able to, or they will miss their chance. This means that if environmental conditions become unfavourable for even a short period of time, the organism may be unable to pass its genes on. Long-lived organisms, on the other hand, can just wait it out.

OK, so where do the dinosaurs come in? Since long-lived species are playing the evolutionary long game, it is essential that members of these species do not simply die young anyway. After all, the fact that you can live 300 years won’t help you pass on your genes if a tropical disease kills you at age 20. If that happens, then your longevity has actually been a hinderance. You wasted a lot of energy to delay the ageing process when you could have spent that energy to produce more offspring instead. It is likely that when an organism is at great risk of being killed before reaching the end of its natural lifespan, this places a selective pressure on it to reproduce faster at the cost of longevity. One theory as to why humans don’t live longer than they do is that our evolutionary ancestors frequently met an early death. What could have posed such a threat to our ancestors that it left a lasting impact on our longevity?

This paper by João Pedro de Magalhães proposes that dinosaurs could have played a major role in the evolution of mammalian lifespan – a hypothesis termed the ‘longevity bottleneck’. Mammals diverged from reptiles somewhere between 200 and 250 million years ago. When they first emerged, they were small and nocturnal, and coexisted with dinosaurs for over 100 million years. João Pedro proposes that early mammals initially shared reptilian genes necessary for long lifespans. However, these genes were lost during this long period of predation, since they would not have conferred an evolutionary advantage in such a dangerous environment. After the extinction of the dinosaurs, certain mammals evolved to have longer lifespans once again, but under the constraint of having lost reptilian longevity genes millions of years earlier.

The ‘longevity bottleneck’ hypothesis is not the first to propose that dinosaurs had a drastic impact on our evolution. There’s also a ‘nocturnal bottleneck’ hypothesis, which proposes that during the reign of the dinosaurs, all mammals evolved to become nocturnal, and that evidence of this exists within the biology of diurnal (daytime) mammals today. For example, there’s genetic evidence that mammals lost their photolyase enzymes, which repair damage caused by ultraviolet light, during the time of the dinosaurs.

Of course, the dinosaurs are not the only factor that could have pressured mammals into ageing more rapidly. As João Pedro mentions, body temperature could also be an important factor. Mammals are warm blooded, which confers the evolutionary advantage of being able to control body temperature without relying on environmental conditions. However, there’s some evidence that warmer body temperatures also result in shorter lifespans. Ultimately, we can only know for certain that a trait did evolve, not exactly how or why. Still, it’s fascinating to speculate how even though mammals now rule the Earth, the tyranny of the dinosaurs may still be reflected in our DNA.