Evolution and aging: do the dinosaurs have anything to do with human senescence?
How senescence evolves
Aging is a fact of life. Any human old enough to pay attention to people around them is familiar with this reality. But it is most salient to those of us over 40. We can list things that have changed since our youth: less flexibility, slower reactions, slower healing, and various things sagging. But the single most important measure of aging is that older individuals have a higher probability of dying. This last point provides one definition of senescence.
We’ll get to the dinosaurs in a bit, but first let’s think about how senescence evolves. Biologists have examined it in many species. To do this, they look at how an individual’s probability of dying changes over time. With small, short lived species like fruit flies, this can be done in the laboratory. For other species, it has to be done with data from zoos or from the wild. One thing biologists have found is that animal species vary widely in how long they live. Fruit flies live a matter of weeks. There are also species such as the Greenland shark which can live hundreds of years. What explains these differences?
The first basic fact underlying senescence is that animals (and other multicellular organisms) must invest resources to maintain their bodies. All organisms do some maintenance. Senescence reflects the fact that they do not do enough to keep their bodies “as good as new”.
The second basic fact underlying senescence is that the world is a dangerous place. No matter how much maintenance an animal does on its body, it may nevertheless die due to external factors. Imagine an antelope on some savannah in Africa. It can invest all it wants in keeping its body youthful and healthy and, nevertheless be killed by a lion.
These two facts bring us to a tradeoff in terms of how senescence evolves in different species. Evolutionarily speaking the goal of every organism is to have as many descendants as possible in future generations. The question is how to do that. One evolutionary strategy is for an organism to invest a lot in keeping its body youthful and healthy and thus have a long life. Spending its resources in this way, it will reproduce less in the short term. But if it has a long life, it will more than make up for this with future reproduction. Another evolutionary strategy is for the organism to forgo most maintenance on its body, and instead use its resources to reproduce as much as possible immediately. This will result in more offspring now, but also a short life and no offspring in the future. Some animals have evolved to follow the first strategy, others the second. And many take strategies that fall somewhere in between.
An example of a species that falls in between is the wildebeest, a large African antelope. This animal weighs hundreds of pounds but only lives about 20 years. Living 20 years clearly requires some upkeep and maintenance of the body. However, the animal doesn’t live nearly as long as a human which is much smaller. This reflects the dangerous environment the wildebeest lives in. Given that it has a relatively high probability of dying due to predation, the wildebeest has evolved to forgo some maintenance on its body, and instead invests these resources in having more offspring right away.
This principle of a tradeoff between reproduction and maintenance is useful in explaining how animals age. For example, it has been observed that animals which have some special protection against predation tend to live longer. Examples of such animals are turtles with their shells, or salamanders with poisonous skin. This makes sense in terms of the tradeoff principle. An animal with protection against predation is less likely to die due to external factors. For this reason, it makes sense for it to invest in the maintenance of its body, delaying senescence.
The fundamental association between longevity and body size is also related to this idea. Having a large body is one sort of protection against predators. A field mouse needs to worry a lot more about predation than an elephant. Beyond this, building a big body is also a sort of investment, and energy put into this is energy not spent on immediate reproduction. Evolution will only build such a body if the animal can live a long life and reproduce many times in the future.
Having taken this digression, let’s now get back to the dinosaurs.
A recent paper speculated that our evolutionary history might have a big effect on how mammals, including humans, age. The paper argued that the longest-lived vertebrates do not include mammals. (It is certainly true that as far as we know, there is no mammal as long-lived as a Greenland shark). The question is, why?
The paper proposed something called the longevity bottleneck hypothesis as an explanation. One of the figures in the paper is helpful in explaining the idea:
This figure is a phylogenetic tree illustrating the history of reptiles, birds and mammals. The root of the tree is on the left, and represents the common ancestor of these groups, an animal which existed about 300 million years ago. As we move right in the diagram, we go forward in time, with different lineages branching off from each other. The first split is between synapsids (ancestors of mammals) and sauropsids (ancestors of birds and reptiles). As we go forward in time, these lineages undergo further splits until we eventually arrive at the right of the figure and living animals.
Consider the lineage leading to mammals (shown on top). One distinctive thing about this lineage is that there was a long period between about 200 and 65 million years ago, when its animals were all small. This is because dinosaurs (part of the sauropsid lineage on the bottom) were the dominant large land animals. Mammals were forced to be small and nocturnal. And almost certainly short lived.
The speculation is that during this long period of small, short-lived animals, our ancestors lost genes and cellular mechanisms that enable very long life. With the extinction of the dinosaurs 65 million years ago, mammals were able to become large again. They also evolved somewhat longer lifespans. However, according to this hypothesis, the loss of certain genes during the reign of dinosaurs constrained how long-lived they could be. And the idea is that this evolutionary history still constrains humans today.
This is currently just a hypothesis. Biologists will have to look for evidence — for example by identifying mechanisms for aging that were lost during the time our ancestors were small. (In principle, such things can be identified by comparing living species).
My own opinion is that while this is an interesting hypothesis (I always like ideas based on history), there are some reasons to be skeptical.
First, it seems to me that the 65 million years since dinosaurs went extinct is a very long time. If mammals with 300-year life spans were evolutionarily advantageous, I think they would have evolved during that long span of time. After all, 65 million years was enough time to go from a shrew-like ancestor to things as different as blue whales and humans.
Beyond this, I’m not so sure that there aren’t mammals with very slow aging. Many researchers think naked mole rats fit this bill. These rodents live underground, are about the size of mice, and live for 30 or more years. While 30 years isn’t much compared to a Greenland shark, it is quite a long time for such a small animal.
It is worth saying, though, that even if the longevity bottleneck hypothesis is not correct, it nevertheless fulfills an important role in the scientific process. It will motivate future work — for example, looking for genes and other cellular mechanisms that mammals may have lost during the reign of dinosaurs — and in that way, it will contribute to a better understanding of aging.
