Over a hundred million years ago, in the Mesozoic Era, a small early mammal dashed through the undergrowth, running for her life from a dinosaur.

She zig-zagged through ferns and scrambled over fallen branches, desperately trying to shake the predator and make it back to the safety of her burrow where her young were waiting.

Sadly, she didn’t make it.

Like basically all early mammals, she was pretty low on the food chain.

And a peaceful death at the end of a long life was a rare luxury for our ancestors and close relatives in the age of reptiles.

Most of the time, their lives were cut tragically short, with their final moments spent as a dinosaur’s snack.

This game of cat and mouse or proto-mouse, I suppose played out countless times every day, all over the Mesozoic world, for well over a hundred million years.

And it’s possible that the legacy of this era, when mammals lived fast and died young, still affects us now having been carved into our very DNA.

Even though the dinosaurs that once hunted us are long gone, we still, in a sense, may be living in their shadows Because, it turns out, those terrible lizards might be the reason that we age faster than any other vertebrate group.

The process of aging can often seem like an inevitable law of biology.

As we get older, our bodies begin to deteriorate, our fertility declines, and, eventually, we die.

But while we take it for granted, from an evolutionary perspective, aging is actually something of a paradox.

Like, why would a process like this evolve in the first place, seeing as it is so detrimental to survival and reproduction, and comes with no obvious upsides?

You’d think that natural selection would have solved it by now, right?

Over 2000 years ago, the Roman poet and philosopher Lucretius argued that aging brings one key benefit that makes it a necessary feature of life: it makes way for future generations.

Space and resources are limited, after all, and if there wasn’t a mechanism to eliminate older individuals, there’d be no room for younger ones.

And this idea that aging doesn’t exist for the good of the individual, but instead for the good of the group remained popular for a long time.

As we learned about how evolution actually works, though, it became clear that this idea was missing something.

You see, natural selection doesn’t act in the long-term interest of the group.

It acts only on individuals, and their relative likelihood of passing on their genes.

And it wasn’t until the mid-20th century that a trio of scientists were finally able to explain the enigma of aging in evolutionary terms.

They realized that the power of natural selection fades over the course of an individual's lifetime.

Harmful mutations that kick in early in an individual’s life and reduce their chance of surviving to reproductive age are quickly weeded out by natural selection.

But harmful mutations that only manifest later in life, after individuals have already started reproducing, are much less visible to selection.

Since those mutations have already been passed on to the next generation, it’s too late for selection to effectively filter them out.

This is especially true of species that tend to die of things like predation, starvation, or disease before they even have a chance to get old.

In those cases, there’s even less selection pressure to weed out the harmful mutations that kick in later in life because it’s not normal for individuals to survive that long in the first place.

So a big part of aging is the accumulation of these late-acting mutations that are hidden from natural selection.

But even though we’ve understood the basic evolutionary theory behind aging for the best part of a century now, at least one deeply personal mystery has remained Why do we mammals do it so differently?

See, in recent years, scientists have noticed that, in other vertebrate groups like reptiles, fish, and amphibians, aging doesn't always happen in the same way, or at the same rate.

Some species can regenerate damaged limbs, teeth, and tissues.

Some lay just as many or even more eggs when they’re older versus when they’re younger.

And some age so slowly that there’s really no obvious sign that they’re aging at all.

In contrast, mammals basically all age rapidly and markedly.

Our bodies deteriorate in very clear ways: we become frail and less able to reproduce successfully, and we experience cognitive decline, cancer, and tooth erosion.

And even in mammals that have long lifespans, we still clearly age along the way.

Those other non-mammal species seem to have protective mechanisms against aging that we mammals lack or, that we’ve lost.

But why, and, as always, how?

One idea was that our aging is a result of us having a higher body temperature than reptiles and amphibians.

Because, while this higher temperature comes with some advantages, it also leads to more wear-and-tear on our cells that eventually catches up with us as we get older.

But there’s a problem with this idea.

Birds have high body temperatures, too.

Yet they have longer lifespans for their size than us mammals.

So body temperature alone can't be the whole story.

Then, in 2023, a researcher proposed a radical new hypothesis to explain why we’re cursed with such unusually rapid and pronounced aging A hypothesis that ties in evolutionary theory with our unique natural history And it lays the blame on perhaps our first ecological enemies, from far back in deep time: the dinosaurs.

See, as we explained, the evolution of aging is shaped mainly by things like when species begin reproducing, and how long they’re likely to survive before something in their environment kills them.

And the Longevity Bottleneck Hypothesis, as it’s known, essentially proposes that for our first 100 million years-plus, we mammals lived fast and died young.

Those early mammals from the Mesozoic Era the age of reptiles were mostly small, shrew-like nocturnal insectivores that lived in the shadows of the dinosaurs that preyed on them.

We mammals spent the first two-thirds of our history this way: as short-lived prey in a dangerous world.

And the Longevity Bottleneck hypothesis argues that spending so much of our history like this changed how we age, in ways that we’re still constrained by today.

For one, spending over a hundred million years being forced to reproduce early, and generally dying pretty young, meant that a lot of aging-related mutations could accumulate that were invisible to natural selection.

Plus, there was very little reason to keep any of the regeneration and repair tricks that we may have once had, that are still used by other vertebrate species today.

After all, if we were rarely surviving long enough to get old, any genes and genetic pathways we had to keep us healthy and fertile into old age just weren’t maintained.

When random mutations disrupted them, there wasn’t much pressure to restore them.

And basically all Mesozoic mammals went through this bottleneck, while not all fish, reptiles, and amphibians did.

Which potentially explains why there’s a lot more diversity in how those groups age, and why some age so much more gracefully than we do.

Now, it's important to note that while the Longevity Bottleneck Hypothesis is plausible, it’s still very new and very speculative.

But there are some pieces of evidence that seem to support it.

For example, we know that, sometime during the Mesozoic, our direct mammal ancestors lost a DNA repair mechanism that’s found across the rest of the tree of life, from bacteria, to plants, to vertebrates.

Specifically, it repairs damage to DNA caused by exposure to ultraviolet radiation from the sun.

And the dinosaurs forced mammals to be active mostly at night where exposure to UV radiation was quite low, while also usually killing us before the effects of gradual DNA damage could start causing serious problems.

So, in theory, because of this, there was no need to keep this repair mechanism.

And if the longevity bottleneck hypothesis is correct, we should expect to find similar examples of lost repair and regeneration tricks in mammals as we dig deeper and deeper into our genomes.

Plus, on the fossil side of things, we also see tantalizing evidence that is consistent with the idea.

Because, right after the non-avian dinosaurs went extinct, mammals were finally able to diversify into larger forms that no longer had to live in constant fear of predation.

Like the pantodonts for example, who show up around 65 million years ago.

They were the first known mammals to get big, reaching up to 42 kilograms in size as the planet recovered from the asteroid impact.

Despite their size and relative safety in their environment, analysis of their bones and teeth shows that they still lived and died way faster than expected for their size.

This suggests that this aging pattern was a potential holdover from having only recently evolved from small, short-lived ancestors in the Mesozoic.

And since then, mammals have continued to diversify and occupy all sorts of niches in all sorts of environments, and some species like us even have lifespans that can reach or exceed a century.

I'm halfway there.

Yet, none of us can resist the process of aging in the same way that many other non-mammals can.

While the Longevity Bottleneck Hypothesis is still speculative for now, it stems from the undeniable fact that we’re shaped by long-gone ecosystems, predators, and lifestyles, in ways that still manifest today.

Evolutionary constraints can last far longer than the pressures that created them.

And the way we age today may well be the result of one simple fact You can take the mammal out of the Mesozoic, but you can't take the Mesozoic out of the mammal.