Our bodies have evolved for a single purpose: reproduce. The human body has been shaped by evolution throughout millions of years and is the product of this remarkably long history. While we don’t need to retrace the full story of human evolution, understanding its fundamental mechanics is crucial. Health is intrinsically linked to these mechanisms: our bodies evolved to maximize our reproductive abilities, which are, in many ways, impacted by how healthy we are.
Though more complex in details, evolution centers around the simple concept of natural selection. Let’s break this down.
The Biological Imperative
The biology of all living species is designed for a single goal: to maximize the number of offspring who, in turn, reach reproductive age and have offspring of their own. As much as we’d like to think otherwise, biologically, we are single-minded towards this goal. Everything else–happiness, health, feelings–is harnessed as means to get us to behave in ways that maximize our chance of success.
Humans need to grow old strong & healthy
The bad news for us is that our offspring need a long time to grow into individuals able to reproduce. In addition, they require a lot of care and attention to shield them from threats and grow them into mature specimens. This need for extended care is more significant than what parents could provide 300,000 years ago.
Enter the grandparents: they take care of the children while parents gather resources for the tribe. Since early humans reproduced from about 16 years old all the way to about 30 years old, grandparents are needed for an extended period. What’s more, tribes were constantly on the move, so physically limited grandparents were of dubious utility.
A specie needing older generations string & healthy is far from universal in the animal kingdom, so we can thank our evolutionary path for it. Our bodies evolved to grow healthy and then maintain their health and physical capabilities well into old age.
How inheritable traits are transmitted
Since raising offspring who themselves reproduce is the common goal of each animal (and plant) species, they compete for access to the resources enabling them to achieve this goal: primarily energy (food), shelter, and so on. Most animals spend their days simply trying to get enough energy to survive another day. Some species coexist peacefully because they do not compete for the same resources. Others do compete, and some become the favored energy source of others. Over time, one or more species might go extinct. Conversely, new species appear from time to time by branching out of other species.
What’s driving this game of appearance and disappearance at the species level? Inheritable traits.
This is the final fact that makes natural selection a reality (and the main driver of evolution): each generation transmits a set of inheritable traits to its offspring. Over time, not all traits are successfully passed down to the next generation. Inevitably, some offspring die before they reach reproductive age, either because they cannot get enough energy to survive or because they are vulnerable to predators or environmental threats.
In some cases, this will be down to pure (bad) luck. But sometimes, it will be because they are particularly maladapted to the demands of their environments. To picture such maladaptation, think about cows on their own in the wilderness of the African savanna: how many generations would it take before cows go extinct? Maybe a couple…
Conversely, some offspring will be better adapted and able to reproduce themselves with higher probabilities and produce more offspring. Think about a cheetah family in which all individuals can run 10% faster than other cheetahs. Chances are, more offspring from that family will survive to adulthood and reproduce. Apply this concept across many generations and, over time, some inheritable traits will disappear while others will become the norm.
If the stars align (there are many possible bumps in the process), then after enough generations, all cheetahs might be 10% faster than the previous ones. They might even qualify as a new species, with the slower cheetahs labeled as a different and extinct one.
New species emerge when their traits are sufficiently different
There are many reasons why inheritable traits might change from one generation to the next, from gene mutations to epigenetic mechanisms. However, natural selection is the great equalizer. If the newly inherited traits give an advantage in terms of reproductive abilities, they’ll have a higher probability of being passed on successfully to the next generation. If they’re unfavorable, the chances of them being passed on decrease dramatically.
New species emerge when a set of inheritable traits becomes common and different enough, over many generations, that the population sharing them exhibits enough differences in their appearance, biology, and behavior compared to the species they evolved from. Sometimes, the two species coexist, either because they no longer compete for the same resources or because enough resources are available for both. If they do compete for the same resources, the new species might be so superior to the previous one that it essentially crowds it out to the point where the other becomes extinct.
Evolution is a tough game.
Critically: natural selection favors the healthy
Let’s pause for a moment on this important fact: a new specie emerges because, at that point in time, it’s so well adapted to its environment and the demands of its lifestyle that it reproduces consistently with a much better probability than the former specie. And what is the one variable that largely determines this reproductive ability? Health. The healthier the individuals are and the more resilient to the rigors of life, the more they’re able to reproduce.
So when a new specie emerges, we can only conclude that, given the environment and the lifestyle practiced at that time, almost all individuals of that specie enjoy optimal health. If most individuals were in poor health and with poor reproductive prospects, there would literally be no new specie to emerge.
This capacity for optimal health is intrinsically linked to the environment though, and to the lifestyle adopted by a specie to adapt to it. Change the environment and health can decrease or even plummet to zero. A fish out of the water loses pretty quickly the capacity for optimal health it enjoyed just moments before.