The human genome is the complete set of genetic information for humans. This genetic blueprint consists of information encoded as DNA sequences within 23 pairs of chromosomes located in the nuclei of your cells: 23 chromosomes from mum and 23 chromosomes from dad. In addition to this main reservoir of genetic information, each one of us also have a small DNA molecule located in the mitochondria; DNA that are passed on purely from mum to offspring.
A gene is a hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome. It’s these sequences we’re primarily concerned with, as it is the instructions in our genes that tell our cells how to make various proteins. It’s important to note though that most of the genes in the human genome are noncoding DNA that don’t have any important functions (that geneticists know of) or are involved in other processes than the synthesis of protein, such as regulation of gene expression and transcription of functional non-coding RNA molecules.
The last mile on the evolutionary road of the human genome
Natural selection, genetic drift, gene flow, and mutations are the mechanisms that shaped the human genome through millions of years of evolution. Of all these processes, natural selection is generally considered the most “important” one. The term natural selection was coined by Charles Darwin in his book On the Origin of the Species and describes the gradual process by which heritable biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment.
When the human and chimpanzee lineages split about 5 to 7 million years ago, hominins started out on a very different evolutionary path than the chimps. The first initial adaptation in the human lineage that set the stage for additional changes was our ability to walk on two legs, a trait that was advantageous for a number of reasons, one of which being that it improved our ancients ancestors’ ability to walk long distances in the search for food.
In the types of ancestral habitats our distant ancestors occupied, those who were not physically fit enough to evade predators, hunt, gather food, and otherwise handle a demanding lifestyle were quickly weeded out of the gene pool. Over time, various traits that improved our ancestors’ ability to survive and reproduce within the hunter-gatherer niche in which they lived accumulated via natural selection. This explains why hominins gradually became better at running long distances in the heat, grew larger brains, and so on.
The first anatomically modern humans appear 200.000 years ago in Africa, and if we are to believe the fossil record, we didn’t colonize the rest of the world until approximately 100.000-150.000 years later. As our ancestors traveled into new areas of the world, they were forced to adapt to habitats that were dissimilar to the African savanna. The combination of cultural evolution (e.g., clothing, housing) and Darwinian evolution (e.g., skin pigmentation, body size) helped our ancestors adjust to these new conditions.
The migration out of Africa marks a major event in the evolutionary history of man, an event that spurred changes to the human way of life. With that said, it’s important to note that it took tens of thousands of years before our ancestors reached the far corners of the world, and that hunting-gathering was the subsistence mode for all human populations up until the Agricultural Revolution.
The Agricultural Revolution approximately 10.000 years ago initiated profound changes in our environment. The rapid cultural evolution that has occurred ever since has completely changed how we eat, move, sleep, and work. Hence, it has changed the expression of our genomes, which are still largely composed of genes selected in preagricultural environments.
The genetic code is often thought of as a static machinery that stays with us throughout life, and few probably pay much attention to how their daily lifestyle choices impact how their genes are regulated and expressed. It’s easy to forget that everything is controlled by genes.
The way our genes express themselves (the process by which genetic instructions are used to synthesize gene products) constantly changes. Besides actual changes in the DNA sequence (think mutations, natural selection, gene flow, and genetic drift), the human body can adapt in at least two other ways to its environment. The first one is epigenetic modifications, a process where decoration of nucleotides results in altered gene expression (1, 2). The second is changes in gene expression in a period of hours, days, or weeks (1).
A western lifestyle adversely affects gene expression
Several studies have found that insufficient sleep, physical inactivity, consumption of highly processed food, and many other factors associated with modern lifestyles promote abnormal and suboptimal gene expression (1, 2, 3, 4, 5). When you think about it, this is just what one would expect, given that our genes were primarily selected when we lived as foragers in the wild, a way of life that involved regular physical activity, consumption of nutrient-dense whole foods, regular sun exposure, etc. We are still – to a significant extent – genetically adapted to a hunter-gatherer lifestyle (6, 7).
You don’t have to look further than at the people around you on the street to understand that the gene expression pattern of people in the modern world is very different from that of hunter-gatherers and healthy non-westernized populations. In affluent nations such as the U.S., the majority of people are either overweight or obese, and a long list of other chronic disorders – many of which are related to obesity – have increased dramatically in prevalence recently.
Not only will a lifestyle that is at odds with your ancient biology promote suboptimal gene expression, but it will also leave epigenetic tags on your genes, which might be passed on to your children (2, 3).
The bottom line is that a western lifestyle can produce changes in gene expression that often pass a clinical threshold into a chronic disease phenotype.
Normalizing gene expression
Hunter-gatherers and traditional, isolated people (both contemporary and prehistoric) don’t know about DNA, gene expression, or natural selection – and in a sense, they don’t have to, seeing as they live in environments that closely resemble those our primal ancestors evolved in for millions of years. Their lifestyles retain most of the features that characterize the lifestyle of our preagricultural ancestors (6, 8). Perhaps most importantly, they eat a Paleolithic or traditional diet, engage in a lot of physical activity, spend plenty of time outdoors, and get sufficient sleep.
So, how can we upregulate our ‘good’ genes and downregulate our ‘bad’ genes? The obvious answer is that we should look at nutrition, exercise, and health through the lens of evolution, so as to get a better understanding of what type of environment we are adapted to live in; an understanding we can use to sculpt our bodies and health.