Two key ideas pervade the book: 1. Our bodies didn't evolve to be healthy or to live long lives but to make as many children as possible; 2. Today our bodies, shaped by millions of years of interaction with the natural environment, have to survive in the artificial environment that humans created with the agricultural and industrial revolutions. The fact that we live longer lives and the fact that we live in unnatural environments are jointly the main causes of today's chronic diseases that affect an older population: obesity, heart diseases, cancer, strokes, diabetes, etc. As Lieberman puts it, "lower mortality is being replaced by higher morbidity". The simple remedy of going back to the lifestyle of the cavemen is not practical (who wants to give up the convenience of the car and of office work?) and sometimes just impossible (we eat plants and animals that we have shaped through artificial selection, that didn't exist just a few hundred years ago).

Lieberman begins with a general discussion on natural selection. Lieberman has a simple definition of life: life is the process by which living things use energy to make more living things. He doesn't mention it explicitly, but the "energy" in that definition is mostly living things: living things eat living things to make more living things. Natural selection is an optimization process that tries to create optimal features for living beings based on the natural environment in which they live, but natural selection can never achieve perfection because it depends on adapting to an ever changing environment, i.e. the target is continuously changing. Natural selection is about survival of the species, not of the individual: it generally selects features that increase the chances of having children who will in turn survive and procreate. It does not necessarily produce long-living animals, i.e. longevity is not a goal of natural selection. It does not necessarily produce healthy individuals. Our ancestors were not any healthier than us. They just had different diseases to worry about.

Lieberman identifies five transitions as crucial for the evolution of the human body. First the early hominids diverged from apes when they started walking upright. Then the australopiths started eating a wide range of foods, not just fruit. Then came the hunter-gatherers. Then brains got bigger and finally humans invented language and culture. To these fundamental stages he adds two more recent ones: the agricultural revolution, during which humans domesticated plants and animals, and the industrial revolution, during which machines replaced human work. These two recent revolutions radically changed what we eat and what we do with our bodies. Lieberman thinks that cultural evolution is now the dominant pressure acting on our bodies.

Indirectly the book is about the many oddities of the human body which would make it unlikely to survive. For example, humans are slow runners. They can easily be captured by beasts who are way faster (and stronger). At the same time, we walk upright. Lieberman thinks that the latter (bipedalism) explains why the former didn't matter for the survival of humans. In 1871 Darwin already speculated that bipedalism was a fundamental feature to explain the success of humans: it freed the hands (that most animals use to run) which could then be devoted to making and using tools, which in turn triggered the expansion of the brain which in turn led to language. This, like many other hypotheses, cannot be proven or disproven. Lieberman adds another virtue of walking upright and freeing the hands: the ability to find new foods other than low-hanging fruit in the face of climate change. Of course, there could be simpler explanations: for example, being able to see over tall grass without being seen (most animals are either too big, i.e. easily identified, or too small, i.e. unable to see what's out there). Clearly, bipedalism has its own disadvantages: not only humans are slow animals but pregnant women are not even stable on their feet; and of course it increases the chances of knee and feet problems that quadrupeds don't have. Lieberman doesn't mention that early upright hominids also lost the ability to swing from tree to tree like other apes, which is a pretty effective way to escape a fast predator.

Four million years ago the australopiths were born. Lieberman thinks, again, that climate change was the cause of the bodily transformation (e.g. the teeth) that allowed australopiths to expand their diet (in particular to tubers, roots and bulbs, i.e. to plants that store their energy underground instead of aboveground like fruit trees do). Here one can appreciate the advantage of having freed the hands: they can now be used to dig. (There are, however, quadrupeds that are much better than humans at digging). Lucy is the most famous australopith.

About two and half million years ago the Ice Age began. Again, climate change was the cause (according to Lieberman) for another momentous transformation: the emergence of the hunter-gatherer Homo Erectus, which first emerged in Africa 1.9 million years ago and quickly spread to the rest of the planet. A calculation of energy needs leads Lieberman to conclude that, especially mothers, could not survive simply on plants and therefore started eating meat in larger quantities (meat was first consumed by humans 2.6 million years ago), which implies that some had to become hunters and some had to become "cooks" (meat is not easy to chew and digest if it is not processed in a more digestible form, e.g. by cutting it in small pieces). Lieberman speculates that the need to process new kinds of food led to the creation of the first primitive tools. He views some changes in the body of Homo Erectus (not only the legs but even the nose and sweating) as implying that those people were able to walk long distances and to run after preys. Lieberman mentions scavenging only as a secondary activity, but clearly the easiest way to get meat, for a relatively slow and weak animal like Homo Erectus, is to wait for a stronger animal to kill a faster prey. Scavenging may have been the original way to get a meal of meat. The individuals who became hunter-gatherers are the ones who survived and reproduced in the changing environment of the Ice Age, and those individuals were hunter-gatherers because they had acquired those bodily features. Furthermore, these individuals had to invent tools to hunt, butcher and process food.

Homo Erectus also had a bigger brain than the australopiths. Lieberman mentions the hypothesis first discussed by Leslie Aiello and Peter Wheeler (in "The Expensive-Tissue Hypothesis", 1995), that meat-eating caused an energetic shift: spending less energy digesting food making more energy available to grow a big brain. This theory is not very credible because of all the drawbacks of eating meat (which collectively result in a waste of energy) but, whatever the cause, Homo Erectus had "small guts and big brains". Larger brains have a huge energetic cost and they might have provided a significant advantage to hunter-gatherers otherwise they would have gone extinct right away. So Lieberman spends a whole chapter discussing the energy needs of the Ice Age. Humans (and in general apes) are exceptions to a simple rule of nature: most animals spend as little energy as possible for growing and as much energy as possible for reproducing. Again, that's because natural selection is about reproducing, not about living long and healthy life. What matters is only reproduction. Humans, however, spend a lot of energy growing their bodies (and particularly their brains) and make relatively few babies. Lieberman speculates that humans violate that fundamental rule because they invest more in making sure that children will survive and reproduce. Most animals make many children and quickly but few survive. Human children are not many but more likely to survive. Human babies also require a lot more care than the offspring of other species.

Nonetheless, big brains suck more calories, are more vulnerable to concussions, and complicate birth. It takes 18 years for the human brain to reach its final form. Lieberman calculates the total cost of growing up to be 18: 12 million calories. Such a huge investment must have a purpose. Robin Dunbar calculated (in "The Social Brain Hypothesis", 1998) that the size of the neocortex correlates with group size. Extrapolating one can conjecture that the large human brain enabled humans to create bigger societies, probably useful for hunter-gatherers.

Between the energetic demands of a large brain and the energetic demands of running around, Homo Erectus therefore had a desperate need of energy and was craving for fat and sugar (which is stored as fat in the body).

About 400,000 years ago Homo Erectus also mastered fire. Humans had been making fire since much earlier, and it's another mystery why it took so long for humans to realize the benefits of fire: keeping warm in cold climates, keeping animals away, work at night, and... cook. Cooked food provides more energy than uncooked food and greatly decreases the chances of getting sick when eating meat. Cooking food, especially meat, was one solution to obtain the energy needed by the hunter-gatherer.

About 50,000 years ago humans started doing something funny: changing their habits. Most animals, and also ancient hominids, keep repeating the same habits, generation after generation. In the Upper Paleolithic humans started making a variety of tools, and each location made different things (so that we recognize a "culture" by their artifacts). Lieberman calls it a "technological revolution". Richard Klein (in "The Dawn of Human Culture", 2002) speculated that this revolution was triggered by the emergence of genes for cooperation and planning. The diet further expanded. The human population grew exponentially. By 15,000 years ago hunter-gatherers had occupied almost every corner of the world, thanks to local innovations to deal with different climates and environments. Then about 12,000 years ago hunter-gatherers invented farming and set in motion the agricultural revolution. Farming is another oxymoron: it produces more food, but it also creates the conditions for new diseases, for famines, for a less nutritional diet. New diseases include any mosquito-borne disease because farming requires irrigation, an ideal habitat for mosquitoes, as well as several diseases that we acquired from our domesticated animals, and diseases that spread because of density of population. The flu is a typical example of a disease that we acquired from domesticated animals, as are (were) the plague and smallpox, as are tuberulosis, measles, diphteria, typhus, and, recently, covid. Farmers also created stable communities and those present another problem that hunter-gatherers didn't have: poor sanitation. Agriculture was a real curse on humankind: it generated a large set of diseases that previously were unknown or rare. At the same time, the diet became less nutritional because farmers focused on a small number of crops, and slowly engineered varieties that had higher yields: they produced a lot more food but a smaller variety of foods compared with the diverse diet of hunter-gatherers, resulting in a diet less rich in vitamins and minerals; quantity at the expense of quality. This also exposed them to the danger of famines, whenever a natural disaster hit their region. The storage of food further deteriorated the diet. For example, farmers figured out how to "refine" cereals, but white rice has only a fraction of the nutrients of brown rice. Overall, the diet of farmers, compared with the diet of the hunter-gatherers, was deficient in fibers, proteins, vitamins and minerals. One has to wonder whether farming spread because of real benefits or for other reasons. Farming also caused a genetic revolution that is hard to estimate. In the notes Lieberman mentions several studies from Benjamin Voight ("A Map of Recent Positive Selection in the Human Genome", 2006), Scott Williamson ("Localizing Recent Adaptive Evolution in the Human Genome", 2007) and others that have identified more than one hundred genes that have become prevalent in recent humans, and Lieberman suspects that many have been favored by farming.

Then came the industrial revolution that drastically reduced how much physical activity we perform because it invented cars, elevators and desk work. At the same time the industrial revolution produced huge quantities of fat, sugar, starch and salt that consumers craved because of the way their Paleolithic bodies are programmed. The age of the industrial era (i.e. of processed food) is deficient in fibers, vitamins and minerals (except for salt). Luckily, at the same time the industrial and scientific revolutions have created medicine which has extended our lifetime, mainly by saving the lives of children. But it's important to notice that the scientific progress of the 20th century didn't make us healthier: it simply chased after all the new diseases caused by our new (wildly unnatural) lifestyle.

Finally, the book deals with the causes of our health problems. Here Lieberman reminds us of the two fundamental problems: 1. human bodies were not shaped to live long and healthy lives, but to procreate, and the hunter-gatherer was not a particularly healthy species, so whatever we inherited from our distant ancestor was not a recipe per perfect health; 2. human bodies are maladaptive for the artificial environment that we have created through the agricultural and industrial revolutions (which also implies that what was healthy for them in their natural environment is not always healthy for us in our artificial environment). For example, why do we crave fat foods and sugary drinks? Because millions of years shaped our bodies to absorb energy-rich foods like fat and sugar. The cavemen were using all that energy for energy-intensive tasks like walking and hunting, and some of those energy-rich foods were rare (like sugar). Many modern chronic diseases can be related to "evolutionary mismatches" due to the fact that our Paleolithic bodies now live in a completely different environment. Most of us will die of a disease caused by one of these mismatches.

Lieberman focuses on obesity, such a widespread disease that most obese people don't realize they are obese (until they travel to a poor country where they suddenly realize they are twice bigger than the local population). Lieberman explains that it's all about "energy balance". What we eat is turned into energy as either carbohydrates (that contain less energy but are easier available when needed) or fats (that contain a lot more energy but harder to retrieve). Lieberman introduces the reader to subcutaneous and visceral fats (fats that are stored under the skin and fats that are stored in the organs and muscles) and to fructose and glucose sugars (sugars from fruit and sugars that make up starch like flour). The reason our body and the bodies of many other animals store fat is that being hungry used to be normal. We're programmed to like the taste of fatty foods because evolution programmed us to store fat for rainy (hungry) days. This is particularly true of women, who in fact are "better" than men at storing fat: again, natural selection is about procreation, the ability of the individual to make children, and a woman needs to feed herself and her foetus, or the baby will die. James Neel came up with a genetic explanation for why our bodies store as much fat as possible, the "thrifty genotype hypothesis" ("Diabetes Mellitus", 1962), and later Nick Hales and David Barker came up with the "thrifty phenotype hypothesis" ("Type 2 non-insulin-dependent diabetes mellitus", 1992). Neither passed the test of empirical evidence but it is a fact that humans are incredibly good at storing fat. Early hominids had plenty of fibers and so didn't develop a similar craving appetite for fibers. We are therefore programmed to crave sweets but not salads.

Our diet has evolved not according to the natural world but according to the logic of capitalism: capitalists produced and marketed foods with lots of (fattening) sugar and little fiber because these are the foods that we find tasty, and we find them tasty because in ancient times sugar was rare and fibers were plentiful: a typical case of supply and demand. Eating a lot of fat but little fiber compounds the problem of obesity because fibers are needed to absorb fat properly. Furthermore, today we eat plants that have been domesticated, i.e. selected over the centuries since we started farming: often the varieties that have been selected are the sweeter ones, again because those are the ones that taste better for sugar-craving humans. And furthermore, today we eat domesticated animals that are fed with foods that are not natural for them: the meat of grass-fed animals is several times lower in saturated fats than the meat of corn-fed animals. At the same time more and more humans are being less and less active: farming reduced the physical activities of the hunter-gatherers, factories reduced the physical activities of farmers, and

Lieberman cites studies that blame obesity on genetics, but the evidence is hardly convincing: people with the exact same genes who live in other countries don't get fat, and those genes have been around for thousands or millions of years without causing epidemics of obesity. Lieberman's notes at the end of the book are more honest: "This is a big, fast-changing topic". Blaming obesity on genetics is an excuse, period. Blaming stress is also unfair: who said that people were less stressed a thousand year ago, when they were constantly under threat from invading armies, natural disasters, and plagues? I find more convincing the hypothesis that our medicines may indirectly cause obesity: antibiotics kill bacteria and most bacteria are actually useful to digest the food we eat. In the notes Lieberman mentions an important study by Peter Turnbaugh ("The core gut microbiome, energy balance and obesity", 2009), but a study by John DiBiase came earlier ("Gut Microbiota and Its Possible Relationship With Obesity", 2008), and countless studies have flooded the literature in the 2010s. There have been experiments of transplanting the gut microbiome from slender people to obese patients ("Transfer of Intestinal Microbiota from Lean Donors Increases Insulin Sensitivity in Individuals with Metabolic Syndrome", 2012).

From the point of view of heart diseases, not all fats are bad, and Lieberman is honest when he admits that the story is not clear yet. Traditionally, saturated fats (and particularly "trans fats") have been considered a leading cause of heart diseases because they increase LDL, but the empirical evidence is not so clear. It appears that low levels of HDL ("good cholesterol") may be more dangerous than high levels of LDL ("bad cholesterol"). William Castelli published many studies about this in the 1980s ("Incidence of Coronary Heart Disease and Lipoprotein Cholesterol Levels", 1986). The link between high cholesterol and heart disease has been disputed by many studies: a study by Lise Bathum found that people aged 50+ with higher cholesterol (total, HDL+LDL) lived longer ("Association of Lipoprotein Levels with Mortality in Subjects Aged 50 + Without Previous Diabetes or Cardiovascular Disease", 2013) and Uffe Ravnskov's study found that people aged 60+ with high LDL lived longer ("Lack of an Association or an Inverse Association between Low-density-lipoprotein Cholesterol and Mortality in the Elderly", 2022). Unfortunately, any study that disputes the evil of bad cholesterol immediately gets a lot of media coverage and becomes very popular among conspiracy theorists who then spread it on social media, with the net result that such studies are more read by the general public than the opposite studies (which are still more numerous). The jury is still out.

On the other hand, there is little doubt that salt is bad for your heart: it causes high blood pressure. We eat enormous quantities of salt compared with past generations.

Lieberman points out that today we often even cause "dysevolution", a feedback loop triggered by treating only the symptoms of a "mismatch disease" instead of its causes, so that the causes are passed on to future generations. Type 2 diabetes is a typical case of dysevolution: the treatment does little to remove the causes.

More or less related to natural selection is the fact that we have made our environment (waters, air and surfaces) as germ-free as possible, and antibiotics routinely kill the bacteria in our bodies, and in many developed countries parents keep children from playing with dirt: all of this deprives the body of the "training" it needs to become resilient against germs. For example, allergies are on the rise everywhere. The most likely explanation is something along the line of David Strachan's "hygiene hypothesis" ("Hay Fever, Hygiene and Household Size", 1989) that allergies (which arise when T-cells, and therefore antibodies, target harmless substances) are spreading because we are not as exposed to germs as we used to be due to higher standards of hygiene.

Lieberman is a pioneer of a new field of medicine, evolutionary medicine. Some other pioneering books in this field:
Randolph Nesse & George Williams: "Why We Get Sick" (1996)
Koella Stearns: "Evolution in Health and Disease" (1999)
Peter Gluckman: "Mismatch" (2006)
Wenda Trevathan, E.O. Smith & James McKenna: "Evolutionary Medicine" (2009) (2009)
Peter Gluckman, Alan Beedle & Mark Hanson: "Principles of Evolutionary Medicine" (2009)
Wenda Trevathan: "Ancient Bodies, Modern Lives" (2010)

I dislike the practice of stacking all the notes at the end of the book, separating them from the main text, and this book is a good example of what the reader misses: most readers will not read the notes, but the notes contain a lot of interesting information. I curse whoever invented this practice of moving the notes from the page where they should be to the end of the book.

TM, ®, Copyright © 2022 Piero Scaruffi