Harvard Professor: They’re Lying To You About Running, Breathing & Sitting! - Daniel Lieberman

Conditions like heart disease, type 2 diabetes, many cancers, osteoporosis, back pain, and chronic stress arise because our bodies evolved for a very different environment—physically active, food-scarce, pathogen-rich—than the sedentary, calorie-dense, sanitized world we inhabit now. Heart disease alone kills about a third of people in modern societies, yet is rare to non-existent in hunter-gatherers and traditional small-scale populations. This means these diseases are not inevitable but largely preventable with changes to behavior and environment.

Natural selection only ‘cares’ about reproductive success—“food in, babies out.” We didn’t evolve to be happy, nice, or optimally healthy past reproductive years; we evolved to survive long enough to reproduce successfully. Traits like strong cravings for sugar and fat, a tendency to store body fat, and hormone responses that push early reproduction when energy is abundant all make sense in this light. Assuming that whatever our ancestors did is automatically ‘healthiest’ today is a logical error; it was optimal for reproduction under very different conditions.

Humans have adapted—biologically and technologically—to eat an enormous range of foods, from nearly all-meat Arctic diets to plant-rich Kalahari diets with hundreds of plant species and many animal sources. Our digestive system and cooking/processing technologies let us turn “almost anything into anything” (carbs into fat, fat into carbs). Meat has been part of our diet for at least 2–2.5 million years and played a major evolutionary role, but that doesn’t mean eating meat today is automatically optimal for health, nor that vegan diets are ‘unnatural’. The key problems now are highly processed, energy-dense, low-fiber foods and excessive energy intake relative to expenditure.

Acute cortisol spikes evolved to help us escape threats, making energy quickly available. But modern chronic psychosocial stress (exams, commuting, racism, poverty) keeps cortisol elevated, driving hunger, sugar cravings, and visceral fat deposition around the abdomen. Overfilled fat cells rupture and trigger immune responses, releasing inflammatory molecules that promote insulin resistance, atherosclerosis, Alzheimer’s pathology, and systemic chronic inflammation. This is a major link between stress, belly fat, and diseases like heart disease and diabetes.

Growing skeletons respond to mechanical loading—“we match capacity to demand.” Weight-bearing activity in youth builds higher peak bone mass; without it, you reach adulthood with smaller, weaker bones and are more likely to develop osteoporosis as you inevitably lose bone from ~25–30 onward. Evidence from tennis players shows up to 40% thicker humerus bones in the racket arm versus the non-dominant arm. Similarly, softer, highly processed foods reduce chewing load, shrinking jaws by ~6% and increasing dental crowding and wisdom-tooth problems. Children need vigorous daily physical activity and harder-to-chew foods to develop robust skeletons and jaws.