The obligatory and natural linkage between food acquisition and physical activity ended during the 19th century due to the Industrialized Revolution. During the 20th and 21st century, the circumstances of human existence changed dramatically, especially during the last few decades. There is no doubt that living standard and health standard has improved during the 20th and 21st century. Neonatal and childhood mortality dropped down, life expectancy increased dramatically. Infectious diseases declined and improved hygiene, nutrition and medical care led to the so called third epidemiologic transition. While infectious diseases declined, so called non-communicable diseases and diseases typical of old age increased dramatically. The new burdens are diabetes type 2, cardiovascular disease, cancer, osteoporosis, sarcopenia, frailty, psychological problems and dementia [
35]. As pointed out above, these diseases are highly related to a physically inactive lifestyle [
2,
3]. We have to be aware that our recent environment is completely different from that in which our species evolved. An increasing number of people live in urban environments, many of them in so-called mega cities of more than 10 million inhabitants. Recent urban
Homo sapiens live alone or in small nuclear families in a quite anonymous society. Technical advances and modernization of lifestyle have resulted in a marked transition in human lifestyle [
1]. Exemplary, medical interventions and practices have significantly changed human morbidity and mortality. The daily energy effort to gather and prepare enough food is reduced nearly to zero, since only few individuals are working in food production. Mechanized transportation, sedentary jobs and labor-saving household technologies reduce physical activity too. A sedentary, completely physically inactive lifestyle prevails in all industrialized postmodern societies. This is also true of children and adolescents although modern children still have a significant desire for physical activity. Western societies however hinder them to act out their desire for physical activities. Children are taken to kindergarten and school by car, and have to sit still in kindergarten and classrooms. They have to stay in the house instead of playing outside. While 26% of 8–16 year old US children watch TV for at least 4 hours per day and 67% watch TV for at least 2 hours per day, only 19% of high school students are physically active for 20 minutes or more in daily physical education classes. Sixty percent of US adults are not regularly active and 25% are not active at all [
77]. However, how different are these recent activity patterns from that of our ancestors or that of recent traditional societies?
In order to compare physical activity patterns, standardized physical activity levels can be used. Physical activity levels (PAL) can be calculated by the ratio of total energy expenditure (TEE) to basal metabolic rate (BMR) or resting energy expenditure (REE) [
7]. PAL is a commonly used indicator of changing activity patterns.
Table 1 presents physical activity levels (PAL) and total energy expenditure (kcal/d) of non-human primates through our hominid ancestors to contemporary
Homo sapiens. Before we start to discuss the impact of subsistence patterns on PAL and TEE, we have to consider the weakness of both parameters. Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure [
89]. Physical activity induced energy expenditure however is determined by body movement, body weight and body size [
89,
90]. In general, it requires more energy to move a large and heavy body than a small and lean one. This is one of the reasons why obese persons move less than lean ones. In the present study, PAL and TEE of different populations are compared. It must be taken into account that traditional populations, in particular hunter and gatherers, living under poor socioeconomic conditions in marginalized environments today are generally shorter and leaner than recent people following a Westernized way of life. These somatic differences should not be forgotten when interpreting differences in PAL and TEE between different recent populations. As to be seen in
Table 1, PAL was nearly always higher among males than among females. This was also true of total energy expenditure (TEE). Males nearly always surpassed their female counterparts. These gender differences may be due to gender typical differences in daily workload but also differences in body size. In every recent population, males are taller and heavier on average than females. These differences in body size and body composition may contribute to gender differences in PAL and TEE. Among recent traditional populations, physical activity levels ranged from 1.54–2.26 among males and between 1.42 and 2.26 among females. Concerning total energy expenditure, highest values were found for recent hunter and gatherer populations such as the Ache and forager horticulturalists such as the Huli and Machiguenga. Among these populations, the average PAL was also high. Among contemporary people in modern societies with a low Human Development Index (HDI), the average physical activity levels were 1.88 for males and 1.70 for females. Among modern societies with a high Human Development Index (HDI), average PAL was 1.79 for males and 1.71 for females [
78]. At a first glimpse, differences in PAL as well as total energy expenditure (TEE) between populations following different subsistence patterns are quite low. It seems surprising that lowest PALs and TEEs were found among two hunter gatherer groups, in particular among !Kung San in Southern Africa and Shuar, indigenous forager-horticulturalists in Ecuador. These observations consequently may implicate that physical activity levels have not much changed with modernization. This view is corroborated by the PALs and TEEs estimated for fossil hominids. Concerning fossil hominids, we should not forget, however, that PALs and TEEs are not based on real physiological parameters and are only estimated. Furthermore, fossil hominids and also recent forager populations are significantly shorter and leaner than Western populations. This may explain the low differences in PAL and TEE between people following a traditional lifestyle and Western populations. Furthermore, concerning recent so called hunter-gatherer populations, we should bear in mind that most of them live in marginalized environments and have adopted Westernized lifestyles to some extent, which might have affected physical activity levels. This is true of the Shuar in Ecuador [
84]. Modernization and Westernization has resulted in a decrease in physical activity levels and also in an increase in obesity rates and the prevalence of non-communicable diseases. The Bolivian Tsimane still living as forager-horticulturalists in the Amazon region display relatively high average PALs (1.85 among females, 2.15 among males) which are typical of other subsistence populations (see
Table 1). Unfortunately, there are no TEE values of Tsimane people available. Nevertheless, increasing socioeconomic change has increased obesity rates although physical activity seems not to be affected by this trend [
83]. We have to be aware that the process of modernization affects first nutritional habits, and later on physical activity patterns. As pointed out above, typical hunter-gatherer lifestyles became more or less extinct during the 20th century. Traditional farmers and pastoralists have also undergone a process of Westernization. Nevertheless, the PALs of pastoralists and traditional farmers were generally higher than PALs of affluent societies. Therefore, the conclusion that can be made from studying recent traditional societies is that physical activity plays only a minor role in the development of non-communicable diseases is not correct. We can assume that
Homo sapiens is adapted to a physically active lifestyle; our indoor, overfed sedentary existence, however, is maladaptive and may result in an increased risk of many diseases as mentioned above. However, why have
Homo sapiens changed physical activity patterns? For the most part of our evolution and history, physical activity was essential for surviving. Except for children under age 5 and those who were disabled, sick or too old, everyone did a wide variety of physical activities every day in order to fight against hunger and thirst [
58]. As to be seen in
Table 1, even after the adoption of agriculture, physical activity levels remained high. Additionally, energy was needed to maintain homeostasis, in particular a stabile body temperature. Among human females, reproduction required considerable energy. Without any doubt, this lifestyle was exhausting and therefore every possibility to conserve energy by reduction of physical activity was taken. This was true of not only Paleolithic times, but also of the Neolithic period up to now. This behavior seems to be coded in the genome of
Homo sapiens. In our recent environment, however, this behavior is clearly maladaptive. From a Darwinian viewpoint, the tendency to reduce physical activity can be understood as a result of the mismatch between our behavioral heritage and our recent environment.