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Age of Homo sapiens

The Homo sapiens lineage may be as old as 500,000 years. Although the oldest fossil remains only date to about 200,000 years, there is growing evidence that the H. sapiens lineage may be much older. Assuming that H. neandertalensis is its own species (and evidence for this assumption grows), then the last common ancestor of both terminal species of hominoid evolution may have lived around that time. Paleoanthropology has not yet sufficiently resolved the taxonomic status of earlier Homo fossils such as H. antecessor (more than 800,000 years from Spain), which might as well be a grade as H. heidelbergensis. The oldest Neandertal lived at least 250,000 years ago.

A caveat of paleoanthropology and the reconstruction of the evolutionary history of H. sapiens is first the decoupling of fossilized information (such as skull anatomy) from the crucial processes (sexuality, social organization, cognition, cultural evolution) in H. sapiens evolution. And second, the transition from archaic sapiens to modern sapiens was a continuous process.

Scenarios of Hominization

For quite a while, two contrasting models, the “out-of-Africa hypothesis” and the “multiregional hypothesis” and their modifications, dominated the discussion of the origin of H. sapiens. The out-of- Africa hypothesis (synonym, “Arche Noah-model,” “replacement hypothesis,”“Eve-hypothesis,” or “Lucky- Mother–hypothesis”) assumes a common origin of all modern humans in Africa and thus that their morphological divergence of all recent human population is the result of a relatively recent differentiation. According to the “multiregional hypothesis” (or “candelabrum model”), modern human populations are derived from regional precursor populations, and Asians and Europeans are thus descendants of earlier endemic humans. The “assimilation model” tries to reconcile both diametrically opposed hypotheses, assuming a somewhat “mixture” of modern immigrants with endemic populations. Very recent findings even suggest a genetic bottleneck more the 50,000 years ago, which would even more contradict a multiregional origin of all modern human populations.

The discussions in all models are limited to the last 150,000 years of H. sapiens evolution.

The Fossil Record

The indisputable fossil record of H. sapiens actually starts in Ethiopia with the specimens Omo I and Omo II from the Kibbish formation now dated at 195,000 years, followed by H. sapiens idalto from Afar, Ethiopia (160,000 years), and by the remains from Border Cave, South Africa (> 100,000 years). Evidence gathered from both molecular genetics (for example, mitochondrial genes) and linguistics support an African origin of all modern humans. This does not necessarily imply that the origin of early sapiens is also found in Africa.

Short-term climatic changes in the Pleistocene led to a habitat fragmentation in Africa. The cooling effect of the climate change fragmented forests, whereas the warming effect expanded savannahs. Humans, by having to react to these relatively short-term climatic alterations, learned to adapt to changing environments. This later enabled their universal dispersal (not to be confused with migration) seen in no other mammalian species. Occurrence of H. sapiens outside Africa is documented as being younger than 100,000 years in the Middle East (Shkul V, Israel, 90,000 years; Qafzeh IX, Israel, 90,000–100,000 years). Note that recent redating of the sites contradicts a simple succession of Neandertal and modern humans. The dispersal out into the rest of the world started more than 60,000 years ago. Humans reached Asia about 60,000 years ago, Australia around 50,000 years ago, Europe 40,000–35,000 years ago (“Cro-Magnon”), and America 35,000–15,000 years ago. The splitting of human ethnic groups is also documented by a similar splitting of their parasitic bacterium, Heliobacter pylori, into various subgroups.

Group size has become a key word in recent discussions on human evolution. Although we have no data whatsoever on the size of human groups during their dispersal, many anatomical traits indicate “genetic drift,” the fixation of randomly distributed alleles in relatively small original groups. Many traits used for ethnic distinctions have continuously escaped all functional arguments.

Anatomy of Homo sapiens

There is no purely anatomical-based definition of H. sapiens due to the difference in modern anatomy and behavior. Linnaeus distinguished between H. diurnus, corresponding to humans, and H. nocturnus, referring to the orangutan. It is the only Linnean species without a holotype of Linnaeus himself.

Diagnostic (not necessarily autapomorphic) cranial characteristics (or even better, trends) of H. sapiens are as follows:

Appearance less robust than in other members of the genus Homo
Reduction of tooth surface
Tooth arcade with parabolic shape
Rounded occipital region
Straight forehead, therefore less prominent supraorbital ridges (but still present in early members)
Face orthognathous
Orbitae more rectangular than rounded
Deep zygomatic fossa
Prominent chin probably due to posterior displacement of dentition indicated by the For. mentale below P2 instead of M1

By summarizing cranial and dental characteristics, it becomes evident that changes in mastication affect most of the above-mentioned characters. It led to reduced tooth surfaces, bite force (herewith supraorbital ridges), and reshaping of the chin. Only after these dominant constraints onto the skull shape had been reduced was the inflation of the temporal and forebrain that led to the straight forehead and orthognathous face possible.

Diagnostic postcranial characters follow.

Body Shape

During their evolution, humans have increased in body size by approximately 150% and doubled their body weight.Most probably already in early members of the genus Homo (if not earlier), a new mechanism of bipedal locomotion evolves, using a torsional mechanism of the trunk and spine in walking. Amplitudes of these torsions depend on walking speed and indicate the use of resonance mechanisms (energy saving), which at higher speeds show significant gender differences. Torsion is driven and countered by the oblique abdominal muscles. The human body forms a hyperboloid, the “waist.”

According to an idea of Weber and Weber, the legs and arms act as physical pendula during walking. Their resonance occurs at the energetically optimal speed of about 1,1 m/s—just the speed at which trunk torsion shows a relative minimum, indicating an adapted mechanical interaction between extremities and trunk. To increase the energetically optimal speed, muscles of the legs are concentrated near the trunk. The consecutive cone shape of the legs allows for great step length with short pendula and thus short swing times, leading to maximized speed. The concept was summarized under the term inverted pendulum. Human arms are shorter than the legs, with a more cylindrical shape and more mass distally, thus comparable pendulum length and thus dynamic properties as legs.

In walking, hip joints create about twice the force as shoulder joints, and mechanically relevant shoulder width (biglenoidal width) is double that of pelvic width (biacetabular width). More mass is concentrated in the pelvic segment than in the (air-filled: lungs) thoracic segments. Thus, the center of mass is situated in the pelvis. In the front and side views of the long trunk (in relation to our nearest primate relatives extended) with mass concentrations at its end has an asymmetric dumb bell shape. Seen from the top, the cross-sectional area of the trunk is stretched into an ellipsis instead of the more rounded shape of, for example, chimpanzees. Changes in body shape are a necessary prerequisite for an important action of hominids in throwing.

Hands

A comparison of modern human hands with those of nonhuman primates reveals features unique to humans that are related to two main functions: from power grip to precision grip, and the importance of gestures. But the basis of the manipulatory abilities that involve structural changes as well as demands for a high speed in the execution of movements should be seen more in brain development than in anatomical changes of the hand itself. Unique human hand movements are first the ulnar opposition (moving the ulnar side toward the thumb) forming the “cup of Diogenes.” Second, the precision and squeeze grip (and many more grips) between the thumb and the fourth and fifth finger is made possible (“basket grip”). (Lucy already had a grip between the thumb and the second and third finger, according to M. Marzke.) The latter involves reorganization of the thumb muscles, especially the long flexor muscle.

Nakedness

The nakedness of humans is the result of a miniaturization of the body hairs (not loss) to a maximum size of 1–2 mm. In addition, the body hairs have lost their pigmentation. In correlation, the sweat glands, formerly restricted to the palm and sole in primates, are now covering the whole body, providing a maximum area of cooling by transpiration. The human ability of sweating is unique within mammals. But nakedness cannot only be seen in the context of thermoregulation but also of sexuality and postnatal development (see below). The naked skin and touch play an outstanding role in the development of emotion. The taboo of skin contact in many different recent cultures indicates the high importance of such skin contacts for the stabilization of group-adherence but also for the development of self-consciousness.

Third, nakedness together with the enormous development of hairs on the head and subcutaneous body fat has reshaped the outer appearance of humans. While subcutaneous body fat makes only 3% of body mass in primates, it makes 16% in men and up to 25% in females. Its function is related to the inflation of the brain. Normally, the brain only consumes glucose, but in the state of starvation it may cover up to 85% with ketones, which are mobilized from the fat tissue.

Already on the naked skin of chimpanzees, melanocytes (pigment-producing cells) produce melanin under UV radiation. Melanin molecules bind free radicals. Any color in humans is only due to the amount of melanin in the skin, which itself is controlled by only a few genes. They regulate the activity of an enzyme (tyrosinase) that catalyzes the first step in the biosynthesis of melanin and control the package and size of melanosomes, which finally cause the difference between the individual pigmentation.

Human colors are a subtle balance between two vitamins. The first vitamin is folate (folic acid), a B vitamin, which is highly sensitive to sunlight. Especially during gestation, a lack of folate can cause multiple malformations. It is also indispensable for rapid mitosis such a sperm production.

In contrast, vitamin D biosynthesis needs UV radiation. Vitamin D is necessary for calcium and phosphate metabolism, for calcium uptake from the gut, and therefore for osteogenesis. In the epidermis, cholesterine is transformed into provitamin D, which is synthesized to vitamin D in the kidneys. To ensure this biosynthesis, the skin gets lighter the more northern individuals live, and even, theoretically, there is not enough sunlight for this biosynthesis beyond 50° of latitude. In the latter case, it has to be compensated by food.

Sexuality

The sexuality of H. sapiens differs dramatically from their closest living relatives, the chimpanzees. The subcutaneous fat tissue has increased to as much as 16%–25% of total body weight. It now shapes the human body in both sexes. The breasts of adult women are permanently developed independent of their function for lactation. In nonlactating periods, the breasts are formed by fat tissue and serve only as a sex signal. The external female genital organs are completely hidden by the major wings of the vulva as well as by pubic hair. These hairs are so different from cephalic hairs that even the lice living in the respective regions are of a different species. In contrast to chimpanzee and bonobo females, women do not signal their estrus in any way, neither by swollen anogenital skins nor by any specific secretion. The estrus of women is cryptic, and their sexual activity independent of it. Together with the unique sensibility of a naked body, and of specific erogenous zones, humans of both sexes are probably the only mammals able to orgasm, a type of brief unconsciousness (“little death” in East-Asian literature). Sexuality plays a crucial role in partnerships even beyond the period of the female reproductive age. A milestone in human evolution is the separation of sexuality from reproduction. Homosexuality as a constitutive character of all human cultures can also be considered an effect of this clear distinction.

One of the possible evolutionary consequences of the new sexuality of humans is the “liberation” of males from their sex-dominated role of achieving access to females, or defending them in harem-organized species (for example, gorilla, chimpanzee, and to a lesser degree bonobos). Men are the only primates who cooperate to a large extent with nonrelated individuals. This male cooperativeness, together with the plesiomorphic cooperation among primate females, is one of the key structures in social organization, which preceded cultural evolution. It also allows for the first time in primate evolution a group size that is well above any biologically constrained size.

Birth

Humans have an unusually long gestation period relative to their body weight. Especially in the last 6–8 weeks, when fetal body growth ceases, the brain and subcutaneous body fat develops, of which the latter can be seen as a supply for the former in case of postnatal starvation. Human birth is unique in that the head is rotated 90° within the birth channel and the newborn is facing dorsally. The complexity of birth and the enormous risk for the mother—also unique within mammals—and for the newborn has led to the idea that assistance during birth by other women is a key factor in human evolution.

Postnatal Development and Social Behavior

Human social behavior develops from an already extremely advanced behavior of their ape sistergroups. In particular, two innate abilities are highly evolved within these groups. Apes can detect the social and emotional status of their counterpart from very small, almost nonvisible signals such as changes in body posture, and they can anticipate its implication. They can recognize other individuals as intentional agents whose attention can be shared and even manipulated. Together with a high degree of vocalization, apes have herewith evolved the highest emotional intelligence among all existing mammals.

By building upon these abilities, human evolution is characterized by a unique “sociability,” an extreme behavior directed toward immediate relatives but also psychological parents (“primary intersubjectivity”). Newborn primates have the well-known reflex of grasping at their mother, but they start very early to move on their own, and within only a few weeks or, rarely, months they are independent from their mother in regard to their locomotion and mobility. In contrast, human babies—although their gestation lasts 6–8 weeks longer than in chimpanzees—are highly retarded in their locomotory abilities. The neocortex and motor tracts descending from the cortex to the spinal cord are extremely immature at birth, for example, axon diameter of the corticospinal tract is 10 times smaller than in adults. Descending brain control does not develop before the end of the 1st year of age. Simple spinal central pattern generators cause the action of stepping.

Babies are carried for at least the first half year and often much longer. In contrast, this extraordinary retardation in the first postnatal year allows the baby to fully concentrate on his or her mother’s actions. While being carried or lying, the child’s brain is highly active in following verbal and nonverbal indications, and long before babies can speak by themselves, they have learned to discriminate their mother’s language from other sounds. Language establishes in mother-child dyads with the communication of simple needs and desires by one- to two-word utterances. Typically, not only in humans, the way of investigating the surface and 3-D haptic properties of an object is with the lips and tongue. From haptic to optic inventory, the baby’s world changes from being purely sensomotoric to an increasingly abstract comprehension. Together with the gradual understanding of words and their meaning, abstract thinking capability is gained. It is worth mentioning that the individual constituents of humans acquired within the first 4 years cannot be remembered.

Only in its second year does the child start to develop motor abilities, including ultimately bipedal locomotion and speech by again imitating the surroundings. Note that the adult appearance of myelin is not attained up until 2 years of age. And it is only at the age of 4 that children recognize their surrounding persons as acting, thinking, and feeling in the same way as themselves, and that they develop self-consciousness (theory of mind). As a consequence, children are able to act responsibly and develop a feeling of guilt. A unique feature of humans is the extraordinarily long period of adolescence, which might even last beyond puberty. It will always remain a matter of debate whether cosmological-religious thinking was a constitutive feature of becoming human. The remains of doubtless buried people 100,000 years ago are a strong argument of the very early occurrence of such a belief in the other world. A second strong argument is the ubiquity of such a belief in human cultures. The human brain mass has increased by 3.5-fold as compared to the chimpanzee. Whereas the cerebrum increases fourfold, the brain stem remained almost unchanged. But only a closer look reveals the real changes. Some regions, for example, the optic and acoustic area of the cortex as well as the primary motor area, are almost not enlarged. The secondary areas of association are doubled in terms of their extent, but the tertiary area of association is formed almost de novo, occupying about half of the cortex surface. Especially the frontal and frontobasal cortex show the major increase with respect to the temporal, parietal, and occipital regions. Accordingly, the appropriate parts of the basal ganglia are enlarged, such as the nucleus caudatus and putamen.

The inflation of the human brain is accompanied by an extremely long development of the intracortical connections, even beyond puberty. While all neurons and interneurons are present at birth, cytologically differentiated and in their proper place, they show a continuous growth until late adolescence. During postnatal growth, the thickness of the cortex doubles too among others due to the enormous growth of pyramide cells and their basal dendrites. Primary and secondary areas highly depend on multiple inputs in their postnatal development, for example, the differentiation of the visual cortex lasts at least 3 months before a baby can recognize persons clearly. It is probably the unique development of the tertiary areas that makes us humans, because their differentiation is only controlled and supported by imitation, socialization, and the deliberate, long-term learning within an intact social group. Increasing neurobiological insights have led to a profound understanding of the developmental processes in ontogenesis as well as right- and left-side hemisphere differentiation. Left-right cerebral hemispheric asymmetries exist in extant pongids and also the australopithecines, but neither the pattern nor direction is as strongly developed as the genus Homo.

The Evolution of Speech and Language

Vocalization is known in many different animals and mammals. In chimpanzees, more than 35 different sounds are used but in a strongly defined context. All such vocalizations are controlled by the midregion of the brainstem. In humans, the baby’s vocalizations and all sorts of sounds during fights or sex in adults are of such a type. Unique vocalizations of H. sapiens are crying and laughing, which are also controlled by the brainstem but initiated cortically. Vocalization should not be confused with language.

The informational content of language is exponentially higher than in vocalization. In a second, four to six syllables, each consisting of up to six consonants or vowels, can be communicated. Production and reception of language is situated exclusively in the newly evolved associative tertiary areas of the cortex. In particular, the left hemisphere is specialized to conduct complex arbitrary actions in gesture and the vocal domain. This may have created a left hemispheric bias not only in gestural communication but also in the execution of all technical ideas.

The development of the Broca’s area (the “motor speech” area) in the gyrus frontalis is intimately correlated with the evolution of language. It is unilateral in the left hemisphere. Note that a well-developed Broca’s area is present in early members of Homo more than 1.6 million years ago, for example, KMNER 1470. The Wernicke’s area, which is involved in the interpretation of spoken language, is not a speech center but the secondary auditory cortex and is always bilateral. The Wernicke’s area of the dominant hemisphere (mostly the left side) integrates sounds more rationally, whereas that of the nondominant hemisphere “understands” the melody, which is of the highest importance in early postnatal development.

For a long time, any definition of language, as the basis of speech, has been restricted to audible sounds produced by the action of the vocal, laryngeal organs. Consequently, speech was restricted to human communication through spoken language. Only recently the possible gestural origin of language has been seriously debated. Seen in this light, for most of our evolution language would have been gestural, although more and more punctuated by (vocalizations) the production of speech phonemes. The discovery of a mirror system in primates by Rizzolatti and Arbib and its probable homology with the Broca’s area in the human brain, together with the unique human ability for imitation, has strongly supported the idea that language originated in a complex system of gestures and phonemes. Articulate speech would have been completed relatively late in the evolution of Homo. If we accept that the Neandertals are fully capable of language, this would have occurred before both species split. It remains unclear whether humans are the only ones who used speech alone for vocalization of language and to communicate ideas and feelings. Some scientists suggest that syntax emerged only very late as a result of cultural evolution, whereas other scientists argue that language evolved exclusively in the vocal domain.

Gestural and vocal communication implies anatomical changes in the hand (see above) and vocal system, which can be followed in ontogeny. Set free from thermoregulation due to the extension of sweat glands over the body, the upper respiratory tract changed dramatically. Motor activity in ordinary language comprises the activation of laryngeal muscles, including vocal cords, throat muscles, soft palate, tongue, jaw, and facial muscles in a highly ordered but fast sequence.

Evolution of Culture

By defining culture as the difference between learned and vested behavior of different populations, one of the keys of human evolution becomes obvious: risks are minimized and advantages are maximized by imitating successful behavior. Cultural history began when the survival of the fittest took in tow the imitation of the fittest (Eckhard Voland). This process inevitably leads to cultures and not culture—and to competition of cultures as a driving force of evolution.

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