Symmetry Breaking of Hominin Locomotion, and Migration Feedback

1. Introduction

Why did hominins start walking on two legs? Inevitable seasonal migration between distant habitats may had been a systematic reason for that. As a mental model, the scenario of hominin evolution presented in this paper is speculative but consistent with empirical and fossil findings, and it obeys Lehninger’s (1972) heuristic criteria of plausibility and continuity. In terms of kinetic phase transitions and their symmetries, it proposes a logical prehistoric migration framework, enclosing the emergence of bipedalism and childhood, the ritualisation of human sexual behaviour, and the final hominin spreading by emigration pressure, resulting from amplified reproduction rates (Feistel 2025). A brief and compact summary is given in Section 6.

Physical kinetic phase transitions far from thermodynamic equilibrium, or mathematical bifurcations in nonlinear systems, possess universal qualitative properties. Such transitions found in complex systems may be studied by means of simple mathematical models (Glansdorff and Prigogine 1971; Haken 1977; Prigogine and Stengers 1981; Nicolis and Prigogine 1987).

From its very beginning, the history of life has been a succession of symmetry-breaking transitions (Schrödinger 1944; Oparin 1963; Eigen 1971; Prigogine et al. 1972; Haken 1977; Ebeling and Feistel 1982, 1994; Margulis and Sagan 1995). In particular, the origin of life itself is a symmetry-breaking phase transition of the 2nd kind (Feistel 2023a, 2024) by which structural information of catalytic networks transformed into the first arbitrary symbols of a primordial genetic code. According to Landau and Lifschitz (1966), involved in a transition of the 2nd kind, the two phases possess different symmetries, coincide at the transition point but cannot stably coexist elsewhere. A typical such transition is the Hopf bifurcation for the onset of self-sustained oscillations (Wiki 2026f). In finite-size systems, the vicinity of this sharp transition point is weakened to a transition region with amplified fluctuations (Feistel and Ebeling 1978, 1989). The morphogenetic differentiation of a homogeneous embryo nucleus into tissues and organs belongs to transitions of the 2nd kind, as well as breaking the sexual symmetry of multicellular organisms.

By contrast, such as the physical evaporation of water, the two phases of a transition of the 1st kind may possess the same symmetry and they, within a finite transition interval, may stably coexist in space with a mutual interface. That coexistence region has a broken spatial symmetry as compared to both pure single-phase regions. In spatially extended systems, the transition starts from a nucleation process (Oxtoby 1998; Kashchiev 2000; Schmelzer 2005) accompanied by strong fluctuations. While symmetry-breaking kinetic transitions of the 2nd kind far from thermodynamic equilibrium are frequently encountered in the physics of self-organisation and evolution (Haken 1977; Prigogine and Stengers 1981; Nicolis and Prigogine 1987; Feistel and Ebeling 2011; Feistel 2023b; 2024), those of the 1st kind with a two-phase transition region are less often investigated. Here, however, as a transition model of the 1st kind in anthropogenesis, the hominin evolution from great apes to Homo will be considered in detail as such a special case. For comparison, as an alternative example with similarities to the hominin evolution, a conceptual model of the phase transition of swarming locusts is briefly described in Appendix D.

“The probably most relevant trait which unifies all representatives of the human lineage and which, to our current state of knowledge, did not emerge in parallel, is the upright gait. Locomotion on two legs, also known as bipedalism, is the proper revolution at the beginning of the human evolution. … Even well-documented skeletons, such as for example the one of »Lucy« (Australopithecus afarensis), raised a heated scientific dispute … about the question in how far … modern-like bipedalism had been developed, or whether »Lucy« had mostly lived on trees”2 (Böhme et al. 2019: p. 116, 121). “Hominin fossil discoveries as old as 6 Ma and advances in DNA research have now documented that humans evolved in Africa and then migrated to other parts of the world starting as early as 2.0 Ma” (Ashley 2009). “Apparently, the success story of humans started with some Pliocene species that developed a habitual bipedal way of life, while the shortening and shrinking of the mastication apparatus and the typically human brain expansion had followed only much later with the appearance of the genus Homo … It was not before [the begin of the Quaternary] about 2.6 million years ago that populations of the early Homo – or of Australopithecus – had changed to permanent bipedalism”3 (Henke and Rothe 2003: p. 29, 68).

Apparently, hominin evolution had at least two different evolutionary stages (Donges et al. 2011). The initial stage, from the LCA up to »Lucy« (Wiki 2025a), see Figure 1 for artist reconstructions, is coined by the development of bipedal gait without advanced manual or mental skills, with a small ape-like brain and skull. West of the Awash River of the African Rift Valley, “Australopithecus and early Homo co-existed as two non-robust lineages in the Afar Region before 2.5 million years ago” (Villmoare et al. 2025).

Only after »Lucy«, apparently, the physical traits had arrived at an advanced level that made permanent survival possible in the open grass land without arboreal refuge (Gursky and Nekaris 2007). There, the selective pressure became redirected to mental progress along with social cooperation and advanced spoken language, reduced forelimbs, jawbones and teeth as required for cooperative hunting, habitual use of fire and consuming cooked food (Organ et al. 2011). Fossil brain mass started to exceed 500 g only after 2.5 Myr BP (de Sousa and Wood 2007).”The genus Homo and the earliest stone tools date back to between 2.4 and 2.6 Ma” (Trauth et al. 2010: p. 2981). There are, however, also some 3.4 Myr old cut marks on bones, and 3.3 Myr old stone tools (Roberts 2011; Callaway 2015).

Onward from »Lucy«, along with close social cooperation necessary for survival, rapid human cultural evolution emerged and interfered progressively with the biological adaption to the newly unlocked ecological niche. Supported by communication, fast and cumulative cultural experience added a new quality to the already existing slow, long-range phylogenetic experience and the fast, short-range ontogenetic one. The road to incomparable human success went along the invention, accumulation and versatile exploitation of increasingly complex causal mental models (Feistel 2023b; Marijuan et al. 2026). “Although the reasons for the accelerated evolution of the human brain remain unclear, it is apparent that this acceleration encompasses at least the entire Homo genus” (Rosales-Reynoso et al. 2018). The brain volume increased, for instance (Holloway et al. 2004), from 446 ml (Australopithecus afarensis, 3.11 Myr) during the bimodal transition phase, via 801 ml (Homo ergaster, 1.74 Myr) shortly after it, see Appendix C, up to a typical modern human size such as 1266 ml (Homo heidelbergensis, 0.27 Myr).

In the hypothetical scenario presented here, the famous, 3.2 million years old fossil of »Lucy« may serve as a demonstrative representative, as a culmination point of a metastable, transitional two-phase interval of occasionally enforced bipedal gait of hominins between the single-phase quadrupedal LCA before and the single-phase bipedal Homo population after that, as schematically displayed in Figure 2. Apes and humans may stably coexist, each of both in its respective, spatially separate Lebensraum.

»Lucy« was discovered by Johanson and Gray in 1974 at Hadar, located at the Awash river in the Afar region of the Ethiopian Rift Valley (Johanson et al. 1978). Fossil evidence indicates that between 4 and 2 Myr BP, hominins became adapted similarly to arboreal habitats with ape-like forelimbs as well as to bipedal locomotion on the ground with human-like feet and hind limbs. Such a design is neither perfectly adapted to a permanent life in the trees nor to a permanent life in the open savannah, but fits as a compromise to either conditions prevailing alternatingly. Naturally compelled seasonal migration may constitute a plausible explanation for the evolution of such a compromise.

By contrast to several existing hypotheses for the origin of bipedal gait, which mostly assume that apes more or less voluntarily and optionally tested their permanent life out of arboreal conditions and experienced it as successful and advantageous (Suhr 2018; Smith and Reynolds 2025), the seasonal migration model mandatorily enforces temporary but skilled motion on the ground for a short time interval regularly each year. Selective pressure is high then for gradual improvement of bipedal speedy locomotion. Motion on the dangerous ground, however, reduces the population’s reproduction rate and may take it to the brink of extinction. Upright gait also affects the hominin’s reproductive behaviour and enforces its transformation (Feistel 2025), as described in Section 4.

The swing-climbing quadrupedal Last Common Ancestor (LCA) of chimpanzees and humans is commonly assumed to have lived in arboreal habitats while bipedal Homo is permanently living on the ground in open territories. In this paper, the transition from the LCA to the genus Homo will be considered as a kinetic phase transition of the 1st kind, see Figure 2. The time interval between LCA and Homo may then be understood as a (metastable) two-phase composite during which hominins spent their lives partly in the trees and only in part on the ground, see Section 3 and Appendix C. The related broken symmetry of the hominin skeletal constitution and behaviour is assumed here to have been produced with high selective pressure by a broken spatial and temporal symmetry of the habitat due to regional climatic change, Section 3 and Appendix A.2. Animal migration, Section 2, may serve as a paradigm for a similar, genetically implemented behaviour of hominins. Required for successful migration between separate, only seasonally beneficial arboreal habitats, the increasingly upright gait resulted in early weaning, hazardous childhood and reduced reproduction rates of the LCA, Section 4.

Reproduction rates could even substantially exceed the previous ones due to shorter interbirth gaps of younger mothers, after the sexual traits had dramatically transformed from LCA to very distinct, human-like ones. Increasing population densities are assumed to had caused migration pressure to explore new distant habitats on the ground, such as in natural caves or manually erected shelters. This expansion was supported by advanced bipedal locomotion and forelimbs and hands liberated for the use of tools and weapons by the upcoming Homo for permanent cooperative social life, Section 5.

When water evaporates from the sea surface, for thermodynamic reasons, the fraction of light isotopes becomes larger in the vapour than in the remaining liquid (Wei et al. 2019). It is reasonable to assume that a similar separation of individual properties may also take place during substantial emigration processes. Likely, the migrating individuals were mostly younger, healthier, more curious, courageous and innovative than the remaining resident ones. At least in part, such properties may be hereditary. Ultimately, step by step, in the course of many millennia this radiation of humans conquered the entire globe, starting from a local nucleus where Homo had emerged.

2. Animal Migration

In the morning, marine autotroph phytoplankton is ascending to the light water surface that is exhausted of nutrients. At the end of the day, plankton returns to deep, dark, nutrient-rich levels (Wasmund and Siegel 2008). This simple, practically observed biological example from a different context demonstrates clearly that migration behaviour can, at least in principle, offer selective advantage over resident competitors. Such kind of argument may similarly apply to other conditions.

Regular or occasional migration of animals is widespread in numerous species, from insects to vertebrata. “On the basis of the recent ratio between migrating and resident large animals in East Africa, the number of migrating species exceeds that of the resident ones by a factor of ten”4 (Reichholf 2004: p. 175). “We recognize migration as an adaptation to resources that fluctuate spatiotemporally either seasonally or less predictably, and note that it is often preemptive” (Dingle and Drake 2007). “In the common sense, migration is considered by many authors as a mechanism for avoiding unfavorable environments by moving to expectedly more auspicious locations at different times. In other terms, migration may represent the seasonal movement of organisms from place to place owing to the change in the environmental conditions. Consequently, two important questions arise to mind, why do some animals migrate? How would migration affect the gene pool?” (Elawa 2005).

Typical migration behaviour is instinctive and genetically controlled. Although little is known about the origin of migration, the conditions causing it must have prevailed a long enough period of time in order to propagate the advantageous genes across the entire given population. Migration must have been such a successful strategy that not-participating individuals lost the competition for survival and sufficient reproduction rate, even though migration comes at a cost and with substantial hazards. Here, three kinds of animal migration may be distinguished,

(i)

seasonal migration, such as that of ruminants in the African savannah, following the local rainy season, or of geese and cranes (Weßling 2020) that escape the dark polar winter periodically, driven by external forcing,

(ii)

ontogenetic migration, such as that of salmons (McCleave et al. 1984), whales or turtles which spend their lives on certain feeding grounds but return to their place of birth for the breeding period, driven by internal forcing of individual ontogenetic development, and finally,

(iii)

density migration, such as that of swarming locusts (Dirsh 1974) that leave their habitat when it becomes overcrowded, driven by climate fluctuations and population dynamics, without return to the starting site.

There are multiple examples for regularly migrating animals, such as Monarch butterflies in Mexico, about 1800 bird species, in particular at high latitudes, or ruminants of the “Great Migration” in the tropical African Serengeti (Wiki 2026b). However, “little is known about the conditions under which migration is expected to evolve from an ancestral resident population” (Griswold et al. 2010). “We lack answers to broad questions such as what ultimate factors generally drive animal migration” (Shaw and Couzin 2013). Typically, various animals move from a “breeding” habitat to a “feeding” habitat and back.

Genetically controlled migration may evolve only if the required environmental conditions persist sufficiently longer than the time needed for establishment of related mutations in the population’s gene pool by “evolutionary invasion” (Otto and Day 2007). “A species capable of persisting as residents in two separate sites is vulnerable to the invasion of a migratory genotype if there is a difference in geometric mean fitness between habitats” (Griswold et al. 2010; Fryxell and Holt 2013). Despite the quadratic ecological growth law of a sexually reproducing population, the invasion of a beneficial mutation follows a linear dynamics with exponential increase of the allele frequency in the genetic pool (Feistel 1977; Feistel and Ebeling 2011: Ch. 6.9).

As a biblical plague, swarming grasshoppers such as Locusta migratoria or Schistocerca gregaria are a particularly well investigated case of animal migration (Uvarov 1921, 1955; Dirsh 1974; Song 2004; Stige et al. 2007). Transitions from solitary to gregarious behaviour of locusts typically depend on the population density, which in turn is strongly affected by dry or wet environmental conditions. In China, “for the periods 200 B.C. to A.D. 1900 and A.D. 957-1956, it has been shown that locust outbreaks typically occurred in years reported as drought years or in years after floods” (Stige et al. 2007).

Modelling the bimodal behaviour of locusts is possible in the form of a kinetic phase transition of the 1st kind (Feistel and Feistel 2015), see Appendix D, from the solitary to the gregarious phase. Qualitative similarities of that locust transition are assumed in this paper to exist with the transition from quadrupedally climbing apes to bipedally walking Homo, passing a bimodal hominin stage in which both forms of locomotion coexist. Dirsh (1974) had emphasized that swarming locust Schistocerca is of very recent origin, possibly from the second half of the Tertiary. Continental lowlands fell dry repeatedly on a large scale, and grasslands expanded 20 Myr ago (Van Soest, 1994). Varying environmental conditions that favoured migrating locusts may similarly had also been responsible for hominin migration. Evidently, unlike locusts, to implements migration genetically, such conditions must had prevailed over numerous generations of apes or hominins, that is, through many centuries or millennia.

3. Hominin Migration

“As soon as some ancient member in the great series of the Primates came to be less arboreal, owing to a change in its manner of procuring subsidence, or to some change in the surrounding conditions, its habitual manner of progression would have been modified: and thus it would have been rendered more strictly quadrupedal or bipedal. … If it be an advantage to man to stand firmly on his feet and to have his hands and arms free, of which, from his pre-eminent success in the battle of life, there can be no doubt, then I can see no reason why it should not have been advantageous to the progenitors of man to have become more and more erect or bipedal. They would have thus have been better able to defend themselves with stones or clubs, to attack their prey, or otherwise to obtain food” (Darwin 2004: p. 70-72).

“The role of African savannahs in the evolution of early hominins has been debated for nearly a century. … Data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years” (Cerling et al. 2011). On the other hand, “Australopithecus sediba is a two-million-year-old hominin fossil from South Africa … Stable isotope, dental microwear and plant microfossil analyses … show that A. sediba consumed a diet high in tree leaves, fruits and bark, indicating a woodland environment” (Henry et al. 2012).

„After Darwin’s … early speculations about the evolution of bipedalism and environmental change, the classic savanna hypothesis of Henry Fairfield Osborn and Raymond Dart attempted to link the evolutionary divergence of hominins and other great apes, and the emergence of bipedalism, with the proposed forest-savanna transition in Mio-Pliocene time” (Trauth et al. 2011: p. 2981). The “Savannah hypothesis” for the transition of quadrupeds in trees to bipeds on the ground became known later as the “East Side Story” that was supported also by the leader of the Lucy expedition, Yves Coppens (Henke and Rothe 2003; Suhr 2018). However, already Owen Lovejoy (1981) was in doubt about its plausibility. For example, as a potential model for the hominid origin, modern gelada baboons (Theropithecus gelada) do feed on grass on the ground, „yet geladas … are not bipedal and show no significant adaptations to bipedality” (Owen Lovejoy 1981: p. 342). Several alternative related hypotheses are reviewed by Suhr (2018). In this paper, a migration hypothesis is favoured as a trigger for temporary bipedal locomotion.

„The Hominidae probably originated in Africa sometime between 14 my and 4 my ago. … The origin of Hominidae, or at least bipedalism, has been conjecturally associated with a regional environmental change from tropical forest to widespread grassland. Evidence accumulating from various parts of Africa … suggests this was not an abrupt transition. The pattern of habitats was probably patchy in space and time” (Hill and Ward 1988). Only little is known about details of the variability of climate and geology at the Horn of Africa during the Pliocene (5.3 to 2.6 Myr BP) and the beginning Pleistocene (2.6 to 0.012 Myr BP). However, harsh and fluctuating conditions for survival in the Ethiopian Rift Valley and the Afar Depression (see Appendices A and B) may reasonably motivate speculations about nomadic life and seasonal migration of hominins at that time and place.

“If seasonality is understood as an alternating succession of periods of either shortage or abundance of food, then seasonality can be regarded as a factor that contributes to more intense selection and ultimately to evolutionary change”5 (Henke and Rothe 2003: p. 87). Seasonal animal migration is behavioural normality rather than exception in Africa, and this may also apply to hominins. For example, much more recently, “changes in [Atlantic meridional overturning circulation] AMOC influenced North African climate and, at times, contributed to amenable conditions in the central Sahara/Sahel, allowing humans to cross this otherwise inhospitable region … during a wet phase in the Early Holocene known as the African Humid Period (AHP) … [that] ended at ≈5.5 ka” (Castañeda et al. 2009).

Gradually, climate change may have spatially separated different seasonal habitats from one another. Between those, the regular relocation trek could have included dangerous areas on the ground such as open savannah or rivers to cross. If such a situation prevails for a sufficiently long period of time, selective pressure results in bimodal adaptation, on the one hand for feet and hind limbs to permit safe and fast motion on the hazardous ground, on the other hand to retain the abilities of swing-climbing in the safe arboreal habitats. The skeleton of Lucy exhibits such a compromise, human-like legs and feet are combined with long ape-like arms and hands.

A recent investigation of “directional selection in ancient DNA” has demonstrated that, under sufficient selective pressure, allele propagation in humans may proceed fast enough to change phenotypic traits already within a few millennia. However, while “this includes … increases in measures of cognitive performance [,] these effects were measured in industrialized societies, and it remains unclear how these relate to phenotypes that were adaptive in the past” (Akbari et al 2026). “The best known example of such ‘directional selection’ is a genetic variant that maintains production of the lactose enzyme into adulthood, which enables many people of European ancestry to digest milk throughout their lives” (Callaway 2026). Probably, genetically implemented lactose intolerance of adults had originally emerged to ensure milk supply for hominin offspring, saving those from starvation and keeping the population’s reproduction rate above subcritical values (Feistel 2025).

“Walking on two legs allows for greater energy efficiency compared to quadrupedalism, especially over long distances. This efficiency would have been advantageous for early hominins traveling between scattered resources” (Jakobus 2024). Some “researchers argue that the australopithecine pelvic morphology only makes sense for a biped who may still spend some time in the trees or whose bipedalism differs in style or frequency from that of modern humans” (Gruss and Schmitt 2015: p.5). “Scapulae [of the Australopithecus afarensis skeleton from Dikika, Ethiopia] display several traits characteristic of suspensory apes, ... and their presence in australopith fossils supports the hypothesis that their locomotor repertoire included a substantial amount of climbing” (Green and Alemseged 2012).

Regular migration treks certainly attracted predators, such as the giant bear Agriotherium, hunting hyena Chasmaporthetes or the sabre-toothed cats Machalrodus and Dinofelis (Treves and Palmquist 2007) at the time between 7 and 3 Myr BP of bipedal evolution. Old, sick individuals or toddlers run the highest risks of being killed or lost by accidents. However, increasingly upright bipedal gait prevents mothers from carrying older and heavier infants on their back. This way, the regular loss of offspring reduces the population’s average reproduction rate and selective value. The famous Laetoli footprints from the Kilimanjaro region (Facchini 2006; Roberts 2011) are 3.6 Myr old and suggest that an adult (likely, the mother) and a child may had walked along in lockstep side by side, such as if they went hand in hand.

Similar to the ritualisation hypothesis for the initial emergence of human reproduction traits (Feistel 2025) as described in Section 4, the seasonal migration assumption suggested here does not presume any advanced manual skills, mental capabilities or social cooperation beyond those commonly observed in chimpanzees. Also here, such conditions are well consistent with Lucy’s fossil skeleton. “The pelvis of the skeleton known as »Lucy« from Afar Locality 288 has been fully reconstructed. One of its most salient features is a birth canal whose shape and dimensions show little or no effects of selection for passage of enlarged fetal crania, adaptations that so clearly dominate the form of the modern human pelvis” (Owen Lovejoy 1981: p. 341).

According to current knowledge, the LCA transition to bipedalism occurred successfully only once in the evolution history. This singular event of anthropogenesis may had been caused by rather exceptional conditions, or it may had been so difficult to survive that eventually only one group of hominins was able to overcome its obstacles and dangers through adaptive radiation. It seems impossible to determine in hindcast the exact local geological and climatic conditions of that time, but certainly it was a period of dramatic geological and climatic unrest (see Appendix A, B) that likely had favoured the emergence and evolution of exceptional traits and behaviours such as genetic adaption to regular migration.

4. Emerging Childhood

„The unique sexual and reproductive behavior of man may be the sine qua non of human origin” (Owen Lovejoy 1981). Fossil and genetic evidence is consistent with the plausible hypothesis that the last common ancestor (LCA) of humans and great apes lived about 7 million years ago and was similar to recent chimpanzees (Pilbeam and Lieberman 2017). The sexual behaviour of the latter, however, is very distinct from that of humans. Female chimpanzees breastfeed their offspring, carried on their back, for about five years (Kennedy 2005). Premature weaning poses a high lethal risk to the helpless infant. After weaning, females develop a prominent anogenital swelling as a sex symbol that invites males to mate (Goodall 1991). It is exclusively then that males show relevant sexual interest, and preferably in old “ugly” females. How may human sex life have evolved from such foreign roots during a relatively short period of history? Likely, the radical change was enforced by violent selective pressure.

When the LCA gradually turned to bipedalism, carrying older, heavier infants on the back became impractical and weaning occurred earlier, with increasing risks for the offspring and generally reduced reproduction rates. Females suppressing the fertility swelling protected the toddler by preventing early pregnancy. However, males necessarily responded with permanent sexual interest also in non-swollen females (Diamond 1997; Feistel 2025). Consequently, females reacted with repulsive frigidity, males in turn with coercive mating. Already from a distance, ostentatious adipose breasts, perfectly imitating lactating ones, prevented coercive male approaches, who in return started closer visual, manual or oral inspection of the nipples in order to check fertility and reveal the possible fake. Although there is no scientific evidence available for hominin females like Lucy to possess already permanent breasts, as the reconstructing artist of Figure 1 had imagined, the narrative of undulating sexual conflicts suggested here renders likely the emergence of such breasts already before Lucy.

The permanent adipose breast of human females is a sex symbol that is unique among any animals, and in particular among primates. However, “the possession of permanent, adipose breasts in women is a uniquely human trait that … remains an unresolved conundrum” (Pawłowski and Żelaźniewicz 2021). It is often assumed that capricious male sexual and/or reproductive interest, “according to their standard of beauty” (Darwin 1911: Ch. IV therein), was the main driving force for the emergence of persistent female breasts (Barash and Lipton 2009), so that “the existence of permanent breasts in women is likely an aesthetic trait that has evolved by male choice” (Prum 2017: p. 256). In qualitative distinction from the smooth, gradual increase of fitness by male or female choice of Darwinian sexual selection, it is suggested in the model scenario of this paper that the formation of the permanent breast was an essential step of alternating sexual conflicts between hominin males and females (Pizzari and Snook 2003; Palombit 2010; Feistel 2025), see the schematic of Figure 3. The symbolic breast as a part of courtship habits evolved by ritualisation from a previous contraception trait. Rather than as a preceding cause, male sexual interest in it arose only after females had originally developed it for an opposite purpose, namely, to refuse rather than to attract male mating approaches.

Successful contraception by fertile females caused periodic futile ovulation and subsequent regular menstruation. Both phenomena are known to be only exceptional in chimpanzees (Emera et al. 2011). The resulting ovary depletion lowered the menopause age into the lifespan, so that old females became infertile and could take care of their grandchildren when the mother became pregnant too soon. This grandmother effect raised the reproduction rate substantially, from about 5.5 years in chimpanzees to about 3.5 years in native human cultures (Galdikas and Wood 1990), supporting enhanced migration pressure on the younger generation. The previous inspection of female nipples and genitals was no longer a necessary use-activity and turned into a courtship habit of humans by a ritualisation transition, similar to that of waterfowls originally discovered by Huxley (1914). This is a hypothetical narrative of how the ritualisation of human sexual behaviour was possibly caused by the transition to bipedal gait (Feistel 2025), and how in turn the novel reproduction traits affected the migration behaviour.

5. Migration Feedback

Climatically enforced, an enduring need for seasonal migration may likely had triggered initial attempts of hominin bipedal gait with its various severe consequences for the anatomy and the behaviour of the LCA (Jakobus 2024). This was a bifurcation point for a symmetry-breaking transition from former unimodal apes to then bimodal hominins, the latter equally capable of living under arboreal conditions as well as of migrating across the open savannah with its natural obstacles, predators and various other risks.

In the beginning, the implications created by that first transition were responsible for a critical decline of the reproduction rate due to increased mortality of infants. High selective pressure accelerated the evolution of novel, alternative forms of sexual behaviour which in the end raised the reproduction rates significantly above former values. The resulting population growth was followed by a new substantial migration wave, this time, however, exploring completely new habitats rather than returning periodically to the familiar ones. The perfection of bipedal locomotion achieved during the bimodal phase permitted emigration to more distant habitats with increasing accommodation to living condition in open territories. The succession of a first, reversible seasonal migration by a subsequently caused, irreversible migration is understood here as a nonlinear evolutionary feedback effect.

Supercritical population density is a possible reason for locust migration (Uvarov 1955; Stige et al. 2007), see Section 2 and Appendix D, and may as well had similarly been one for the expansion of Homo. Note, however, that such emigration processes due to competition for resources are not necessarily harmonic and peaceful ones, but may take place in conjunction with violent conflicts between primate population groups (Sandel et al. 2026).

Evidently, survival in the open savannah with its predators and various other dangers is possible for bipeds if those use weapons, build shelters, cooperate closely in social groups or have learnt to use fire, based on intelligence, language and a developed brain. None of those abilities may reasonably be attributed, however, already to the last common ancestor (LCA) of chimpanzees and humans at the very beginning of bipedalism. “It is more likely that hominids venturing into open habitats were already bipedal and that their regular occupation of savannahs was not possible until intensified social behavior was well developed” (Owen Lovejoy 1981; Wiki 2026a). Due to the second migration wave of hominins, survival out of arboreal refuges turned possible and increasingly advantageous. The bimodal hominin population became gradually replaced (Callaway 2015) by unimodal Homo who exclusively settled and fed on the ground as a vast new habitat with exotic new evolution opportunities (Darwin 2004; Engels 2000; Klix 1980; Bramble and Liebermann 2004; Morgan et al. 2015; Marijuan et al. 2026).

6. Summary

Narrative in brief: Miocene/Pliocene environmental change forced LCA to migrate periodically. Changing knuckle-walk to more upright gait improved survival of adults but caused lethal offspring hazard. Implied decreasing reproduction rates recovered only after grave sexual conflicts had been overcome by successively alternating changes to a more human-like behaviour. As soon as hominin skeletons like Lucy’s had adapted bimodally to habitually both living on trees and on the ground, shortened birth spacing owing to novel groomed childhood rapidly raised the population density, and caused in turn emigration pressure into the open savannah without return. Hands and arms, then liberated from climbing, could adapt there to tool use, division of labour and social cooperation, eventually ending up in modern humans. This way, via feedback by bipedalism and innovated reproductive traits, the enhanced and irreversible spatial spreading of upcoming Homo had resulted from original, reversible seasonal LCA migration. Periodic migration had modified reproduction, and reproduction in turn commenced irrevocable migration out of arboreal habitats.

Scenario in more detail: At the end of the Miocene (until 5.3 Myr BP) and during the Pliocene (between 5.3 and 2.6 Myr BP), geological processes in and around Africa (Appendices A,B,C) had fundamentally varied its climate, in particular that of the Afar triangle where today the East African Rift Valley opens to the Red Sea and the Gulf of Aden. In a hypothetical scenario of beginning anthropogenesis in that region, it is assumed that the symmetry of certain large and persistent habitats of the LCA (the Last Common Ancestor of great apes and humans) was spatially and temporally broken into mutually isolated arboreal islands with conditions suitable for LCA survival prevailing only seasonally.

Individuals capable of safely migrating between distant seasonal refuges, such as by gradually more upright knuckle walk, gained selective advantage. If modern chimpanzees may represent former LCA, mothers breastfeed their infants for about 5 years. During that time, offspring is helpless, mostly riding on the mother’s back and cannot survive without assistance and protection. Habitual promiscuous mating prevents paternity information and any childcare by males. Upcoming upright gait physically shortens the time span during which elder offspring can be carried and breastfed. Early weaning prematurely resumes the mother’s oestrus cycle, and pronounced female swellings invite males to mate. Early pregnancy raises the lethal risk for the immature toddler, in particular during repeated seasonal migration moves across open territory.

Protecting infants from early pregnancy required concealed oestrus of females which in turn unavoidably caused permanent mating attempts of males. Alternating sexual conflicts continued until female menstruation, caused more frequently by contraception, had lowered the menopause age down into the life span, and infertile grandmothers may had taken care of toddlers as soon as the mother gave another birth. Birth spacing, typically about 5.5 years in chimpanzees, could reduce to about 3.5 years, a typical rate in native human cultures (Galdikas and Wood 1990). Such a nearly doubling of the reproduction rate implied an exponential growth of the population and high emigration pressure to explore additional habitats.

Evolution of reproductive traits proceeded in parallel to physical adaption of feet, legs and hips to perfect intermediate migration locomotion on the ground while forelimbs remained adapted to climbing in arboreal habitats. Symmetry of morphology required for life on trees was broken in favour of bimodal symmetry for surviving alternately in arboreal home and during migration transits on two legs.

Rising population numbers eventually required permanent survival of emigrated groups away from arboreal regions. While feet and legs were already adjusted to such a life, hands and arms became liberated for new activities that could provide selective advantage for survival on the ground, such as division of labour in hunting, tool production, and childcare in combination with improved communication supported by increasing brain size and appropriate cuisine. After having passed the bimodal hominin evolution phase of commuting between distinct habitats, upcoming Homo refrained entirely from climbing in favour of efficient endurance running on two legs with powerful novel social, mental and manual abilities.