Hominin Migration: Bipedalism, Sexual Conflict, and Spreading Homo in a Phase Transition Model

1. Introduction

In their anatomy and behaviour, humans evidently differ significantly from their next animal relative, the chimpanzee (Muller 2017). Despite numerous fossil finds, there remain various open questions yet regarding why, how and when in detail certain particular transitions occurred between the Last Common Ancestor (LCA) of apes and humans and their present-day phenotypes. “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”¹ (Böhme et al. 2019: p. 116, 121). Here, a phase transition model of anthropogenesis is suggested, based on Lucy’s bimodal anatomy, see Figure 1.

As a mental model, the novel scenario of hominin evolution presented in this paper intends to explain Lucy’s half-ape, half-human body shape as being transient under the prevailing specific environmental conditions at that time and place. This hypothesis is inevitably speculative but consistent with empirical and fossil findings as far as available, and it obeys Lehninger’s (1972) heuristic criteria of plausibility and continuity. As a review, this scenario is an attempt to continuously bridge between scattered anchor points of hard evidence and widely accepted assumptions published elsewhere. For this reason, in order to appreciate previous discoveries and ideas, and to avoid plagiarism, statements of other authors will often be quoted literally here, smoothly immersed in the text flow as integrated parts, rather than being critically commented or arguably rephrased.

Since Darwin’s plausible savannah hypothesis, anthropogenesis has often been imagined as a continuous, gradual, perhaps intentional transformation process from quadrupedal life in the trees to bipedal locomotion on the open ground. By contrast to that common picture, this paper assumes the broken locomotion symmetry as a result of a naturally compelled, kinetic phase transition from quadrupedal apes to bipedal humans, crossing a metastable bimodal hominin regime, as schematically presented in Figure 2. Additional symmetry aspects are discussed in Section 5 and Appendix F.

Hominin evolution consisted of at least two different evolutionary stages (Donges et al. 2011). In this model, starting from the LCA, the bimodal hominin regime commenced with an externally imposed need for regular migration between spatially separate, temporarily inhabitable arboreal biotopes, possibly caused by regional climate change about 7 Myr ago in the African Rift Valley (compare Appendix A, B, C). Replacing ape’s knuckle-walking, this migration must had required upright walk over longer distances, perhaps for rapid passing of dangerous terrains, perhaps for a better view atop of the savannah vegetation. Migration frequency, such as seasonal, is incompatible with LCA birth spacing of 5-6 years, and synchronisation between those periods was of no avail. Increasingly bipedal locomotion implied early weaning, enhanced offspring mortality and grave sexual conflicts, as outlined in detail elsewhere (Feistel 2025). This transition phase ended between about 3-2 Myr BP with hominins like »Lucy« (Wiki 2025), see Figure 3 for an artist’s reconstruction. Those bimodal, regularly migrating hominins had both, hind limbs adapted to upright gait, and forelimbs still suitable for swing-climbing in trees. They had a small ape-like brain and skull, and did not yet possess advanced manual or mental skills. This bimodal hominin stage is further outlined is Section 2 and Section 3.

The second stage of anthropogenesis started about 3 Myr BP, see Appendix C. It includes the emergence of the genus Homo with increasing brain size, novel mental and manual capabilities, and various forms of close social cooperation for survival on the open ground, sometimes denoted as human “self-domestication” (Blumenbach 1798; Bednarik 2008; Price 2019; Sauer 2023). This second, Homo stage has already been subject to numerous former publications. It is very briefly described in Section 5, primarily for presenting consequences of the evolutionary changes that had occurred in the bimodal stage before.

As background information of the hominin migration scenario, the geological and climatic conditions in the Afar region are briefly reviewed in Appendix A and B. The fragmentary knowledge available may well be understood as supporting the migration hypothesis as a consequence of the dramatic changes that occurred in the African Rift Valley during the past 10 Myr. Regional fossil finds from that period are summarised in Appendix C.

There is little known yet about hominin migration. Some introduction to animal migration is presented in Appendix D. As a special case, presenting similarities with the hominin transition, irregular migration of locusts is modelled mathematically as a kinetic phase transition in Appendix E. With respect to the physics of self-organisation, the conceptual ideas behind kinetic phase transitions are mentioned in Appendix F.

2. Putative Hominin Migration

»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 similarly well adapted to arboreal habitats with ape-like forelimbs as well as to bipedal locomotion on the ground with human-like feet and hind limbs (see Figure 1). Such a design is neither perfectly adapted to a permanent life in the trees nor to an exclusive residence in the open savannah, but it fits as a compromise to either conditions prevailing alternatingly.

If early hominins had ultimately moved away from the trees to a life on the ground, why should they retain their climbing abilities? Otherwise, if apes had spent their nights safely in the tree tops but had returned daily to the ground, why should they develop bipedal legs and feet? Recent 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). The only plausible explanation for Lucy’s anatomy seems to be that those hominins spent significant amounts of time, including nights, walking or running on the ground, and as well a significant share of their life times in their familiar arboreal habitats. Naturally compelled seasonal migration may constitute a reasonable motivation for the evolution of such a compromise between both genetically implemented climbing and upright bipedalism. Recent animal migration, see Appendix D and E, may serve as a suitable paradigm for such an inherited bimodal behaviour to emerge also in hominins.

„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, however, 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 of the Pleistocene (2.6 to 0.012 Myr BP), see Appendix A and B.

There is no empirical evidence available that may in fact attest hominin migration in the Afar region, neither from fossil nor from geological records. Research has revealed dramatic changes of landscape and climate in the past, as well as the long traditions of nomadic tribes surviving there, which underpin that possibility of migration. Harsh and strongly fluctuating conditions for survival prevailed in the Ethiopian Rift Valley and the Afar Depression. “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 serious doubt about its plausibility. Several alternative related hypotheses are reviewed by Suhr (2018). In this paper, by contrast, a novel migration hypothesis is favoured as a trigger for temporary bipedal locomotion.

“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”² (Henke and Rothe 2003: p. 87). Seasonal animal migration is behavioural normality rather than exception in Africa, and this may had also applied to hominins. Considering for comparison a much more recent example of climate-driven human mobility, “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 terrain or rivers to cross. If such situations prevail repeatedly and systematically 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 demonstrates 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 to 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).

At the time between 7 and 3 Myr BP of bipedal evolution, regular migration routes had certainly attracted predators, such as the giant bear Agriotherium, hunting hyena Chasmaporthetes or the sabre-toothed cats Machalrodus and Dinofelis (Treves and Palmquist 2007). Old, sick individuals or toddlers run the highest risks of being caught or lost by accidents. Increasingly upright bipedal gait prevents mothers from carrying older and heavier infants on their back, which comes along with a raised lethal risks for those. 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. This way, regular loss of offspring had likely reduced the population’s average reproduction rate and selective value.

Similar to the ritualisation hypothesis for the initial emergence of human reproduction traits (Feistel 2025) as described in Section 3, 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, Figure 1. “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 retrospect 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.

3. Reproduction as Transformed by Hominin Gait

„The unique sexual and reproductive behaviour 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 distinct foreign roots during a relatively short period of history? Assumingly, that radical change had been 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 concealing their oestrus by reduced fertility swellings could protect their toddlers by avoiding 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 3 had imagined, the narrative of undulating sexual conflicts suggested here renders likely the emergence of such breasts already before »Lucy«. An additional, different “sexual revolution” in favour of pair bonding followed only after »Lucy«, see Section 5.

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 this model scenario 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 4. The symbolic female breast as a part of human courtship habits evolved by ritualisation from a previous contraception trait. In order to refuse rather than to attract male mating approaches, females had originally developed adipose breasts imitating breastfeeding and infertility. Male sexual interest in those arose only later during their existence, rather being the cause of their emergence.

Successful contraception by fertile females caused periodic futile ovulation and subsequent regular menstruation. Both phenomena are known to happen only exceptionally 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. “Distinctive postmenopausal life stage may have evolved in our lineage when grandmothers’ subsidies for weaned dependents allowed mothers to have next babies sooner” (Coxworth et al. 2015: p. 11806). This grandmother effect raised the reproduction rate substantially, from interbirth gaps about 5.5 years in chimpanzees to merely about 3.5 years in native human cultures (Galdikas and Wood 1990), supporting enhanced migration pressure on the younger offspring generation. The previous male 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, enhanced reproduction traits affected the migration behaviour in a different manner.

Systematically increased hominin reproduction rates resulted in rapid population growth, and soon also in conflicts for limited resources in restricted arboreal habitats. As a consequence, groups of excess population had to emigrate from those refuges to survive ultimately in the open savannah, without returning to the safe life in the trees. Those emigrants, being subject to another kind of selective pressure than before, were the candidates for the upcoming new genus Homo. 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). “In the [Omo-Turkana] basin the relative abundance of Paranthropus and Homo (2/3 and 1/3, respectively) during their long period of coexistence” (Marchal et al. 2025) is evident, consistent with a model of diverging residential and emigrated hominin groups that existed and evolved then in parallel.

4. Emigration and Evolution of Homo

In the very beginning of bipedalism, the implications created by the related first gait transition had been responsible for a critical decline of the reproduction rate due to increased mortality of infants (Feistel 2025), as described in Section 3. High selective pressure accelerated the evolution of novel, alternative forms of sexual behaviour which in the end raised the reproduction rates significantly above the original 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 former bimodal phase permitted emigration to more distant habitats with increasing accommodation to living condition in open territories. 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). The succession of a first, reversible seasonal migration by a subsequently caused, irreversible migration is understood here as a nonlinear evolutionary feedback effect.

“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).

Only in the second migration phase 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; Sauer 2023). 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).

It is only this secondary migration phase after »Lucy« to which to Darwin’s original savannah hypothesis may fit better. “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).

An isolated human all alone in the African landscape may not stay alive for long. In order to successfully conquer the so-far unpopulated and challenging ecological niche, a supercritical group size is required for an expanding nucleus of the new bipedal phase of hominin life off the wood. 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 behaviour was well developed” (Owen Lovejoy 1981; Wiki 2026a). In the course of the second migration wave of hominins, survival out of arboreal refuges turned out to be possible and increasingly advantageous. The resident bimodal population became accompanied and gradually replaced (Callaway 2015) by unimodal Homo who exclusively settled and fed on the ground as a vast new habitat with unprecedented new evolution opportunities (Darwin 2004; Engels 2000; Klix 1980; Bramble and Liebermann 2004; Morgan et al. 2015; Marijuan et al. 2026). The irreversible spreading of Homo likely took place by separate emigration groups initiating different evolutionary branches.

Onward from »Lucy«, rapid human cultural evolution had 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).

The sexual behaviour of hominins changed again in the second migration phase from Australopithecus to Homo. Chimpanzee females mate opportunistically with as many males of the group as possible, assumingly to obscure paternity and to avoid infanticide by males (Tutin 1979; Muller and Pilbeam 2017). This behaviour is possible as long as mothers may live in a self-sustained manner with sufficient food available for all individual group members, which is supposedly the case also for hominins like Lucy. Away from arboreal habitats, however, food of emigrated groups may be rare and difficult to get. If food is available only by cooperative activities such as hunting, mothers may not participate and need support from other group members.

Unlike chimpanzees, hominin females conceal their oestrus, and those males, for successful siring, need to have a permanent sexual interest in females, see Section 3. “If a female satisfied the permanent male desire for mating, she obtained better supply for herself and her offspring”⁴ (Mohrig 1980: p. 74). Indeed such an exchange of food for sex is not exceptional in the animal world. Even “male chimpanzees, which in contrast to humans do not maintain pair bonds, were suggested to share food with females to increase their mating opportunities” (Harten et al. 2019: p. 1). If such deals became systematic in a hominin population, a female may had taken advantage of mutually competing with other females for the most successful hunter or the most powerful male. In return, a male may had taken selective advantage of feeding exclusively the mothers raising his own offspring. As a child may have only one father, any female should have chosen just one male partner to mate with. Vice versa, a successful male may be the biological father of several infants at the same time, so he may act as the same sexual partner of several females, of his “harem”.

This way, expected food shortage and necessary food sharing of the emigrating hominins may had naturally replaced the previous promiscuous mating system with obscured paternity by a system in which a female mates only with a selected male, revealing offspring paternity, similar to the mating habits of modern humans (Walum and Young 2018). The formation of individual families with cooperating members, sexual division of labour in raising offspring, vigilance of exclusive mating partners and mutual jealousy (Owen Lovejoy 1981) has likely been the result of this “2^nd sexual revolution” of migrating hominins after »Lucy«.

5. Broken Symmetries

Symmetry breaking upon a kinetic phase transition is a phenomenon by which basic qualitative properties of a system passing a bifurcation point are systematically different before that point from those behind that point. As an example, a Hopf bifurcation separates continuous dynamics on one side from oscillating dynamics on the other one, see Appendix F. “Symmetry breaking constitutes a precondition for complex behaviour which must have been involved already in processes like the appearance of the first living cells” (Nicolis and Prigogine 1987: p. 113).

For convenience of referring to the three subsequent, qualitatively distinct model stages of the hominid phase transition from LCA to Homo, as schematically displayed in Figure 2, those stages will be denoted here as

(a)

the preceding LCA phase, in which the quadrupedal LCA lived predominantly in the trees with only short knuckle-walking excursions on the ground,

(b)

the bimodal phase, during which hominins developed partial bipedal gait, suitable for regular effective upright migration on the ground between separate arboreal refuges, and

(c)

the spreading phase, in which the upcoming Homo spent the entire life on the ground, conquering this novel niche, and losing most of its climbing abilities in favour of manual skills.

See Appendix E and F for some universal properties of kinetic phase transitions. Model details of the bimodal phase (b) are outlined in a previous article (Feistel 2025). The spectacular fossil skeleton of »Lucy« (Figure 1 and Figure 3) is about 3 Myr old, see Appendix C, and indicates the culmination point of the bimodal phase (b) toward the beginning of the spreading phase (c). Far beyond this paper’s scope, the subsequent spreading phase (c) extended perhaps until the appearance of Homo sapiens about 0.2 Myr BP (Vidal et al. 2021) or the cultural revolution by the first domestication of plants and animals, which is believed to have happened about 12 000 years ago (Bergström et al. 2021; Marchi et al. 2022).

During the evolutionary course from the LCA (a) to the spreading phase (c), several symmetries had been broken at the related bifurcation points:

(i)

Symmetry of terrain and habitat

It is the hypothesis of this model that, similar to contemporary cases of animal migration, originally united habitats became split or displaced by climatic or geological impacts. In a gradual evolution process, the inhabitants got accustomed to the need to regularly cross between distant habitats. Breaking a previously homogeneous Lebensraum into fractions is a symmetry-breaking process which induces associated broken symmetries in biological traits and behaviour.

(ii)

Symmetry of locomotion and anatomy

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 LCA phase, the four limbs of suspensory apes are anatomically and functionally largely equivalent. The change to bipedal hominins broke that symmetry in favour of hind limbs adapted to walking and forelimbs to climbing. Still, the limbs had similar size and strength. After »Lucy«, in Homo, climbing became unnecessary and forelimbs were reduced for novel manual activities, while asymmetrically, hind limbs became stronger and longer for effective endurance running.

(iii)

Symmetry of sexual behaviour

In both the LCA and the bimodal phase, promiscuous mating of apes and hominins up to »Lucy« obscures paternity of offspring. Fathers are not involved in raising infants to maturity, this is the lone task of females.

In the spreading phase, after their ultimate exit from arboreal habitats, Homo’s survival on the ground required intense social cooperation and food sharing. Previous occasional mating with many male group members became replaced by individual pair bonding including paternal responsibility for feeding mother and child. Only in this phase, the nucleation emerges of later human sexual relations with lasting family structures, social rules for restricted mating with other group members, jealousy, shame, and new cultural forms of sexual and social conflicts.

• (iv) Symmetry of beauty

In the LCA phase and in the initial bimodal phase, males prefer mating with old females and sexually ignore young ones. Males mate only with fertile females whose oestrus is indicated by a rosy swelling. Fertile females mate with all available males.

In the spreading phase and at the end of the bimodal phase, males prefer mating with young females and sexually ignore old ones. Males mate with all available females at any time, regardless of oestrus, except females that are breastfeeding or menstruating.

In the spreading phase, females start competing mutually for their chosen successful male.

• (v) Symmetry of ovulation

In the LCA phase, after some preliminary menstruations, females become fertile at about 12 years of age. From then on, with an ovulation period of about 5-6 years, they are either pregnant or breastfeeding for the rest of their lives.

In the bimodal phase, childhood emerges, breastfeeding is gradually reduced to perhaps 2 years. Ovulation starts again in a monthly rhythm, accompanied by menstruation as long females may prevent next pregnancy by contraceptive traits, until infant maturity at an age of 5 years. In this model, pregnancy and breastfeeding is followed by 36 ovulations within 3 years, until the whole cycle repeats. Ovary depletion lowers the menopause age.

At the transition to the spreading phase, females become pregnant again after 2 years of breastfeeding. Females have an ovulation cycle of about 3 years without relevant menstruations until menopause, that is, the onset of the remaining unfertile span of life without ovulation.

(vi)

Symmetry of reproductive success

In the LCA phase, reproduction rates are assumed to be balanced for a stable population.

In the bimodal phase, due to bipedal gait, reproduction rates get out of balance and become adjusted in an undulating transformation process as schematically indicated in Figure 4.

In the spreading phase, owing to the grandmother effect, the assumingly doubled reproduction rate enforces emigration of the excess population and adaptation to new ecological niches.

(vii)

Symmetry of maternal mobility

In the LCA phase, similar to males, chimpanzee mothers may freely move around, regardless of infants clinging to their fur.

In the bimodal phase, upright mothers are restricted in moving range and speed by taking care of infants, as in the case of fossil Laetoli footprints.

In the spreading phase, mothers are strongly restricted in moving with infants across open terrain, remaining preferably in the immediate vicinity and protection of the social group.

(viii)

Symmetry of food access

In both the LCA and bimodal phases, individual food access is assumed to be abundant in the arboreal habitats.

In the bimodal phase, food becomes temporarily scarce in arboreal habitats and enforces migration. Along the migration route, food access may be difficult.

In the spreading phase, requiring new mental and manual capabilities, novel collective efforts are necessary to get sufficient amounts of food, likely only irregularly and of varying kind. Food sharing within the social group becomes indispensable.

• (ix) Symmetry of migration dynamics

In this model, the bimodal phase was started by the need for periodic migration between distant arboreal habitats, assumingly caused by geological or climatic changes, see Appendix A and B.

The spreading phase was started by aperiodic aimless migration from arboreal habitats into the open terrain, caused by supercritical reproduction rates and population density.

For comparison, supercritical population density is a possible reason for locust migration (Uvarov 1955; Stige et al. 2007), see Section 4 and Appendix D and E, and may had similarly been one for the spreading of Homo as well.

• (x) Symmetry of mental demands and brain size

In the LCA and bimodal phases, way of life and mental challenges were similar and the brain size did not increase substantially.

Survival in the spreading phase posed a severe mental challenge regarding food supply, social cooperation and protection against natural hazards. Brain size increased significantly, female pelvis and birth canal adapted accordingly. Tool production, language and division of labour paved Homo’s road to modern humans.