Hylobatian Model

The Hylobatian Model is a scientific hypothesis first advocated by Keith in the early 20th century and revisited recently that deals with the evolution of bipedalism. The model holds that bipedalism evolved from a common gibbon-like ancestor, and was therefore arboreal.

Model History

The first mentions of an ancestor more similar to hilobatids (generic term to refer to the family Hylobatidae, whose main representative is the gibbon) can be found in the works of Keith, who analyzed structural similarities in the vertebral column and anatomical organization of several primates. This same author coined the term "brachiation" to refer to the very common movement of locomotion along tree branches, swinging through the forelimbs (arms), a mode of locomotion especially common in Hylobatidae. Morton was the first to discuss the model in a manner more like what it is today. In 1924, he extensively discussed the evolution of the human foot from anatomical comparisons with other primates and realized a relationship between the human foot and that of arboreal primates. This author further argued that "The gibbon establishes a very apparent connection of the great primate group with the Old World ape group, by reason of its many simian features." In 1928, the author William Gregory conducted a review of the bipedal posture of humans, discussing its evolution from aquatic ancestors. His conclusion reinforced the proposal that the common ancestor of the great primates moved by brachiation. In the late 20th century, advances in the understanding of DNA and the development of sequencing techniques allowed molecular data to be used to understand the evolution of living things. One of the findings of this era is that the human lineage is most closely related to chimpanzees, bonobos, and gorillas. Such a finding was interpreted as clear evidence that the common ancestor among the great primates was not hylobatid-like, and that bipedalism would have evolved from a nodopedalic ancestor (nodopedalism is the habit of locomotion supported under the knuckles, the main method of terrestrial locomotion in chimpanzees, bonobos and gorillas). This weakened the Hilobatid Model, which remained little explored until recently. In 2009, Kivell and Schmitt, following the line of other research that demonstrated significant differences between nodopedalism in chimpanzees and gorillas, analyzed the nodopedalism of these two lineages and concluded that the peculiarities of the movement of each could be explained by independent origins of this mode of locomotion throughout evolution. The authors also point out that several anatomical features, especially the bones of the hands, which were treated as unequivocal evidence of nodopedalism, were in fact evidence of arboreal habit. The independent origin of nodopedalism in the two lineages most closely related to humans weakens the idea of a terrestrial, nodopedalistic ancestor and rekindles the hypothesis of an arboreal, brachiating ancestor. However, it should be mentioned that other authors, such as Williams (2010), argue that the nodopedalism of gorillas and chimpanzees is too integrated to have originated from independent evolutionary events.

Frequency of bipedalism in present-day Hilobatids

Rosen, Jones, and DeSilva (2022) analyzed the frequency of bipedalism in 496 individuals of eight primate species located in 46 zoological institutions. It was found that the frequency of bipedalism was dramatically higher in hilobatids, which obtained an occurrence of bipedalism of 82.8% (compared to 55.7% of the second-place finisher, the gorilla). In addition, the number of steps was higher in Hylobatidae in each bipedalism event. 35.1% of bipedalism events had more than 8 steps. All primate species followed practiced some degree of bipedalism. The authors point out that bipedalism could arise from either species if the selection force was strong enough to select for bipedalism. However, they point out that this force is significantly less in the case of hylobatids, in which the frequency of bipedalism is much higher.

Bipedalism

Bipedalism is the main characteristic of Homininia, a group that includes humans and their extinct close relatives. Among the more than 200 primate species living today, only the human species is obligately bipedal, i.e., it has bipedalism as its only viable mode of locomotion. Many experts, however, prefer not to use bipedalism as a defining characteristic of the human family, precisely because this mode of locomotion occurs in other animal groups, such as dinosaurs (avian and non-avian) and other non-primate mammals. Bipedalism is a mechanically complex mode of locomotion associated with several anatomical modifications in humans. For these and other reasons, several authors have sought to explain the evolution of bipedalism in significantly different ways. In the early 20th century, the consensus was that bipedalism arose in arboreal ancestors. The molecular revolution in the late 20th century, however, generated an explosion in the use of molecular data to understand evolution and allowed conclusions such as that the human lineage is much more closely related to the great African primates (chimpanzees, bonobos and gorillas) than to any other lineage of living primates. This gave strength to the idea that bipedalism would have evolved from an ancestor who walked on his knuckles (nodopedalism). Although very common in the popular imagination, the hypothesis that bipedalism arose from nodopedalism never reached consensus. Several authors have pointed out flaws in this idea and proposed other ways to explain the evolution of bipedalism. One of these ways, elaborated in the early twentieth century and long discussed recently, is the "Hylobatian Model".

The Biomechanics of Bipedalism

In general, human gait is divided into two phases: support phase and swing phase. The support phase refers to the period when the foot is effectively supported on the ground, which is equivalent to a little more than 60% of the gait cycle. On the contrary, the swing phase is when the foot is suspended, without contact with the ground. The support phase begins with the foot touching the ground, specifically the heel region, which contacts the ground first. The whole foot is then placed in contact with the ground, and the body weight is mainly borne by the sides of the feet. The body is propelled forward by its own body weight and by the action of the so-called plantar flexors, which cause the more distal portion of the foot ("ball of the foot") to push the substrate and force the body forward. This phase ends with a final thrust provided by the big toe, the so-called "toe-off." As for the swing phase, its relative time is less than 40% of the gait cycle. Historically, this phase was interpreted as a passive movement, where the leg would function as a pendulum that is carried without significant energy expenditure until it touches the ground again and begins the next support phase. This, however, was contested by other authors, who sought to reconcile the pendulum mechanics of the swing phase with significant energy expenditure. Umberger (2010), for example, found that the swing phase accounts for 29% of the total metabolic energy consumed by the lower limb muscles.

Effects of bipedalism on human anatomy

It is apparent that the unique way humans walk is strongly related to anatomical features associated with bipedalism. Harcourt-Smith (2007) points out, however, that it can be difficult to separate which features are a result of bipedalism and which facilitate bipedalism , even though all indicate bipedalism. One importance of these modifications concerns the recognition of bipedalism in fossils: if a fossil has features that are recognizably indicative of bipedalism, there is good reason to argue that that fossil individual was locomoting on both lower limbs. Among the key features is the orientation of the foramen magnum, the hole where the spine fits. The most anterior position of the foramen is associated with controlling the balance of the head on the spine during bipedal walking. Russo & Kirk (2013) analyzed the orientation of the foramen magnum of three clades (marsupials, rodents, and primates) and concluded that indeed bipedal posture is associated with the most anterior position of the foramen magnum. This study corroborated many other previous studies that advocated the same correlation. Furthermore, bipedal posture is associated with radical changes in the pelvis and lower limbs. Some examples include a narrower pelvis, wider sacrum, a larger acetabulum accompanied by a larger femur head as well, more laterally positioned ilia, etc.

Other Hypotheses

The unquestionable importance of bipedalism for mankind and its complex evolution allowed for several explanations regarding its evolution. It is important to point out that the hypotheses are not necessarily mutually exclusive, so that many of them can be reconciled.

Evolution from nodopedalism: The main hypothesis for the evolution of bipedalism and that is more fully present in the popular imagination is the evolution from a nodopedal ancestor. This explanation gained strength with molecular findings and has been defended until today by many experts. As early as 1967, Washburn discussed the possibility of evolution from nodopedala ancestors and disputed a possible arboreal ancestor. According to the author, orangutans and gibbons rarely come down from trees and practice terrestrial locomotion. Richmond and Strait (2000), argued for a nodopedal ancestor, reinforcing the idea that nodopedalism would have arisen only once and that differences in the locomotion of chimpanzees and gorillas would be due to body size-related adaptations that would have arisen later. In the following year, Richmond, Begun and Strait discuss the main models regarding the evolution of bipedalism, emphasizing that the main way to test the hypotheses is to analyze the characteristics that would be retained in living primates and in fossils. That is, if the human lineage evolved from an ancestor more similar to chimpanzees, it is expected that certain characteristics have been retained in the body structure of our lineage. In 2008, authors Richmond and Jungers discussed the mode of locomotion of the fossil Orrorin tugenensis, concluding that it was bipedal and defending the idea of a nodopedal ancestor. This hypothesis is considered the best known by the lay public, and is widely disseminated by the media. However, there is no consensus among academics.

Savanna Hypothesis: The Savanna Hypothesis argues that bipedalism arose as an adaptation to open environments, where hominins are found. The bipedal posture in an open environment would facilitate the detection of potential predators as well as prey. The first mention of something like this hypothesis is in the work of Lamarck, who in 1809 defended the passage from an arboreal and quadrupedal ancestor to a bipedal ancestor. A little later, in 1871, Darwin would defend the same idea in a very similar way. The main force that would have pushed pre-human ancestors to abandon the arboreal habit would be the increase of aridity, accompanied by an expansion of savannas and a shrinking of forests. The formulation of the hypothesis in a more modern way and its effective popularization would come with Dart. In 1925, this author described a new species, Australopithecus africanus, and argued that the bipedal posture of this species would be associated with the savannic environment in which it supposedly lived. His ideas did not produce some effect in scientific academia until more than 20 years later, when A. africanus was effectively accepted as a hominin fossil. For Dart, the evolution of bipedalism on deforested sites was strongly associated with behavioral characteristics of the hominin lineage. For him, pre-humans would be primarily hunter-like beings who would exhibit aggressive and cannibalistic behavior, including strong violence among members of their own species. Although present in the popular imagination, the idea of a hunter ancestor has been disputed by several researchers. Later, the savannah hypothesis was associated with other elements, such as that an upright posture would be associated with better body temperature control. Criticism of the savanna hypothesis is based primarily on evidence that bipedalism would have evolved in forested environments. Australopithecus afarensis, for example, was a bipedal species with arboreal habits.

Tripedalism as an intermediary stage: Kelly (2001), based on the idea that bipedalism would have originated from an ancestor that walked on the knuckles, theorized that the intermediate stage between a nodopedalic animal and a fully bipedal animal would be a type of locomotion called "tripedalism". The author argues that tripedalism would be evolutionarily favored, since the nodopedalic ancestor would have a free limb to handle tools and perform other activities. The main evidence brought by the author is a body asymmetry on the right and left sides of the body, which would have been caused by an asymmetric locomotion behavior, which is the case of tripedalism. Tripedalism is observed in chimpanzees. The aforementioned hypothesis has been little discussed and has not reached consensus.

Conclusion

The relevance of bipedalism to the hominin clade makes understanding the evolution of this trait one of the most important in human evolution. Several hypotheses, most proposed many years ago, need an adjustment regarding the most recent evidence on bipedalism presence in fossils, paleoenvironment reconstruction, etc. Some authors explore the possibility of biped evolution driven by several factors that would have acted together to select this mode of locomotion. Good conclusions will only be obtained by combining what is already known with new evidences, respecting the interdisciplinary nature of the subject.

References

This article "Hylobatian Model" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:Hylobatian Model. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

---