Chapter 2: Bipedalism

Animals that move by using only one of their rear (or lower) limbs or legs are said to be bipedal. Bipedalism is a classification of terrestrial movement. The term "biped" refers to an animal or machine that often travels in a bipedal fashion. The definition of the term "biped" is "one foot," which originates from the Latin words "one" and "pes," which means "foot." Bipedal gaits, such as walking or running, as well as hopping, are examples of different types of bipedal mobility.

There are several groups of current species that are habitual bipeds, which means that their regular form of locomotion takes place on two legs. During the Triassic period, certain groups of archosaurs, which includes dinosaurs and crocodiles, evolved the ability to walk on two legs. Among the dinosaurs, all of the early forms and many later groups were habitual or exclusive bipeds. The birds are representatives of a clade of dinosaurs that were exclusively bipedal, known as the theropods. There have been several instances of habitual bipedalism emerging among mammals. This includes the macropods, kangaroo rats and mice, springhare, jumping mice, pangolins, and hominin apes (including humans), as well as a number of other extinct taxa that have separately evolved the trait independently.

There is a greater number of current species that walk on two legs for short periods of time or intermittently. During their sprints, a number of different species of lizards travel on two legs, typically in order to avoid danger. Although there are a few instances in which they walk solely on their hind limbs, the majority of ape and bear species will adopt a bipedal gait in order to reach food or explore their habitat. There are a few species of arboreal primates, such as gibbons and indriids, that walk solely on two legs for the few periods of time that they spend on the ground. When an animal is fighting or copulating, it is common for them to rear up on their hind legs. In order to access food, keep watch, threaten a competitor or predator, or pose in courting, certain animals frequently stand on their hind legs. However, these species do not move in a bipedal manner under any circumstances.

To contrast with the word quadruped, which means "four feet," the word comes from the Latin words bi(s) which means "two" and ped- which means "foot."

It is possible for a species to benefit from limited and exclusive bipedalism in a number of ways. Bipedalism gives animals the ability to elevate their heads, which enables them to have a wider field of vision, which improves their ability to detect threats or resources that are further away, provides access to deeper water for animals that are wading, and enables animals to reach higher food sources with their mouths. While the animal is standing, its non-locomotory limbs are free to perform other functions, such as manipulation (in primates and rodents), flight (in birds), digging (in the huge pangolin), battle (in bears, great apes, and the large monitor lizard), or camouflage.

Both the ostrich and the red kangaroo are capable of reaching speeds of 70 kilometers per hour (43 miles per hour), but the cheetah is capable of exceeding 100 kilometers per hour (62 miles per hour). The greatest speed of bipedal movement appears to be slower than the maximum speed of quadrupedal movement with a flexible backbone. According to the endurance running hypothesis, despite the fact that bipedalism is slower at start, it has enabled humans to outrun the majority of other animals over extended distances. It has been postulated that the ability of kangaroo rats to walk on two legs improves their locomotor performance, [clarification needed], which may help them sneak away from potential predators.

In the field of zoology, behaviors, such as bipedalism, are frequently classified as either "facultative" (meaning they are choice) or "obligate" (meaning the animal has no rational alternative). Even this distinction is not quite clear-cut; for instance, humans other than infants typically walk and run in a bipedal manner, but almost all of them are able to crawl on their hands and knees when it is necessary to do so. There have also been accounts of people who walk on all fours with their feet but not their knees on the ground. However, these occurrences are thought to be the consequence of illnesses such as Uner Tan syndrome, which are extremely uncommon genetic neurological disorders rather than typical behavior. There are numerous examples that are not clear, including the fact that "normal" persons are able to crawl on their hands and knees. This is true even if one disregards exceptions that are brought on by medical conditions or injuries of some kind. Consequently, the labels "facultative" and "obligate" are not utilized in this article; rather, the attention is placed on the extensive variety of locomotion methods that are typically utilized by different groups of animals. There is a possibility that normal humans are deemed to be "obligate" bipeds due to the fact that the alternatives are extremely painful and are typically only utilized when walking is not possible.

When it comes to mobility, there are a few different states that are typically linked with bipedalism.

Only a small number of living taxa demonstrate bipedalism due to the fact that the vast majority of living terrestrial vertebrates are quadrupeds. By elevating one foot at a time, mammals such as humans, gibbons, and huge birds move by. The majority of macropods, smaller birds, lemurs, and bipedal rodents, on the other hand, move by simultaneously hopping on both legs with their feet together. It is possible for tree kangaroos to walk or hop, with the most common behavior being to hop on both feet simultaneously when they are on the ground and to hop on both feet simultaneously when they are traveling among trees.

During high-speed, sprint locomotion, many species of lizards, notably the spiny-tailed iguana (genus Ctenosaura), which is the fastest lizard in the world, transform into bipedal animals.

The bones of the bolosaurid Eudibamus, which date back to 290 million years ago, are the first known creature to walk on two legs. There is evidence that it is a biped, as evidenced by its long rear legs, short forelegs, and unusual joints. Around the beginning of the Permian period, the species went extinct.

It is true that every single bird, as well as every single theropod dinosaur, is a biped. Hoatzin chicks, on the other hand, have claws on their wings, which they utilize to climb.

The archosaurs, which include both dinosaurs and crocodilians, have been known to have undergone multiple instances of bipedalism's evolution. Each and every dinosaur is assumed to have descended from a completely bipedal predecessor, possibly one that was comparable to the Eoraptor.

A divergence between dinosaurs and their archosaur forebears occurred approximately 230 million years ago, during the Middle to Late Triassic period. This occurred approximately 20 million years after the Permian-Triassic extinction event, which was responsible for the demise of approximately 95 percent of all species on Earth. The presence of the Eoraptor dinosaur in the fossil record at this time can be established by the use of radiometric dating using specimens from the early dinosaur genus Eoraptor. In the event that this is the case, the characteristics of Eoraptor indicate that the early dinosaurs were small, bipedal predators. Paleontologists have a strong suspicion that Eoraptor is similar to the common ancestor of all dinosaurs. This theory is supported by the discovery of primitive ornithodirans that resemble dinosaurs, such as Marasuchus and Lagerpeton, in Argentinean Middle Triassic strata. The examination of fossils that were collected reveals that these creatures were definitely small, bipedal predators.

Other dinosaur lineages, such as the iguanodonts, also re-evolved into bipedal creatures at some point in their evolutionary history. Some extinct members of the Pseudosuchia group, which is a sister group to the areetatarsalians (the group that includes dinosaurs and their cousins), also evolved bipedal forms. For example, Effigia okeeffeae, a poposauroid that lived during the Triassic period, is believed to have been bipedal for its time. In the past, it was believed that pterosaurs walked on two legs, however new trackways have all demonstrated that they walked on four legs.

There are several groups of extant animals that have independently acquired bipedalism as their primary mode of locomotion. These groups include humans, ground pangolins, the extinct gigantic ground sloths, and various species of jumping rodents and macropods. Humans, whose ability to walk on two legs has been the subject of intense research, are documented in the following section. During the course of their evolution, macropods are thought to have only ever exhibited bipedal hopping once, at some point in time that occurred no later than 45 million years ago.

On the other hand, the majority of mammals are quadrupedal, hence bipedal movement is not as common among them. Although the majority of primates are typically quadrupedal when they are on land, all primates have some degree of the capacity to walk on two legs. On the other hand, macropods, which include kangaroos, wallabies, and their cousins, as well as kangaroo rats and mice, hopping mice, and springhare, are able to walk bipedally through the use of hopping. There are only a handful of non-primate mammals that walk frequently on two legs and alternate their leg gait. Some exceptions include the ground pangolin and the tree kangaroo, depending on the specifics of the situation. One black bear, known as Pedals, gained notoriety in the community and on the internet due...