question: why don’t chimpanzees and other primates have buttcheeks like humans, but instead protruding buttholes?
comment: They do not walk upright on two legs, or at least do not do so often enough to develop glutes. The entire reason that we have butcheeks is that they are our glute muscles that allow us to control balance and power our walking and running on two legs.
passages: In species such as baboons and African great apes, an increase in the available capacity of the infratemporal fossa is simultaneously accompanied by a constriction in the sagittal plane. As such, the anterior and posterior portions of the anterior temporalis muscle are inversely correlated in size, with the anterior being larger. Although the temporalis muscle is used for chewing, there is no evidence that the supraorbital structure of primates is dependent upon their respective chewing habits or dietary preferences.
   Gorillas use the form of walking on all fours with the fingers on the hands of the front two limbs folded inward. A gorilla’s forearm and wrist bones lock together to be able to sustain the weight of the animal and create a strong supporting structure. Gorillas use this form of walking because their hips are attached differently than humans so standing on two legs for a long period of time would eventually become painful. Humans would have to walk on all fours as well if they had the same type of hip placement. Gorillas sometimes do walk upright in instances where dangers are present. Walking upright is a way to improve the ability to watch for danger, those who could do so for longer without any pains have a better survival advantage and evolved into bipeds.
   Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because their femurs are not adapted for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee, thus making human knees closer together and under the body’s center of gravity. This adaptation lets humans lock their knees and stand up straight for long periods of time without much effort from muscles. The gluteus maximum became a major role in walking and is one of the largest muscles in humans. This muscle is much smaller in chimps, which shows that it has an important role in bipedalism. When humans run, our upright posture tends to flex forward as each foot strikes the ground creating momentum forward. The gluteus muscle helps to prevent the upper trunk of the body from "pitching forward" or falling over.
   Section::::Shifting consensus.
  
  In the latter parts of the 20th century, new fossil evidence began to emerge which called the savanna hypothesis into question. These newly-discovered remains showed indications that they were still well adapted to climbing trees, even after they had begun to walk upright. Both humans and chimpanzees tend to walk upright when moving along long branches of trees, increasing their reach.
   Section::::Significance.
  
  Even with much modification, some features of the human skeleton remain poorly adapted to bipedalism, leading to negative implications prevalent in humans today. The lower back and knee joints are plagued by osteological malfunction, lower back pain being a leading cause of lost working days, because the joints support more weight. Arthritis has been a problem since hominids became bipedal: scientists have discovered its traces in the vertebrae of prehistoric hunter-gatherers. Physical constraints have made it difficult to modify the joints for further stability while maintaining efficiency of locomotion.
  
  Section::::See also.
  
  BULLET::::- Happisburgh footprints
  
  BULLET::::- Ileret footprints
   It has been speculated that the loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is a result of sexual selection by females looking for honest signals of good health in prospective mates. The reliance of the human penis solely on hydraulic means to achieve a rigid state makes it particularly vulnerable to blood pressure variation. Poor erectile function portrays not only physical states such as age, diabetes, and neurological disorders, but also mental states such as stress and depression.
   Section::::G-family.:Goblin.
   In the 1960s, chimpanzees did not yet occupy the special place in science on human evolution reserved for them today. Baboons were considered the best model of human evolution, since baboons had descended from the trees to become savanna-dwellers, like our ancestors. These rambunctious monkeys, however, are genetically more distant from us, and many of the characteristics deemed important for human evolution, are either absent or minimally developed, such as tool technology, cooperative hunting, food sharing, territoriality, cultural traditions, and certain cognitive capacities, such as planning and theory-of-mind. Chimpanzees show all of them. 
   Upright bipedal walking is commonly thought to have evolved from knuckle-walking with bent legs, in the manner used by chimpanzees and gorillas to move around on the ground, but fossils such as "Orrorin tugenensis "indicate bipedalism around 5 to 8 million years ago, in the same general period when genetic studies suggest the lineage of chimpanzees and humans diverged. Modern apes and their fossil ancestors show skeletal adaptations to an upright posture used in tree-climbing, and upright, straight-legged walking has been proposed to have originally evolved as an adaptation to tree-dwelling.
   Unlike most other primates, humans lack an os penis or os clitoris; however, this bone is present but much reduced among the great apes. In many ape species, it is a relatively insignificant structure. Cases of human penis ossification following trauma have been reported, and one case was reported of a congenital os penis surgically removed from a 5-year-old boy, who also had other developmental abnormalities, including a cleft scrotum. Clellan S. Ford and Frank A. Beach in "Patterns of Sexual Behavior" (1951), p. 30 say, "Both gorillas and chimpanzees possess a penile bone. In the latter species, the os penis is located in the lower part of the organ and measures approximately three-quarters of an inch in length." In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa. An "artificial baculum" or penile prosthesis is sometimes used to treat erectile dysfunction in humans.
   Climate changes around 11 to 10 million years ago affected forests in East and Central Africa, establishing periods where openings in forest lands prevented travel through the tree canopy. During such times proto-hominins could have adopted upright walking behaviour for ever-increasing ground travel, while the ancestors of gorillas and chimpanzees continued to specialize in climbing vertical tree trunks and lianas with a bent-hip and bent-knee posture—which ultimately lead them also to adopt knuckle-walking for minimal ground travel. This differential development within the larger hominid community would result in "A. afarensis" being adapted to upright bipedalism for extensive ground travel while still using arms well adapted for climbing smaller trees. Still, the proto-hominins and the ancestors of chimpanzees and gorillas were the closest of relatives, and they shared anatomical features including a fused wrist bone, which may suggest that knuckle-walking was used for a time by human ancestors.
   The ancestors of gorillas and chimpanzees are suggested to have become more specialised in climbing vertical tree trunks, using a bent hip and bent knee posture that matches the knuckle-walking posture they use for ground travel. This was due to climate changes around 11 to 12 million years ago that affected forests in East and Central Africa, so periods occurred when openings prevented travel through the tree canopy, and at these times, ancestral hominids could have adapted the erect walking behaviour for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle-walking.
   In "Oreopithecus", a Miocene hominid that became extinct , the orientation of the facet on the second metacarpal is similar to human conditions — an indication that it had the capability of pad-to-pad precision grip. "Oreopithecus" also lacks the waisted capitate associated with apes and climbing still present in "Australopithecus".
  
  Section::::Ossification.
   Though a number of fossil hominin (humans and chimpanzees) clavicles have been found, most of these are mere segments offering limited information on the form and function of the pectoral girdle. One exception is the clavicle of AL 333x6/9 attributed to "Australopithecus afarensis" which has a well-preserved sternal end. One interpretation of this specimen, based on the orientation of its lateral end and the position of the deltoid attachment area, suggests that this clavicle is distinct from those found in extant apes (including humans), and thus that the shape of the human shoulder dates back to less than . However, analyses of the clavicle in extant primates suggest that the low position of the scapula in humans is reflected mostly in the curvature of the medial portion of the clavicle rather than the lateral portion. This part of the bone is similar in "A. afarensis" and it is thus possible that this species had a high shoulder position similar to that in modern humans.
   The only primates to have an APL completely separated from the extensor pollicis brevis are modern humans and gibbons. In gibbons, however, the APL originates proximally on the radius and ulna, whereas it originates in the middle part of these bones in crab-eating monkeys, bonobos, and humans. In all these primates, the muscle is inserted onto the base of the first metacarpal and sometimes onto the trapezium (siamangs and bonobos) and thumb sesamoids (crab-eating monkeys).
   Section::::Evidence of bipedalism.
  
  Bipedalism is a characteristic of modern humans. Fossil evidence reveals that hominins walked on two feet as early as 6 to 7 million years ago. It can be difficult to reconstruct gait evolution due to the inadequacy and scarcity of the fossil record. Often, fragments of bones are discovered and offer little information about hominins’ walking manners. Additionally, the hominin fossil record does not provide information concerning the social patterns of modern humans and other primates—patterns which drove evolution. 
   According to the Savanna-based theory, hominines descended from the trees and adapted to life on the savanna by walking erect on two feet. The theory suggests that early hominids were forced to adapt to bipedal locomotion on the open savanna after they left the trees. One of the proposed mechanisms was the knuckle-walking hypothesis, which states that human ancestors used quadrupedal locomotion on the savanna, as evidenced by morphological characteristics found in "Australopithecus anamensis" and "Australopithecus afarensis" forelimbs, and that it is less parsimonious to assume that knuckle walking developed twice in genera Pan and Gorilla instead of evolving it once as synapomorphy for Pan and Gorilla before losing it in Australopithecus. The evolution of an orthograde posture would have been very helpful on a savanna as it would allow the ability to look over tall grasses in order to watch out for predators, or terrestrially hunt and sneak up on prey. It was also suggested in P.E. Wheeler's "The evolution of bipedality and loss of functional body hair in hominids", that a possible advantage of bipedalism in the savanna was reducing the amount of surface area of the body exposed to the sun, helping regulate body temperature. In fact, Elizabeth Vrba’s turnover pulse hypothesis supports the savanna-based theory by explaining the shrinking of forested areas due to global warming and cooling, which forced animals out into the open grasslands and caused the need for hominids to acquire bipedality.
   Chimpanzees helped Köhler to prove that animals are capable of learning beyond simple trial and error, and that, given the right conditions, many species—particularly the more "human" species of primates—will demonstrate a deeper understanding of the constituents of a problem. For example, several chimpanzees who had proven capable of reaching the banana via a stack of crates found that in a crateless room, a table or chair worked to meet the same end.
   "Oreopithecus" has been claimed to exhibit features that are adaptations to upright walking, such as the presence of a lumbar curve, in distinction to otherwise similar species known from the same period. Since the fossils have been dated to about 8 million years ago, this would represent an unusually early appearance of upright posture. However, a reevaluation of the spine from a skeleton of "Oreopithecus" has led to the conclusion that it lacked adaptations for habitual bipedality 
  
  Section::::Physical characteristics.:Positional behavior.:Semicircular canals.
   The authors argue that many of the basic human adaptations evolved in the ancient forest and woodland ecosystems of late Miocene and early Pliocene Africa. Consequently, they argue that humans may not represent evolution from a chimpanzee-like ancestor as has traditionally been supposed. This suggests many modern human adaptations represent phylogenetically deep traits and that the behavior and morphology of chimpanzees may have evolved subsequent to the split with the common ancestor they share with humans.
  
  Section::::Before "Homo".:Genus "Australopithecus".
   Bipedalism is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids. The earliest hominin, of presumably primitive bipedalism, is considered to be either "Sahelanthropus" or "Orrorin", both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorilla and chimpanzee, diverged from the hominin line over a period covering the same time, so either of "Sahelanthropus" or "Orrorin" may be our last shared ancestor. "Ardipithecus", a full biped, arose approximately 5.6 million years ago.
   There are links between human spinal morphology, bipedality, and scoliosis which suggest an evolutionary basis for the condition. Scoliosis has not been found in chimpanzees or gorillas. Thus, it has been hypothesized that scoliosis may actually be related to humans’ morphological differences from these apes. Other apes have a shorter and less mobile lower spine than humans. Some of the lumbar vertebrae in Pan are "captured", meaning that they are held fast between the ilium bones of the pelvis. Compared to humans, Old World monkeys have far larger erector spinae muscles, which are the muscles which hold the spine steady. These factors make the lumbar spine of most primates less flexible and far less likely to deviate than those of humans. While this may explicitly relate only to lumbar scolioses, it is possible that small imbalances in the lumbar spine could precipitate thoracic problems as well.
   "Australopithecus afarensis" seems to have had the same conical rib-cage found in today's non-human great apes (like the chimpanzee and gorilla), which allows room for a large stomach below and indicates the longer intestine needed for digesting voluminous plant matter. Fully 60% of the blood supply of non-human apes is used in the digestion process, greatly impeding the development of brain function (which is limited thereby to using about 10% of the circulation). The heavier musculature of the jaws—those muscles operating the intensive masticatory process for chewing plant material—similarly would also limit development, here of the skull (braincase). During evolution of the human lineage these muscles seem to have weakened with the loss of the myosin gene MYH16, a two base-pair deletion that occurred about 2.4 million years ago.
   Chimpanzees, gorillas, and humans, when walking, walk upright, and their limbs swing in opposition to one another for balance (unlike monkeys, apes lack a tail to use for balance). Disadvantages related to upright walking do exist for primates, since their primary mode of locomotion is quadrupedalism. This upright locomotion is called "orthograde posture". Orthograde posture in humans was made possible through millions of years of evolution. In order to walk upright with maximum efficiency, the skull, spine, pelvis, lower limbs, and feet all underwent evolutionary changes.
  
  Section::::Evolutionary significance.
   Crocodilians use a "high walk", with a more erect limb posture that minimizes sideways flexing, to cross long distances. However, as they evolved from upright walkers with limited bipedality, this may simply be a remnant of past behavior rather than a specific adaptation to overcome this difficulty. Todd J. Uriona (University of Utah) hypothesizes that costal ventilation may have aided the upright posture in overcoming the constraint.
  
  Section::::Solutions.:Avoiding the constraint.
  