The History of Our Tribe Hominini Textbook Part I An Introduction to Paleoanthropology Chapter 5 What is a Hominim

We are species throug tree ! hon . What is a , as are all those bipedal apes that came before us . Figure shows most of the time , some of which we are descended from and some that are side branches in our tribal It Id . Ma menus . EB TA 72 VI Figure species through time . species distributed through time from in Wikipedia . is licensed ANATOMICAL TERMS In order to adequately understand a discussion of evolution and appreciate changes over time , some basic anatomical information is necessary . It is also necessary in order to distinguish primitive or skeletal characteristics from those that are derived , ie . those that arose later in time . Any discussion of anatomy assumes that one is speaking of a body in anatomical position , ie . facing forward if standing or supine ( face up ) if lying down , with palms forward or up ( see Figure ) When ing particular structures or regions of the body , we make use of the following terms to the soles ( or plantar surfaces ) of the feet . to the midline of the body . Latera to the far right or left of the body , relative to the midline . closer to the top of the head . The following two terms are used in reference to the limbs closer to the base of a limb . 39

to the end of a limb . You may hear your professor use the above terms when describing aspects of human or fossil specimen anatomy . Note If you are thinking about a career in paleoanthropology , get as much training in anatomy as possible , even while an undergraduate . Anthropology and biology departments may offer courses in human anatomy , human osteology , forensic anthropology , and the like . Finds like Lucy ( Chapter 11 ) come along once in a lifetime . Most discoveries consist of little pieces of fossilized bone or teeth and thus knowledge of human anatomy is crucial for recognizing possible landmarks on the remains . HUMAN GROSS SKELETAL ANATOMY The terms gross anatomy and macroscopic anatomy refer to the study of structures that are visible to the naked eye . In this section , we will consider whole bones versus the individual parts of bones and only those bones that are external , as those are most relevant in a discussion of and human evolution . While the anatomy of the lower limb takes precedence in a discussion of , other parts of the body also changed over time . The anatomy of the skull is particularly important because the skulls and faces of species changed over time and geographic space . inner ear ) Figure Human axial skeleton ( shaded ) Axial skeleton diagram by Mariana Ruiz is in the public domain . 40 The History of Our Tribe

Figure Human appendicular skeleton ( shaded ) skeleton dia by Ruiz is in the public domain . We will begin with regional anatomy . The axial skeleton consists of the head and torso . Regionally it is divided into the skull , thorax , and spinal column . The spinal column is divided into the seven cervical ( neck ) twelve thoracic ( chest ) five lumbar ( lower back ) five sacral ( also as sacrum ) and four ( also known as coccyx or tailbone , they may also number three or five ) vertebrae . The limbs and shoulder and hip regions make up the appendicular skeleton . The upper limb is also as the arm or forelimb . It is divided into the arm ( upper arm ) forearm , wrist , and hand and fingers . The lower limb is also known as the leg or and is made up of the thigh , leg ( equivalent of forearm ) ankle , and foot and toes . Individual bones of the axial and appendicular skeletons are labeled in Figure . They will not be discussed here . What is a 41

Cranium Spinal Column Cervical nun ) Thoracic clavicle mum Humerus Ulna Radius Pelvic girdle Carpals Phalanges Phalanges Figure Human skeleton . by Mariana Ruiz is in the public domain . 42 The History of Our Tribe THE EVOLUTION OF Figure 55 Running man . Nude Man Running by is in the public domain . There are a variety of theories as to how evolved and why it proved to be so successful for early . One early idea suggested that by standing up , our ancestors would have been able to see above the grass and thus avoid predation . Baboons and patas monkeys provided living models for hypothesizing the environmental stresses early might have faced on the open plains of Africa . While they likely traveled through open areas , we now know that the earliest were exploiting forest resources , as evidenced by their thinner molar enamel , relative to later . There were also theories that involved the freeing up of the hands to make and use tools and for carrying resources to a safe place or home base . Owen Lovejoy believes that allowed males to provision mates with resources ( Lovejoy , 1981 ) Those males with the most advanced bipedal capabilities would have had an increased chance of mating and possibly offspring survival , and thus would have spread throughout the population . While Lovejoy makes a good case for how a trait could be favored in a population , it is not clear why females would have needed to be provisioned unless their offspring had already lost their ability to hang on with their feet , and hence became a burden to foraging . We now know that , while bipedal on the ground , had a divergent hallux , so that young animals could likely have hung on to their mothers in trees like modern mates . Although we can not know for sure whether they were covered with hair , I can only speculate that when on the ground , young infants may have hung by clasping the fur on their mothers ( anterior aspect of the trunk ) while the mother supported their bottom . As the baby matured , it may have clasped its hands around the mothers neck or chest and hung on to her fur with its feet and later ridden back like modern quadrupedal monkeys and apes . Eventually , it would have walked beside her from place to place . However , if resources had become extremely scarce , bipedal males may have ventured out onto ground for resources with which to provision their mates . Another theory that sees males as being the impetus for suggests that males may have been more terrestrial and females more arboreal , a case of niche partitioning , like gorillas and the mandrill and drill monkeys , where males forage on the ground and females and young spend more time in the trees . Other theories also suggest that was a response to the changing nature of the resource base . For example , and Kevin Hunt . What is a 43

( a theory known as Hunt Postural Feeding Hypothesis , Hunt 1996 ) believe that the ability to stand on two legs for long periods of time would have facilitated picking fruit from the terminal branches of low , scrubby trees in the increasingly open habitats of East Africa . While the aforementioned theories are not mutually exclusive and there was likely a synergistic effect that resulted from our ancestors changing locomotor capabilities , a highly plausible model suggests that it was our ability to break out of the ape habitat that facilitated our evolutionary success . The following items will help us to better understand this great step in our ancestry Apes are descended from an arboreal climber from the early of Africa . The ancestor of the African great apes possessed a suspensory hanging adaptation and thus had an upright trunk that was wide and shallow mobile shoulder and wrist joints long arms relative to leg length long , curved hand and foot bones and an opposable big toe ( hallux ) The next section explains how that thoracic morphology facilitated our ancestors ism . The common ancestor of chimps and was likely a quadruped that was adapted to climbing , feeding , and sleeping in trees , as well as moving and foraging on the ground . The late of Africa was marked by climatic change that spurred floral and hence nal changes . Equatorial Africa was cooler and increasingly drier than in earlier times . The Indian subcontinent continued to move north , resulting in the uplifting of the Himalayan mountain chain that produced a rain shadow , clouds that would have previously drifted down into Africa now lost their moisture on the mountains . Drying winds and cooler temperatures resulted in forest reduction and fragmentation in Africa , splitting and isolating resident faunal populations . The majority of African ape species went extinct due to loss of habitat . is an efficient means of locomotion for covering distance on fairly level ground . A more vertical posture reduces the surface area exposed to solar radiation in a more open environment . It also raises a large percentage of the body away from the hot ground , where it is exposed to cooling breezes . The ancestors of the adapted to the changing environment by becoming bipedal on the ground . Over time they developed a more efficient heat exchange system for their bodies ( sweating ) and brains ( large venous sinuses for rapid blood movement ) and lost much of their body hair . Based upon early teeth , they were like chimps , likely getting the majority of their carbohydrates and fats from fruit , protein from young leaves , and possibly fat and protein from animal matter , social insects ( chimps and gorillas eat a lot of ants and mites ) and animals caught opportunistically . No early exhibit the same degree of canine size or sharpness as chimp and gorilla males . Their canines stay sharp via a honing ( sharpening ) action with the first lower premolar , termed a sectorial premolar due to its cuspid morphology . The combination of the action and morphology of the two teeth is termed a honing complex . Males canines are exceptionally large and , in combination with 44 The History of Our Tribe

their high degree of sexual dimorphism , are used to compete with other males for access to females . Of course , they are also useful for predator defense . Thus , if the common ancestor of chimps , gorillas , and possessed a premolar honing complex , as seems likely , the early fossil evidence suggests that they were losing their fighting teeth . In addition , fossil do not exhibit the same degree of sexual dimorphism as seen in chimps and especially gorillas . Later chapters contain more information on . Most apes went extinct as their habitats dwindled and they competed for limited resources . However , with an efficient means of locomotion to move between forest patches when resources became depleted , could continue to exploit those resources to which they were adapted . They also likely evolved new capabilities for exploiting newly encountered food items as they moved through and between zones . Loss of habitat and resources often leads to local extinctions . By expanding their home ranges and dietary niches , survived while the majority of their close relatives did not . See Figure for individual bones . The majority of bipedal characteristics involve the hip ( or pelvic girdle ) and lower limb . However , as will be seen below , certain skull and trunk characteristics are also adaptations for bipedal locomotion . In addition , we have inherited many aspects of our upper bodies from our ape ancestors and those will all be discussed in the following sections . SKULL The skull consists of the bones of the and face and the mandible ( lower jaw ) The foramen num is the hole in the occipital bone situated in the base of our skulls ( see Figure ) It is where our spinal cord exits the cranial vault . In , the foramen magnum is positioned more anteriorly than in the other apes because our head sits on top of our vertebral column . Thus while the earliest had very faces , the position of the foramen magnum shows that they were bipeds . What is a 45

Figure magnum indicated from inside skull vault . magnum by is licensed under . Figure Bones of the skull . Plate 188 from Gray Anatomy . Side view of the skull by Henry Carter is in the public domain . 46 The History of Our Tribe VERTEBRAE Ape spines are not as flexible as monkeys spines , giving us better upper body support since we are more upright than most other primates . Our vertebrae increase in size and robusticity from top to bottom so that our lumbar vertebrae are very large they sit on the fused vertebrae of the sacrum , which is firmly attached to the hip bones . The sacrum is large and broad and curves inward ( as does the Coccyx ) to help support the organs . Thus our spinal column is a strong supporting structure for the upper body . We have two larger curves in our backs relative to the other apes , the cervical curve and the lumbar curve . The fact that our heads are more upright than nonhuman apes means that the cervical vertebrae must form a more cave curve , the superior aspect of our neck is arched back relative to theirs ( see Figure ) The more pronounced lumbar curve forms when we stand up and begin toddling about . The joints between the bar vertebrae are easily strained and it is thus important to maintain strong back and abdominal muscles throughout life , to aid in the stability of the region . Figure Chimp ( left ) us . human spine ( right ) Illustration by Taylor THORAX The thorax consists of the sternum , ribs , and thoracic vertebrae . The ape thorax is adapted to climbing and swinging in trees . It is wide ( right to left ) and shallow ( front to back ) relative to quadrupedal monkeys , which have a narrow and deep thorax like those of dogs . While the morphology of the thorax was originally adapted to arboreal climbing , the upright trunk also allowed for bipedal locomotion . In addition , the shallow nature of the thorax brings the center of gravity closer to the vertebral column for better weight tion and transfer . Apes are better bipeds than most nonhuman primates . They can walk for short to moderate distances , depending on the species , but it is not efficient and they can not maintain it for very long . SHOULDER GIRDLE The shoulder girdle consists of the clavicle , scapula , and humerus . The shoulder joint of extant nonhuman apes and early is and was angled upward , demonstrating the arboreal ancestry of those . What is a 47

and , in combination with their long , curved fingers , it suggested that they could ascend and climb about in trees . Our stabilize our shoulder joints for swinging and hanging . The triangular shape of the scapula ( shoulder blade ) is more equilateral than that of a quadrupedal monkey , which is more elongated . Our are more mobile than those of a typical quadruped and the articular surface of the scapula , where the round head of the humerus articulates ( makes contact ) is shallow and allows us to rotate our arms at the shoulder . This suspensory hanging complex of clavicle , scapula , and humerus morphology ( also elbow and wrist below ) along with the muscles and connective tissue involved , allows us to climb , hang , and swing by our arms . UPPER LIMB The upper limb consists of the humerus of the arm , the radius and ulna of the forearm , the eight carpal bones of the wrist , the five metacarpals of the body of the hand , and the phalanges of the digits ( three per finger and two per thumb , or pollex ) As mentioned , the head of the ape humerus is round , resulting in a very mobile shoulder joint . We can fully extend our arms at the elbow in order to hang or swing , whereas can not . In addition , we can supinate and pronate our forearm , move our hand palm up or palm down . This movement is possible because the head of the radius is a concave disk that rotates on a structure termed the capitulum on the distal humerus ( see Figure ) Our wrist joints are very flexible , allowing us to rotate and twist our hands in a variety of ways . Early had short legs , long arms , and curved fingers . Over time , legs lengthened so that their intermembral index ( became reduced . The intermembral index is the ratio of to , calculated by the following equation MI ) 100 If an animal has long legs and short arms ( like us ) they have a low IMI and vice versa if and are approximately equal in length , such as in terrestrial , the will be close to 100 . HIP OR PELVIC GIRDLE Our shoulders are somewhat analogous to our butts ! This is because muscles originating from outside the limbs are crossing the joints to insert upon and move the limbs . Because we are bipeds , it is not as apparent as it is in , whose and move similarly . if you compare the humerus and femur and the muscles that cross the respective joints ( deltoids and gluteal muscles , respectively ) you will definitely see similarities . Our pelvis is very unique and interesting . It has changed significantly from an ape pelvis ( see Figure ) The pelvis is made up of three bones the two lateral bones , termed or os , and the sacrum . Collectively , they form a structure that holds our internal organs while providing support for our upper bodies . Each innominate consists of three bones that fuse during development the ilium , ischium , and pubis ( see Figure 513 ) They meet at the hip joint . became shorter and broader , so that the ilium wraps around laterally from an earlier , more posterior position . This changed the orientation and action of our hip muscles , allowing for our striding gait and the ability to balance our weight on one fully extended leg while the other leg is in the swing phase . A portion of the gluteus maximus muscle inserts behind the hip joint in ( versus lateral in chimps ) and thus instead of abducting the femur ( moving it out laterally , as when doing jumping jacks ) it changed to a powerful hip extensor ( ward motion ) for running . 48 The History of Our Tribe

Elbow . Right anterior View Lateral epicondyle Medial epicondyle Coronoid process Ulnar tuberosity Figure Left of chimp ( left ) center ) and human ( right ) Illustration by Taylor . Figures and illustrate the landmarks that are adaptations to . The iliac crest is long and curved , as the bone wraps around laterally . The iliac blade is short but expanded horizontally . The iliac blades are or reinforced to handle the stress of our upper body weight . The thick section of . What is a 49

Figure 511 Human pelvic morphology The three fused bones of the ( ilium , ischium , and pubis ) sacrum , and Coccyx . Plate 241 from Anatomy . Male pelvis by Henry Carter is in the public domain . bone , termed the iliac pillar , can be seen running from the iliac crest ( at the iliac tubercle ) down behind the hip joint . The articular area , termed the , is large and deep , providing a stable socket for the head of the femur . The joints of the pelvis are very strong and relatively immobile ( compared to the shoulder joint ) The sacroiliac joint ( between the ilium and the sacrum ) is large and more posterior and proximal to the hip joint than in quadrupedal apes . Together with the strong pubic symphysis ( anterior joint where the pubis portions of the two meet ) these characteristics make for a very stable supporting structure . LOWER LIMB The lower limb consists of the femur of the thigh , the tibia and fibula of the leg , seven tarsal bones of the ankle , five metatarsals of the body of the foot , and phalanges of the digits ( three per toe and two per big toe or hallux ) The head ( proximal structure ) of the femur is large . The femur angles medially ( inward ) from hip to knee , so that our upper body weight is transferred down through our hip joints to our knees . This is termed the carrying or bicondylar angle . The knees of quadrupedal apes are directly below the hip joint , so there is more strain on the knee joints when they walk ( see Figure ) We have two bulbous structures on the bottom of each femur , termed . The innermost condyle , the medial condyle , has lengthened in so that the femur sits on top of the flat tibial surface . If this were not the case , the medial condyle would not make contact with the tibial plateau , due to the bicondylar angle . Weight is transferred through our innominate , hip joint , and lateral condyle of the femur ( see Figure , Human ) Unlike apes knees that are chronically flexed , our knees are capable of full extension each locks into place when the other leg is in swing phase , giving us a stable supporting leg . Each gluteus alternately supports the opposite side of the torso and pelvis , so that it does not slump on the ported side . 50 The History of Our Tribe

Figure of the bones of a left ( lateral view ) Plate 237 from Anatomy . Note where the , and pubis articulate at the . Plan of of the hip bane by Henry Carter is in the public domain . Figure Pelvic girdle and weight loading on knee joint . Illustration by Taylor . What is a 51

Figure 514 Human foot bones . Plates 290 and Grays Anatomy . Skeleton oot . Medial as and Skeleton of foot . Lateral aspect by Henry Canter are in the public domain . Our feet ( see Figure ) have changed dramatically from a mobile , grasping structure to a rigid , supporting one . The tarsal bones of the human ankle are large and robust for support . The joint between the distal tibia and fibula is fairly immobile , so that the two bones are firmly lashed together . Together , they articulate with the talus ( most superior tarsal bone ) in a hinge joint . We have lost much of the mobility of an ape foot and thus have become less agile in climbing over time . The calcaneus or heel bone is very large and robust and , along with the ball of the foot ( distal end of the first metatarsal ) and the area below the baby toe ( fifth metatarsophalangeal joint ) forms a tripod structure . Our feet have three arches for support , shock tion , and forward propulsion they are the medial and lateral longitudinal arches and the transverse arch . toes became shorter and less curved over time . While shorter , the hallux became more robust and lost its degree of divergence and opposability , by coming into alignment with the lateral four toes , digits II through EVOLUTIONARY TRENDS Just as there are evolutionary trends that characterize primates , there are also trends that characterize the , traits that developed over time and occur to varying degrees in the various species ( see Table ) While all of the morphological changes involved with are trends , there are also relevant characteristics in other parts of the body . 52 The History of Our Tribe

Table EVOLUTIONARY TRENDS and related morphological adaptations . and corresponding prolonged juvenile dependency . Loss of craniofacial robusticity and muscle attachments . Reduced prognathism . Reduction in size of dentition . of . Increased manual dexterity . Elongated legs and increased stature . Increased reliance on culture , technology , and language . Increased reliance on meat in the diet . Most notable , our ancestors and their relatives became increasingly more intelligent . Our brains have increased in size more than , from a more brain ( 400 ) in the earliest to a mean of . This likely occurred in response to environmental stresses as well as competition with other for resources . Skull size and shape changed in response to , increasing brain size . Brains are very costly organs and researchers believe that in order for brain size to have increased , there would have had to have been a corresponding decrease in some other costly organ system . It is that a higher quality diet allowed the gut to shrink and , in turn , the brain to expand . Marked in the lineage began with the first members of our own genus Homo . While there is some evidence that earlier species ( eg . manufactured tools , there is solid that early Homo did , and the archaeological record suggests an increasing reliance on meat in their diet . While the dimensions of the thorax remained wide and shallow , the shape of the rib cage changed from a more conical , thorax that could accommodate a large gut , to our more barrel shape . As our brains became bigger , our ancestors gave birth to increasingly altricial ( helpless ) young that their development outside of the womb . Our offspring require protection and care for a much longer period than those of the other great apes , ie . we have a prolonged juvenile dependency period . At least by the time of the earliest members of the erectus grade at ( see Chapter 27 and subsequent chapters ) it is thought that infant requirements necessitated assistance from mates relatives . Many of the early had pronounced , jaws , termed prognathism ( pro forward jaw ) Over time , became more , or orthognathic . While extant African apes retain primitive prognathism and the dental complex , lost those pronounced canines , as well as the gaps in the corresponding tooth canine diastema ( singular ) or ( plural ) allow apes to close their jaws . The first mandibular premolar into the characteristic ( morphology over time , as opposed to the apes pid sectorial premolar . The loss of the honing complex may have been in response to reduced competition for females as a result of . Once the jaws no longer locked into place , our were free to grind their food , aiding in its digestion . What is a 53

Figure muscle Originates on frontal , parietal , and temporal bones and inserts on mandible . Zygomatic is shown as having been cut to reveal underlying muscle . Plate 382 from Gray Anatomy . The by Henry Carter is in the public domain . The size of jaw and neck muscle attachment sites on the skull became reduced in the lineage over time , along with a reduction in the size of the teeth and craniofacial robusticity . The action of the powerful muscle ( a muscle of mastication ) changed from primarily acting on the front of the jaw , allowing apes to clamp their jaws powerfully shut during fighting , to acting on the molar region for grinding food ( see Figure ) The origin of the muscle moved over time from the midline of the top of the skull to a more inferior position on the lateral aspect of the frontal and parietal bones ( see Figure ) due to the reduction of the sagittal crest and decrease in power in . increased as our ancestors came to rely less on an arboreal environment and became primarily terrestrial . The pronounced increase in and stature seen in the erectus grade ( see ter 27 ) relative to past species , allowed them to cover vast distances efficiently and they moved from Africa to Asia in a relatively short period of time . fingers became shorter and lost their curvature over time . By the time of the , hands had become more dexterous . There is evidence that possessed a power thumb , giving them increased abilities for holding objects in one hand while manipulating or working them with the other hand . This was necessary for our ancestors to have made and efficiently used tools . The first metacarpal was adapted for such activity , and the intrinsic thumb muscles ( within the hand versus ing from the forearm into the hand ) were . There is some controversial evidence that by the time of the later robust , or , expanded fingertips had appeared . Thus they may also have possessed the enlarged tactile pads and increased sensitivity and vascularization that allows for modern humans fine degree of manual dexterity . Finally there is the cultural side of things . We are like no other species that has ever lived . We are unique in our ability to practice , produce , and pass on culture . Our brains are predisposed to the acquisition of language and culture . Thus , the final evolutionary trend seen in the is an increased reliance on culture and language over time . 54 The History of Our Tribe

GROUPS The can be divided into five groups that will be covered in subsequent chapters , based on shared characteristics phylogenetic affinity . Earliest bipeds and possibly . bipeds with relatively thin molar enamel . Bipeds that exploited a more open and drier niche with thick molar enamel gracile , such as and . Descendants of gracile with heavy masticatory apparatus , adapted to exploit tough or hard foods when necessary , also known as robust . that retained the gracile masticatory apparatus of their ancestors and exhibited a trend for and increasingly complex culture Homo species . What is a 55