Chapter 6 Early Hominins I What Makes Us Human A Bipedalism post cranial material for Ardipithecus the earliest widely accepted hominin genus indicates a capacity for upright bipedal locomotion the ardipithecus pelvis appears to be transitional between one suited for arboreal climbing and one modified for bipedalism bipedalism is the key feature differentiating early hominins from the ape more than five million years old traditionally has been viewed as an adaptation to open grassland or savanna several advantages ability to see over long grass to carry items back to home base and to reduce exposure to solar radiation quadrupedalism exposes the body to 60 more solar radiation preceded stone tool manufacture and the expansion of the hominin brain could move bipedally but also preserved enough ape like anatomy to climb B Brains Skulls and Childhood Dependency early hominins had very small brains brain size has increased during hominin evolution especially with genus Homo human children have a long period of childhood dependency during which their brains and skulls grow dramatically larger skulls demand larger birth canals which can affect moms walking and too narrow can kill both mom and baby therefore natural selection has struck a balance C Tools D Teeth it is likely that early hominins shared this ability as a homology with the apes first firm evidence for hominin stone tool manufacture is 2 6 m y a upright bipedalism would have permitted the use of tools and weapons against predators and competitors bipedal locomotion also allowed early hominins to carry things one early trait that has been lost is big black teeth pattern of overall dental reduction has been characterized human evolution it was adaptive to have big black teeth with thick enamel since they adapted to the savanna with its gritty tough and fibrous vegetation II Chronology of Hominin Evolution term hominin is used to designate the human line after split from ancestral chimps hominid refers to the taxonomic family that includes humans and African ape and their immediate ancestors III Who were the Earliest hominins A Sahelanthropus tchadensis 2001 anthropologists working in Chad unearthed the 6 7 million year old skull of the oldest possible human ancestor yet found consists of a nearly complete skull two lower jaw fragments and three teeth dates to time when humans and chimps would have been diverging fossil was known as Toumai a local name meaning hope of life Toumai blends apelike and human like characteristics brain was chimp sized but tooth enamel was thicker than chimps suggested a diet that included not just fruit but vegetation from savanna placement of hole where spinal cord joins brain was more forward than in apes suggesting that it moved bipedally discovery in Chad indicates that early hominid evolution was not confined to East Africa s Rift Valley proof of a more widespread distribution of early hominids 2001 Kenya 13 fossils from at least 5 individuals appears to have been a chimp sized creature that climbed easily and walked on fossilized left femur suggests upright bipedalism while the thick right humerus two legs when on the ground suggests tree climbing skills B Orrorin tugenesis C Ardipithecus lived during the late Miocene between 5 8 and 5 5 m y a first discovered in Egypt and Ethiopia apelike in size anatomy and habitat pelvis appears to be transitional between one suited for arboreal climbing and one modified for bipedal motion Ardi replaces Lucy as earliest known hominin skeleton feet lacked the archlike structure of later hominin feet IV The Varied Australopithecines some Miocene hominins eventually evolved into a varied group of Pliocene Pleistocene hominins known as austrolopithecines for which we have abundant fossil record this term reflects their one time classification as members of a distinct taxonomic we now know that the various species of Australopithecus do not form a distinct subfamily Austrolopithecinae subfamily within the order primates today the distinction between them and later hominins is made on genus level Australopithoceines are assigned to genus Australopithecus and humans to genus Homo A Australopithecus anamensis bipedal hominin from northern Kenya molars have hick enamel and apelike canines are large about 110 lbs larger than earlier and later species B Australopithecus afarensis one sample includes 40 percent of the complete skeleton of a tiny hominin female Lucy although clearly a hominin was so similar in many ways to chimps and gorillas that our common ancestry with the African apes must be very humans and apes have on either side of the upper or lower jaw two incisors one canine two pre molars and three molars 2 1 2 3 recent vegetation Lucy s Baby found before Lucy amazingly complete find with more intact skull and more skeletal material than Lucy skull and upper body are apelike while the lower body confirms bipedal Lucys baby sheds light on the growth process in early humans humans now need a prolonged dependent childhood to grow skull brain lucys baby s brain was smaller than that of a chimp was said to be 3 years old the A afarensis fossils show that as recently as 3 mya our ancestors had a mixture of apelike and hominin features had longer canines than Homo but shorter than apes massive backteeth jaws and facial and cranial structures suggests a diet demanding extensive grinding and powerful crushing grassland skull of aferensis contrasts with later hominins with small cranial capacity structure of pelvic hip leg and foot bones also confirms that upright bipedalism muscle engraved bones are much more robust than ours are with only rudimentary tools and weapons early hominins needed powerful and resistant bones and muscles 3 Gracile and Robust Australopithecines a b c d e f g Two australopithecine groups lived in South Africa between 3 and 1 m y a gracile A africanus and robust A robustus Some scholars believe that a third hyperrobust australopithecine group that existed in East Africa was a separate species A boisei while others believe this group was simply a regional variant of A robustus Both gracile and robust australopithecines probably descended from A afarensis Some scholars believe these groups were separate species and that the graciles lived before and were ancestral to the robusts Other scholars argue that the graciles and robusts were separate species that overlapped in time Yet others view the graciles and the robusts as different ends of a