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Human Evolution. What is a Hominid?. Modern humans & our direct and indirect ancestors after our lineage split from the chimpanzee Until recently, earliest hominids were dated between 3.5 and 2.4 mya & placed in the genus Australopithecus
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What is a Hominid? • Modern humans & our direct and indirect ancestors after our lineage split from the chimpanzee • Until recently, earliest hominids were dated between 3.5 and 2.4 mya & placed in the genus Australopithecus • In last few years, time range of Australopithecus pushed back to 4.2 mya, distribution expanded to include regions outside E. and S. Africa • New finds from 4.5-7 mya are thought to be hominids that predate Australopithecines, although their status is debated
Hominid Sites • Earliest fossil hominid sites are in Africa • They now span the latest Miocene to the early Pleistocene from about 6-7 mya to about 1.6 mya • The major groups of sites are: • Ethiopia = Middle Awash valley & Hadar (Australopithecus afarensis) • Kenya = Lake Turkana • Tanzania = Olduvai Gorge • South Africa = various sites in limestone caverns centered around Sterkfontein
What Makes A Hominid? - Bipedalism • Primary feature distinguishing hominids from other hominoids is walking erect on two legs – erect bipedalism • Adaptations for bipedalism in the the partial skeleton of “Lucy,” an australopithecine ( 3.2 mya) clearly seen in the hip, spine and leg bones
Why did bipedalism become the primary adaptation of hominids? • Carrying behavior • Reduction of overall heat stress - facilitates heat loss through convection by exposing body to air currents, only humans have sweat glands that produce moisture to cool body • Most energy efficient way to travel long distances • Allows for better vision in open environments & defensive action against predators by freeing hands to throw objects
Evidence for Early Bipedalism • The record of bipedalism is most graphically preserved in the fossilized footprints at Laetoli, Tanzania! • 3.6 million year old tracks left by 2 individuals were uncovered in volcanic ash by Mary Leakey (1978-79) • Footprints were left by 2 australopithecines in damp volcanic ash of Laetoli • In the Laetoli trail, prints of the 2 individuals can be seen walking away from us • Notice how close the tracks are!
Laetoli Footprints • Laetoli footprints clearly show that the creatures who made them were fully bipedal • Big toe hardly diverges from the rest of the foot, unlike in apes • Gait = “heel-strike” followed by “toe-off” – the way modern humans walk
Laetoli Reconstruction • In this reconstruction, the 2 early hominids, identified as Australopithecus afarensis, walk bipedally across an open ash field produced by an erupting volcano • Region is wooded, but here trees are absent and the volcanic ash, wetted by a light shower of rain formed a flat shallow layer in which footprints are deeply implanted • Footprints filled up with yet more ash, and were thus preserved • Footprints reveal that, even at this early stage of human evolution, our ancestors walked upright with striding gait very similar to our own
Orrorin: Earliest Evidence for Walking on Two Legs? How far back in time does the record of bipedalism extend? • Fossils from Turgen Hills in Kenya have been dated to about 6 mya • Include: upper portion of a femur, lower portion of the humerus, some lower jaw fragments, & teeth • Arm bone = virtually identical to that of a chimpanzee • Femur = more human-like,most important for showing adaptations for walking on 2 legs • Was Orrorin a direct human ancestor, or close to the common ancestor of chimps and humans?
Ardipithecus: Earliest True Hominid? • Between 4.5 and 5.5 mya from the Middle Awash valley site • Remains very fragmentary: limb bones, toe bones, jaws & teeth • Straight toe bones suggest it may have been bipedal • Ardipithecus & Orrorin are candidates for last common ancestor of chimps & humans • Each have mosaic of features seen in later hominids & modern chimpanzee IT MAY WELL BE THAT THE LAST COMMON ANCESTOR OF CHIMPS AND PEOPLE HAD A MIX OF FEATURES: SOME RETAINED IN HUMANS, OTHERS RETAINED IN CHIMPS!
Australopithecus anamensis • Turkana region of Kenya • Dated to 4.2-3.9 MYA • Probably walked upright • Teeth covered with enamel much thicker than that of Ardipithecus, so diet may have had hard-to-chew foods
Australopithecus anamensis • Lived roughly 4 mya • Only a few anamensis fossils have been found • Those shown here include: jawbone & part of the front of the face, parts of an arm bone (radius), fragments of a lower leg bone (tibia)
Australopithecus afarensis • Eastern Africa 3.9 – 3.0 MYA • Pelvis and leg bones resemble modern humans in some ways • Sexually dimorphic in body size • May have been adept at tree climbing, based on curvature of finger and toe bones • Ape-like features: • Small brain case – averaging 430 cc • Prognathic (jutting out) face • U-shaped palate vs. the parabolic shape of modern humans • Reduced canines
Paranthropus aethiopicus • Eastern Africa, dated 2.7-2.3 MYA • Note widely flaring zygomatic arches (bones arching around side of skull to join below eyes, forming cheeks). Prominent sagittal crest – largest ever discovered in human lineage – and cheek teeth are correspondingly large. Adaptations for heavy chewing • Small cranial capacity (410 cc) and prognathic face • Black skull exhibits features more like A. afarensis
Paranthropus boisei (KNM-ER 406) • Ethiopia, Kenya and Tanzania; dated 2.2 – 1.2 MYA • Absolutely largest teeth found in any hominid species; referred to as hyper-robust due to massive molar and premolar teeth • Skull has extremely broad, short face with flaring zygomatic arches (cheek bones), relatively small brain, pronounced sagittal crest in males • Skull and dental features = adaptations for heavy chewing
Kenyanthropus platyops • Kenya, 3.5 to 3.2 MYA • Ancestral features: • small ear canal more like that of chimps, A. anamensis & Ardipithecus • shape & small size of braincase • Derived features: relatively flat face and small molars • Importance: flat, human-like face appeared early in evolution alongside range of other facial forms; not outcome of progressive, linear evolution
Australopithecus africanus • 3.3 – 2.5 MYA in Transvaal region of South Africa • First australopithecine to be described (1924) • More globular cranium & slightly higher ratio of brain to body size • Teeth & face appear less ancestral (reduced in size relative to earlier forms, face less prognathic) • May represent 2 species or one very sexually dimorphic species • Proportions of arm to leg lengths may be more ape-like than in A. afarensis
Australopithecus africanus Australopithecus africanus Ms. Ples – best known A. Africanus cranium – front view Ms. Ples – best known A. Africanus cranium – lateral view Taung Child Partial skeleton of A. africanus discovered in 1950s STS 71 – Most complete A. africanus skull
Paranthropus robustus • South Africa, 2.0-1.0 MYA • Short, broad face with deep zygomatic arches & large temporal fossa. Larger individuals (males?) have sagittal crests • Very large cheek teeth covered with thick enamel • Wear patterns on teeth indicate herbivorous diet of harder, more resistant, & perhaps smaller food items than A. africanus • Lived in secondary grasslands near rivers and wetlands
Paranthropus robustus This recently discovered (1999) skull of Paranthropus robustus from Drimolen, South Africa is the most complete found so far (reversed image)image) Note the remarkable similarity to the newly found but older skull of A. africanus (above), reinforcing notion that the 2 species represent a single ancestor-descendent lineage Note remarkable similarity
Australopithecus garhi • 2-3 mya = lack of adequate hominid fossil record in Eastern Africa • 2.5 mya = Recently discovered Hominid cranial & dental remains in Ethiopia • Recognition of new species of Australopithecus • Descended from Australopithecus afarensis = a candidate ancestor for early Homo
Australopithecus garhi • Discovered at Bouri, Ethiopia in 1999 • Found with butchered bones – maybe oldest toolmaker • Oldest stone tools are also dated to about 2.5 mya • Ancestral to Homo? Right place at the right time…
Australopithecus garhi One surprise in the A. garhi skull was enormous back teeth, instead of smaller ones seen in later Homo species (Video Image/UC Berkeley)
Australopithecus garhi Not yet clear whether A. garhi falls on the direct lineage leading to modern humans, or lies on an extinct side-branch
Australopithecus garhi • EARLIEST BUTCHERS: Signs that hominids scraped and smashed animal bones, like this tibia, 2.5 mya • Antelope tibia shaft has been shattered, smashed, and cut by stone tools • The earliest documented percussion marks made by hominids, presumably extracting fatty marrow from these bones
Rise of the genus Homo • Earliest come from the same African sites as Australopithecus • Most date between 2.4 and 1.8 mya • Homo habilis means “handy man” • Growing consensus that there may be 2 or more species of Homo by 2 mya
“Homo habilis” (KNM-ER 1813) • East Africa, 2.4-1.6 MYA • 3 traits define H. habilis as transitional species: • Expanded cranial capacity (590-710 cc) • Reduced post-canine tooth size • A precision grip, provides anatomical basis for tool-making • One of many problems = obtaining accurate brain volume estimates from crushed and/or distorted skulls • May represent 2 or more species
“Homo rudolfensis” (KNM-ER 1470) • East Africa, 2.2-1.8 MYA • Distinguished from other specimens of early Homo by: • Slight supra-orbital brow ridge across the forehead with no trough behind • Less prognathic & longer face, squared off maxilla • Discovery of Kenyanthropus, similar facial features, have led some to see link between “platyops” and “rudolfensis,” representing a distinct hominid lineage – placing “1470” in the new genus as “Kenyanthropus rudolfensis”
