HUMAN EVOLUTION: GENUS AUSTRALOPITHECUS & PARANTHROPUS
What is a Hominid? Modern humans & our direct and indirect ancestors (after our lineage split from the chimpanzee) In the last decade, the time range of genus Australopithecus has been pushed back to 4.2 mya & its 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: 1.Ethiopia = Middle Awash valley & Hadar (Australopithecus afarensis) 2.Kenya = Lake Turkana 3.Tanzania = Olduvai Gorge 4.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? 1. Carrying behavior 2. 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 3. Most energy efficient way to travel long distances 4. 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 mya Tracks of 2 individuals were uncovered in volcanic ash by Mary Leakey (1978-79) Footprints were left by 2 australopithecines in damp volcanic ash of Laetoli 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
2 early hominids walk bipedally across an open ash field produced by an erupting volcano. Rain wet the volcanic ash & footprints filled up with more ash, and were thus preserved. Footprints reveal that our ancestors walked upright with a gait very similar to our own. Laetoli Reconstruction
Time-Line of Hominid Evolution: 5 Adaptive Radiations First Adaptive Radiation: 6-7 mya in the late Miocene, potential last common ancestors Second Adaptive Radiation: 4-5 mya in early Pliocene, first true hominids Third Adaptive Radiation: 3-4 mya in middle Pliocene, more hominids Fourth Adaptive Radiation: 2-3 mya in late Pliocene, more & “robust” hominids Fifth Adaptive Radiation: 2-1.8 mya in late Pliocene & first ice age, genus Homo
First Adaptive Radiation 6-7 mya in the late Miocene, potential last common ancestors 1.Sahelanthropus tchadensis 6-7 mya in Chad (North Central Africa) 2. Orrorin tungenensis 6 mya in Kenya (East Africa) Note: We know little about the lifeways of these species. However, we do know that they were forest adapted.
1. Sahelanthropus tchadensis Discovered in Chad (6-7 mya) Most complete cranium from this time period Mosaic of ape & human-like features, but at the “ape grade” of evolution: Cranial capacity (320-380 cc) U-shaped upper jaw Very wide distance between the orbits Large, thick continuous brow ridge Human-like flat face Human-like dentition
2. Orrorin tungenensis Fossils from Tugen Hills in Kenya have been dated to about 6 mya Earliest Evidence for Walking on Two Legs? How far back in time does the record of bipedalism extend?
Orrorin tungenensis (cont.) Fossils 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 a common ancestor of chimps and humans?
Second Adaptive Radiation 4-5 mya in early Pliocene, first true hominids 1. Ardipithecus ramidus 4.5-5.5 mya in Ethiopia (East Africa) 2.Australopithecus anamensis 4.2-3.9 mya in Kenya (East Africa) Note: We know little about the lifeways of these species. However, we do know that they were forest adapted and fully bipedal.
1. Ardipithecus ramidus Earliest True Hominid or Last Common Ancestor? LAST COMMON ANCESTOR OF CHIMPS & HUMANS MOST LIKELY HAD A MIX OF FEATURES: SOME RETAINED IN CHIMPS, OTHERS RETAINED IN HUMANS!
Ardipithecus ramidus (cont.) Between 4.5 and 5.5 mya from the Middle Awash valley site in Ethiopia Fossil Remains very fragmentary: limb bones, toe bones, jaws & teeth Straight toe bones suggest it may have been bipedal A mosaic of features seen in later hominids & modern chimpanzees
2. Australopithecus anamensis Lake Turkana Region of Kenya 4.2-3.9 mya Probably walked upright Teeth enamel thicker than Ardipithecus ramidus, so diet included hard foods
Fossil Remains very fragmentary 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 anamensis (cont.)
Third Adaptive Radiation 3-4 mya in middle Pliocene, many hominids 1. Australopithecus afarensis (“Lucy”) 3-4 mya in East Africa 2. Australopithecus africanus 2.5-4 mya in South Africa 3. Kenyanthropus platyops 3.5-3.2 mya in Kenya Note: We know little about the lifeways of these species. However, we do know that they lived in open woodlands & along wooded streams in the savannas, ate fruits and soft foods, maybe had a tool-culture like modern chimps
1. Australopithecus afarensis 3-4 mya in East Africa Pelvis & leg bones resemble modern humans Sexual dimorphism (males larger with sagittal crest) Tree climbers (curved fingers & toes) Ape-like Features: Small brain case (430 cc.) Prognathic (jutting out) face U-shaped palate (v. parabolic shape)
2. Australopithecus africanus 4-2.5 mya in Transvaal region of South Africa 1924: 1 st Australopithicine to be described by Raymond Dart! Globular cranium, slightly higher ratio of brain to body size than A. afarensis Face less prognathic than A. afarensis Proportions of arm to leg lengths may be more ape-like than A. afarensis
Australopithecus africanusAustralopithecus africanus Taung Child Most complete A. africanus skull best known A. africanus cranium (front & lateral views) Australopithecus africanus
A. afarensis A. afarensis Australopithecus afarensis A. afarensis verses A. africanus
3. Kenyanthropus platyops 3.5-3.2 mya: found in 2001 west of Lake Turkana in Kenya Ape-like features: small ear canal, small brain case Human-like features: flat face, small molars Importance: flat face appeared early in evolution, alongside the range of other facial forms. Evidence that evolution is not linear or progressive
Fourth Adaptive Radiation 1-3 mya in late Pliocene, more & “robust” hominids 1. Paranthropus boisei 2.2-1.2 mya in East Africa 2. Paranthropus robustus 2-1 mya in South Africa 3. Australopithecus garhi 2-3 mya in East Africa Note: We know little about the lifeways of these species. However, we do know that they lived in open dry woodlands & savannas. The robust species are famous for eating hard to chew food, like seeds, nuts, and roots. May have used tools to dig for roots in dry seasons.
1. Paranthropus boisei 2.2-1.2 mya in East Africa: Ethiopia, Kenya, Tanzania Largest teeth found in any hominid! Referred to as “hyper-robust” due to massive molars and premolars Skull: broad, short face with flaring cheek bones, relatively small brain, pronounced sagittal crest in males Skull and dental features are adaptations for heavy chewing!
2. Paranthropus robustus 2-1 mya in South Africa Short, broad face with deep zygomatic arches (cheek bones). Larger individuals have sagittal crests. Large molars covered with thick enamel Wear patterns on teeth: herbivorous diet of hard resistant foods such as seeds, nuts, roots Lived in grasslands near rivers and wetlands
3. Australopithecus garhi 2-3 mya in East Africa: spotty fossil record, cranial & dental remains found in 1999 in Bouri, Ethiopia Bones of antelopes, horses, and other animals with cut marks made by stone tools: butchering animals & smashing bones for marrow. First meat eaters? Molars too large to be early Homo Ape-like long lower arm, human-like upper arm & leg Mixed traits: classified as a new species, maybe ancestor of early Homo
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
EARLIEST BUTCHERS: Signs that hominids scraped & smashed animal bones, like this antelope tibia, 2.5 mya Earliest documented percussion marks made by hominids, presumably extracting fatty marrow from these bones Australopithecus garhi