Lecture 6/ Chapter 6
Lecture 6/ Chapter 6 The first hominins
What’s a hominin?
Sub family – always ends with the nae Ponginae – Orangutans Mosaic Evolution – Fossil data often shows different collections of primitive and derived characters Cranial and Dental morphologies: Increase in the size of the brain and a decrease in the size of the face and dentition. •
A phylogenetic species is one encompassing the smallest set of organisms that share a common ancestor and can be distinguished from other such set of organisms
•
Hominini: tribe within the homininae comprising humans, chimpanzees, and our most recent common ancestors
Morphological trends in hominin evolution •
Reviewing morphological trends in hominin evolution is important for two reasons: 1. We need to understand what features contributed to an adaptive radiation of hominins at a time when contemporaneous fossil apes were disappearing 2. We need to understand that there is no singular feature that resulted in the biological evolution of human beings
•
Mosaic evolution: the evolution at different rates of various related or unrelated features within a lineage or clade
•
Bipedalism: habitual upright locomotion on two feet
Cranial and dental morphologies
•
Robust: physically strong durable
•
The earliest hominins have small molars with thin enamel, larger canines
•
The earliest species in our genus homo evolved a large, globular cranium, a more gracile, slight, slender, jaw and smaller teeth
Increased brain size •
Increased brain size is relative to the social group.
•
A better measure of intelligence may lie in the number of neurons and the speed at which they communicate
•
Human brain occupies 2% of our body mass, consumes about 20% of our total metabolism
•
Thy hypotheses formulated by evolutionary anthropologists for increased brain size falls into three categories: 1. Ecological: improved ability to hunt or forage food 2. Epiphenomenal: increased body size resulted in a concomitant increase in brain size 3. Socialization: to deal with increasing complexity of social organization
Bipedalism Feature
Bipedal Hominins
Quadrupedal Apes
Foramen magnum
Located anteriorly
Located posteriorly
Spinal curvature
Dual
Singular
Pelvis shape
Shorter, broader
Longer, narrower
Femur length
Longer
Shorter
Knee orientation
Valgus (adducted)
Straight
Foot shape
Narrower
Broader
Hallux (“big toe”) placement
Nonopposable
Opposable
•
Longtitunal arch – Bipeds have this
•
Adducted: toward the midline of the body
•
Three most plausible hypothesis for bipedalism:
1. Feeding posture hypothesis: early hominins were preadapted to an upright posture 2. Behavioural theory: to improve the ability of males to carry food resources to their mates and offspring 3. Thermoregulatory theory: improve heat dissipation
The knees of A. afarensis are more like the knees of modern humans than the knees of chimpanzees. Consider the lower end of the femur, where it forms one side of the knee joint. In chimpanzees, this joint forms a right angle with the long axis of the femur. In humans and australopithecines, the knee joint forms an oblique angle, causing the femur to slant inward toward the centerline of the body. This slant causes
the knee to be carried closer to the body’s centerline, which increases the efficiency of bipedal walking. In apes the femur is very closer to 90 degrees.
BIPEDALITY The pelvis of A. afarensis resembles the modern human pelvis more than the chimpanzee pelvis. Notice that the australopithecine pelvis is flattened and flared like the modern human pelvis. These features increase the efficiency of bipedal walking. The australopithecine pelvis is also much wider and narrower than that of modern humans. Some anthropologists believe these differences indicate that australopithecines did not walk the same way as modern humans do.
•
Pelvis is longer in the chimps compared to humans. Orientation of the pelvis is also different in chimps compared to humans
•
Length of the limb (arms) is longer in chimps than humans.
A schematic diagram of the lower body at the point of the stride in which all of the weight is on one leg. Body weight acts to pull down through the centerline of the pelvis. This creates a torque, or twisting force, around the hip joint of the weighted leg. (a) If this torque were unopposed, the torso would twist down and to the left. (b) During each stride the abductor muscles tighten to create a second torque that keeps the body erect. Three different gluteus muscles (your behind) Why did apes go from quadrupeds to bipeds? ( million dollar question) 3 Theories on Evoultion 1)Feeding posture – eating leaves 2)Behaviour – avoiding predators , it allows to use tools. 3) Thermoregulation – Less solar radiation Bipedal locomotion helps an animal living in warm climates to keep cool by reducing the amount of sunlight that falls on the body, by increasing the animal’s exposure to air movements, and by immersing it in lowertemperature air.
Finding and estimating the age of fossil hominins
•
Breccia: a coarse grained rock composed of angular fragments of preexisting rocks
•
Dating is critical in determining a chronological framework for the hominins
•
There is a strong interest in determining the oldest evidence for hominins and their material remains
•
There are two categories of dating methods: relative and absolute
Relative dating •
Involve determining whether a fossil or the sediment it is buried in is younger or older than another fossil or sediment
•
Stratigraphy: the study of vertical sequence of rock layers, the oldest and one of the best relative dating methods (high energy or low energy deposits ) low energy deposits are useful because high energy deposits can break up the fossils.
•
Palynology: study of fossilized pollen
•
Biostratigraphy: study of fossilized animals in vertical sequence of rock layers
Absolute dating •
Provides a direct age listed in years before present (bp), with present referring to the year 1950
•
Radioactive decay: the process of a material giving off particles to reach stable state. When they decay it decay into argon gas. If you the rate it does that, then you can estimate how old is the fossil.
•
Potasium dating 1.4 bilion years of half life
•
Pottasium – argon dating can be dated to really old fossils, such as millions of years ago. Very useful.
Transitional forms: advanced apes or primitive hominins? Sahelanthropus tchadensis •
foramen magnum under the cranium (suggesting bipedality), flat face (more Homolike), but small apelike cranial capacity, and apelike occipital region. Strangely large brow ridge. Environment: lush woodland
•
The oldest known transitional hominin
•
320380 cc
Orrorin tugensis •
femur morphology suggest bipedal locomotion, small molars with thick enamel (Homo like). Environment: mixed woodlandsavanah
•
Is important because it may represent the earliest and first bipedal hominin
Ardipithecus ramidus and ardipithecus kadabba •
foramen magnum under the cranium (suggesting bipedality), smaller canines, no canine sharpening on the lower third premolars (Homolike), the base of the skull is more pneumatized (apelike). Environment: forest
•
Teeth have thin enamel, canines have reduced dimorphism
Kenyathropus platyops •
Thick enamel
Australopithecines -
East African fossils and South African fossils
Gracile •
Ardipithecus ramidus (4.4 mya) One of the oldest fossil
•
Australopithecus afarensis (4.33.0 mya)
•
Australopithecus anamensis (4.23.9)
•
Kenyanthropus platyops (3.5 mya)
•
Australopithecus africanus (3.02.3 mya)
•
Australopithecus garhi (2.5 mya)
Robust Paranthrpous aethiopicus 2.7 – 2.3 mya Paranthropous boisei ( 2.2 – 1.2 mya) Paranthropous robustus (2 – 1 mya)
(Need to know the dates and cc): L
•
Australopithecus anamensis: large molars with thick enamel, Partial tibia provides strong evidence for bipedality..(Cranial capacity unknown)
•
Australopithecus afarensis: small brain (450 cc), intermediate Uparabolic dental arcade, diastema, mediumsized canines, flared and pneumatized cranial bases (primitive traits), subnasal prognathism
•
Australopithecus africanus: slightly larger brains than A. afarensis, less pneumatized cranial base, less subnasal prognathism. (460 cc),
•
Paranthropus robustus (South Africa): large postcanine dentition, broad facial architecture, small sagittal crest, large deep mandibles. (530 cc),
•
Paranthropus boisei (East Africa) even LARGER postcanine dention, dishedshaped face. (490 cc),
•
Paranthropus aethiopicus (East Africa): Much like boisei but more prognathic with larger temporalis muscles and sagittal crest. CHEWING MACHINES! (415 cc)
Australopithecines: the first ‘real’ hominins •
Relative small canines and incisors, and lack sectorial premolar
•
Sectorial premolar: first lower premolar with a shearing edge for the upper canine
•
Adept tree climbers
Australopithecus anamensis •
Parallel dental arch
•
Is the earliest evidence for bipedalism in definitive hominins
Australopithecus afarensis
•
410 cc
•
Sagittal crest: a lengthwise ridge of bone along the top of the skull where strong chewing muscles attach to the top of the skull
•
Nuchal crest: a horizontal ridge of bone where large neck muscles attach to the back of the skull
•
Has the valgus knee of a habitual biped
Australopithecus africanus
•
458 cc
•
Shallower snout, smaller canines, broader incisors, and smaller molars and premolars
•
Lacks nuchal and sagittal crest
Australopithecus garhi •
Prognathic lower face and procumbent incisors
Australopithecus aethiopicus
•
Hyperrobust hominin
•
Large face, large cheek bone, very large sagittal and nuchal crest, and enormous teeth
Australopithecus boisei
•
Hyperrobust
•
498 cc
•
Small anterior and large posterior teeth, sagittal and nuchal crest
Australopithecus robustus •
563 cc
•
Has small anterior and large posterior teeth
•
Has sagittal and nuchal crest
The cranium of A. afarensis possesses a number of primitive features, including a small brain, a shallow jaw joint, a pneumatized cranial base, and subnasal prognathism in the face. Pneumatized – air pockets – looks like a Aero chocolate bars
The teeth and jaws of A. afarensis have several features that are intermediate between those of apes and modern humans. (a) The dental arcade is less Ushaped than in chimpanzees, but less parabolic than in modern humans.
In these two skulls—of (a) P. aethiopicus and (b) a modern human—the area of attachment of the temporalis muscle is shown in red.
Gracile and robust Australopithecines •
Australopithecus are divided into two morphological groups:
1. Gracile: includes Australopithecus afarensis and africanus
Thinner bones
Smaller teeth
2. Robust: includes Australopithecus aethiopithecus, Australopithecus boisei, and Australopithecus robustus
Thicker skull bones and large teeth
Temporal fossa: a large space between the eye orbit and the cheek bone
What’s a hominin?
Sub family – always ends with the nae Ponginae – Orangutans Mosaic Evolution – Fossil data often shows different collections of primitive and derived characters Cranial and Dental morphologies: Increase in the size of the brain and a decrease in the size of the face and dentition. •
A phylogenetic species is one encompassing the smallest set of organisms that share a common ancestor and can be distinguished from other such set of organisms
•
Hominini: tribe within the homininae comprising humans, chimpanzees, and our most recent common ancestors
Morphological trends in hominin evolution •
Reviewing morphological trends in hominin evolution is important for two reasons: 1. We need to understand what features contributed to an adaptive radiation of hominins at a time when contemporaneous fossil apes were disappearing 2. We need to understand that there is no singular feature that resulted in the biological evolution of human beings
•
Mosaic evolution: the evolution at different rates of various related or unrelated features within a lineage or clade
•
Bipedalism: habitual upright locomotion on two feet
Cranial and dental morphologies
•
Robust: physically strong durable
•
The earliest hominins have small molars with thin enamel, larger canines
•
The earliest species in our genus homo evolved a large, globular cranium, a more gracile, slight, slender, jaw and smaller teeth
Increased brain size •
Increased brain size is relative to the social group.
•
A better measure of intelligence may lie in the number of neurons and the speed at which they communicate
•
Human brain occupies 2% of our body mass, consumes about 20% of our total metabolism
•
Thy hypotheses formulated by evolutionary anthropologists for increased brain size falls into three categories: 1. Ecological: improved ability to hunt or forage food 2. Epiphenomenal: increased body size resulted in a concomitant increase in brain size 3. Socialization: to deal with increasing complexity of social organization
Bipedalism Feature
Bipedal Hominins
Quadrupedal Apes
Foramen magnum
Located anteriorly
Located posteriorly
Spinal curvature
Dual
Singular
Pelvis shape
Shorter, broader
Longer, narrower
Femur length
Longer
Shorter
Knee orientation
Valgus (adducted)
Straight
Foot shape
Narrower
Broader
Hallux (“big toe”) placement
Nonopposable
Opposable
•
Longtitunal arch – Bipeds have this
•
Adducted: toward the midline of the body
•
Three most plausible hypothesis for bipedalism:
1. Feeding posture hypothesis: early hominins were preadapted to an upright posture 2. Behavioural theory: to improve the ability of males to carry food resources to their mates and offspring 3. Thermoregulatory theory: improve heat dissipation
The knees of A. afarensis are more like the knees of modern humans than the knees of chimpanzees. Consider the lower end of the femur, where it forms one side of the knee joint. In chimpanzees, this joint forms a right angle with the long axis of the femur. In humans and australopithecines, the knee joint forms an oblique angle, causing the femur to slant inward toward the centerline of the body. This slant causes
the knee to be carried closer to the body’s centerline, which increases the efficiency of bipedal walking. In apes the femur is very closer to 90 degrees.
BIPEDALITY The pelvis of A. afarensis resembles the modern human pelvis more than the chimpanzee pelvis. Notice that the australopithecine pelvis is flattened and flared like the modern human pelvis. These features increase the efficiency of bipedal walking. The australopithecine pelvis is also much wider and narrower than that of modern humans. Some anthropologists believe these differences indicate that australopithecines did not walk the same way as modern humans do.
•
Pelvis is longer in the chimps compared to humans. Orientation of the pelvis is also different in chimps compared to humans
•
Length of the limb (arms) is longer in chimps than humans.
A schematic diagram of the lower body at the point of the stride in which all of the weight is on one leg. Body weight acts to pull down through the centerline of the pelvis. This creates a torque, or twisting force, around the hip joint of the weighted leg. (a) If this torque were unopposed, the torso would twist down and to the left. (b) During each stride the abductor muscles tighten to create a second torque that keeps the body erect. Three different gluteus muscles (your behind) Why did apes go from quadrupeds to bipeds? ( million dollar question) 3 Theories on Evoultion 1)Feeding posture – eating leaves 2)Behaviour – avoiding predators , it allows to use tools. 3) Thermoregulation – Less solar radiation Bipedal locomotion helps an animal living in warm climates to keep cool by reducing the amount of sunlight that falls on the body, by increasing the animal’s exposure to air movements, and by immersing it in lowertemperature air.
Finding and estimating the age of fossil hominins
•
Breccia: a coarse grained rock composed of angular fragments of preexisting rocks
•
Dating is critical in determining a chronological framework for the hominins
•
There is a strong interest in determining the oldest evidence for hominins and their material remains
•
There are two categories of dating methods: relative and absolute
Relative dating •
Involve determining whether a fossil or the sediment it is buried in is younger or older than another fossil or sediment
•
Stratigraphy: the study of vertical sequence of rock layers, the oldest and one of the best relative dating methods (high energy or low energy deposits ) low energy deposits are useful because high energy deposits can break up the fossils.
•
Palynology: study of fossilized pollen
•
Biostratigraphy: study of fossilized animals in vertical sequence of rock layers
Absolute dating •
Provides a direct age listed in years before present (bp), with present referring to the year 1950
•
Radioactive decay: the process of a material giving off particles to reach stable state. When they decay it decay into argon gas. If you the rate it does that, then you can estimate how old is the fossil.
•
Potasium dating 1.4 bilion years of half life
•
Pottasium – argon dating can be dated to really old fossils, such as millions of years ago. Very useful.
Transitional forms: advanced apes or primitive hominins? Sahelanthropus tchadensis •
foramen magnum under the cranium (suggesting bipedality), flat face (more Homolike), but small apelike cranial capacity, and apelike occipital region. Strangely large brow ridge. Environment: lush woodland
•
The oldest known transitional hominin
•
320380 cc
Orrorin tugensis •
femur morphology suggest bipedal locomotion, small molars with thick enamel (Homo like). Environment: mixed woodlandsavanah
•
Is important because it may represent the earliest and first bipedal hominin
Ardipithecus ramidus and ardipithecus kadabba •
foramen magnum under the cranium (suggesting bipedality), smaller canines, no canine sharpening on the lower third premolars (Homolike), the base of the skull is more pneumatized (apelike). Environment: forest
•
Teeth have thin enamel, canines have reduced dimorphism
Kenyathropus platyops •
Thick enamel
Australopithecines -
East African fossils and South African fossils
Gracile •
Ardipithecus ramidus (4.4 mya) One of the oldest fossil
•
Australopithecus afarensis (4.33.0 mya)
•
Australopithecus anamensis (4.23.9)
•
Kenyanthropus platyops (3.5 mya)
•
Australopithecus africanus (3.02.3 mya)
•
Australopithecus garhi (2.5 mya)
Robust Paranthrpous aethiopicus 2.7 – 2.3 mya Paranthropous boisei ( 2.2 – 1.2 mya) Paranthropous robustus (2 – 1 mya)
(Need to know the dates and cc): L
•
Australopithecus anamensis: large molars with thick enamel, Partial tibia provides strong evidence for bipedality..(Cranial capacity unknown)
•
Australopithecus afarensis: small brain (450 cc), intermediate Uparabolic dental arcade, diastema, mediumsized canines, flared and pneumatized cranial bases (primitive traits), subnasal prognathism
•
Australopithecus africanus: slightly larger brains than A. afarensis, less pneumatized cranial base, less subnasal prognathism. (460 cc),
•
Paranthropus robustus (South Africa): large postcanine dentition, broad facial architecture, small sagittal crest, large deep mandibles. (530 cc),
•
Paranthropus boisei (East Africa) even LARGER postcanine dention, dishedshaped face. (490 cc),
•
Paranthropus aethiopicus (East Africa): Much like boisei but more prognathic with larger temporalis muscles and sagittal crest. CHEWING MACHINES! (415 cc)
Australopithecines: the first ‘real’ hominins •
Relative small canines and incisors, and lack sectorial premolar
•
Sectorial premolar: first lower premolar with a shearing edge for the upper canine
•
Adept tree climbers
Australopithecus anamensis •
Parallel dental arch
•
Is the earliest evidence for bipedalism in definitive hominins
Australopithecus afarensis
•
410 cc
•
Sagittal crest: a lengthwise ridge of bone along the top of the skull where strong chewing muscles attach to the top of the skull
•
Nuchal crest: a horizontal ridge of bone where large neck muscles attach to the back of the skull
•
Has the valgus knee of a habitual biped
Australopithecus africanus
•
458 cc
•
Shallower snout, smaller canines, broader incisors, and smaller molars and premolars
•
Lacks nuchal and sagittal crest
Australopithecus garhi •
Prognathic lower face and procumbent incisors
Australopithecus aethiopicus
•
Hyperrobust hominin
•
Large face, large cheek bone, very large sagittal and nuchal crest, and enormous teeth
Australopithecus boisei
•
Hyperrobust
•
498 cc
•
Small anterior and large posterior teeth, sagittal and nuchal crest
Australopithecus robustus •
563 cc
•
Has small anterior and large posterior teeth
•
Has sagittal and nuchal crest
The cranium of A. afarensis possesses a number of primitive features, including a small brain, a shallow jaw joint, a pneumatized cranial base, and subnasal prognathism in the face. Pneumatized – air pockets – looks like a Aero chocolate bars
The teeth and jaws of A. afarensis have several features that are intermediate between those of apes and modern humans. (a) The dental arcade is less Ushaped than in chimpanzees, but less parabolic than in modern humans.
In these two skulls—of (a) P. aethiopicus and (b) a modern human—the area of attachment of the temporalis muscle is shown in red.
Gracile and robust Australopithecines •
Australopithecus are divided into two morphological groups:
1. Gracile: includes Australopithecus afarensis and africanus
Thinner bones
Smaller teeth
2. Robust: includes Australopithecus aethiopithecus, Australopithecus boisei, and Australopithecus robustus
Thicker skull bones and large teeth
Temporal fossa: a large space between the eye orbit and the cheek bone
