Cenozoic Climates First Prosimians Hominid Origins Ecology, Changing Social Patterns, and Bipedalism Anthropoids Hominids Miocene Climates Miocene Habitats The increase in climate variability would have been evident in many regions as increased seasonal variability in temperature and rainfall One result was the increasing spread of grasslands Animals with large bodies to travel long distances and large guts to process the fibrous grasses sprang up on every continent East African Rain Shadow Grass Grass Forest Forest Grass Moderate Phase Modified Cool/Dry from Phase Cool/Dry Modified Phase Conroy, 1990:192b) from Conroy, Modified 1990:192a) from Conroy, 1990:192a) 1
Miocene Adaptations About 18 million years ago we begin to see the expansion of the hominoid apes Fossils occur in more open woodland and woodland to forest transition areas Apes have larger bodies, larger brain to body size ratios Thicker enamel enables a broader diet of soft and hard foods Likely that developmental slow down is becoming pronounced in these animals Longer developmental period enables more learning of complex behaviors to take place in larger brains Provides a behavioral buffer for the extreme environmental variability of the Miocene Late Miocene Climates Relative climatic stability occurs from around 10 mya to about 6.4 mya Cooler temperatures prevailed than earlier in the Miocene, and ice sheets were constant From 6.4 to 4.6 mya, large scale climatic fluctuations returned, larger than 16-12 mya, with glaciations being large enough to isolate the Mediterranean from the Atlantic and to dry the Mediterranean up periodically (the Messinian crisis) A small, insignificant hominoid species began to walk upright during this time Climate Change Last 6,000,000 Years Miocene Habitats The increase in climate variability would have been evident in many regions as increased seasonal variability in temperature and rainfall One result was the increasing spread of grasslands Animals with large bodies to travel long distances and large guts to process the fibrous grasses sprang up on every continent Miocene African Forests Turnover-Pulse Model 2
Early Hominid Patterns The vertebral column articulates at almost a right angle to the base of the skull creating a sharply angled vocal tract Larger group sizes, facilitated by bipedalism, leads to increased opportunity for learning and communication Dunbar (1996) posits this as the time for the transition from grooming to gossip as a means of group solidarity Humans versus Apes To understand the patterning of hominid adaptations, we need to be aware of the differences between humans and our closest relatives, the African anthropoid apes: Gorillas, Chimpanzees, and Bonobos The two key morphological differences include reduced canines and bipedalism Bipedalism changed elements throughout the body and also must have had significant behavioral ramifications Locomotor differences By comparison to apes, humans have: a foramen magnum that points down a curved lumbar spine a short, flared (versus long and thin) ilium (the upper most section of the hip bone or pelvis) a strong, robust talus (ankle bone) a strong, non-opposable big toe a complex two-way arch system in the foot Posture Comparisons Hip Comparisons Knee Comparisons 3-4 mya 3
Foot Comparisons Humans vs. Apes Since we differ in cranial dimensions, dentition, and locomotor skeleton from the living apes, hypotheses about hominid origins must address the adaptive nature of these differences Mosaic Evolution Mosaic evolution refers to evolutionary changes in different body systems at different times and rates In the context of human evolution, we need to consider the sequence of evolutionary events involving changes in brain size and structure, dentition, and locomotor adaptations Hominid Origins Early workers emphasized the cranial differences between apes and man, assuming that brain changes came first This led to the futile search for large brained apes (like Piltdown) More recent fossil discoveries tell us that locomoter changes came first, then dental changes, and much later the characteristic cranial enlargement Environmental Change Many models of hominid origins have focused on environmental change as a driving force in the evolution of bipedalism Our understanding has changed from viewing the Pliocene as a time of drying and cooling resulting in grasslands replacing forests to a view of a much more complex mosaic environment with forests receding and advancing cyclically A few of the Many Hypotheses about Hominid Origins Tool manufacture and use Hunting Seed eating Reproductive Efficiency Aquatic adaptations Walking efficiency Vertical Climbing Terrestrial Quadrupedalism 4
Tool manufacture and use Tool manufacture and use--2 Bipedalism, small canines, and large brains are all linked to the manufacture and/or use of tools for early hominid adaptation to the environment Larger Brains Tool Use Bipedalism Smaller Canines This model views brain, teeth, and bipedalism as systems among which feedback from the use of tools has occurred Thus, all hominid characteristics are considered to be a complex of interrelated traits that evolved simultaneously Tool manufacture and use--3 Darwin (1871) proposed that the hands took the place of the large canines found in apes for fighting with enemies (presumably by the Marquis of Queensbury rules) He was viewing nature red in tooth and fang, and conjectured about the literal struggle for survival, with hominids who could wield a club out-competing those who were limited to fighting with enlarged canine teeth Tool manufacture and use--4 The use of tools requires learning and intelligence, which would result in larger brains Bipedalism frees the hands from locomotion this would permit the carrying of tools also allows an increased precision of grasp with more gracile structure of hands Tool manufacture and use--5 Problems The brain, teeth, and bipedalism evolved at different times Many small brained, non-bipedal species use tools; chimps manufacture and carry tools Bipedalism shows up at least 2 million years before stone tools appear Hunting Washburn focused on the role of men in hunting as an explanation for bipedalism He argued that bipedalism evolved as an enabling mechanism for men to carry food from the hunt to provision their mates and offspring Those most successful would leave the healthiest offspring most likely to survive 5
Seed Eating Jolly (1970) looked to gelada baboons to explain human dentition and posture, based on the assumption that early hominid diets would have been grass seed based in the drier environment Smaller canines and heavy occlusal wear are found for processing seeds Seated, upright posture accompanies the squatting processing of grasses The Prostitute Model Lovejoy proposed a model of hominid origins based on increased reproductive efficiency In this scenario, males who were mobile and successful at hunting and carrying nutritious food back to females were more successful at reproducing than were less mobile males (i.e., those less bipedal) The Prostitute Model--2 Females stayed close to a home base and invested their time in rearing offspring Food that was readily available to females and children near the home base was not usually of high quality Females who were provisioned by males would have better success at raising young The Prostitute Model--3 Provisioning also would have shortened the interval necessary between births Provisioned females would have to expend less of their own energy to sustain infants, both in terms of carrying and in terms of breast feeding Provisioned females would return to an ovulatory state sooner, and be able to produce more offspring on average than nonprovisioned females The Prostitute Model--4 Sexual selection operated to reinforce the provisioning of females by males Fatty storage especially in breasts and buttocks marked fertile, desirable females Cryptic estrus allows females to fake it by copulating throughout the menstrual cycle without external evidence of peak fertility periods to restrict sexual activity Both of these facilitate long-term pair bonding The Aquatic Ape Partly in response to Man the Hunter evolutionary scenarios, Elaine Morgan proposed that hominid evolution was primarily a female phenomenon She says that several female characteristics such as higher body fat levels and less body hair came along with standing bipedally in the surf off East Africa In this model, males splash along behind 6
Walking efficiency Human bipedal running is both slower and less energetically efficient for running than is chimpanzee knuckle-walking However, for locomotion at a normal walking pace, for instance from one food patch to another, human bipedal walking is more energetically efficient than is chimpanzee knuckle-walking Walking Efficiency, 2 Energy Efficiency Walking speed Species Energy cost ml 0 2 /kg/hr Energy cost relative to quadruped 2.9 km/hr Chimp 0.522 149% Human 0.193 86% 4.5 km/hr Chimp 0.426 148% Human 0.170 94% 2.9 km/hr is normal knuckle-walking speed of chimps, 4.5 km/hr is normal bipedal walking speed of humans (Rodman and McHenry 1980) Bipedal Efficiency Walking efficiency, 3 Bipedalism would confer an adaptive advantage especially in times of low food availability and sparse distribution of food resources Bipedal humans would expend less energy to move from patch to patch of food, and would get there faster on average than knucklewalking early pongids Walking efficiency, 4 Social Group Size In the late Miocene environment, forest patches and food resources became more sparsely distributed The Chimp-human common ancestor could have adapted in two different ways: Remain quadrupedal/knuckle-walking and reduce group size so that less travel was necessary for foraging Increase the energetic efficiency of travel from one patch to another by becoming bipedal 7