Cenozoic Climates Human Evolution and Adaptation Life Styles of the Merely Hominid 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 Grass Grass Forest Cool/Dry Phase Modified from Conroy, 1990:192a) Moderate Phase Modified from Conroy, 1990:192b) 1
Grass Warm/Wet Phase Modified from Conroy, 1990:193c) Forest 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 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 2
Hip Comparisons Knee Comparisons 3-4 mya Foot Comparisons Walking Efficiency Why would our ancestors have invested in this awkward way of getting around on the ground? Human bipedal running is both slower and less energetically efficient than is chimpanzee knuckle-walking or quadrupedalism But at a normal walking pace, for instance the way Australopithecus might have gotten from one food patch to another, human bipedal walking is more energetically efficient than is chimpanzee knuckle-walking Walking Efficiency, 2 Energetic 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) Walking efficiency, 3 Bipedalism would confer an adaptive advantage especially in times of low food availability and sparse distribution of food resources Bipedal hominids would expend less energy to move from patch to patch of food, and would get there faster on average than knucklewalking early pongids 3
Walking efficiency, 4 In a changing environment such as that found at the close of the Miocene, food resources became more sparsely distributed and there was less certainty about the location of food resources as forests shrank and expanded The ability to move from one forest/woodland patch to another efficiently would have been adaptive Bipedal hominids would have been well suited to do this Early Australopithecines appeared to have picked up bipedalism while they retained climbing abilities Pliocene This bipedal/climbing adaptation of early australopithecines remained stable through much of the Pliocene, despite the high levels of climatic variability Adaptation includes retention of the Miocene trait of thick enamel in the cheek teeth so that both hard and soft foods were consumable Elaboration of the pattern results of substantial regional and temporal variation and speciation Late Pliocene Around 3 mya the Isthmus of Panama emerged as a permanent separation of the ocean circulation between the Pacific and the Atlantic/Caribbean Several other planetary and continental forces combine so that by around 2½ mya there is a major expansion of the Antarctic ice sheet and major ice rafts are recorded in the Northern Hemisphere There is a critical drop in temperature about this time, along with a nearly doubling of the level of climatic variability Late Pliocene Climates Late Pliocene, 2 Savanna habitats spread into previous woodland settings, along with increased climate variability Some hominids appear to have adapted to this change by increasing dental surface for grinding tough, drought resistant plant foods (e.g., Australopithecus aethiopicus, A. boisei, A. robustus) Other hominids adopted toolmaking, transport of resources, and dietary change including increased use of animal resources (e.g., Homo habilis) The survivor of this transition appears by 1.8-1.6 mya, (Homo erectus) with substantially enlarged brains and large erect bodies Plio-Pleistocene Setting There is little average change in temperature between 2.5 mya and today Dramatic changes in amplitude of changes Breakpoint about 600,000 years ago The Ice Ages with periods of extensive glaciation in high latitudes alternating with brief warm periods The beginning of extreme fluctuation coincides with advanced Homo erectus Continuing fluctuation produces Archaic Homo sapiens 4
Plio-Pleistocene Climates Archaic Homo sapiens Advanced Homo erectus Early Homo erectus Less Ice More Ice Pliocene Lifecycles Australopithecus afarensis and earlier forms appear to have developed much like modern chimps Leigh would add brief adolescent spurt to Bogin and Smith s model Australopithecus africanus may have had slightly extended infancy and childhood Increased adolescence in Leigh s model Homo habilis adds a childhood phase, and probably increased adolescence Pleistocene Lifecycles, 2 Early Homo erectus increases the childhood phase, but according to Bogin and Smith still had no adolescent spurt Based on Leigh s analysis, this group, represented by the oversized adolescent KNM-WT 15000 would be uniquely large bodied among primates to not have an adolescent spurt Later Homo erectus would have increased childhood and exhibits an adolescent spurt in both models Possibility that the extended childhood is beginning to play a role in transmitting communication skills Lifecycles Alternate Lifecycles Pleistocene Lifecycles, 3 Early Homo erectus may have lived long enough for post-reproductive females to experience menopause--a first in mammalian evolutionary history A small fraction of later Homo erectus populations appears to have lived substantially past menopause for females and into old age for males If relatively sophisticated communication systems existed at this point, the later life could have served a valuable adaptive purpose in passing on information about rare events (the once in a century drought or freeze) in the face of unprecedented climatic variability 5
Menopause Late Pleistocene Happenings The warm peak of the last interglacial (ca. 120,000-125,000 years ago) suspiciously coincides with the consensus date estimate of several genetic studies for the origin of modern Homo sapiens The last refinements on language and improvements in technology may have occurred and been a substantial cultural advantage in this time of extreme climatic change The final expansion of childhood to allow the perfecting of language skills may have accompanied this change Consensus date of genetic models for appearance of Modern Homo sapiens Recent Climates 6