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Evolution Evolutionary Process Human Evolution Evolution • Evolution = change in characteristics of organisms as a result of changes in genetic composition • Two important aspects – Descent from a common ancestor – Adaptation to the environment • Adaptation = characteristic that makes it more likely that an organism will survive and reproduce in its environment Evidence for Common Descent From the Fossil Record Living Organisms resemble extinct fossil forms. Evidence for Common Descent From the Fossil Record Progressive changes from simpler to more complex organisms can be seen in the fossil record. Biogeographical Evidence for Common Descent Plants and animals of each continent are distinctive. Biogeographical Evidence for Common Descent Different island species resemble each other. Cactuseater Insect-eater Seed-eater Natural Selection led to beak sizes and shapes suitable for different food sources Biogeographical Evidence for Common Descent Island Populations resemble those on nearby land. The Galapagos finches resembled the grassquit found on the coast of Ecuador. Recent studies suggest that the finch ancestor may have originated from the Caribbean islands. Anatomical Evidence for Common Descent: Homologous Structures Flying Swimming Running Grasping Anatomical Evidence: Vestigial Structures Remnants of hindlimb seen in boa and whale Functional hindlimb in salamander Anatomical Evidence: Vestigial Structures In fish, these pouches form the gill slits. In humans, one of the pouches does not persist past this embryonic stage. Other pouches give rise to structures of the ear, tonsils, parathyroid and thymus. The tail has 12 vertebrae at this stage. Numbers 612 are lost through cell death and numbers 4-5 are reduced. The coccyx is the remainder of vertebrae 1-5. Evidence for Common Descent from Developmental Biology Evidence for Common Descent from Biochemistry A Flowchart of Evolutionary Reasoning Formation of new genotypes leads to phenotypic variation observations inferences Variations exist between members of a population Relatively constant resources and population over time At least some variation is hereditary More offspring are born than survive and reproduce Individuals whose inherited traits give a higher probability of survival and reproduction leave a greater number of offspring Favorable traits will accumulate in the population over generations Adaptations Darwin’s proposal Originally, giraffe neck length varied. Competition for resources causes long-necked giraffes to have the most offspring. Due to natural selection, most giraffes now have long necks. Natural Selection as the Mechanism for Evolution ~7 MYA: There was a common ancestor for African apes and hominids bipedal posture Primate Evolutionary Tree ~15 MYA: There was a common ancestor for all apes and hominids ~45 MYA: There was a common ancestor for monkeys, apes and hominids Common ancestor may have resembled a tree shrew Adaptations of Primates • • • • • Opposable thumb Well-developed brain Nails instead of claws Single birth Extended period of parental care • Emphasis on learned behavior Australopithecus afarensis An early hominid Hominin Evolution In Africa In Asia Possible ancestor of both A. africanus and Genus Homo Possible ancestor of modern humans Human Evolution • Genus Australopithecus – walked upright – product of mosaic evolution = different body parts change at different rates • Smaller brain resembles apes • Bipedal locomotion resembles humans – ancestors of genus Homo (not certain which species led to Homo habilis) Human Evolution • Genus Homo Homo ergaster – Homo habilis = “handy man” • produced tools • teeth supported omnivorous diet • enlarged speech area in brain – Homo ergaster (in Africa) • advanced tools, use of fire • larger brain, taller body • striding gait neck of femur femur – early Homo sapiens (based on Cro-Magnon fossils) • compound tools • larger brain • language and culture, including art femur neck is longer than in modern humans Two Hypotheses for the Origin of Modern Humans Multiregional continuity Out of Africa DNA Analyses Related to Human Origins Visit http://www.geneticorigins.org and choose Mitochondrial Control Region Media and Animations Solving the Mystery of the Neanderthals Other Applications of DNA Analysis can be found at http://www.dnai.org Choose Applications, then Human Origins Homo floresiensis Homo erectus Homo sapiens A Newly-Discovered (but extinct) Species of Genus Homo