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Chapter 7

TEXTBOOK Chapter 7 - The Human Species (PGS. 166-190)

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Esteban Parra

Notes From Reading C HAPTER  7: HE  HUMAN  PECIES  PGS . 166­190) Characteristics of Living Humans Distribution and Environment - Humans are the most widely distributed living primate species - During the course of human evolution, we have expanded into many different environments - Biological adaptations have aided humans in new environments (such as cold weather, high altitudes) - Cultural adaptations of humans have allowed greater expansion o Enabled us to range far beyond our biological limitations o Permitted incredible population growth Brain Size and Structure - Ape’s skull is characterized by a relatively small brain and large face, modern humans have relatively large brains and small faces - The relationship between brain and body size is not linear o As body size increases, brain size increase – but not at the same rate o Allometry – the study of the change in proportion of various body parts as a consequence of different growth rates - Allometric relationship between brain size and body size is quite regular among almost all primates (most notable exception is humans) o The human brain has a higher proportion of neocortex (part of the cerebral cortex), involved in conscious thought and language - The human brain is not only larger than the brain of a chimpanzee, but also structurally different - The increased convolution of the human cerebral cortex (the folding of brain tissue) means that the brain of a human child with the same volume of a chimpanzee has more cerebral cortex Bipedalism - Bipedal – Moving about on two legs. Unlike the movement of other bipedal animals, such as kangaroos, human bipedalism is further characterized by a striding motion - The physical structure of human beings shows adaptation for upright walking as the normal mode of movement o Apes can walk on two legs, however they cannot as well or not as often - Humans bipedalism is made possible by anatomical changes involving the toes, legs, spine, pelvis, and various muscles - No bones are added or deleted; the same bones can be found in humans and in apes o The changes involve shape, positioning, and function - Chimps have a divergent big toe which allows them to grasp with their feet, where the big toe of the human is tucked in next to the other toes - Humans are literally “knock-kneed” where the femurs are sloped inward from the hips which transmits our weight directly underneath us o In contrast, the angle of an ape femur is very slight, where the ape is off balance and tends to fall toward one side - Human spine also promotes balance when we walk upright o Spinal column of humans is vertical, allowing weight to be transmitted down through the center of the body o In knuckle-walking apes, the spine is bent in an arc so that when the apes stand on two legs, the center of gravity is shifted to the front of the body - Human pelvis is shaped differently from an ape pelvis as well Notes From Reading C HAPTER 7: THE H UMAN  SPECIES PGS. 166­190) o It is shorter top to bottom and wider side to side, providing changes in muscle attachment that permit striding bipedalism o Shortness of the human pelvis allows greater stability when we stand upright Canine Teeth - Human canines are small and do not project beyond the level of the other teeth - Human canine teeth serve much the same function as the incisor teeth - Given that canine teeth serve as weapons in many primate species, the lack of large canine teeth in humans seems to imply that we do not need them for weapons anymore Genetic Differences between Humans andApes - Humans and chimpanzees share over 98 percent of their DNAsequences - The FOXP2 gene, located on human chromosome 7, which may have something to do with the evolution of human language o Arare mutant allele of this gene is associated with language impairment, and two copies of the nonmutant functional form of FOXP2 are needed for normal language acquisition o Same form of this gene is found in chimpanzees, gorillas, and rhesus monkeys, all of which differ from human by two amino acid substitutions o This pattern that suggests that these two changes occurred after the hominin line split from the common ancestor withAfrican apes - Some specific human traits have been traced to the loss of regulatory genes during human evolution examined the genomes of humans, chimpanzees, and macaques (an Old World monkey) and identified 510 DNAsequences that had been present in chimpanzees but were lost during the course of human evolution o One sequence acts to regulate a gene that limits tissue growth, and the deletion of this regulatory sequence in humans may be associated with the expansion of the cerebral cortex, possibly representing a genetic mechanism associated with the rapid brain growth seen in human evolution o The other sequence affects an androgen receptor; the deletion of the regulatory gene in humans is associated with the loss of sensory whiskers and penile spines, both of which are absent in humans but present in many other mammals - Apes have large jaws and powerful jaw muscles for chewing o Living humans have rather small, less powerful jaws Sex and Reproduction - The fact hat humans do not have the estrus cycle has often been cited as a unique aspect of human sexuality o Esctrus – Atime during the month when females are sexually receptive - For the most part, temperate-zone domestic animals breed only during certain seasons and mate around the time of ovulation o Human females, in contrast, cycle throughout the year and may mate at any time during the cycle o The human pattern of reproduction is basically the same as that of most other primates: single births - The pelvic anatomy of living humans means that a new baby has to rotate through a narrow and twisting birth canal o Childbirth in humans is more difficult and more complicated than in apes, due to our enlarged brains and the changes in pelvic anatomy that accompanied bipedalism Notes From Reading C HAPTER 7: THE H UMAN  SPECIES PGS. 166­190) o Childbirth is also made difficult by the fact that the baby is born facing backward relative to the mother, making it difficult for the mother to guide the infant from the birth canal without assistance Social Structure - Acommon Western assumption is that the “normal” social structure of human beings is the nuclear monogamous family group: mother, father, and children - The majority of human societies studied have a stated preference for polygyny (one husband and several wives), and a few cultures practice polyandry (one woman and several husbands) - Humans show a great deal of variation in other aspects of their culture as well, such as economic systems, political systems, and legal systems The Human Life Cycle Prenatal Growth - Prenatal life is the period from fertilization through childbirth o After fertilization, the fertilized egg, or zygote, develops into a cluster of identical cells deriving from the initial fertilized egg o During the first week, the fertilized egg multiplies as it travels into the uterus o During the second week, the outer layer of this ball forms the beginning of the placenta - Embryo – The stage of human prenatal life lasting from roughly two to eight weeks following conception; characterized by structural development o During this time, the basic body structure is completed and many of the different organ systems develop o The embryo has a recognizably human appearance, although it is still not complete - The fetal stage lasts from this point until birth o Fetus – The stage of human prenatal growth from r
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