EVE100_F24_609-622 PDF: Human Evolution

12/3/24 Humans are apes 609 609 FK 20.4 610 610 1  12/3/24 Early hominids...

12/3/24 Humans are apes 609 609 FK 20.4 610 610 1  12/3/24 Early hominids included Sahelanthropus, probably bipedal 7 mya, and Ardipithecus. - Bipedalism is a key event in hominid history - Reduced galloping speed and tree-climbing ability - But greatly increased efficiency of walking - Climate cooling and drying in Africa, leading to pressure to forage more widely might have been the agent of selection - Humans are very unusual in their ability to run for long periods of time 611 611 Early bipedal hominids (see FK 20.7) Lucy – A. afarensis Ardi– Ardipithecus ramidus 612 612 2  12/3/24 See also FK 20.11 613 613 FK 20.5 614 614 3  12/3/24 Brain size evolved fast relative to body size 615 615 What selection pressures drove brain size evolution? 616 616 4  12/3/24 Childbirth is risky – big brains and bipedalism Hiwi hunter-gatherers death rates due to childbirth (Hill et al. 2006, J. Human Evol.) 4.4 per 1000 reproductive aged women per year Based on pregnancy rates = death-in-pregnancy rate of 1 per 55-75 pregnancies We are the only apes that help each other during birthing. 617 617 High risk (for women) reproduction may be a trade-off resulting from optimizing the conflicts between big brains and bipedalism. Indeed, infants are born about 0.5 kg less than optimal birth weight for survival after birth (pre vs. post-natal conflict/trade-off?) Infant brains grow quickly with significant metabolic demands - a newborn uses about 85% of resting metabolic rate for brain. 618 - the 5-yr old still uses 45% of resting metabolic rate for brain. 618 5  12/3/24 Figure 21.17 Humans have a higher metabolic rate relative to fat-free body mass than do chimpanzees (Pan), gorillas (Gorilla), and orangutans (Pongo) 619 We are the fattest great apes, but our babies are unusually fat 620 620 6  12/3/24 Selection would strongly favor infant fat storage during early development as a buffer for lean times. 621 621 An extended childhood may be an unavoidable indirect effect of these human traits: 1) Human babies must be born relatively “undeveloped” to get through the mom’s pelvis 2) But human brains are very big, requiring many years to reach final size 3) We are the only great apes that tend to have overlapping juveniles that cannot provide for themselves. This has major implication for social organization (e.g., alloparenting, male resource provisioning) and human interbirth interval. 622 622 7