A Useful Question • What is the difference between valgus angle and bicondylar angle? Week 7: Lecture 7: Hypotheses about the origins and elaboration of our genus • Introduction to Life History Theory • Several hypotheses related to appearance of H. ergaster • Return to the Mid-Pleiostocene hominin fossil record Transitions Change in cranium, face, teeth, and body shape as well as size! Big Question! • What processes led from a small bodied australopithecine-like species to a larger bodied, larger brained Homo ergaster, and then eventually to us? • Useful questions sought within the framework of life history theory Introduction to Life History Theory • How do members of a species allocate energy through life to accomplish: • survival to and through their reproductive period • reproduction, and offspring care (if any) • growth and development • maintenance of organ systems • Tasks clearly interlinked and involve trade-offs Some Common Life History Parameters • Metabolic requirements • Gestation length • Number of offspring / litter size • Aspects of growth and development • Age at maturity / age at first birth • Inter-birth interval • Duration of reproductive period • Age mortality profiles etc. Life History links to Socio-ecology • Links between ecology, anatomy, and behaviour: – Niche(s) – Size of home range – Anatomy: body size, brain size, related to locomotion, dentition, gut, and patterns of growth – Diets (quality and quantity) – Social characteristics etc. Several life history parameters characterizing humans: • long gestation relative to most mammals, but… • usually born singly and very dependent (secondarily altricial) • weaned relatively early • an extended period of dependency on kin • relatively late puberty and even later first reproduction • relatively short inter-birth intervals & relatively high infant and child survival rates • long life spans that extend well beyond reproductive period in women, and most men Accounting for changes in brain size, body size and body shape • Several important hypotheses – Heat adaptations including radiator hypothesis – Expensive tissue hypothesis – Assisted birthing, alloparenting and changes in the nature and course of growth and development – Mental mapping – Social group size Hominins Adapting to Environmental Change? • Late Pliocene environments – Cooling – Aridification – Patchier environments – Increased Seasonality • More grazers Heat adaptations • Bipedal posture reduces body surface exposed to solar radiation • Hair on head and shoulders to keep heat out • Greater applicability in accounting for appearance of Radiator Hypothesis Homo - Related to patterns of blood flow through brain - A. africanus and Homo have pattern that is different than in Paranthropus Stone Tools • Earliest: Gona, Ethiopia 2.6-2.5 MYA • Percussion flaking • OLDOWAN INDUSTRIAL COMPLEX • Cores, Flakes, Manuports • Who were the first stone tool- makers? – A. garhi? Homo habilis? – Maybe even P. boisei?? Tool Use • Animal bones and stone tools associated at Olduvai • FLK Zinj site: bordering ancient Olduvai lake – 2500 Oldowan artifacts – 60,000 bones from small mammals to large • 1970’s: Hunters / home-base concept Hunting or Scavenging? Shipman in 1980s finds … • No evidence for disarticulation of skeletons • Kinds and proportions of bones found • Specimens with both cut marks and carnivore tooth marks – 5/13 cut marks overlay carnivore tooth marks! Tool Use (cont’d) • Rick Potts: “butchery” sites – Stone tools “cached” at sites around landscape – Like chimps: stone caches for nut-cracking – Temporary processing areas Significance of these data? • Expensive tissue hypothesis and brain evolution • Changes in diet represent continuation of trends in hominin evolution • Early Homo: – Stone tools, butchery sites: more meat in diet, but… – Importance of increasing technology to access things like underground storage organs Expensive Tissue Hypothesis (continued) • Constraint on evolution of brain – The brain is energetically expensive! – Selection pressures may act, but only if benefits outweigh costs – Within living species of primates, EQ associated with dietary quality – This suggests that foraging strategy, diet and brain evolution may be linked Basal Metabolic Rate (BMR) and Body Weight in Mammals Allometric relationship between brain size and body weight Kleibers Law BMR = kW0.75 How does our basal metabolic rate compare with other mammals? • Humans have similar BMR as other mammals our size • but a higher percentage of energy used to maintain brain • ~ 20 to 25% in adult humans; higher in infants and children • ~ 8 to 10% in adult chimps • ~ 3 to 5% in many other adult mammals The Expensive Tissue Hypothesis (Aiello and Wheeler) The Expensive Tissue (continued) Mental mapping (Milton) • Spider monkey (above) • Howler monkey (to right) Sociality, Development, & Brain Evolution • How to grow a bigger brain when you • Processes that face anatomical & energetic constraints? complement changes in foraging strategies • Advantages of early weaning and alloparenting • Need for changes in patterns of growth – Insertion of childhood phase in human evolution • Intelligent care-givers and increasing survival of infants and children • Increasing group size and neocortex size • Reduced aggression, more cooperation Evidence for changing development? • Bipedalism + encephalopelvic disproportion (Trevathan) • Dental development & eruption patterns – link between M1 eruption, adult brain size, weaning age, sexual maturity across primate species (Holly Smith) Development of modern human sociality • Improved conditions for individual survival & growth, longer lifespans, reduced inter-birth intervals – higher net reproductive success! • increased cortical control of sexual behaviour • family - divisions of labour • recognition of socially-defined kinship • economic cooperation based on sexual bonds and kinship • primate dominance hierarchy replaced by kinship • broader social groups with economic and territorial basis For Self-Review • What is life history theory? • Describe important life-history parameters that distinguish humans from other apes • Describe the expensive tissue hypothesis • Describe archaeological evidence of early Pleistocene hominin scavenging. How does this relate to hypotheses about human evolution? • How are greater cooperation and allo-parenting hypothesised to influence the net reproductive success of earlier members of our genus? Return to the Pleistocene hominin fossil record • When does the Pleistocene begin? – I have been reporting the wrong date! Up until 2009 it was 1.806 mya (~1.8 mya). – Now it is defined as 2.588 mya (~2.6 mya) – So our genus (Homo) probably appeared toward the very end of the Pliocene, or early Pleistocene • Earliest representatives? Ledi-Geraru, Ethiopia (2.8 mya) • Hadar, Ethopia, 2.3 mya; Koobi Fora, Kenya, 1.9 mya H. Erectus vs. H. sapiens sapiens Features of “Classic” H. erectus • Classic from Java and China • Ave cranial capacity ≈ 1000 (range 850-1225 cc) • Large supra-orbital torus / sulcus • Face moderately prognathic • Angled occipital region • Thick cranial bones • Nuchal torus • Greatest breadth of cranium is low Features of Classic H. erectus (continued) • Sagittal keel /parasagittal depression • Some post-orbital constriction • Nasal bones project: beginnings of modern nose! • No chin • Some with shovel-shaped incisors African H. ergaster (or H. erectus?) • East Lake Turkana: KNM-ER 3733 and (~1.8 mya) • ~850 cc cranial capacity • East Lake Turkana: KNM-ER 3883 • (~1.6 mya) • ~ 800 cc cranial capacity Homo ergaster: differences from classic H. erectus • Generally: thinner supra-orbital torus • Less sagittal keeling • Occipital region less angled • Somewhat thinner cranial bones • Smaller average cranial capacity Homo erectus to Homo sapiens • Middle Pleistocene: ~780,000 - 125,000 ya • Transitional forms in Africa, Europe and Asia • Not H. erectus but not H. sapiens sapiens either • ‘Archaic Homo sapiens’ ??? – includes Homo antecessor, Homo heidelbergensis, Homo rhodesiensis, Homo neanderthalensis, Homo helmei, Denisovans, and early Homo sapiens? Homo erectus in Europe? • Numerous sites of Acheulian tools – but perhaps made by “archaic” H. sapiens (aka H. heidelbergensis), not H. erectus • Ceprano “Man” - 850,000 yrs old from Italy Gran Dolina - H. antecessor? • Found in northern Spain (as part of the Atapuerca sites), the crania from Gran Dolina date to 780 kya. • The best preserved individual is a juvenile • Estimated cranial capacity ~1000 cc • Broader frontal than H. erectus • Separated brow ridges and more modern looking mid- face of later humans Mid-Pleistocene Homo in Europe • Gran Dolina, Petralona, Arago, Steinheim, Swanscombe and a mandible from Mauer (others too) • Some argue this grouping represents new species: Homo heidelbergensis Europe – Mauer • 500,000-400,000 ya • Type specimen for Homo heidlebergensis Same Location – Different Deposit Sierra de Atapuerca, Spain • ~ 500-350 kya • Sima de los Huesos • 28+ individuals • H. heidlebergensis • “Young” population • Deliberate burial? African Middle Pleistocene Discoveries • Evidence of divergence from an H. erectus like species early in the Mid-Pleistocene • Probably in either Africa or western Eurasia • Represented by African fossils such as Bodo (600 kya) and Kabwe (600 – 400 kya ?) • Jebel Irhoud (~315 kya) • Omo Kibish 1 and 2 (200 kya) Africa – Kabwe • ~300,000 ya • Broken Hill/Kabwe, Zambia • 1280 cc cranial capacity • Homo rhodesiensis or heidelbergensis • Extremely robust