Chapter Outline
I. Introduction
a) Definite hominin fossil material has been found in Africa that dates to just after 5 mya.
i) The hominin nature of these remains is indicated because of the way they behaved.
ii) Hominin biocultural nature of human evolution, this chapter focuses in the methods scientists use to explore the secrets of early hominin behavior and ecology.
II. Definition of a Hominin
a) Hominin origins date to the end of the Miocene.
b) Hominins have been variously defined as having: a large brain, bipedal locomotion, and/or tool-making behavior.
i) It is clear that these characteristics did not evolve simultaneously. The phenomenon of different physiological systems evolving at different rates is called mosaic evolution.
ii) Bipedal locomotion is the key indication that a fossil was a hominin.
c) What's in a name? we refer to members of the human family as hominins.
i) Molecular evidence clearly shows that the great apes are not a monophyletic group.
ii) Hominoid classification has been significantly revised adding two further taxonomic levels (subfamily and tribe).
iii) There is very close evolutionary relationships between humans and African apes (particularly chimpanzees and bonobos)
(1) The former term hominid has a quite different meaning in this revised classification referring to all great apes and humans.
d) Biocultural evolution: the human capacity for culture
i) The most distinctive feature of humans is our dependence on culture.
(1) Human culture is an adaptive strategy that includes cognitive, political, social, and economic aspects as well as the capacity to make and use tools.
ii) The earliest hominins did not regularly manufacture stone tools.
(1) They probably had the tool-making capabilities of living chimpanzees.
(a) Stone tools appear in the archaeological record about 2.6 mya.
(2) By 7 to 6 mya, hominins had developed bipedalism and could carry and transport objects from place to place..
iii) The dynamics between neuronal reorganization, tool use, changing social organization, and communication form the core of biocultural evolution.
III. The Strategy of Paleoanthropology
a) Paleoanthropology, the study of ancient humans, is a multidisciplinary approach that includes geologists, archaeologists, physical anthropologists, and paleoecologists.
i) The earliest artifact date back to 2.6 mya and were found in sites from the Gona and Bouri areas in northeastern Ethiopia
ii) Paleoanthropologists must synthesize information regarding:
(1) Dating of the site.
(2) The paleoecology of the site.
(3) Any archaeological traces of behavior.
(4) Any anatomical evidence from hominid remains.
iii) The ultimate goal is to "flesh out" the hominids to produce a more complete and accurate understanding of human evolution.
IV. Paleoanthropology in Action-Olduvai Gorge
a) Olduvai Gorge, located on the eastern branch of the Great Rift Valley of Africa, has yielded an immense quantity of high-quality data on early hominid behavior.
b) Olduvai has a well-documented sequence of geological, paleontological, archeological, and hominid remains that span the last 2 million years.
i) The earliest hominid site dates to about 1.85 mya, and is accompanied by the Oldowan tool industry.
ii) The most famous hominid fossil from Olduvai is probably the Zinjanthropus skull, discovered by Mary Leakey in 1959.
V. Dating Methods
a) The two types of dating methods are relative dating and chronometric, or absolute dating.
i) One relative dating method is based on stratigraphy.
(1) The law of superposition states that (in an undisturbed deposit) a lower layer is older than a higher layer.
ii) Another relative dating method is fluorine analysis.
(1) Groundwater contains fluorine, and the longer a bone is in a deposit; the more fluorine will accumulate during the fossilization process.
(2) The fluorine method can only be used to compare bones found at the same location.
iii) Chronometric methods are based on the phenomenon of the radioactive decay of unstable isotopes.
(1) Depending on the half-life of the isotope used, the time range for this method varies from the age of the earth (billions of years) to less than 1,000 years.
(a) Paleoanthropologists use the K/Ar method extensively to determine the age of volcanic deposits (and therefore the associated fossils) in East Africa.
(i) The 40Ar/39Ar method can be used on smaller samples and provides more accurate results.
(b) The C-14 method is used on organic materials and has a useful time-range of 75,000 to less than 1,000 years.
(c) Thermoluminescence dates can be obtained from burnt flints.
b) Applications of dating methods: examples from Olduvai
i) Olduvai has several reliable K/Ar dates for the underlying basalt and the tuffs in Bed I, and the Zinj site has been dated to 1.79 + .3 mya
(1) K/Ar dates must be cross-checked, since all dates have errors associated with them.
(2) One important cross-check is to use the fission-track method.
(a) In fission-track dating uranium 238 (238 U) decays regularly by spontaneous fission.
(3) Another way of cross-checking is to use paleomagnetism.
(a) With this technique, the orientation of magnetic sediments is checked to determine whether they were deposited during a period of normal or reversed magnetism.
(4) Biostratigraphy, or faunal correlation, is another cross-check.
(a) This technique matches faunal remains, such as pigs, elephants, rodents, and antelopes at the site in question with the known evolutionary sequences of the same faunal species.
(5) Cross-checking with different methods lends confidence to the results because each method has a different source of error.
VI. Excavations at Olduvai
a) There are many hominin sites found at Olduvai.
i) Mary Leakey excavated close to 20 of these sites.
ii) There is much controversy regarding hominin activities at these sites ("campsites").
(1) These are general-purpose areas where hominins carried out daily activities.
(2) The exact nature of the activities carried out at these sites is not fully understood.
(a) Initially, they were interpreted as true campsites. Alternately, the accumulations could have been produced by nonhominins, or the accumulations could be the result of hominin gathering and scavenging activities.
VII. Experimental Archaeology
a) Archaeologists try to reconstruct prehistoric techniques of tool-making, butchering, and other activities in order to answer fundamental questions about hominin behavior.
i) Stone tool (lithic) technology is the most commonly preserved aspect of hominin cultural behavior.
(1) Initially, archaeologists thought that the Oldowan industry consisted of deliberately fashioned cores and flakes.
(a) Richard Potts believes that only the flakes were being deliberately produced, and the "core tools" were merely byproducts of flake manufacture.
(2) Flakes can be removed from cores by direct percussion and/or pressure flaking.
(a) The nodules found in Bed I at Olduvai (1.85-1.2 mya) have been unifacially flaked by using direct percussion.
(b) Later, in Bed IV (400,000 ya), most tools are flaked bifacially with direct percussion, but they could only have been produced with a "soft" hammer, most likely a piece of bone or antler.
(c) The microliths found in the upper beds (17,000 ya) must be produced using the pressure flaking method in which a pointed piece of bone, antler, or wood is pressed firmly against the stone.
(3) Microwear analysis is performed by observing stone tools under high magnification to detect patterns of polish and striations characteristic of tool use on various materials such as bone or wood, as well as the way the tools were used (such as cutting versus scraping).
(a) Tools observed under a scanning electron microscope may reveal the presence of the phytoliths of the different species of plants on which the tool was used.
ii) Analysis of bone is undertaken to determine how humans and natural forces affect bone.
(1) Taphonomy is the branch of paleoecology concerned with the influence of natural forces on bone deposition and preservation.
(a) Research such as observing human butchering practices, or analyzing scavenger bone accumulations is conducted to determine the extent to which these factors can account for the composition of bone accumulations at hominin sites.
(b) Another research concern is to determine the effects of natural forces, such as running water, on patterns of bone accumulations.
(2) Bones may also be examined for evidence of hominin activity in the form of cut marks.
VIII. Reconstruction of Early Hominin Environments and Behavior
a) The behavioral interpretation of archaeological and skeletal evidence is speculative and not as amenable to rigorous testing as are the original data.
i) Nevertheless, behavioral and environmental reconstructions (or scenarios) give us a broader view of evolution and adaptation which can be known from the paleoanthropological record.
(1) Environmental explanations for hominin origins focus on the environment as a major factor in evolution.
(a) It is possible to oversimplify the complex interactions between the organism and the environment by making sweeping generalities about environmental determinism.
(b) There may have been major ecological changes occurring in Africa at or around the same time that hominins were evolving (i.e. a trend towards cooler, drier, and more seasonal environments).
(i) However, the causal connection between environmental change and the initial hominoid/hominin transition has not yet been convincingly demonstrated.
(c) The evolutionary pulse theory proposes a causal connection between periods of increased aridity in South and East Africa and the evolution of new species of hominids.
(i) In Indonesia, scientists investigate the flora and fauna enticing early hominins, evident at Sangrian Dome where life there included the extended habitation of Homo erectus from 1.6 to .9 million years.
(ii) Some experts believe that hominins evolved due to their flexibility to adapt to many types of environments.
(2) Why did hominins become bipedal?
(a) The shift to bipedality is the fundamental hominin adaptive shift.
(b) Hominins are adapted to a more mixed and open-country habitat than are the chimpanzees, yet no other mixed to open-country adapted mammals are full-time bipeds.
(i) There can be no simple environmentally-driven explanation for the shift to full-time bipedalism.
(ii) Jolly's seed-eating hypothesis proposes that early hominins, like living gelada baboons, were highly dependent on eating seeds and nuts but they stood upright while feeding.
1. Fossil evidence indicates that the dental adaptations predicted by Jolly's hypothesis are not restricted only to the early hominins.
(iii) Lovejoy has devised a scenario in which hominins are tied to a "home base" that requires female provisioning by pair-bonded males.
1. The skeletal evidence suggests early hominins were not pair-bonded because they are highly sexually dimorphic. A recent study, however, questions the high level of sexual dimorphism in early hominins.
2. Additionally, notions of food sharing, home basis, and long distance provisioning are questioned.
(iv) Falk has proposed the "radiator theory" which proposes a link between bipedalism and brain expansion.
1. Falk argues that the hominins that exploited the hot savannah evolved a venous drainage system that better cooled the brain. This particular venous drainage pattern also removed the constraints for encephalization because it efficiently cooled larger brains too.
2. This hypothesis does not explain the origins of bipedalism, nor does it explain why brain size increased in the genus Homo.