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Chapter 10

Chapter 10 How Humans Evolved - Bio 1M03.docx

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James S Quinn

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How Humans Evolved Part 3: The History Of The Human Lineage Chapter 10: From Tree Shrew To Ape Textbook Caveats  “Primitive" o Recall that no organism should be considered “lower” or “higher” (“primitive” or “advanced”) than any other  “Population” o Occasional use of this word in the textbook may be better described as a “community” (eg/ p 259) Overview  Human evolution in its entirety began with the origin of cellular life ~3.8 billion years ago  Here we will focus on a recent part o this history: the time from our shrew-like insectivorous ancestors scurried through leaf litter in forests ~100 mya until the present Life’s Timeline  Major events in the history of life can be summarized in a timeline broken into four segments  Mesozoic Era – 251 to 65.5 mya o Known as the Age of Reptiles because it saw the rise and dominance of the dinosaurs o Ended with the extinction of the dinosaurs except birds  Cenozoic Era – 65.5 mya to the present o Known as the Age of Mammals, since during this time the mammals diversified after the disappearance of the dinosaurs  Permian and Early Triassic Periods o Much of the worlds fauna was dominated by therapsids  Therapsid – diverse group of reptiles that possessed traits such as being warm blooded and covered with hair; reptiles with mammalian traits  End of Triassic Period (~190 mya) o Dinosaurs radiated to fill all of the niches for large terrestrial animals o Most therapsid groups disappeared o One therapsid lineage evolved and diversified to become the first true mammals  Probably mouse-sized, nocturnal creatures that fed mainly on seeds and insects  Had internal fertilization, but laid eggs  End of Mesozoic Era (65 mya) o Placental and marsupial mammals that bore live young had evolved\  Cenozoic Era o Extinction of dinosaurs – came the radiation of mammals o Evolved from creatures like the contemporary shrew o This was coupled in time, and perhaps associated with the diversification of angiosperm plants Continental Drift and Climate Change  Climate has changed substantially during the last 65 million years – first becoming warmer and less variable, then cooling, and finally fluctuating in temperature  Evolution produces adaptation, but what is adaptive in one environment may not be adaptive in another environment  If the environment o Remained the same over the course of human evolution – then the kinds of evolutionary changes observed in the hominin fossil record would have to be seen as steady improvements in the perfection of human adaptations  Evolution would have progressed toward a fixed goal o Varied through time – then evolution would have to track a moving target  New characteristics seen in the fossil record would not have to represent progress in a single direction – adaptations to changing environmental conditions  Continental Drift – movement of continents o Contributed to environmental change o Continents are not fixed in place – the enormous, relatively light plates of rock that make up the continents slowly wander around the globe, floating on the denser rock hat forms the floor of the deep ocean o Pangaea – a single huge land mass ~200 mya  All of the land making up the present-day continents was joined together  ~125 mya, Pangaea began to break apart into separate pieces  Laurasia – northern half of Pangaea  Includes what is now North America and Eurasia (minus India)  Gondwanaland – southern half of Pangaea  Had broken into several smaller pieces ~65 mya, when dinosaurs became extinct  Africa and India separated o India headed north, eventually crashing into Eurasia o Remainder of Gondwanaland remained in the south  Separated into South America, Antarctica and Australia  ~ 5 mya South America drifted north to join North America o Important to the history of human lineage for two reasons 1. Oceans serve as barriers that isolate certain species from others – the position of continents plays an important role in the evolution of species 2. Continental drift is one of the engines of climate change – climate change has fundamentally influenced human evolution  The size and orientation of the continents have important effects on climate o Large continents tend to have severe weather  When continents restrict the circulation of water from the tropics to the poles, world climates seem to become cooler  How we know that climate has changed o Analyze the ratio of oxygen isotopes in dated strata from ocean cores o Isotopes are variants of atoms that have a different number of neutrons in the nucleus 16 18 o O evaporates more readily than O o Snow and rain have a higher O concentrations than oceans o When there is extensive glacial coverage (cold climate) this causes the ratio of O: O in the ocean to drop because O is trapped in glaciers 18 o The concentration of O is higher in the oceans during periods with cold climate The Methods of Paleontology – Relating Changes in Climate to Biological Diversification  Fossil – natural copies of bones; the bones of dead organisms may be preserved long enough for the organic material in the bones to be replaced by minerals (mineralized) from the surrounding rock o In order to assign a fossil to a particular position in a phylogeny, we must know its age  Radiometric Dating – based on the fact that isotopes of certain elements are unstable; they change spontaneously from one isotope to another of the same element or an entirely different element o Isotopes – elements have the same number of protons but different numbers of neutrons in the nucleus o Potassium Argon Dating  Used to date the age of volcanic rocks found in association with fossil material  Molten rock emerges from a volcano at a very high temperature and all argon gas is boiled out of the rock – after this, all argon present in the rock must be due to decay of potassium  Argon is absent from lava so any argon in a volcanic rock is derived from the decay of potassium  Works for ages of 500 000+ years o Carbon-14 Dating (radiocarbon dating)  Live animals have the same level of C and C as the atmosphere  After death, C begins to decay into C; the ratio of these two can be used to estimate age  Works for cells or tissues younger than ~40,000 years  Radiometric Methods o Thermoluminescence Dating  High energy particles from decay of radioactive material in and around a rock and from cosmic rays that bombarded Earth from outer space pass through a rock  Particles dislodge electrons from atoms – electrons get trapped elsewhere in the rocks crystal lattice  High energy nuclear particles from cosmic rays and radioactive decay are trapped by particles  These particles/electrons are released when heated – gives off light  Measure the light given off – if the density of high-energy particles currently flowing through the site is known, the length of time that has elapsed since the object was burned can be estimated  Because burning “resets” the particle composition, the age of burned material can be quantified by measuring the particle composition o Electron-Spin Resonance Dating  Determines the age of apatite crystals (component of tooth enamel)  Similar to thermoluminescence dating; measures age according to the presence of trapped electrons  Problems of Radiometric Dating o A particular site may not always contain material that is appropriate for radiometric dating o Radiometric methods can have large confidence intervals or large margins of error  Radiometric Dating is often supplemented with relative methods:  Magnetic Reversals  The Earths magnetic field reverses itself every once in a while  Pattern of magnetic reversals is not the same throughout time – pattern fir a given time is the same  We know the pattern because when rocks are formed, they record the direction of the Earths magnetic field at that time  By matching up the pattern of the magnetic reversals at a particular site with the well-dated sequence of reversals from the rest of the world, scientists are able to date sites  Extinct and distinct species of animals The Evolution of the Early Primate  Vascular Plants o Vascular plants have a circulation system (xylem and phloem) for conducting water, minerals and photosynthetic products o This allows them to be much larger than non-vascular plants o Ferns, gymnosperms and angiosperms are all vascular plants  Plants with seeds vs. Plants without seeds o Seeds are an innovation that evolved in the ancestor of gymnosperm and angiosperm plants o Before seeds evolved, all plants used spores (ferns, for example, use spores not seeds) o Spores, unlike seeds, have very little stored food resources  Gymnosperm vs. Angiosperm o Gymnosperm – produces ovules that lack a cover whereas angiosperms produce ovules that are covered by the ovary  Dominated the forests of the world during the first two thirds of the Mesozoic o Angiosperm – vascular plants that form flowers  Appeared and spread during the Cretaceous when Pangaea broke up  Seeds can also have a fruit coat around the seed coat (testa)  Have flowers  Most have endosperm within the seeds (eg/ flour is made from endosperm of wheat)  Most produce fruit that contain the seeds (eg/ an apple)  Most diverse group of land plants  Experienced an explosive diversification during the Cretaceous  By the late cretaceous angiosperms dominated habitat formerly occupied by ferns and cycads  By the end of the Cretaceous 65 mya, large canopy-forming angiosperms replaced conifers as the dominant trees  Angiosperms are not the first plants to establish mutualistic interactions with animals (eg/ some pines require animals to open the pine cones)  These interactions did become more elaborate as compared to conifers, ferns and cycads  Relationships between angiosperms and animals  Pollination  Defense  Dispersal  What is a Primate? o Adaptations for climbing and other forms of locomotion (leaping, walking on 2 or 4 limbs, knuckle walking, brachiation) o Most have opposable thumbs and prehensile tails o Increased reliance on stereoscopic vision o 3-color vision in some species o Nails on toes and fingers o Typically nails instead of claws  Claws are typically curved, pointed and compressed sideways  Primates hands generally have 5 digits, usually with nails  Presence of nails may be correlated with small branch foraging o Colugo – “flying lemurs”  Arboreal gliding mammal from Southeast Asia  Has flaps of skin between legs used for gliding  Early Primate Evolution o With Pangaea breaking up in the Cretaceous, a revolution occurred in the plant lineage, flowers evolved o The evolution of flowering plants created new niches; primates were among the first animals that evolved to fill these niches o The ancestors to modern primates were small, nocturnal quadrupeds, similar to contemporary shrews  Plesiadapiformes o A taxon containing extinct animals o Fossil specimens have been found on North America, dating to the Paleocene, 65 to 54 mya o Possess some primates-like traits but experts disagree as to whether or not should be considered part of the primate order o Solitary quadruped with well developed sense of smell o Teeth are variable – suggests a wide range of dietary specializations o Did not have binocular vision o Claws on hands and feet o Carpolestes simpsoni – a small creature that lived ~56 mya  Possibly an ancestral primate  Opposable big toe with a flat nail instead of a claw  Claws on other digits – used for climbing and to gasp small supports  Molars consistent with frugivory  Low-crowned molars, suited for eating fruit  Eyes on side of head – fields of vision do not overlap  Early Primate Evolution: What Traits Were Favored by Natural Selection o Binocular vision, grasping hands and feet, nails on fingers and toes may have evolved (together or sequentially) to:  Enhanced visually directed predation in insects in the terminal branches of trees  Support: many arboreal predators (owls, ocelots) have eyes in the front of his head  Problems for this hypothesis: Discovery that grasping hands and feet evolved in a frugivorous plesiadapiform species before the eyes were shifted forward  Facilitate locomotion (leaping)  Problems: C. simpsoni had grasping hands and feet but evidently didn’t leap from branch to branch  Enhance the ability to exploit a new array of plant resources on terminal branches of angiosperms  Including fruit, nectar, flowers and gum (as well as insects)  Middle Eocene (54 – 34 mya) o During the early Eocene, primates occurred in North America, Europe, Asian and Africa, but not in South America o ***  Eocene Primates o The ancestor of “wet-nosed”, “flat-nosed”, “downward-nosed” and primates, lived during the Eocene o It was wetter and warmer back then o At the beginning of the Eocene – North America and Europe
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