The Fossil Record – Chapter 4
Rock types
• Three major rock classifications:
– Igneous
• Solidified magma
• Most common type of rock
– Sedimentary
• Sediment that becomes compacted into rock
• Usually distinctly layered
– Metamorphic
• Rocks modified by heat, pressure and chemical agents
• In which type of rock are most fossils found?
A dynamic planet
• A Layered Sphere
– Core
• Interior composed of solid, intensely hot metal
• Generates magnetic field enveloping the earth
– Mantle
• Hot, pliable layer surrounding the core
• Less dense than core
– Crust
• Cool, lightweight, brittle outermost layer
• Floats on top of mantle
Tectonic processes
• Upper layer of mantle contains convection currents that break overlaying crust into a mosaic of tectonic plates
– Slide slowly across earth’s surface
Tectonic processes
• Earthquakes are caused by grinding and jerking as plates slide past each other
• Mountain ranges pushed up at the margins of colliding plates
Geological time
Geologic time
• Absolute ages are determined by radiometric dating
Dating a Fossil
• A living mollusk takes up trace amount of 14C and 12C
• Mollusk dies; buried in sediments and is fossilized
• Radioactive decay of 14C changes ratio of 14C to 12C
• Unearth fossil and measure carbon
Radioactive decay
Geologic time scale
• Boundaries determined by geologists and are subject to revision
• Boundaries based on abrupt transitions in fossil record
– Correspond to mass extinctions
• Focus on eras after complex animals appeared
– Paleozoic
– Mesozoic
– Cenozoic
Division of time
• Eras are divided into periods, which are divided into epochs
Fossilization
• Organism becomes buried in ash or sediments
• Rapid burial and a lack of oxygen aid in preservation
• The organic remains become infused with metal and mineral ions
Stratification
• Fossils are found in sedimentary rock, which is formed in layers
• Deepest rock strata are assumed to be the oldest, surface layers the youngest
Fossil record is incomplete
• Fossils have been found for about 250,000 species (less than 1% of all species)
• Most species weren’t preserved
• Record is biased toward the most accessible regions and hard-bodied species
Evolutionary change is gradual within a species
Origins of higher taxa
• Read about the fossil evidence of origins of higher taxa in your text, including:
– Amphibia
– Birds
– Mammals
– Cetacea
Hominin evolution
What is a primate?
• Hands and feet adapted for grasping
• Relatively large brains and short jaws
• Forward looking eyes
• Nails (rather than claws)
• Opposable thumb in anthropoids only (all but prosimians)
Order Primates
Suborder Prosimii
Prosimians
• Mostly nocturnal insectivores
• Large eyes/greater brain capacity than mammals from which they were derived
• Evolved ~ 55-65 Mya
Platyrrhini
New World Monkeys
• Split off from Old World monkeys ~ 40 Mya
• Arrived in New World via rafting or island hopping
• Diurnal, insectivorous and frugivorous
• Grasping tails; strictly arboreal
• Limited color vision (2 types of cones)
Catarrhini
• Old world monkeys + Hominoidea
Cercopithecoidea
Old World Monkeys
• Diurnal, frugivorous, folivorous, insectivorous
• Tails not prehensile
• Ground dwelling or arboreal
• Excellent color vision (three types of cones)
Hominoidea
Hominoids
• No tails
• Only gibbons and orangs are arboreal
• Variable feeding preferences (gorillas folivores, orangs vegetarian, rest fairly omnivorous)
• Split from Old World Monkeys ~ 25-30 Mya
Hominoidea
Resolving the trichotomy
Morphological evidence
• Gorilla, Pan share skeletal features, primarily associated with knuckle-walking
Morphological evidence
• Most parsimonious: humans diverged first (if knuckle-walking is derived trait)
Morphological evidence
• Shared traits between Homo and Pan: skull and tooth traits, delayed sexual maturity, genital traits
• Which traits would surprise us more if they evolved multiple times?
Morphological evidence
• Gorilla shares skull traits with extinct European ape; suggests skull traits in Homo and Pan are derived
Morphological evidence
• Knuckle-walking either lost in human lineage, or evolved twice (Gorilla and Pan)
• Involves a spur on a wrist bone that allows it to be supported without bending backwards
Knuckle-walking in human ancestors
• A. anamensis and A. afarensis wrist bone morphologically similar to gorilla, Pan; later fossils (A. africanus) more similar to modern Homo
Molecular evidence
• Different methodologies all suggest that humans and chimps share most recent common ancestor
Molecular evidence
Hominid evolution: fossil evidence
• Very poor fossil record
– No record of gorilla, chimp lineages: they lived in moist tropical forests, not conducive to fossil formation
– Lots of gaps: makes it difficult to know which lineages are ancestral, how many evolving lineages (species) present at once
Homeland of humanity
• where do we come from?
• closest relatives (C,B,G) all from Africa
• same for oldest hominid fossils
• specifically east Africa
• but v. ancient fossil from Chad, found 2002
• contradicts theory of “rift valley” origins
Migration of Homo sapiens
Fossil hominids
• many species
• many intermediate forms
• documents trends
• show mosaic evolution
• oldest hominids
– until recently: 4.4 myo
– finds since 2001: 6+ myo
– close links with apes
A timeline for some hominid species
Australopithecus
• “Southern ape”
• Robust form (robustus, boisei, aethiopicus)
– Big teeth, sagittal crest, may have used tools; did not contribute to ancestry of modern humans
• Gracile form (afaransis, africanus)
– 4 feet tall, may have walked upright (but see knuckle-walking, before), chimp-sized brain
Australopithecus
Homo
• habilis: similar to gracile Autralopithecus, but had larger brain, tools
• erectus: larger body, brain, heavy brow ridge, used tools, had fire; someconfusion with ergaster (more on that later)
Homo
• neanderthalensis: Largest brains (larger than sapiens), diverse tools, burials
• sapiens: smaller brow ridge, flat face, high cranial vault
Phylogeny of Homo sapiens and relatives
Hobbits of Indonesia
Evolutionary trends
• Increased body size
• Retention of generalized limb, 5 digits
• Increased digit mobility
• Claws to flat nails
• Increased bipedalism
• Changes in skull, e.g. decreased snout length
• Change in dentition, e.g. decrease canine length
• Increase brain size
• Increase vision, binocular vision
• Decrease in olfaction
Brain size and body size
Brain size
Increasing Brain size
• Costs
– Large brain is energetically expensive: 2% of weight, 18% of energy
– Mortality risks of trial and error learning
– Generalization trades off with specialization (e.g. strength, speed)
Increasing Brain size
• Benefits
– Outsmart predators, prey; tool use
– Social interaction, parental care, cooperative liaisons
– All select for intelligence, learning, communication
Increasing Brain size
• Comparison of primates, other mammals
Increasing Brain size
• Change in eating habits correlated with change in brain size in primates
• Frugivores have larger brains than folivores