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History of Life
Measuring the Past
Humans and all other organisms are part of the natural world
 Natural processes shape the Earth and the living things that reside on the Earth
 All living things are the products of evolution
Measuring the Past
Humans share features with other animals due to shared evolutionary ancestry
 e.g., Our genetic code is identical to that of virtually every living organism
 Same relationship between codons and amino acids
Measuring the Past
The Earth was formed about 4.6 billion years ago
 This massive time frame is divided into multiple “eras”
 Precambrian, Paleozoic, Mesozoic and Cenozoic
 Eras divided into “periods”
 Further subdivided into “epochs”
Historic time encompasses only the last 10,000 years of this time frame
Measuring the Past
 The divisions between eras periods, etc., represent times of transition in life forms
 Many of these transitions are due to major extinction events
 e.g., The Cretaceous Extinction marks the boundary between the Cretaceous and
Tertiary periods
 Largely caused by the impact of a giant asteroid
 Extinction of the dinosaurs
Measuring the Past
 We will discuss “notable events” in the history of life on Earth
 Our choice of what is “notable” is subjective
 We will discuss many events in our own evolutionary past
 Evolution IS NOT a march toward the formation of humans
 We could just as easily discuss events in the evolutionary history of the common
dandelion
19.2 How Did Life Begin?
 The early Earth was very hostile
 Covered with a layer of molten rock
 Bombarded by comets, asteroids, etc.
 Contained gasses such as methane and ammonia
 Spewed from volcanoes, released from deep-sea vents, etc.
 By 3.8 billion years ago, the Earth’s environment became less hostile
 Black Smoker Video
 Black Smoker Video
Black Smoker
How Did Life Begin?
 The basic organic molecules present in life can be spontaneously assembled from methane,
ammonia, and similar gases
 This can be recreated in the laboratory
 Similar to what is seen at Yellowstone
How Did Life Begin?
 Where this assembly took place is the only question
 Life may have arisen in the “prebiotic soup” of a hot water system
 Life may have arisen within the sand and silt of ancient beaches
 Yellowstone Thermal Features
 Yellowstone Thermal Feature
Early Life
How Did Life Begin?
 Life did not originate in a single step
 The ability of a molecule to self-replicate is one critical feature of life
 RNA was most likely the earliest self-replicating molecule
 Served as the genetic material
 Errors in self-replication produced genetic variation
How Did Life Begin?
 Once life was established, forms of life evolved
 The earliest branchings produced three domains
 Bacteria, Archaea, and Eukarya
 Each of these domains branched further to form multiple kingdoms, etc.
 e.g., Eukarya branched into four kingdoms (Protista, Plantae, Fungi, and Animalia
The Precambrian Era
 The Precambrian Era stretched from the origin of the Earth until 542 million years ago
 Many notable events occurred during the Precambrian Era
 Origin of life
 Origin of photosynthesis
 Origin of eukaryotes
 Origin of multicellular life
The Precambrian Era
 The Earth was formed 4.6 billion years ago
 4,600 million years ago
 The earliest evidence for life is 3.7 – 3.8 billion years old
 3,700 – 3,800 million years ago
 Chemical signatures of life exist in ancient rocks
The Precambrian Era
 For the first 2 billion years in the history of life, all life was either archaea or bacteria
 2,000 million years
 The earliest life subsisted mainly on organic matter from their surroundings
 This supply was limited
The Precambrian Era
 Photosynthesis arose in bacteria by 3,400 million years ago
 Energy from the sun was used to produce organic molecules
 A large amount of energy-rich food was made available
The Precambrian Era
 Cyanobacteria produced oxygen as a product of photosynthesis
 Oxygen gas was virtually absent in he atmosphere until 2,400 million years ago
 Production of this oxygen drastically changed the Earth’s environments
 “Oxygen holocaust”
 Organisms unable to adapt to this changed environment died
The Precambrian Era
 Many organisms did adapt to the presence of oxygen
 Many ancient bacteria were able to metabolize oxygen
The Precambrian Era
 Some of these bacteria took up permanent residence in early eukaryotic cells
 “Endosymbiosis”
 These bacteria became mitochondria
 Organelles of eukaryotic cells
 Oxygen is used to harvest energy from food
The Precambrian Era
 Some cyanobacteria also took up residence within eukaryotic cells
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 “Endosymbiosis”
 These bacteria became chloroplasts
 Organelles of eukaryotic cells
 Perform photosynthesis
The Precambrian Era
The oxygen produced by photosynthesis reacted to form ozone
 Rose through the atmosphere to form the ozone layer
 Protects life from the damaging effects of ultraviolet radiation
 This protection was important in allowing the colonization of the land
The Cambrian Explosion
Throughout much of the Precambrian Era, many forms of life were present
 Archaea
 Bacteria
 Many types of protists
Early animals appeared near the end of the Precambrian Era
 600 million year-old animal fossils have been found
The Cambrian Explosion
The Cambrian Period began 542 million years ago with the “Cambrian Explosion”
 Lasted only approximately 6 million years
 Produced incredible diversity of animal life
 Many new animal forms appeared in the fossil record during the Cambrian Explosion
 First fossil evidence of 35 of the 36 currently existing animal phyla
 Some extinct phyla appeared then also
The Cambrian Explosion
 Did the Cambrian Explosion represent an explosion of forms large and hard enough to leave
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fossils?
 Some divergence of animal forms occurred prior to the Cambrian Explosion
What caused the Cambrian Explosion?
 The rise in atmospheric oxygen is a likely cause
 Larger organisms would require more oxygen
Movement onto Land:
Plants First
Plant-fungi combinations were the first multicellular life to colonize land
 Occurred between 460 million and 1,300 million years ago
Most modern plants have a mutually beneficial relationship with fungi
 Plants supply food through photosynthesis
 Fungi aid in water and mineral absorption
Movement onto Land: Plants First
Primitive land plants evolved from algae
 “Bryophytes”
 Represented by today’s mosses
 No vascular tissue to transport water and nutrients
 Cannot grow very tall
Movement onto Land: Plants First
 Early vascular plants evolved from these bryophytes
 Represented by today’s ferns
 Possess a vascular system
 Transports water and nutrients
 Affords support and allows taller growth
Movement onto Land: Plants First
 Some seedless vascular plants grew quite tall
 These were the dominant large plants during the reign of the dinosaurs
 Seed plants evolved from seedless vascular plants beginning 350 million years ago
Movement onto Land: Plants First
 Gymnosperms are the living descendents of these early seed plants
 e.g., Pine and fir trees
 Decreased dependence on water
 Sperm are packaged into pollen grains
 Carried by wind instead of swimming through dew to get to the egg
 700 species alive today
Movement onto Land: Plants First
 Flowering plants evolved from these early seed plants beginning 165 million years ago
 “Angiosperms”
 Flowers aided in reproduction of these plants
 260,000 species alive today
Animals Follow Plants onto the Land
 Vertebrates evolved in the oceans
 Gnathostomes evolved 450 million years ago
 First jawed animals
 Ancestral to all modern fish
 New foods became available with the evolution of jaws
Animals Follow Plants onto the Land
 Early fish were ray-finned
 Lacked bones in their fins
 Typified by walleye, etc.
 Lobe-finned fish evolved from ray-finned fish
 Possessed bones two pairs of fins
 Lobed fins are precursors of four limbs
 Amphibians evolved from lobe-finned fish
 Amphibians are “tetrapods”
 Possess four limbs
Animals Follow Plants onto the Land
 Amphibians evolved from lobe-finned fish
 Retained a major dependence on water
 Amphibian literally means “double life”
 Spend some of their life in the water and some of their life on land
 Notably, their eggs are laid in water or other moist environments
Animals Follow Plants onto the Land
 Reptiles evolved from amphibians
 Reptiles produce an “amniotic egg”
 Membranes within amniotic eggs supply nutrients and remove wastes
 Hard outer casing protects them from drying
 Amphibian ties to water were severed
 Reptiles could move inland
Animals Follow Plants onto the Land
 Dinosaurs evolved from one branch of reptiles about 220 million years ago
 220 million years ago
 Dominant vertebrate for 155 million years
 Mammals arose from another branch of reptiles shortly after the dinosaurs arose
 210 million years ago
 Possessed fur and mammary glands
Animals Follow Plants onto the Land
 Early mammals were rather small
 Lived in the shadow of dinosaurs for over 100 million years
 Began an adaptive radiation near the end of the Cretaceous
 The Cretaceous period ended with an asteroid impact that killed the dinosaurs 65 million
years ago
 Mammals radiated into many recently vacated niches
Animals Follow Plants onto the Land
 Primates evolved from ancestral mammals between 55 and 90 million years ago
 Key characteristics of primates
 Large, front-facing eyes allowing binocular vision and enhanced depth perception
 Limbs with opposable first digits
 Tree-dwelling existence
The Evolution of Human Beings
 Humans and chimpanzees share a common ancestor approx. 6 –7 million years ago
 It took more than a single speciation to give rise to humans from this common ancestor
 Numerous human-like species have been discovered
 The human evolution tree is more of a bush
The Evolution of Human Beings
 Taxonomic grouping Hominini
 The relationships between these species are not perfectly understood
 Major disagreement over which are ancestors and which are “cousins”
 Nearly all of the evolution of hominins occurred in East Africa
 Some species migrated from Africa to other continents (e.g. Homo erectus, Homo sapiens)
 Toumai fossils
The Evolution of Human Beings
 Helped push root of hominin family tree back 2 million years
 Oldest fossils previously found dated to 4.4 million years old
 Date shortly after the divergence of human and chimpanzee lineages
 May actually belong in chimp lineage
The Evolution of Human Beings
Australopithecus afarensis
 Ancestral to Homo lineage
 Best known through “Lucy”
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Largely intact skeleton discovered in Ethiopia in the 1970s
Bipedal (known from pelvic structure)
Long arms, short legs, grasping feet
Brain size similar to chimpanzee
 450 cc (Homo sapiens is 1,400 cc)
The Evolution of Human Beings
Homo ergaster
 Ancestral to Homo erectus and Homo sapiens
 Brain over half volume of Homo sapiens
 Modern face, limbs, height, advanced tool technology
 “Turkana boy”
 Best-preserved remains of Homo ergaster
 9 years old at death
 1.6 million years old
The Evolution of Human Beings
Homo neanderthalensis
 Lived 200,000 – 27,000 years ago
 First extinct hominin fossils found (1856)
 Short, stocky, powerfully built
 Heavy brow ridge, receding chin
 Brain size slightly larger than Homo sapiens
 Tool technology, burial of dead
 Likely descended from Homo ergaster
 Not ancestral to Homo sapiens
The Evolution of Human Beings
Modern Homo sapiens
 Evolved modern anatomical form in Africa before migrating
 Arose from 100,000 – 200,000 years ago
 Migrated to Indonesia, etc. by 46,000 years ago (coexisted with Homo erectus)
 Arrived in Europe 40,000 years ago (coexisted with Homo neanderthalensis)
 Homo sapiens replaced them – likely through competition
The Evolution of Human Beings
Homo floresiensis
 “Hobbit people”
Discovered in 2004 on the Indonesian island of Flores
Lived as recently as 18,000 years ago
Three feet tall as adults
Cranial capacity of 380 cc
 Bain size similar to “Lucy”
 Used fire and sophisticated tools
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The Evolution of Human Beings
 Appeared in the literature 2006
 Nothing like it before
 Debate:
 Diseased population
 Preserved older
 Normal variation
The Evolution of Human Beings
Homo floresiensis
 This species raises some interesting questions
 How did Homo floresiensis avoid extinction for so long?
 How could they be so sophisticated in their tool and fire use with such small brains?
 Did they evolve from Home erectus?