6. the History of Life on Earth

7/30/2019 6. the History of Life on Earth

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The History of Life

on EarthJ.T.II Olivar

Faculty of Arts and Letters

2nd semester AY 2012-2013



Outline of the Lecture

• The Geologic Time Scale

• Tracing the History of Life on Earth: How

Did Life Begin?

• The Tree of Life

• The Precambrian

•The Cambrian Explosion



• The Movement Onto the Land: Plants First

• Animals Follow Plants Onto The Land

•

The Evolution of Human Beings



The Geologic Time Scale

I. Geological Time Scale

A. Dividing Earth’s History

1. Eras

– Long periods of time subdivided by periods

– Periods are subdivided into epochs

2. Dating the Eras

– Modern method uses radiometric dating (quantitative)

– Fossils define the ages of Earth – Appearances of new life-forms define new era

– Boundaries between eras often marked by extinctions



B. Extinction Events

1. Demise of species opens up niches

– Adaptive radiation fills niches

2. Cretaceous extinction – End of dinosaurs

– Aided by asteroid impact

3. Permian extinction

–

End of Paleozoic era – 96 percent of all species on Earth died out



II. Transition Does Not Always Mean Death:

The Cambrian Explosion

– Cambrian Explosion

1. Appearance of ancestors of all current life-forms

2. Occurred only 544 million years ago (Mya)

3. Contrast with life in Precambrian era –

Precambrian has just single-celled organisms



III. What Is “Notable” in Evolution Hinges on

Values

– Choosing Notable Events

1. Humans choose to plot evolution focusing on the

path leading to humans

2. Only one of the many possible, interesting paths

– not more important or “notable” than any other

path



IV. The Kingdoms of the Living World Fit into

Three Domains

– Organizing Major Life-forms

1. Archaea – single-celled organisms from

“extreme” environments

2. Bacteria – single-celled organisms from familiar

environments

3. Eukarya – plants, animals, fungi, and protists



Tracing the History of Life on

Earth: How Did Life Begin?

I. In What Kind of Environment Did Life

Begin?

– Theories about life’s origins

1. Cold Earth

– Life arose in cold seawater beneath thick ice layer

2. Hot Earth

– Life began in hot seas

– Earliest organisms may have lived in extremely hot

water



II. What Was the Source of the Raw

Materials for Life?

– Chemical raw material

• Gases

– Methane and Ammonia (Source: volcanic activity)



III. Life May Have Begun in Boiling Water

– Organic raw materials

1. Hydrogen sulfide and methane from deep-sea

vents – Oldest organisms, Archaea, tolerate extreme heat

2. More complex molecules had to be made from

basic materials

– How did simple molecules combine to form complexones?



IV. The RNA World

A. Dilemma of Reproduction

1. DNA codes information but can’t copy or

translate it

2. Proteins can copy and translate codes, but must

be produced from RNA instructions



B. The role of RNA in early history of life

1. Ribosomes

– Enzymes made of RNA

–

Molecules encode information and act as enzymes – Identified in 1983

2. Suggests and “RNA world” – these molecules

would be self-perpetuating



C. The step at which life begins

• Critical step is replication

– Copying of molecules would produce errors

–

Errors provide source of evolving units for naturalselection



D. “Directed” evolution of ribozymes

• Experimental models

a. Synthetic ribozyme chains selected for replication

behavior

b. Goal is to select ribozymes that can replicate without

human intervention

c. If successful, will have “created” life



V. Adding Life’s Elaborations to Replication

– More to life than replication

1. Making protective membranes

2. Extracting energy

3. Removing waste



The Tree of Life

• The Pattern of Evolution

– Organization of the Tree of Life

1. Universal ancestor

– Gives rise to all life-forms

– Probably successor to self-replicating molecules

2. Three major branches on tree

– Bacteria domain

– Archaea domain – Eukarya domain

• All creatures with nucleated cells

• All developed from Protista kingdom



The Precambrian

I. The Slow Pace of Change in the

Precambrian

– Time Frame

1. First bacterial microfossils – 3,460 Mya

2. First single-cell algae – about 1,900 Mya

3. Must wait another 10 million years for

multicellular organisms



II. Notable Precambrian Events

A. Initiation of Photosynthesis

• Developed at same time as first bacteria

• Critical for development of life – provided food

B. Oxygen in the atmosphere

• Cyanobacteria – first creatures to produce O2 as

waste product

• Oxygen appears – about 200 Mya

– Deadly to most organisms – oxygen holocaust

– Extreme selective pressure – adapt or die



C. Adapting to oxygen: bacteria and eukaryotes

strike up a relationship

• Earliest organisms to adapt O2 probably similar to

mitochondria• Mitochondria-like bacteria establish symbiosis

with early eukaryotic cells

D. One more effect of oxygen: The blockage of

UV light• Oxygen forms ozone

– Shields Earth from ultraviolet light

– Land “safe” for developing organisms



The Cambrian Explosion

I. Precambrian Era

– Appearance of animals

1. Occurred about 600 Mya

2. Defining an animal

– Multicellular

– Does not photosynthesize food



II. The Cambrian Explosion

– Huge increase in diversity of life

1. Occurs about 70 My after the appearance of the

first animals2. All 30 modern phyla of animals appeared then –

and many others that are now extinct

3. Occurrence of all new organisms takes place

within 5 million years



III. “Post Cambrian” Analysis

– Explaining the explosion

1. Animals that appeared were finally big enough

and hard enough to leave fossils2. “Explosion" was triggered by threshold level of

oxygen



The Movement Onto the Land:

Plants First

I. Adaptation of Plants to the Land

A. Leaving an aquatic environment

1. Water loss – develop water-resistant cuticle

2. Gravity – stay low; mosses are the earliest landplants

3. Reproduction – protect gametes from desiccation

B. Another plant innovation: A vascular system

1. Vascular system allows plants to grow against

gravity

2. “Development” time was 100 million years



II. Plants With Seeds: The Gymnosperms

and Angiosperms

A. The gymnosperms

1. Next evolutionary development was seeds

2. Eliminate water requirement for gametes

– Impact on dispersal of plants

– Could use mechanisms other than water: wind

3. First appearance about 350 Mya



B. The Angiosperms

1. Evolved 140 Mya

2. Currently the most dominant plants on Earth

–

260,000 angiosperms species vs 70,000 gymnosperms – More diverse in character



C. A Relationship Between Plants and Animals

1. Contact with animals provides pollen dispersal

2. Encase seeds in fruit to attract animal pollinators

3. Plants and animals are in a mutually dependentrelationship

– Plants provide food

– Animals provide reproductive services



Animals Follow Plants Onto The

Land

I. Animals Colonize the Land

A. Favorable environmental conditions

• Plants provide food and shelter

B. First colonizers

• Arrived 20 million years after first plants

• Arthropods arrive first, at 440 Mya – protected by

exoskeleton



C. Insects onto land

1. Wingless insects arrive about 50 million years

after arthropods

2. Insects develop flight about 10 million years later – dominated air for 100 million years

3. Most primitive flying insect is the dragonfly

4. Folding wings were a later development



II. Vertebrates Move Onto Land

A. Primitive fish first

1. Lobe-finned fish moved out of water onto land

about 400 Mya – Ancestral creatures for all tetrapod vertebrates

2. Order of divergence

a. Lobe-finned fish give rise to amphibians

b. Amphibians give rise to reptilesc. Reptiles give rise to dinosaurs and mammals

• Dinosaurs give rise to birds



B. Amphibians and reptiles

1. Amphibians

a. Land dwellers still linked to water environments

b. Example – frogs

• Live on land as adults

• Depend on water to reproduce

2. A critical reptilian innovation – the amniotic egg

a. Hard outer casing prevents egg from drying out

b. Inner material protects embryo

c. Membranes supply nutrients, eliminate waste



3. Amphibian appear about 350 Mya

– Only land vertebrates for 30 million years

4. About 220 Mya, dinosaurs (one line of reptiles)

appeared

– Lasted about 155 million years



C. From reptiles to mammals

1. First mammals appeared at same time as the

dinosaurs

– Small, rat-like insectivores

– Fed young with milk from female mammary glands

2. Remained minor creatures until dinosaurs

became extinct

– 65 Mya in Cretaceous extinction

3. Some mammals returned to sea – ancestors of

whales and seals



III. The Primate Mammals

– Rise of the Primates

1. Five million years after the end of dinosaurs

2. Characteristics of early primates – Binocular vision

– Opposable first digit

– Tree dwellers

3. Today, 230 primates species exist – minority of 4,600 species of mammals

Th E l i f H B i



The Evolution of Human Beings

I. Paleoanthropology – Study of Humans

Based on Fossils

A. Hominids

1. Four genera on evolutionary tree

a. Ardipithecus

• One species (ramidus)

• 4.4. Mya

• Closect to common, primate ancestor b. Australopithecus

• Give rise to Homo and Paranthropus genera

• Paranthropus and Australopithecus extinct



2. Bipedal locomotion characterizes hominids

3. Brain size increases

B. Human beings emerged in Africa –

GreatRift Valley and South Africa



C. The descent from the trees

1. Hominids were the first primates to abandon a

tree-dwelling lifestyle

2. Lucy – Bipedal

– Long arms for tree climbing



D. Homo ergaster – a form more like ours

1. 1.6 Mya – “Turkana Boy”

2. Face and limbs similar to modern humans

E. The Neanderthals1. Appeared in Europe and Asia 200,000 years ago

2. Disappeared 30,000 years ago

3. DNA analysis sorted out H. neanderthalensis

place on treea. Evolved from H. hiedelbergensis

b. Separate lineage from H. sapiens



F. Homo sapiens

1. Appeared 100,000 years ago in South Africa

2. Unique qualities

a. Brain sizeb. Tool-making abilities

c. Speech

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6. the History of Life on Earth