Early Earth and the Evolution of Life
(This is intended to present a brief sketch of the evolution of our planet. It is not, nor is it intended to be, an all inclusive history.)
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The Beginning
Big Bang- Creation of the Universe
Earth formed
4.6 billion years ago
cloud of gas and dust condensed into a sphere
heat and radioactivity caused the planet to melt
Early Earth
Some cooling-- solid rocks formed~4 billion years ago-- Earth's Crust
Volcanic activity-- violent and planet-wide
Meteor shower bombards Earth
Earth's Atmosphere 4- 3.8 billion years ago
Volcanoes- CO2, N2 and other gases
Meteorites- carry water in the form of ice, as well as other substances
Composition: CO, CO2, H2, N2, NH3, CH4 and water vapor (H2O)
No free oxygen!
Oceans
Earth cooled enough for liquids to remain
Thunderstorms drenched planet for thousands of years- forming oceans
Earliest sedimentary rocks dated to this period
Chemosynthesis Hypothesis
Methane (CH4), ammonia (NH3), hydrogen (H2) and water vapor (H2O) from
the earth's atmosphere,
Combined with UV rays and lightening,
Formed organic compounds, such as amino acids
Thus, leading to the "Organic Soup".
Evidence-1
Stanley Miller
Simulated Chemosynthesis Hypothesis in the laboratory
Yielded amino acids, as well as, many other organic compounds
Evidence 2
Alexander Oparin and Sidney Fox
Demonstrated that the organic soup would not remain a mix of simple
molecules, but that
More complex molecules (such as proteins) would be formed.
Evidence-3
Collections of the more complex molecules in the "organic soup" tend to
gather into droplets.
Some of these grow, and some reproduce.
Called proto-life forms, because they have begun some tasks necessary for life.
First Life
Microfossils found near Lake Superior have been dated to 3.5 billion years old
Prokaryotes- resembling bacteria
Anaerobic- can live without oxygen
Heterotrophs- obtained food and energy from the organic molecules in the soup
Evolution of Photosynthesis-1
Complex molecules in organic soup begin to run out.
Some organisms have developed a method of making complex organic
compounds using H2S and CO2.
Competition for the food in the organic soup gives these autotrophs an advantage.
Evolution of Photosynthesis-2
Organisms who can make their own "food" have a better chance of survival.
Evolution of autotrophs was very successful.
Autotrophs became very commonplace all over Earth.
Evolution of Modern Photosynthesis
Organisms begin using H2O instead of H2S.
The waste product, oxygen gas (O2), is formed instead of sulfur.
Free oxygen is released into the atmosphere for the first time.
Effects of Atmospheric Oxygen Aerobes
Oxygen is poisonous to most anaerobes
Therefore, the majority of anaerobes that filled the oceans died.
Evolution of aerobes-- Most anaerobes that survived began to use
oxygen in their respiration.
These organisms were more energy efficient than the remaining anaerobes,
and so they proliferated.
Effects of Atmospheric Oxygen Ozone
Oxygen accumulated in the atmosphere.
Oxygen in the upper atmosphere was split by UV rays into single atoms.
These single atoms recombined to form O3, ozone.
Effects of Atmospheric Oxygen Life on Land
Ozone accumulated in the atmosphere.
Ozone absorbed most of the harmful UV rays, making land a possible
habitat for organisms.
Organisms came out of the oceans and onto the land.
Some Problems Encountered on Land
How to keep from drying out?
How to get oxygen?
How to move?
How to reproduce?
Evolution of Monerans
First to evolve
All are prokaryotic
Includes blue-green algae (cyanobacteria), and other bacteria
Evolution of Protists
Evolved from Monerans.
Very diverse.
Gave rise to all other eukaryotic organisms.
Includes the green algae.
Evolution of Fungi
Most are multicellular heterotrophic eukaryotes
Absorb dissolved organic substances from the surrounding environment
A few are unicellular, such as yeast.
Evolution of Plants -1
All evolved from the green algae.
Multicellular photosynthetic eukaryotes
Adapted for life on land
Evolution of Plants-2 Why Was It Easy For Plants To Adapt to Land?
Light was abundant and not blocked by water
CO2 was plentiful and circulated more freely
Initially the land was unoccupied by competing forms of life
Evolution of Plants-3 Plant Adaptations to Land
A waxy coating to prevent water loss
Stomata for gas exchange
Reproductive organs protected from drying out by surrounding layers of cells
Retention of fertilized egg and the development into an embryo
Vascular systems evolved for transporting substances throughout the plant.
Evolution of Animals
All are multicellular heterotrophic eukaryotes
All lack cell walls
Mobility (of the entire organism or some of its parts) was required for obtaining food
Sequence of Animal Evolution-1
Sponges (Phylum Porifera)- did not evolve into any other group
Jellyfish (Phylum Cnidaria)- radially symmetrical
Flatworms (Phylum Platyhelminthes)
may have evolved from Cnideria
bilateral symmetry and ganglia
Ribbonworms (Phylum Rhynchocoala)
evolution of 1 way digestive tract
evolution of primitive circulatory system
Roundworms (Phylum Nematoda)- presence of pseudocoelom
Mollusks (Phylum Mollusca)-
most have shells (except octopus)
coelom
Segmented worms (Phylum Annelida)
body divided into segments
coelom
example: earthworm
Arthropods (Phylum Arthropoda)
most numerous and diverse
three major Classes:
Class: Arachnids-spiders, ticks, horseshoe crabs
Class: Insects- grasshoppers
Class: Crustaceans- lobsters
Echinoderms (Phylum Echinodermata)
body divided into 5 segments
example: starfish, sanddollar
Chordates (Phylum Chordata)
presence of a notochord
bilaterally symmetrical
Subphylum: Vertebrata
segmented vertebral column-- a backbone
distinct head with a skull and brain
nerve chord protected by vertebral column.
Classes of Fish- evolution of the first jaws
Class Agnatha- lampreys and hagfish
jawless;
eel-like (no fins)
Class Chondrichthyes- sharks, rays,
skeleton of cartilage;
scales pointed
Class Osteichthyes- bony fish
bony skeletons;
fins;
scales;
air bladders.
Class: Amphibians
egg laying
have gills in larvae stage; lungs in adult stage
depend on moist environment for at least one part of their life cycle.
water evaporates rapidly from their skins
evolved from air-breathing lunged fish
ectothermic
Class: Reptiles (reptiles)
evolved from amphibians
developed the amniotic egg
scales
ectothermic
Class: Aves (Birds)
evolved from reptiles (specialized reptiles)
feathers
forelimbs are modified into wings
all lay amniotic eggs
endothermic
Class: Mammals
hairy
mammary glands
endothermic
descended from reptiles
Order: Monotremes
transitional forms
duckbill platypus and spiny anteater
Order: Marsupials
pouched mammals
kangaroo and opossum
Order: Placentals- mammals with a placenta
Primates
New World Monkey- grasping tails
Old World Monkey- brachiation
Hominoids
Gibbon
Orangutan
Gorilla
Chimpanzee
Hominid
MAN- Homo sapien
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©1998 Linda Pellegrini. All rights reserved.