History of Life on Earth Coach Fults Chapter 12

The Age of Earth When earth formed 4.6 billion years ago it was a a fiery ball of molten rock Eventually the molten rock cooled and formed the crust Water vapor in the atmosphere condensed to form the oceans Most scientists think that life arose out of these early oceans Rocks have been dated back, so this supports the idea of a gradual change

Measuring Earth’s Age Radiometric dating- is the estimation of the age of an object by measuring its content of certain radioactive isotopes An isotope is a form of an element whose atomic mass differs from that of other atoms of the same element Radioisotopes- are unstable isotopes that break down & give off energy in the form of charged particles (radiation)

Measuring Earth’s Age The breakdown, called radioactive decay, results in other isotopes that are smaller and more stable The time it takes for ½ of a given amount of a radioisotope to decay is called the radioisotope’s half- life By measuring the proportions of certain radioisotopes and their products of decay, scientists can compute how many half-lives has passed since the rock was formed

Formation of the Basic Chemicals of Life Most scientist think that life on earth developed thru natural chemical and physical processes Ppl think that the path to development of living things began when molecules of non-living matter reacted chemically during the 1 st billion years of Earth’s history These chemical reactions produced many different simple, organic molecules

Formation of the Basic Chemicals of Life Energized by the sun and volcanic heat, these simple, organic molecules formed more- complex molecules that eventually became the building blocks of the 1 st cells The hypothesis that organic molecules necessary for life can be made from non-living molecules has been tested and supported by experiments

The “Primordial Soup” Model 1920’s, the Russian scientists A.I. Oparin and the British scientists J.B.S Haldane both suggested that the early earth’s oceans contained large amounts of organic molecules This hypothesis became known as the primordial soup model They hypothesized that these molecules formed spontaneously in chemical reactions activated by energy from sunlight, lightning, & volcanoes

The “Primordial Soup” Model Oparin, together with American scientists Harold Urey, & others proposed that’s earth’s early atmosphere lacked oxygen Rich in nitrogen, hydrogen, and hydrogen-rich gases such as ( water vapor;H 2 0, ammonia NH 3 ; and methane CH 4 ) They reasoned that electrons in these gases would have been frequently pushed to higher energy levels by light particles from the sun or lightning

The “Primordial Soup” Model Today, high energy electrons are quickly soaked up by the oxygen in the earth’s atmosphere b/c oxygen atoms have a great “thirst” for electrons So w/out oxygen the electrons would have reacted with hydrogen molecules, forming a rich variety of organic compounds

The “Primordial Soup” Model: Urey/Miller Experiment 1953, this hypothesis was tested by Stanley Miller, who was working with Urey Miller placed the gases they thought was abundant on early earth into a device To simulate lightning, he used sparks

The “Primordial Soup” Model: Urey/Miller Experiment After a few days, Miller found a complex collection of organic molecules in his device He found: amino acids, fatty acids, and other hydrocarbons These result support the hypothesis that some basic chemicals of life could have formed spontaneously under conditions like those in the experiment

Reevaluating the Urey/Miller Model We now know that a mixture of gases used in Miller’s experiment couldn’t have existed on early earth 4 bya, earth didn’t have a protective layer of ozone gas, O 3 Today ozone protects from harmful UV Rays from the sun W/out ozone the UV rays would destroy any ammonia or methane in the atmosphere

Reevaluating the Urey/Miller Model W/out these gases, no key biological organic molecules were made If the chemicals needed to form life were not in the atmosphere, where did they come from? Some scientists argue that chemicals came from ocean bubbles or deep sea vents

The Bubble Model 1986, geophysicist Louis Lerman suggested that the key processes that formed the chemicals needed for life took place within bubbles on the ocean’s floor.

The Bubble Model Step: 1 Ammonia, methane, and other gases resulting from the numerous eruption of undersea volcanoes were trapped in underwater bubbles

The Bubble Model Step: 2 Inside the bubbles, methane and ammonia needed to make amino acids might have been protected from damaging UV rays. Chemical reactions would take place much faster in bubbles (where reactants would be concentrated) than in the primordial soup proposed by Oparin & Haldane

The Bubble Model Step: 3 Bubbles rose to the surface and burst, releasing simple organic molecules into the air

The Bubble Model Step: 4 Carried upward by winds, the simple organic molecules were exposed to UV rays and lightning, which provided energy for further reactions

The Bubble Model Step: 5 More complex organic molecules that formed by further reactions fell into the ocean with rain, starting another cycle “Thus the molecules of life could have appeared more quickly than the primordial soup model alone”

Precursors of the 1 st Cells There are enormous differences between simple organic molecules & large organic molecules found in living cells In labs, scientists haven’t been able to make either proteins or DNA from spontaneously in water However, short chains of RNA, the nucleic acid that helps carry out DNA’s instructions, have been made to form on their own in water

A Possible Role as Catalysts In the 1980s, American scientists Thomas Cech and Sidney Altman found that certain RNA molecules can act like enzymes RNA’s 3-D structure provides a surface on which chemical reactions can be catalyzed mRNA acts as an info-storing molecule By Cech’s & Altman’s work on other experiments showing that RNA molecules can form spontaneously in water; a hypothesis was formed next slide

A Possible Role as Catalysts RNA was the 1 st self-replicating information-storage molecule and it catalyzed the assembly of the 1 st proteins. More important, such a molecule would have been capable of changing from 1 generation to the next

Microspheres & Coacervates Observations show that lipids, which make up cell membranes, tend to gather together in water By shaking oil and vinegar in a bottle you see small spheres of oil form in the vinegar Certain lipids, when combined with other molecules, can form a tiny droplet whose surface resembles a cell membrane

Microspheres & Coacervates Similarly, experiments have shown that, in water, short chains of amino acids can gather into tiny droplets called microspheres Another type of droplet called a coacervate is composed of molecules of different types, including linked amino acids and sugars Scientists think that formation of microspheres might have been a 1 st step towards cellular organization

Microspheres & Coacervates According to this hypothesis, microspheres formed, persisted for awhile, and then dispersed Over millions of years, those microspheres that could persist longer by incorporating molecules and energy would have become more common than shorter lasting microspheres were

Microspheres & Coacervates Microspheres couldn’t be considered true cells, however; unless they had the traits of living things, including heredity

Origin of Heredity Many agree that DNA evolved after RNA and that “RNA enzymes” catalyzed the assembly of the earliest proteins Some agree that microspheres with RNA were able to transfer their traits to offspring But we really don’t know

The Evolution of Prokaryotes When did life form? We study fossils Fossil- is the preserved or mineralized remains or imprint of an organism that lived long ago Oldest date back 2.5 billion years

850 million year old fossil “cyanobacteria”

The Evolution of Prokaryotes Recall that prokaryotes are single-celled organisms that lack internal membrane-bound organelles Among the 1 st were marine cyanobacteria Cyanobacteria- are photosynthetic prokaryotes B4 cyanobacteria appeared, oxygen was scarce on earth, but the cyanobacteria gave off oxygen as a bi-product This oxygen eventually escaped to the air

2 Groups of Prokaryotes Early in the history of life, 2 different groups of prokaryotes evolved- eubacteria (which are now known as bacteria) and archaebacteria Eubacteria contain a chemical called peptidoglycan in their cell walls Archaebacteria lack peptidoglycan in their cell walls and have unique lipids in their cell membranes

2 Groups of Prokaryotes Archaebacteria are thought to closely resemble the 1 st prokaryotes to have existed on earth Chemical evidence indicates that the first eukaryotic cells were more likely to have evolved from archaebacteria than from eubacteria

E. Coli Eubacteria

Archaebacteria

The Evolution of Eukaryotes ~ 1.5 bya, the 1 st eukaryotes appeared Almost all eukaryotes have mitochondrias and chloroplasts are found only in protists and plants Mitochondria and chloroplast are the size of prokaryotes, and they contain their own DNA

The Origin of Mitochondria and Chloroplast Most scientists think these originated as described by the theory of endosymbiosis It was proposed in 1966 by Lynn Margulis It proposes that mitochondria are the descendants of symbiotic, aerobic eubacteria and chloroplasts are the descendants of symbiotic, photosynthetic eubacteria

The Origin of Mitochondria and Chloroplast According to Lynn Margulis’s theory of endosymbiosis, bacteria entered large cells either as parasites or as undigested prey Instead of being digested, the bacteria started to live in the host cell, where they performed either cellular respiration or photosynthesis

Evidence for the theory Size and structure: mito and chloroplast are the same size as most eubacteria. Both are double membraned. The smooth inner membrane of mito is thought to be derived of from the ER of a larger host cell, also the inner membrane is folded much like the membranes of many aerobic bacteria

Evidence for the theory Genetic material- mito and chloroplast have circular DNA similar to chromosomes found in bacteria; mito and chloroplast contain genes that are different from those found in the nucleus of the host cell

Evidence for the theory Ribosomes- mito and chloroplast ribosomes have similar size and structure to that of bacterial ribosomes Reproduction- like bacteria, mito and chloroplast reproduce by simple fission. This replication takes place independently of the cell cycle of the host cell

Multicellularity Many biologist group all living things into 6 broad categories called kingdoms The 3 oldest kingdoms, eubacteria and archaebacteria, are made up of single-celled prokaryotes The 1 st eukaryotic kingdom was the kingdom Protista Protists are a group that consist of multicellular and unicellular organisms

Multicellularity The unicellular body plan has been very successful, ½ of earth’s biomass is unicellular But multicellular has had the opportunity to arise b/c many times cells get specialized Almost every organism that can be seen with the naked eye is multicellular Fungi, plants, and animals evolved independently from protists

Origin of Modern Organisms Most phyla that exist today probably originated during the Cambrian period from 540 million to 505 mya. Many unusual marine animals appeared at this time, that there are no current living relatives A very rich collection of Cambrian fossils was uncovered in Canada called the Burgess Shale in 1909

Origin of Modern Organisms The Ordovician Period, which followed the Cambrian lasted from about 505 to 438 mya During this time many marine animals continued to thrive in the seas until about 250 mya

Mass Extinctions The fossil record indicates that a sudden change occurred at the end of the Ordovician Period About 440 mya, a large percentage of Earth’s organisms became extinct Extinction- death of all the members of a species This was the 1 st of 5 major mass extinctions

Mass Extinctions Mass Extinction- is an episode during which large numbers of species become extinct Another M.E. of about the same size happened about 360 mya The 3 rd and most devastating of all occurred at the end of the Permian Period about 245 mya About 96% of all living species died

Mass Extinctions About 35 mya, a fourth, less devastating M.E. occurred Although the specific causes of these extinctions are unknown, evidence indicates that worldwide geological and weather changes were likely factors The 5 th M.E. occurred about 65 mya; and killed 2/3 of all land based animals

The Ozone Layer The sun provides both life-giving light and dangerous ultraviolet radiation Early life on earth originated in the seas, so how did life get past the UV radiation on land to colonize the land?

Formation of the Ozone Layer ~ 2.5 bya, photosynthesis by cyanobacteria started to release oxygen into the atmosphere As oxygen (O 2 ) began to reach the upper atmosphere, the sun’s rays caused some of the molecules of oxygen, to chemically react and form molecules of ozone (O 3 ) In the upper atmosphere ozone blocks the UV radiation, and after millions of years enough accumulated to make earth safe

Plants and Fungi on Land The 1 st multicellular organisms to live on land are thought to have been plants and fungi living together. Such paired organisms were able to live on land b/c each grouped possessed a quality needed by the other Plants can use photosynthesis, Fungi can absorb minerals- even from bare rock

Plants and Fungi on Land Early plants and fungi formed biological partnerships called mycorrhizae which allowed them to live in harsh habitats Mycorrhizae- exist today, are symbiotic associations between fungi and roots of plants The fungus provides minerals to the plant, and the plant provides nutrients to the fungus This kind of partnership is called mutualism

Plants and Fungi on Land Mutualism- is where both parties benefit from the relationship Plants and fungi began living together on the surface of the land ~ 430 mya

Arthropods ~ 100 million years later the fungus/plants provided food for land-dwelling animals The 1 st animals to invade land successfully were the arthropods An arthropod is an animal with an exoskeleton, a segmented body, and paired jointed limbs Ex: crabs, spiders, insects Biologists think that a type of scorpion was the 1 st arthropod to live on land

Arthropods The insect evolved from the first land dwellers Insects are the most plentiful and diverse group in history The insects’ success is probably b/c of their ability to fly Insects were the first animals to have wings, which also formed relationships with flowering plants Oldest flowering plant fossil are ~ 127 million years old

Vertebrates Vertebrate- is an animal with a backbone Usually anything bigger than your fist is a vertebrate

Fishes The 1 st vertebrates were small, jawless fishes that evolved in the oceans about 530 mya Jawed fishes appeared about 430 mya Jaws enabled fishes to bite and chew their food instead of sucking up their food For hundreds of millions of years the sea is where vertebrates stayed Make up ½ of modern vertebrates

Amphibians The 1 st vertebrates to inhabit the land didn’t come out of the sea until 370 mya Amphibians are smooth-skinned, 4 legged; that today includes frogs, toads, and salamanders Several structural changes in the bodies of amphibians occurred as they adapted to life on land They had moist-breathing sacs-lungs-which allowed them to absorb oxygen from air

Amphibians The limbs of amphibians are thought to have derived from the bones of fish fins The evolution of a strong support system of bones in the region just behind the head made walking possible This system of bones provided a rigid base for the limbs to work against B/c of their strong internal skeleton, they can get much larger than insects

Amphibians While Amphibians were well adapted to their environment, a new group of animals more suited to a drier environment evolved from them

Reptiles Reptiles evolved from Amphibian ancestors ~ 340 mya Reptiles skin reduces the loss of moisture, can lay eggs on dry land “Oldest fossil of a snake”

Mammals & Birds Birds apparently evolved from feathered dinosaurs Therapsids, reptiles with complex teeth and legs positioned beneath their body, gave rise to mammals about the same time dinosaurs evolved, during the Triassic Period 65 mya, the 5 th M.E.

Mammals & Birds All of the dinosaurs except for the ancestors of birds became extinct The smaller reptiles, mammals, and birds survived Both extinctions and continental drift played important roles in evolution Continental drift- plate tectonics

Mammals & Birds Evidence for continental drift: There a large number of marsupials in Australia & South America b/v they were once connected