Biology Life On Earth Notes 1r2e2b

Biology life on earth notes THE ORIGINS OF LIFE ON EARTH  

The Earth is about 4.6 billion years old It was very different than of today

How do we know? 

Conditions and chemicals on other planets in our solar system The chemicals that erupt from volcanic vents today, and probably always have

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Spontaneous Generation of life 

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Throughout history until about 150 years ago, many believed that life could just appear spontaneously… maggots just happened in rotting meat, things would just arise This concept was proven wrong in 1862 by Louise Pasteur

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By the 1880’s cell theory was established (all cells (life) come from pre-existing cells

Chemicals first, then cells  Chemicals present in the atmosphere of primitive earth contained exactly the same chemical elements that organic chemicals of life are made from (mainly CO2, H, O, N)  In in the 1920s, it was suggested by 2 scientists that conditions on earth 4 billion years ago:  Caused chemical reactions that made complex organic molecules  Then these chemicals somehow came together to form a living organism  Once life got started, it evolved into the species we see in the fossil record and those alive todays The Urey Miller experiment: was designed in the 1950’s to test the first part of this hypothesis (conditions of primitive earth could spontaneously produce complex organic molecules necessary for life to start Significance of Urey Miller experiment 1) Demonstrates how Science works, formulating a hypothesis, testing

2) Didn’t prove how life got started on Earth, the experimental results the hypothesis by showing that life-forming chemicals could have been produced naturally under the conditions of the primitive Earth 3) In combination with evidence from: • Space Exploration, Vulcanology & Earth Chemistry • Ancient Rocks & Fossils • Study of “Primitive” Life Alive Today and other areas of scientific study, the Urey-Miller experiment is just one part of a “package” of evidence that seems consistent with the idea that life formed naturally on the primitive Earth 4 billion years ago, and has evolved into what we see today

Life from outer space? 

The hypothesis that Urey & Miller tested is not the only idea for the origins of life

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It has been suggested that the first living things on Earth (or at least the chemicals they formed from) could have come from outer space

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so far the weight of evidence s the hypothesis that Earth life arose on Earth, and not somewhere else

Technologies That Helped Us Understand

   Radiometric Dating 

It is the method that allows us to measure the age of rocks & fossils

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Atoms of each chemical element are not all exactly the same They have the same number of protons & electrons (that’s what makes them that element), but the number of neutrons in the atom can vary (making them an isotope) Some isotopes are “radio-active” and give off nuclear radiations... hence “radioisotopes”. Radio-isotopes “decay” at a predictable rate

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By measuring the amount of radiation present now, and knowing the “half-life” of the isotope, the age of objects can be calculated most famous example is “Carbon Dating” which uses an isotope of carbon (called “carbon-14”) to find the age of artifacts from human history It has a half-life of about 5,700 Carbon-14 is not much use beyond about 40,000 years, but there are other isotopes (e.g. potassium and uranium) with half-lives of many millions of years These can measure the ages of fossils and rocks formed millions/ billions years ago This “Radiometric Dating” is how we know

Electron microscope  Allows scientists to study very ancient fossil cells in rocks and to make comparisons with some types of “primitive” cells  Gives us further clues about how ancient life-forms lived and evolved Biochemical Analysis & DNA Technology    

Used to identify the remains of victims of war or natural disasters; By comparing DNA samples from a body part with samples from the relatives of missing people same technique can be used to find the “relatedness” of different living things, and even give estimates of how long ago 2 related species divided E.g. (DNA studies suggest that humans and chimpanzees are closely related; our DNA is 99% identical) Biochemistry and DNA studies have shown: • All life forms on Earth are related. (means that all living things today evolved from 1 original type). • Which types are more closely, or more distantly, related. (complements the evidence of the fossil record, gives us a clearer picture of the exact sequence of evolution)

History of life on earth Summary of the Main Events   

following time-line identifies the major stages in the evolution of life on Earth main evidence for this has come from fossils preserved in sedimentary rocks The times suggested are approximate, but based on radio- metric studies of the rock

1) Formation of organic molecules 

Began as soon as the Earth was cool enough forthe molecules to existwithout being torn apart

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The Urey-Miller experiment (and many others) prove sugars, amino acids, lipids and the building blocks of DNA and RNA could form spontaneously in the chemical conditions of the primitive Earth

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4 billion years ago, early oceans must have become a chemical “soup”, highly concentrated in organic molecules

2) Molecules Formed Membranes 

It is thought that the next crucial step was the formation of membranes

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In watery environment of the oceans, hydrophobic molecules naturally tend to cling together, like oil forming droplets in water

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Experiments have shown that some lipid molecules in water will, quite naturally, form “microspheres” with other chemicals trapped inside

3) The first living cells Microspheres which trapped inside themselves a mixture of chemicals that could attract other molecules in through the “membrane”, they became bigger (i.e. feeding & growing) and also cause copies of their own molecules to be built, (e.g. small RNA) 

thesphere wouldsplit in two,(reproduction)each part witha share of theessentialchemicals tomake it all happen over again

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We can’t be sure when this happened, but by about 3.8 billion years ago we find chemical evidence of living cells in some rocks, and by

3 billion years ago there are fossils of bacteria-like cells, in various forms... they were evolving into new types already 

These cells lived without oxygen (“anaerobic”) and were probably feeding on the “soup” of organic molecules in oceans

4) A Billion Years of Scavenging 

For 1,000 million years the most advanced organisms on Earth were bacteria-like cells which lived without O2, and scavenged the organic “soup” of organic molecules in oceans

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Probably the production of organic molecules (“food”) was still occurring as in the Urey-Miller experiment, but the whole regime (scientific process) was about to change...

5) Chemosynthesis & Photosynthesis 

About 2.8 billion years ago, a new type of bacteria appears in the fossil record

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The fossils appear similar to autotrophic bacteria alive today, which make their own food, using energy from chemicals in the environment. This process is called “Chemosynthesis”

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2.3 billion years ago, fossils of cells recognisable as cyanobacteria appeared. These are bacterial type cells, but use chlorophyll to trap sunlight, and 

They grew in shallow seas in structures called “stromatolites” which we find as common fossils in rocks from this time. Living stromatolites still grow in some places today.

produce OXYGEN as their waste product

6) The World Goes Oxic 

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Up until

this time the Earth was

“anoxic”(completely without any uncombined oxygen (O ). 2 There were plenty of oxygen atoms , they were all chemically combined in (H O) and CO and 2 2 various other compounds.

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Now the cyanobacteria began “modern style” photosynthesis in

millions of stromatolites, for millions of years 

We find huge deposits of evidence for what happened next... the world went rusty!

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In many parts of the world (including Western Australia) we find huge deposits of “Banded Ironstone”; rock containing layers of iron oxide (Fe O ). The iron mineral is very fine grained as if precipitated from a 2 3 water solution.

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The explanation is that, in the anoxic early conditions a lot of iron was 2+ dissolved in the oceans in the soluble form of Fe ions. As the cyanobacteria began releasing vast quantities of O oxygen, it 2 reacted with the iron forming the insoluble Fe O iron oxide (which is 2 3 RUST).

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Eventually, after about 200 million years, all the iron was precipitated, and now the oxygen began building up in the atmosphere... the air became “OXIC”.

This had four important consequences: 1. The natural production of organic chemicals by the “Urey-Miller process” stopped forever. Oxygen is chemically active enough to destroy organic molecules as fast as they could form. Life could never again start up the way it once did. 2. The old-type anaerobic bacteria found oxygen poisonous, so many became extinct. A few survived in environments where there is no oxygen, and there they live to this day... you will study them soon. 3. Atmospheric oxygen allowed development of an ozone layer. This absorbs UV rays and was vital for the later development of life on land. 4. The oxic environment encouraged a new, more efficient way to use food energy... cellular respiration. By 2 billion years ago the familiar modern cycle was operating:

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ATP is the energy compound which powers all life processes... cell division, moving, growing etc.

7) The First Eucaryotic Cells 

“More advanced” living things on Earth today are characterized by cells containing many organelles built from and/or surrounded by membranes. Such cells are called “eucaryotic”.

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Prior to about 1.5 billion years ago, all life on Earth was “procaryotic” meaning that the cells lack a true nucleus, mitochondria, chloroplasts, etc. The living procaryotes of today are the bacteria and cyanobacteria

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Without true organelles to organize their cell functions better, the procaryotes have to remain very tiny, single cells in which diffusion distances are small, and the SA/Vol ratio is high

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that’s how life on Earth might have remained forever, except some cells ate some smaller cells, but failed to digest them

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The small cells lived on inside their “host” in a relationship that soon became mutualism, and after millions of years, the ingested cells evolved to became “organelles” of the larger cell

What’s the evidence for this? • Both mitochondria & chloroplasts contain their own DNA, and it is bacteria-like DNA in a loop.These organelles reproduce independently of the rest of the cell in a mini-version of a cell division. • Mitochondria contain their own ribosomes for making their own proteins. • Mitochondrial enzymes (which control cellular respiration) are attached to the inner membrane in a very bacteria-like way. 

This whole idea is known as the “Endosymbiotic Hypothesis” and is

our best explanation of where the first cells came from that later evolved into the plants and animals. 

(“Endo-” = inside,“symbiosis” = to live together)

8) Sex Speeds Things Up 

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So far in the history of life, all the living things probably reproduced by

simple cell division. This produces identical daughter cells, The only way a new variation could occur was by occasional genetic “accidents” ... mutations. So the evolution of new types was very slow. A little over 1 billion years ago, some cells began exchanging bits of DNA with each other Fossils have been discovered (using the electron microscope) of 2 cells ed by a thin tube apparently in “conjugation”, in which the cells swap DNA fragments in a kind of simple sexual fertilisation of each other. CELLS in CONJUGATION Small fragments of DNA are ed through the tube, increasing the genetic variations within a population

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The result is more genetic variations and more differences between individuals. Evolution had more opportunities, and sure enough, the fossil record shows an accelerating increase in new, more complex forms appearing

9) The First Multicellular Organisms 

appeared about 800 million years ago

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It is often an advantage for an organism to be large. A larger organism deters predators and gathers more of the resources of the environment, so its chance to survive and thrive is better

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a single cell cannot grow too large because the SA/Vol ratio gets less

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About 800 million years ago some eucaryotic, photosynthetic cells became “colonial”. When the cells divided, they didn’t separate, but stayed attached to each other, forming filaments or flat sheets

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These simple, multicellular associations evolved into the algae group

and from one type of them, came (eventually) the plants 

Some eucaryotic, heterotrophs became colonial to evolve into the first multicellularanimals, similarto modernsponges. Latercame flatworms and jellyfish-likecreatures withvery simple bodyplans

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About 600 million years ago there came an “explosion” of life. We find a huge increase in fossil numbers and forms, partly because some types developed shells and other hard body parts that fossilized well

Science Clashes with Culture? 

Darwin’s Theory of Evolution caused tremendous controversy when published in 1859 because it was not consistent with the Biblical story of “divine creation” and many people saw this as an attack on their religion and their culture

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Even today, there are some religious groups who reject the entire concept of the “Evolution of Life” because they interpret their traditional, cultural or religious stories of creation very literally

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Most mainstream religions however, accept that Science is not trying to attack any belief, culture or tradition, but only to understand and explain the natural world

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Most religious organizations now accept the “Facts of Evolution” that life on Earth has existed for billions of years, and has undergone progressive change

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Many Christian churches, for example, accept the scientific evidence for the age of the Earth, the beginnings and history of life, and recognise that the creation stories in “Genesis” are not literally true, but are allegories to the power and benevolence of the JudaicChristian-Islamic God

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The belief is that evolution happened, but under God’s control and supervision, along a pathway he ordained

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It is quite possible to reconcile religious belief and faith with scientific enquiry and knowledge

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Each contributes in its own way to human culture, and to each individual’s “humanity”