Chapter 12 Section 1 How Did Life Begin? Grade 10 Biology Spring 2011

Objectives Summarize how radioisotopes can be used in determining Earth’s age Compare two models that describe how the chemicals of life originated Describe how cellular organization might have begun Recognize the importance that a mechanism for heredity has to the development of life

The Age of Earth Earth formed 4.5 billion years ago ◦ Was a fiery ball of molten rock ◦ Eventually cooled and formed a rocky crust ◦ Water vapor cooled and formed oceans ◦ Scientists think life first evolved in oceans and evolution of life occurred over hundreds of millions of years

Measuring Earth’s Age Radiometric dating: estimation of the age of an object by measuring the content of certain radioactive isotopes ◦ Scientists have measured the earth this way

Measuring Earth’s Age Isotope: form of an element whose atomic mass (mass of each individual atom) differs from that of other atoms of the same element Radioisotopes: unstable isotopes that break down and give off energy in the form of charged particles (radiation)

Measuring Earth’s Age Radioactive decay: results in other isotopes that are smaller and more stable Half-life: the time it takes for one-half of a given amount of a radioactive to decay

Formation of the Basic Chemicals of Life It is thought that the path to the development of living things began when molecules of nonliving matter reacted chemically during the first billion years of Earth’s history Chemical reactions produce many different, simple organic molecules

Formation of the Basic Chemicals of Life Sun and volcanic heat caused simple organic molecules to form more complex molecules that became building blocks of first cells

Primordial Soup Model Primordial Soup Model: early Earth’s ocean contained large amounts of organic molecules ◦ Oceans filled with many different organic molecules like a soup ◦ Hypothesized that these molecules formed spontaneously in chemical reactions activated by energy from solar radiation, volcanic eruptions, and lightning

Primordial Soup Model Scientists proposed Earth’s early atmosphere lacked oxygen and rich in nitrogen gas, hydrogen gas, and hydrogen containing gases such as water vapor, ammonia, and methane

Primordial Soup Model Electrons in these gases would have been frequently pushed to higher energy levels by light particles from sun or electrical energy in lightning

Primordial Soup Model Today, high energy electrons are socked up by oxygen, without oxygen high energy electrons would have been free to react with hydrogen rich molecules, forming organic compounds

Miller-Urey Experiments Placed gases they thought existed on early Earth into a device Provided electrical sparks to simulate lightning After a few days, found complex collection of organic molecules ◦ Amino acids, fatty acids, hydrocarbons

Miller-Urey Experiments Results support hypothesis that some basic chemicals of life could have formed spontaneously under conditions like those in experiment

Reevaluating Miller-Urey Model Now know that the reductant molecules used in Miller’s experiment could not have existed in abundance on early Earth No ozone present, without ozone UV light would have destroyed any ammonia and methane present When these gases are absent, key biological molecules are not made

Reevaluating Miller-Urey Model If chemicals needed to form life were not in the atmosphere, where did they come from? Could be produced in ocean bubbles, or arose in deep sea vents

Bubble Model

1. Ammonia, methane, and other gases resulting from numerous eruptions of undersea volcanoes were trapped in underwater bubbles

Bubble Model 2. Inside bubbles, methane and ammonia needed to make amino acids might have been protected from damaging UV radiation. Chemical reactions would take place must faster in bubbles (where reactants would be concentrated) than in primordial soup.

Bubble Model 3. Bubbles rose to surface and burst, releasing simple organic molecules into the air 4. Carried upward by winds, simple organic molecules were exposed to UV radiation and lightning, which provided energy for further reactions

Bubble Model 5. More complex organic molecules that formed by further reactions fell into the ocean with rain, starting another cycle

Precursors of First Cells In the lab, scientists have not been able to make either proteins or DNA form spontaneously in water Short chains of RNA, have been made to form on their own in water

Precursors of First Cells Inorganic moleculesRNA nucleotidesRNA macromolecules RNA molecules catalyze protein synthesis Proteins Self-Replication

Possible Role As Catalyst RNA molecules can act like enzymes RNA’s 3-dimensional structure provides a surface on which chemical reactions can be catalyzed

Possible Role As Catalyst RNA was the first self-replicating information storage molecule and it catalyzed the assembly of the first proteins Would have been capable of changing from one generation to the next

Microspheres and Coacervates Lipids which make up cell membranes, tend to gather together in water Certain lipids, when combined with other molecules can form a tiny droplet whose surface resembles a cell membrane

Microspheres and Coacervates Microspheres: in water, short chains of amino acids can gather into tiny droplets Coacervates: composed of molecules of different types, including amino acids and sugars, gather into tiny droplets

Microspheres and Coacervates Formation of microspheres may have been the first step toward cellular organization Microspheres formed and dispersed Those that could persist longer and incorporate molecules and energy would become more common than shorter lasting ones Need to have characteristic of heredity to be considered living things

Origin of Heredity Double stranded DNA evolved after RNA and RNA “enzymes” catalyzed the assembly of earliest proteins Hypothesis that some microspheres or similar structures that contained RNA developed a means of transferring their characteristics to offspring

Origin of Heredity Do not yet understand how DNA, RNA, and heredity mechanisms first developed