Chemical Evolution: The Atoms and Molecules of Ancient Earth

2.1 The Ancient Earth Studying the Formation of Planets When Did Chemical Evolution Take Place? Atomic Components and Isotopes Radioactive Decay How Old Is the Earth? When Did Life Begin? 2.2 The Building Blocks of Chemical Evolution What Atoms Are Found in Organisms? Some Simple Molecules Formed from H, C, N, and O 2.3 Chemical Evolution and Chemical Energy Chemical Evolution: A Model System Energy Inputs and the Start of Chemical Evolution How Did Chemical Energy Change during Chemical Evolution? 2.5 The Early Oceans and the Properties of Water What Was Water’s Role in Chemical Evolution? Outline

The Atoms and Molecules of Ancient Earth The chemical evolution hypothesis explains how complex carbon-containing compounds (and eventually life) could have formed from simpler molecules.

Studying the Formation of Planets During the formation of the solar system, dust particles and debris began to accumulate into larger masses; i.e., small meteors and asteroids; These small bodies continued to collide and larger bodies several hundreds of kilometers wide formed, called proto- planets; As the new proto-planets increased in size so too did their gravitational force leading to further accretion of material and larger sizes; Computer simulations are a valuable tool and support the condensation theory of planet formation;

How do scientists estimate when chemical evolution occurred? Radiometric dating can be used to estimate the age of the Earth and when life first appeared. Atoms are composed of negatively charged electrons orbiting a nucleus made of positively charged protons and uncharged neutrons.

Isotopes (“same place”) Each element has a characteristic number of protons. The number of neutrons can vary; forms of an element with different numbers of neutrons are called isotopes. Radioactive isotopes have unstable nuclei that emit particles of radiation (energy) to form new daughter isotopes. This is known as radioactive decay. Each radioactive isotope decays at a constant rate quantified as its half-life.

How do scientists age a rock or a bone? Uranium 235 (radioactive) decays to Lead 207 (Stable); T 1/2 = 4.5 billion years Carbon 14; T 1/2 = 5,700 years Tritium H3; T 1/2 = 12 days N=(N o )(e -.693t/T 1/2 ) where N the amount of the parent isotope at a given time (t), and N o is the original amount of the isotope, T 1/2 is the half-life To age an object like a rock or a bone one needs to know: 1)How much of the parent isotope was originally present 2)How much of the parent isotope is present now 3)The half-life for the isotope Solve equation for t: t = [ln(N/N o )/-.693 ] (T 1/2 ) For information on how to measure half-lives:

How Old Is the Earth? Oldest dated rocks on earth are about 4.0 Ga (billion years ago), but we think the earth is about 4.5 Ga Meteorites formed 4.58 Ga, and the Moon formed 4.51 Ga. Earth must be about the same age, but no direct radiometric dating is possible because Earth was initially molten.

Chemical Evolution: A Model System Ancient Atmosphere was dominated by H 2 O, CO 2, CO, N 2, but H 2 and NH 3 (ammonia) and CH 4 (methane) in smaller, but still significant amounts. Most cells are 96 percent hydrogen (H), carbon (C), nitrogen (N), and oxygen (O).

In order for chemical evolution to take place, energy had to be put into the system Figure 2-20 Potential Energy Potential Energy and Order (Entropy) High potential energy Low potential energy High potential energy, more order Low potential energy, less order

Where did the energy inputs come from to start chemical evolution? High-energy photons can knock electrons away from atoms, breaking apart molecules to form free radicals— highly reactive atoms with unpaired electrons.

Free Radical Formation in the Ancient Atmosphere Figure High-energy photon Relatively unreactive molecules Hydrogen molecule Highly reactive free radicals Hydrogen atom

Figure High-energy photon Carbon dioxide molecule Carbon monoxide radical Oxygen atom

Highly Reactive Atoms and Molecules Formed the Building Blocks for Further Chemical Evolution Computer models show that significant amounts of H 2 CO (formaldehyde) and HCN (hydrogen cyanide) form under temperature and concentration conditions that were likely on ancient Earth about 4 Ga. The formation of H 2 CO and HCN is a critical step in chemical evolution because energy from sunlight has been converted to chemical energy (potential energy in chemical bonds). The chemical energy stored in H 2 CO and HCN is now available for the production of larger, more complex organic molecules, some of which are found in organisms today.

Figure 2-24 CHEMICAL EVOLUTION HYPOTHESIS 1. Simple molecules were present in the atmosphere of ancient Earth, including carbon monoxide (CO), carbon dioxide (CO 2 ), hydrogen (H 2 ), ammonia (NH 3 ), water (H 2 O), and nitrogen (N 2 ). 2. The energy in sunlight drove reactions among the simple molecules, resulting in compounds such as formaldehyde (H 2 CO) and hydrogen cyanide (HCN). 3. When heated, compounds containing single carbon atoms reacted to form more complex molecules containing carbon-carbon bonds, including acetaldehyde, glycine, and ribose (a sugar). Light energy Hydrogen cyanide Formaldehyde Ribose Glycine Acetaldehyde Heat

Life originated in and is based on water because water is a great solvent (substances dissolve easily in it). The H–O bonds in water are polar because oxygen has a high electronegativity. The O has a partial negative charge (  – ), and the H atoms carry partial positive charges (  + ). Water is Central to the Origin and Existence of Life Water also has several striking physical properties: it expands as it changes from a liquid to a solid, and it has an extraordinarily large capacity for absorbing heat.

Figure 2-12 Water is polar. Hydrogen bonds form between water molecules. Electrons are pulled toward oxygen

Figure 2-13a Polar molecules and ions dissolve readily in water. Salt in absence of water Salt dissolved in water

Important Properties of Water Figure 2-24-Table 2-2

What Was Water’s Role in Chemical Evolution? Chemical evolution proceeded in the ocean, and the simple organic molecules that formed from reduced-carbon* compounds were readily dissolved in water and once dissolved, they were preserved from destructive energy sources by water’s high specific heat. *For our purposes, think of reduced carbon as carbon compounds that have stored chemical energy, which can then be used to build more complex organic molecules, the building blocks of life.