How did life originate?

To ask this question using science (as opposed to other “ways of knowing”):

What natural processes could account for the origins of life?

A primary question that can be asked about life:

"We are Stardust"The atoms that form our genetics, which form us,...such as carbon, oxygen, ironand calcium...were formed by fusion inside the cores of massive stars billions ofyears ago. We then evolved from these elements here on earth. Our home. Theatoms that we are made of are 4.6 billion years old. We are stardust.

"Stars have died that we might live." -Preston Cloud, Geologist

"We are star stuff." -Carl Sagan

H OH

H C C O

H

C

C CC

H H

H H

HO OH

OHHOCH2

HH H

HH N

O

N NO

OCC

O

CHEMICAL EVOLUTION

Light energy

OC

HC

H

O

H C O

H

HC

OH

Heat

1. Simple molecules inatmosphere of ancient Earth

2. Reduced carbon-containing compounds

3. First carbon-carbon bonds

Chemical Evolution

Early earth atmosphere and surface was composed of inorganic atoms and simple molecules.

Chemical evolution of these molecules produced the complex organic compounds that are the basis for life.

Electron

Proton

Neutron

CarbonHydrogen

-

--

- ---

+ ++

The Structure of Atoms

When the number of protons = the number of electrons:The positive and negative charges cancel each other out and the atom is electrically neutral

1 proton ≈ 1 dalton; 1 neutron ≈ 1 dalton

Atomic number =# of protons

Mass number = of protons+ neurons

11

2311

73

H

Li

Na 4018 Ar

2412Mg 27

13Al 2814Si

3115P 32

16S 3517Cl

94 Be

115B 12

6C 147N 16

8O199F 20

10Ne

42He

The Structure of Atoms

Atomic and mass numbers indicate the identity of atoms

The Structure of Atoms

All atoms of the same element have the same number of protons, but they may have different numbers of neutrons

Atoms of the same element with different numbers of neutrons are called isotopes

Example:

• 12C

• 14C

Radioactive Isotopes allow us to determine the age of certain materials

Radioactive Parent

Stable Daughter Half life

Thorium 232 Lead 208 14 billion yrs

Uranium 235 Lead 207 704 million yrs

Uranium 238 Lead 206 4.47 billion yrs

Carbon 14 Nitrogen 14 5730 years

• Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

• Once the rate of break down is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements

Parentisotope(red)

Daughterisotope(gray)

Number of half-lives10 2 3 4

100

50

25

136

Per

cen

tag

e o

f p

aren

t is

oto

pes

rem

ain

ing

This graph shows how a sample of radioactive atoms decays with time. At the time a sample forms, the ratio of parent to daughter isotopes is 100:0. After the amount of time represented by one half-life passes, the ratio is 50:50; after two

half-lives it is 25:75, after three half-lives it is 12.5:87.5, and so on. 

How electrons are distributed in an atom

+

= electron

= empty space

electron shells

1 2

3

electron orbital

the valence shell

Energy is neither created nordestroyed; it simply changes form.

Ek

Ep (higher)ENERGY TRANSFORMATION IN AN ATOM

Ep (lower)

Heator light

3. Once the electron arrives at a lower electron shell, the kinetic energy is converted to light or heat

2. As the electron falls to a lower energy shell, its potential energy is converted to kinetic energy, Ek-

1. An electron in an outer shell has a defined amount of potential energy, Ep-

Na+Na+

Loss ofelectron

A sodium ion being formed

Ions are atoms that have a charge

Cation

AnionCl-Cl

Gain ofelectron

Cl Cl

A chloride ion being formed

Ions of opposite charges are held together by ionic bonds

Table salt is a crystal composed of two ions.

Cl-

Na+

Water molecule

Ionic solids dissolve readily in water

Na+

-

-

-

+ +

+

+

Cl-Cl

The number of electrons in the valence shell of an atom determines its chemical reactivity.

Distribution of electrons in shells of these elements

Hydrogen Helium

Lithium Beryllium OxygenBoron Carbon Nitrogen NeonFluorine

Sodium Magnesium SulfurAluminum Silicon Phosphorus ArgonChlorine

H H H H

Hydrogen atoms each haveone unpaired electron

H2 molecule hastwo shared electrons

+

Sharing electrons between two atoms, in the form of a covalent bond, produces molecules.

Methane Ammonia Water Oxygen

CH4 NH3 H2O O2

H

H

C H

H

H

H

N O H O O

Molecular formulas:

Structural formulas:

Ball-and-stick models:

Space-filling models:

H H

How do chemical reactions occur?

Chemical reactions = the formation and breaking of chemical bonds:

Reactant + Reactant Products

AB + CD AC + BD

When the forward and back reactions occur at the same rate, the system is stable and is called a chemical equilibrium

Chemical Reactions on Early Earth

It is thought that the early atmosphere was made of gases ejected from volcanoes

Volcanoes eject carbon dioxide, water, and nitrogen, along with some H2 and methane (CH4)

• CO2 + H2O H2CO3

(carbonic acid)