CHAPTER 18: ORIGIN & HISTORY OF LIFE
In order to reconstruct life’s history, scientists use evidence from both the fossil record and
contemporary organisms to determine what the Last Universal Common Ancestor (LUCA) must have
been like.
It has been determined that the Earth is 4.6 billion years old, and that the first life forms did not appear
until about 2.5 billion years ago. The first 2.1 b.y. are believed to have been spent undergoing change
and experiencing a chemical evolution, which paved the way for the creation of living things.
STAGE 1: EVOLUTION OF MONOMERS
Primordial Soup Hypothesis

(1920’s) A.I. Oparin and J.B.S. Haldane independently postulated that the origin of life was the
evolution of simple organic molecules (monomers) from inorganic compounds present in the
Earth’s early atmosphere  “Primordial Soup” Hypothesis

It is believed that early Earth had:
o Little to no free O2
o Lots of CO2, H2, methane (CH4), and ammonia (NH3)

Methane and ammonia are reducing agents (give H to others)  so early Earth
had a reducing atmosphere
o Lots of free E (volcanism, unfiltered UV rays, heat, meteorite bombardment, and
lightning)

Stanley Miller and Harold Urey tested the Oparin/Haldane hypothesis, by constructing an
apparatus containing H2O, H2, CH4, and NH3.
o Produced many amino acids and other organic molecules

Additional experiments with modifications to the environment (less NH3 and less reducing) have
still produced all 20 amino acids, ATP, some sugars, lipids and purine and pyrimidine bases of
RNA and DNA.
Iron-Sulfur World Hypothesis

Thermal vents at the bottom of the oceans provided all the elements and conditions necessary to
synthesize organic monomers.

Dissolved gases (CO, NH3, and H2S) emitted from thermal vents pass over iron and nickel
sulfide

The Fe and Ni sulfide molecules act as catalysts to create organic monomers
Extraterrestrial Origins Hypothesis

Some believe that the comets and meteorites that have constantly bombarded the Earth brought
organic monomers with them, which “seeded” the formation of more
STAGE 2: EVOLUTION OF POLYMERS

Lab simulations of early Earth conditions have produced organic polymers

Polymers = chains of similar building blocks or monomers

Synthesized by dehydration reactions (Recall Unit 1)

Must have been done on pre-life Earth:
o Without the help of enzymes
o In dilute solutions of monomers
Iron-Sulfur World Hypothesis

Researchers have shown that organic molecules will react and amino acids will form peptides in
the presence of iron-nickel sulfides under conditions found at thermal vents.
Protein-First Hypothesis

Dilute solutions of organic monomers may have dripped onto hot sand, clay, or rock, which
helped water vaporize and concentrated the monomers on the substrate.

Sidney Fox abiotically produced polypeptide proteinoids called microspheres (small
polypeptides that exhibit some catalytic properties) by simulating this in the lab.

Hypothesized that these encased some enzymes that gave it a selective advantage metabolically
o “Protein-First” refers to the idea that protein enzymes arose prior to the first DNA
molecules
RNA-First Hypothesis

Suggests that only RNA was needed to progress toward formation of the first cells

RNA can act as both an enzyme and a substrate

Some viruses only use RNA

Hypothesis is that RNA was the first form of genes
STAGE 3: EVOLUTION OF PROTOCELLS
Living cells may have been preceded by protocells (or “protobionts”)
Protocells = Group of abiotically produced molecules able to maintain an internal environment different
from their surroundings and exhibiting some life properties such as metabolism and excitability

First plasma membranes were probably originally made of fatty acids instead of phospholipids
 formed micelles = single layer of fatty acids

Under right conditions form vesicles  larger and with a bilayer of fatty acids
o Have potential energy in the form of a membrane potential
o Can divide

A.I. Oparin showed that under the right conditions, mixtures of macromolecules give rise to
coacervate droplets
o Undergo osmotic swelling and shrinking

Alec Bangham showed that lipids extracted from egg yolks will form double-layered bubbles the
size of cells in water = liposomes
o Perhaps these liposomes then began to engulf molecules with enzymatic, replicative
abilities

Membrane-First Hypothesis = the 1st cell had to have a plasma membrane before any of its
other parts

This protocell would need to acquire nutrition to grow
o One hypothesis: 1st cells were heterotrophic (consumed preformed organic molecules)
o Other hypothesis: 1st cells were chemoautotrophic = synthesized organic molecules from
inorganic molecules by using the oxidation of inorganic compounds (like H2S) for energy

Especially if they lived near thermal vents
o Eventually would have had to develop glycolytic/fermentation pathways to regenerate
ATP from ADP
STAGE 4: EVOLUTION OF A SELF-REPLICATION SYSTEM

RNA molecules were probably the first genes
o Has ability to self-replicate and direct production of polypeptide chains

RNA is autocatalytic
o Produced short polypeptides that catalyzed RNA replication

Eventually must have been replaced by DNA, which is more stable
o Most likely the development of reverse transcriptase would have made this possible
o After this, the evolution of the genetic code had to occur

Some propose that RNA and polypeptides evolved simultaneously

Once this simple machinery for replication and translation of genetic information became
sequestered into membrane-bound protobionts, molecular cooperation could be refined as natural
selection acted on the level of the entire protobiont.
o Began to make proteins for the cell which were selectively advantageous

This made Darwinian evolution possible
o Perhaps this hypothetical membrane-bound protobiont:

Incorporated genetic information

Selectively accumulated monomers

Used enzymes

Grow and split