CH 20 ORIGIN & HISTORY OF LIFE
1. Origin of Life on Earth
2. History of life on Earth
- Biological evolution is a process that involves genetic changes that occur over one to many generations.
1. Origin of Life on Earth
- Occurs in 4 overlapping stages:
1. Nucleotides and amino acids were produced before cells existed
2. Nucleotides and amino acids became polymerized to form DNA, RNA, and proteins
3. Polymers became enclosed in membranes
4. Polymers enclosed in membranes evolved cellular properties.
Stage 1: Several Scientific Hypotheses Have Been Proposed to Explain the Origin of Organic Molecules
- Oparin and Haldane independently proposed that organic molecules – nucleotides and amino acids – arose
spontaneously under conditions that occurred on ancestral Earth. promordal soup living cells
- Little oxygen was present and atmosphere contained inorganic CO , organic molecules, and water vapour.
- There are many hypotheses on how nonliving matter originated:
Reducing Atmosphere Hypothesis
- Miller simulated conditions on ancestral Earth:
- Put water vapour from boiling water into chamber of H , CH NH and inserted 2 electrodes that sent electrical
2 4, 3
discharges to simulate lightning bolts.
- Condenser jacket cooled some gases and droplets formed were tested for chemical analysis.
- Found formation of HCN and CH O, prec2rsors of organic molecules – like amino acid glycine.
- Recently, scientists have suggested that the atmosphere on ancestral Earth wasn’t reducing and was a neutral
environment composed mostly of CO, CO , N , and H O.
2 2 2
o Based on studies of volcanic gas.
- Organic carbon in the asteroids and comets that reached the surface of early Earth entered the prebiotic soup.
- Meteorites contain a substantial amount of organic carbon – amino acids, and nucleic acid bases.
Deep-Sea Vent Hypothesis
- Cracks in Earth’s surface where superheated water rich in metal ions and H S mixes abruptly with cold sea water.
- Hot gases are released from the interior of Earth at temperatures in excess of 300 C.
- Reaction between Fe and H S yi2lds pyrites and H which 2s the energy necessary to reduce N to NH . 2 3
o Ex. Fish, worms, crabs, cams, shrimp and bacteria are found in abundance in those areas and these
organisms receive energy from chemicals in the vent and not the sun.
Stage 2: Organic Polymers May have formed on the surface of Clay
- Scientists speculate synthesis of polymers didn’t occur in a prebiotic soup but took place on a solid surface or in
evaporating tidal pools.
- During the prebiotic synthesis of RNA, the purine bases of nucleotides interact with the silicate surfaces of the
clay. Divalent cations, such as Mg2+ bind the nucleotides to the negative surfaces of the clay, thereby positioning
the nucleotides in a way that promotes bond formation between the phosphate of one nucleotide and the ribose
sugar of an adjacent nucleotide. Stage 3: Cell-Like Structures May Have Originated When Polymers Were Enclosed by a Boundary
- Formation of a boundary that separated the environment from internal polymers, such as RNA
- Protobiont is used to describe the first nonliving structures that evolved into living cells.
- 4 characteristics that put them on path to living cells:
1. Boundary (membrane) that separates external environment from the internal contents of the protobiont
2. Polymers inside contained information
3. Polymers inside had enzymatic functions
4. Capable of self-replication.
- Hypothesis: living cells evolved from coacervates – droplets that form spontaneously from the association of
charged polymers such as proteins, carbohydrates, or nucleic acids.
o Coacervates have osmotic properties and are surrounded by tight skin of water molecules = selective
- Enzymes trapped within coacervates perform ancestral metabolic functions.
- Microspheres: small water-filled vesicles surrounded by a macromolecular boundary.
- Liposomes: vesicles surrounded by a lipid bilayer
o Clay catalyze formation of liposomes that grow and divide and if RNA was on surface of clay,
liposomes were formed that enclosed RNA.
Stage 4: cellular characteristics may have evolved via Chemical Selection, Beginning with and RNA World
- RNA is favoured as the first macromolecule found in protobionts:
o RNA stores information in nucleotide sequence
o Base pairing – nucleotide sequence has the capacity for replication
o RNA has lots of functions – act as ribozymes to synthesize macromolecules in living cells.
- Chemical selection caused RNA molecules to evolve:
o Chemical selection: when a chemical within a mixture has special properties that cause it to increase