Chapter 4
From Molecules to
Cells and the Origin
of Selection
The History of Microbes
Continuing Abiogenesis to
Living Systems
• Important Transitions
1. Abiotic organic molecules – sheet/droplet membranes
2. Concentrating molecules – protocells
3. Protocells leading to the first true cells, prokaryotes
• Life became based in cells with an inheritance
system that could pass their life properties to future
generations
Early Earth Atmosphere
• First Atmosphere ― a reducing atmosphere
– 4.6 to 4.2 Bya
– Hydrogen and Helium lost to space because Earth's
gravity is not strong enough to hold lighter gases
• Second Atmosphere ― a reducing atmosphere
– 4.2 to ~2.3 Bya
– Produced by volcanic out gassing
– Gases produced were probably similar to those
created by modern volcanoes (H2O, CO2, SO2, CO, S2,
Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane)
– No free O2 (not found in volcanic gases)
Earth’s Atmosphere(s) and the First
Cells/Organisms
• Rise of early organisms – Methanogens
– Molecular fossils (methane in rocks) provide the
evidence for support of their presence
– Addition of methane (CH4) to the atmosphere by
methanogens begins ~3.8 Bya
– Methane, a greenhouse gas, helped warm the
earth which would increase the rate of
metabolism
Methanogens
Methanosarcina acetivorans is
unique among archaea in forming
multicellular structures or colonies.
Anaerobic fermentations/reduction reactions yielding energy
with methane and carbon dioxide as waste
The Third (Oxidizing) Atmosphere (Current)
• Nitrogen, Oxygen predominate
– O2 from cyanobacterial photosynthesis accumulated
slowly
– From 3.3 to 1.8 Bya, most of the oxygen combined
with metal ions, especially iron, in rocks and soils
– Free oxygen (O2) only gradually accumulated in the
atmosphere
– Ozone (O3) layer forms gradually
• Ozone blocks uv radiation which reduces mutation
rates in DNA
– Photosynthetic Autotrophs consume CO2
Oxygen and Cyanobacteria
Increased O2 levels encourage evolution of new forms!
• Photosynthetic
cyanobacteria
produced O2
• Oxygen began
accumulating in the
atmosphere 2.3 Bya
Figure 01: Percentage of oxygen in the
atmosphere over the past 600 million years
Reproduced from Berner, Robert, A., et al., Science 316 (2007): 557
[http://www.sciencemag.org]. Reprinted with permission from AAAS.
Cyanobacteria
Let’s Go Back to the RNA World
• Is there evidence that the RNA World
consisted of living cells? Not really.
• For cells to evolve, cell membrane material
had to arise.
• How might that have happened?
Alexander I. Oparin (1894-1980)
“There is no fundamental
difference between a living
organism and lifeless
matter. The complex
combination of
manifestations and
properties so characteristic
of life must have arisen in
the process of the
evolution of matter.”
Phospholipids Formed in the
Organic Soup
Figure 02A: Phospholipid molecule, lecithin
Phospholipids Self-Assemble
Into Lipid Bilayers
Proto-Membrane!
How to get there?
Figure 02B: Bimolecular sheet-like double layer of phospholipid molecules
Membranes → Protocells
• Membranes
– Phospholipids:
polar end
Figure 03A: Molecules oriented with one end
pointed away from water (hydrophobic) and
hydrophilic
the other toward the water (hydrophilic)
– Form a monolayer
on water
– Vesicles may then
form
Figure 03B: Formation of droplets and
the bilayered vesicles shown
Membranes → Protocells
Figure 03C: Formation of droplets
and the bilayered vesicles
• Wave action and surf at shores
could produce these droplets and
vesicles
Figure 03D: Formation of droplets
and the bilayered vesicles
• Experimental evidence
demonstrates that such vesicles
could concentrate replicating
systems
•Such systems might have been
present in the RNA World, and if
not, then certainly in the
subsequent DNA World
The First Living Systems?
• Protocells
– Membranous droplets
– The first protocells, life forms that could maintain
their internal environments in the face of a
different external environment
• Laboratory Studies
– Dehydrating solutions of molecules produced
droplets that aggregated into what were named
coacervates
Coacervates
Alexander I. Oparin proposed the importance of coacervates in the 1930s
Coacervates From Colloidal Particles
Figure 04: Coacervates by the exclusion of water molecules (dots) from
associated colloidal particles
Adapted from Kenyon, D. H. and G. Steinman. Biochemical
Predestination. McGraw-Hill, 1969.
Coacervates Can Facilitate Catalysis
Figure 05A: Phosphorylase enzyme acts to polymerize phosphorylated glucose into starch
Figure 05B: Starch formed this way is hydrolyzed into maltose by the amylase enzyme
Microspheres
• Sidney Fox (1912-1998)
developed membranebound microspheres
from organic polymers
in the laboratory
Figure 06A: Proteinoid
microspheres
Figure 06B: Proteinoid microspheres bud spontaneously
Selection at the Molecular and
Protocellular Levels
• The origin of chemical selection in the RNA World
– Non-homogeneous populations
• Molecular selection – competition among catalytic systems
• Protocell selection – competition among vesicle populations
• Malthusian selection – when energy or raw materials are in
limited supply
• Selection and inheritance
– Selection at these molecular and protocellular levels
does not involve a system of genetic inheritance
• Selection and the cellular basis of life
Life
Robert A. Wilson concluded that living systems:
• have parts that are heterogeneous and specialized
• include a variety of internal mechanisms
• contain diverse organic molecules, including nucleic
acids and proteins
• grow and develop
• reproduce
• repair themselves when damaged
• have a metabolism
• exhibit environmental adaptation
• construct the niches that they occupy
The First Living Systems?
• The Molecular “Missing Link”
– Membranous droplets or protocells that had some
sort of catalyzed metabolism but without a functional
inheritance system to code for the catalysts, whether
RNA or protein
• The First Living System
– Protocells that had some sort of catalyzed
metabolism with a functional inheritance system
(RNA or DNA) to code for the catalysts, whether RNA
or protein
The First Fossilized Cells
• Prokaryotic cyanobacteria are known from
fossilized stromatolite reefs at 3.5 Bya
• Similar reefs exist today in Australia as well
Emergence of Cellular Life
• Based on fossils, chemical evidence, and extrapolation
from molecular clocks, the first appearance and
relationships of the major kingdoms of life are indicated
against a 4.6-billion-year time scale
Prokaryote Characteristics
•
•
•
•
•
•
unicellular
single circular DNA and plasmids
no cytoplasmic membrane-bound organelles
prokaryotic cytoskeleton and ribosomes
most have a rigid external cell wall
asexual reproduction and prokaryotic forms of
gene recombination
Generalized Bacterial Cell
• Note the circular DNA, prokaryotic ribosomes,
flagellum (for movement), pili (grappling lines to
attach to structures), capsule (complex
carbohydrate/glycoprotein protective coat present in
many bacteria), and the plasma membrane
contained within a cell wall
Biogeochemistry:
Early Organisms' Contributions
• Successful in Anoxic/Anaerobic conditions
– Probably chemosynthetic, producing H2S, CH4, CO2
• Sulfate-reducing archaebacteria
• Methanogenic archaebacteria
– Heterotrophs
• Fermentative anaerobes - bacteria and archaebacteria
consumed simple organic nutrient compounds from the
“organic soup” of the early oceans
The Problem of Bacterial Taxonomy
• Most bacterial taxonomy is pragmatic and for
medical, industrial, and ecological purposes
• Most bacterial taxonomists have avoided
organizing bacteria into the higher taxa (Order
and above)
• Molecular characteristics suitable for a natural
classification have only been available for a few
decades
• Most of the higher level phylogenetic analysis is
still to be done
Prokaryote Life Forms
• We will return to the origins and
diversification of the prokaryote lineages in
Chapter 8
• Once cellular life developed on earth, then
issues of reproduction become important
• Prokaryotes reproduce asexually
• This chapter reminds you that reproduction
becomes more complex in eukaryotes and in
multicellular life forms
Chromosomes and Cell Division
• Genetic code enables the results of selection
to be passed to future generations
• The transmission of genetic information as
sets of nucleotides (genes) arrayed along
chromosomes accompanies cell division
– Binary fission – asexual - prokaryotes
– Mitosis – asexual - eukaryotes
Prokaryote Life Cycle
Figure 07: Differences Between the
Three Types of Eukaryotic Cell Division
(mitosis)
Figure 08: Separation of Chromosomes
across the Mitotic Spindle
© AbleStock/Alamy Images
Mitosis - Nuclear Division
• Prophase
P
M
• Metaphase
• Anaphase
A
• Telophase
T
Life Without Cells? Prions
1. A prion is a proteinaceous infectious particle and can induce spongiform encephalopathies in
animals, not only sheep (scrapie), cows (BSE) and humans (CJD and Kuru), but also rodents,
minks (TME), elks (CWD) and probably pigs, chicken and some farmed fish
2. BSE (bovine) was first recognized in 1986, whereas Kuru had already been identified in 1957
(not the disease-causing agent itself, but a source where it�was residing in the harmful
conformation), scrapie has been known for at least 200 years
3. The harmless variant of the prion is called PrPc (Prion Particle cellular), the harmful one PrPsc
(from scrapie)
Life Without Cells? Viruses
– A virus is an acellular entity that can only replicate
inside the cells of a host organism
– DNA- and RNA-viruses exist and may infect
prokaryotes or eukaryotes
Life
Your author, Brian Hall, concluded that living systems:
• have DNA, RNA, and proteins
• nucleic acids are the essential components for transfer
of heritable information
• are based on carbon chemistry
• transform external matter and energy into processes
such as metabolism, growth and development
• membrane-bound cells are the units of life (prions and
viruses are non-living)
• perform
• have a metabolism and all necessary processes for
survival and replication
• exist as individuals in populations
The Tree of Prokarotic Life
Aquifex, is genetically positioned as the oldest
living fossil on the earth today
Chapter 4
End