SCIE 1120 Chemistry and Life

Origin of Life on Earth

Prof. Guang ZHU

Division of Life Science

Email: gzhu@ust.hk
Phone:8705
Mar.21, 2025, 15:00-16:20
LT-J
 Life in the universe
220px-Universe_expansion2

1024

13.3 to 13.9 billion (109) years ago

Mars has ice

There are plenty of planets in the universe, many are like our own.
Formation of Earth

Ave temp 4000 160 41 35 29 C

The Earth formed over 4.5 Billion Years Ago
◼ A planetesimal is an object formed
from dust, rock, and other materials.
The word has its roots in the concept
infinitesimal, which indicates an object
too small to see or measure.
◼ Early Earth heats up due to
radioactive decay, compression,
and impacts. Over time the
temperature of the planet interior rises
towards the Fe melting point creating
earth magnetic field.
◼ Volcano
Stable hydrosphere

carbonate acid

Delivery of water by icy planetesimals and comets: After condensation of

water vapor produced the earth's oceans, thus sweeping out the carbon
dioxide and locking it up into rocks, our atmosphere was mostly nitrogen and
some CO2.
◼ SO₄²- , anaerobic microorganisms used
sulphates instead of oxygen to meet their
energy needs (next to hydrothermal vent
in the bottom of ocean)
◼ CO2+sun light, anaerobic microorganisms
thrive with photosynthesis and release
O2.
anaerobic microorganisms : Any organism that does not
require molecular oxygen for growth.
Organism:is any living system that functions as an individual
entity
Living system: growth, energy transformation, reaction Cyanobacteria (2.4BYA)
to stimuli, and reproduction. obtain energy via Ave. oxygen concentration
photosynthesis 21% in 1 BY
 Stable hydrosphere

0.1-.3mm

Tiny zircons (zirconium silicate crystals) found in ancient stream deposits indicate that
Earth developed continents and water -- perhaps even oceans and environments in
which microbial life could emerge -- 4.3 billion to 4.4 billion years ago, remarkably soon
after our planet formed. The presence of water on the young Earth was confirmed
when the zircons were analyzed for oxygen isotopes and the telltale signature of rocks
that have been touched by water was found: an elevated ratio of oxygen-18 to oxygen-
16. (because 18O is more abundant in ocean due to its mass)
 Theories about the origin of life
◼ Special creation: life
forms may have been put Spontaneous origin:
on earth by supernatural or life may have evolved from
divine force. inanimate matter, as
◼ Extraterrestrial origin: association of molecules
life may not have originated became more and more
on earth at all; instead, life complex.
may have infected earth
from other planets
Theories about the origin of life
Lightning

Before life evolved, the simple molecules in the

earth’s atmosphere combined to form more
complex molecules.

The energy that drove these chemical reactions

may have come from lightning and forms of
geothermal energy.
 Miller-Urey Experiment (1953)

Prebiotic Chemistry

Hydrogen cyanide (HCN) Cyanoacetylene(HC3N)

Formaldehyde (HCHO) is the most important carcinogen
Atmosphere from volcanic outgassing
Problem: The assumed atmospheric composition
The experiments only give large yields of
interesting organics (amino acids, nucleic acids,
sugars) if the gas is H-rich (highly reducing). If the
early atmosphere was CO2+ N2 (mildly reducing),
as many suspect, the yields are tiny.

This would give atmosphere rich in CO2, N2, and

H2O. Not the composition that favors Miller-Urey
synthesis.
 Prebiotic Chemistry

After condensation of water

vapor produced the earth's
oceans, thus
sweeping out the carbon
dioxide and locking it up
glycine into rocks, our
atmosphere was mostly
HCHO formaldehyde nitrogen.
Bubble Hypothesis

Bubble Hypothesis

In 1986, geophysicist Louis Lerman proposed that the chemical processes

leading to the evolution of life took place within bubbles on the ocean’s surface
 Evolution of Small Organic Molecules

Some scientists
plume of hot water
rich in iron-nickel sulfides hypothesize life
began in
hydrothermal
Some scientists vents deep in the
hypothesize life ocean
began in tidalpool hydrothermal
vent

anaerobic microorganisms

CO2+sunlight: photosynthesis
 Requirements for a life
How to create a life,
(1)this system must be able to synthesize needed building constituents of living
materials from environment (Metabolism),
(2)The maintenance of a relatively stable internal (local) chemical environment
(Cell),
(3)pass on genetic material from one generation to next generation,
(4)the system can harness energy from environment. All these require some
enzymes if you want to build a molecular life form

DNA RNA Protein

 Four Types of Biopolymers

DNA-protein system: Too DNA RNA Protein

complex for the first life
So which came 1st?
◼ However, only two types seem crucial for primitive biological processes:
◼ amino acids/proteins and nucleotides/nucleic acids
 Theories about the origin of life
◼ Protein-first Hypothesis
◼ RNA-first Hypothesis ◼ Amino acids can form
◼ Ribozymes are RNA molecules polypeptides when exposed to
that can function as both a dry heat
genetic substrate and an enzyme
◼ Form microspheres when
◼ If RNA evolved first it could introduced back into water
function as both genes and Complex enzymatic processes may
enzymes
◼
have been necessary for formation
◼ Some viruses have RNA genes of DNA and RNA
◼ Reverse transcriptase produces ◼ Enzymes may have been needed to
DNA from RNA produce nucleotides and nucleic
acids
◼ Suggests a mechanism as to how
cells evolved to have DNA genes
Pre-RNA world
So now we have some amino acids
(monomers) loosely mixed in the
oceans. Liquid medium is important: Most amino acids have a mirror
◼ Ease of transport and Interaction image (L and D):
◼ Next goal is to combine Monomers • L and D both found in meteorites
into Polymers (peptide chains) • L only in organisms on the earth
why is D selected against?(*)
Protects molecules from UV photon (*) We believe that Earth life's "choice" of
disruption chirality was purely random, and that if
carbon-based life forms exist elsewhere in
the universe, their chemistry could
theoretically have opposite chirality.
 What came before DNA and proteins?
RNA world
◼

Almost certainly: RNA

◼ RNA looks a lot like DNA, but is single stranded. The big
difference is that RNA is a molecule that can carry
information like DNA, but can also fold itself into complex
three-dimensional shapes like proteins,
◼ so RNAs can be their own enzymes (proteins). Because
RNA is ribonucleic acid, but can act like an enzyme (protein),
these primordial RNAs are called “ribozymes” and are the
most important candidate for the origin of life.

◼ When naturally occurring ribozymes were discovered

in present-day organisms (including humans), the
idea that there was once an “RNA world” became
easily the most plausible scenario for the transition to
life.
Theories about
the origin of life:
One proposed sequence
of events that led to
the first self-
replicating
system and, later, to the
first living cells.

chemical concentration
How self-
replicating RNA
could have led to
the DNA/protein
world
◼

1989 Nobel Prize in Chemistry Cech&Altman

Encapsulation: Prerequisite for RNA
world?
Prebiotic membranes (vesicles) are easy
◼ The production of RNA
polymers at fast enough rate
is usually considered a
problem, but there are many
ways to enhance it. One is
to confine the reactants to a
compartment of some kind;
a lipid vesicle, forerunner of
today’s lipid membranes.
Evolution of the Membrane System
Plasma membrane → protocells
phospholipids
 Cairns-Smith hypothesis
(DNA/Protein)
Clay was helpful in Clay is a type of fine-grained
polymerization of both natural soil material Mud hot spring
proteins and nucleic Clay crystal masses of a
acids at the same time particular external form may
happen to affect
May have served as a their environment in ways that
catalyst and energy affect their chances of further
source replication
 Formation of synthetic complex
(protein synthesis)
RNA genes could replicate
because proteins were
already present to catalyze
the reactions

But this supposes that two

unlikely spontaneous
processes would occur at
once - formation of RNA
Primordial genetic molecules adsorbed on clay minerals would
have been protected against degrading agents present in the and formation of protein
environment and would have been in the right conditions to
undergo evolutionary processes.(Gene 2005)
 True Cells When did life begin?
◼ Stromatolites: Bacterial colonies that used
Photosynthesis (Cyanobacteria)
◼ Microfossils: Difficult! Controversial….
Photosynthesis and / or cellular
respiration ◼ Isotope ratios: carbon-12 to carbon-13
abundance is affected by metabolism in
living things.
True cell
◼ Best estimate: 3.5 to 4.0 Byr ago
Biological Evolution

DNA
origin of RNA
◼ When organisms ingest carbon, they
genetic code preferentially use 12C over 13C. (14C is
radioactive, and thus won’t remain over a
Protocell
long time period.) Carbon with a high
aggregation
ratio of 12C compared to 13C is therefore
an indicator of living processes (diffusion
/reaction rates of 12CO2 >13CO2).
Macromolecules Plasma membrane
Carbon enriched in 12C has been identified
in rocks from Greenland dated at 3.85
billions of years ago. This is the earliest
evidence for life on Earth.
 Establishing biological nature of
fossils: stromatolites
◼ stromatolites found in Precambrian rocks as the earliest
known fossils, and still being formed in lagoons in
Australasia.
◼ The Earliest Trace of Life? This fossil from Western
Australia is 3.5 billion years old and shows carbon traces
that indicate life. Its form is similar to that of modern
filamentous cyanobacteria (inset).
◼ Oldest stromatolites are about 3 Byr, but photosynthesis is so
complex that it could not have been available near the
beginning of life.
◼ Photosynthesis from algae in the ocean produced most
Science 8 March 2002 :
“Earliest Signs of Life Just oxygen in earth atmosphere
Oddly Shaped Crud?”
 Where did life begin?
◼ Land?
Problem: No protection from intense UV, or from sterilizing impacts.
Additional reason for excluding origin on land:
Hard to imagine life not in an aqueous solution.
◼ Ocean?
How to concentrate the molecules so they polymerize in a reasonable
time? One possibility: Encapsulation of molecules in cell-like
membrane.
◼ In tidepools or lagoons?
Evaporation concentrates monomers,
but unfortunately exposes to UV.
◼ Hydrothermal deep-sea vents?
A present-day favorite
 Life on Earth
◼ Earth forms over a time of 50Myr more
than 4.5 Byrs ago
◼ Pre-biotic chemistry somehow leads to
first replicating macromolecules (maybe
RNA)
◼ RNA leads to DNA and first life form(s)
◼ Best estimate: life on Earth is between
3.5 and 4 Byrs old!
◼ For >2 Byrs we have simple prokaryotes,
they start photosynthesis
◼ 1.5 Byrs ago Eukaryotes evolve
◼ ~600 Myrs ago complex, multi-cellular
life evolves
Lucy, the mother of
mankind
Perhaps the world's most famous early human
ancestor, the 3.2-million-year-old ape "Lucy" was the
first Australopithecus afarensis skeleton ever found, Lucy (2014 movie)
though her remains are only about 40 percent
complete (photo of Lucy's bones)

There is broad consensus for the ‘Out-of-Africa'

hypothesis that states that modern humans arose
∼200 000 years ago in Africa and spread throughout
the continent ∼100 000 years ago.

In human genetics, the Mitochondrial Eve (more

technically known as the Mitochondrial-Most Recent
Common Ancestor (MRCA). Estimates of the MRCA
of even closely related human populations have been
much more than 5000 years ago
 The Cambrian Explosion
◼ 530 Myrs ago, fossil record of animals and other complex organisms
“explodes”
• Major diversification of life on Earth
• Explosion took many millions of years (organisms before 580 Myrs
were much simpler)
Several hypotheses:
– Increased oxygen levels (more energy)?
– Earth was recovering from a Snowball event ( several frozen period)?
– Evolution of eyes (sensing far distance in 3D space) ?
– It wasn’t an explosion at all!
Nature530, 268–270(18 February 2016)