Chapter 5: Origins of life
Synthesis of monomers and polymers and the
beginnings of life on young Earth
 Research shows that life has showed up on Earth
about 3.8 billion years ago, only a few hundred
million years after the epoch of heavy
bombardment of the planet by the interplanetary
debris
 At that time, the Earth’s surface was oceans and a
little bit of dry land, a neutral or mildly reducing
atmosphere and a variety of energy sources
including the UV radiation
Reducing atmosphere  contains substances that
give up electrons easily, e.g. H
Oxidizing atmosphere  contains substances that
incorporate electrons easily, e.g. O
Neutral atmosphere is neither reducing nor
oxidizing
Synthesis of monomers
 The U-M experiment assumed a strongly reducing
atmosphere (CH4, NH3), in which the amino acid
synthesizing reactions occur easily ( also the ones
producing monomer components of the nucleic
acids
 In a more realistic atmosphere of the early Earth
(N2, CO2, H2O), the efficiency of these reactions is
greatly reduced
 Alternative environments: (1) hot underwater
sources (CH4, NH3) and (2) the interstellar medium
Synthesis of monomers (continued)
 Hot underwater sources are currently under a
thorough investigation, because of discoveries of
numerous organisms, which live in these
environments (extremofiles). Their genetic makeup
suggests that they are very old
 Analysis of high C content meteorites (the so-called
chondrites) shows that 1-2% of their mass is made
out of organic compounds, including „nonbiological” amino acids  proof of a presence of
monomers of the cosmic origin on Earth
 Interplanetary dust falling down on Earth
represents a rich source of organic molecules.
About 3 mln kg/year of such dust does reach the
Earth. Organic molecules make up about 10% of
its content.
Transition to life
 Emergence of life is a transiton from chemical to
biological evolution
 Requirements related to biological evolution:
Precise (but not ideal) mechanism for organism
reporduction  genetic code
Controlled chemical reactions  enzymes
A „container” to separate an organism from environment
 a cell
A method to acquire and utilize energy immediate or
intermediate utilization of solar energy(e.g. absorption of
light by pigments)
Theories of life
 Proteins, nucleic acids and cells (protocells) were
created separately and somehow joined together to
produce life as we know it today
 One of these ingredients initially dominated  first
lifeforms were based on just one polymer and have
reached their current sophistication as a result of
gradual evolution
 „Protolife”  a state that was more primitive than
the most primitive known lifeforms. Statistical
arguments favor a protolife based on one
particular type of a molecule: protein or nucleic
acid („naked” genes, „the RNA world”)
If not the „RNA world”, then (among other ideas) …
 Panspermia (Hoyle and Wickramasinghe)  „life
seeds” transferred to Earth on the interstellar
grains. Problems: no access to water before making
it to Earth, difficulties in making a transition from
molecules to life
 „Seeding” of Earth by cosmic intelligent beings
(Crick i Orgel). Problem: this simply moves all the
difficulties elsewhere!
 Creationism  a religious concept of the
beginnings of life depicted by means of science.
Problem: too many disagreements with facts
resulting from the scientific research
“Exotic” life forms
 Silicon-based life  (1) Si-Si bonds 2 times weaker
than C-C bonds; (2) Si-H, Si-O bonds stronger
than Si-Si bonds; (3) Reactions between Si and O
make silicates (solids) rather than CO2 (gas)
 Alternative solvents  must remain liquid in a
temperature range, in which life-related chemical
reactions can occur
Molecule
Water (H2O)
Freezing point
(K)
273
Boiling point (K)
373
Ammonia (NH3)
Methyl alcohol (CH3OH)
Methane (CH4)
Ethane (C2H6)
195
179
91
90
240
338
109
184
More on exotic life forms
 Alternatives for chemical (electromagnetic) energy:
“bodiless” electromagnetic energy (e.g. a living
molecular cloud)  lifespan of such clouds (~10
million years) can be too short
nuclear energy  life at the level of subatomic
particles  but how do we discover such lifeforms
and communicate with them?
Gravitational energy  gravitational force is
weak, meaning that time needed for creation of a
gravity based life may be longer than age of the
Universe