Origin of Life (Wellman) Flashcards
4,600Ma
- Earth formed by the gravitational accumulation of dust and larger objects. The mass melts forming a molten ball of rock and debris. It begins to differentiate into the core (heavy metals), mantle (lighter metals) and crust (lightest metals, e.g. aluminium).
- Water vapour and various gasses are outgassed but do not accumulate due to the great heat and continual bombardment as new material is accumulated.
- The Moon forms during a major collision with other debris.
3,750Ma
- Age of oldest rocks on Earth (Isua Supercrustal Group from Greenland). This suggests the Earth has cooled to the extent that a crust begins to solidify.
- As temperatures continue to fall, the oceans and gases can potentially begin to condense out. Gravity doesn’t let the gases and water vapour escape so the atmosphere is formed.
> 3,800Ma
- Progress retarded by continued bombardment of large objects, e.g. meteorites, causes oxygen to break down quickly.
- Released energy is sufficient to boil off the oceans and atmosphere (along with any prebiotic organic compounds).
- Moon becomes a dead planet (no atmosphere), but craters remain, so can be used to date. There are no tectonic plates.
<3,800Ma
- Meteorite bombardment decreases n intensity and the plane cools below a threshold that allows oceans and atmosphere to condense out. Organic compounds can begin to be synthesised and accumulate.
- Sand is washed into oceans, settles at the bottom.
-Early atmosphere forms by volcanic outgassing of water vapour and N2 (and its oxides), CO2, CH4, NH3, H2 and H2S. The water vapour gradually condenses to form the oceans and the H2 is lost into space.
Eventually we have an Earth hospitable for life;
- Less collisions
- Solid and oceanic crust
- Atmosphere formed (but still very different from todays).
3,500Ma
The earliest fossil evidence for life on Earth.
How long did it take for Earth to become habitable for life?
Earth became habitable around 3,800 million years ago and the earliest fossil evidence arises from 3,500 million years ago. This means it took approximately 300 million years from when the Earth became habitable to when life evolved.
Panspermia
The theory that life on Earth originated from microorganisms or chemical precursors of life present in outer space. However highly unlikely due to the harsh conditions and the distance needing to be travelled through space.
Fundamental similarities between prokaryotes and eukaryotes
- The method of transmitting information in triplet code in DNA and translating it into proteins through RNA. (Genome works the same).
- In living organisms, all amino acids are laevo-rotatory (rotate light to the left) and in nucleic acids all the sugars are dextro-rotatory (rotate light to the right).
Approaches to solving the origin of life
- Analyse (Cladistics) living prokaryotes and attempt to reconstruct their common ancestor (essentially the simplest conceivable prokaryote).
- Compare duplicated genes potentially enabling us to reach back beyond that ancestor and estimate some of the earliest components of genetic machinery.
- Reconstruct conditions that existed on Earth in these remote times and stimulate these experimentally and see what is produced.
Why are prokaryotes believed to have originated before eukaryotes?
- They appear earlier in the fossil record
- They are simpler in virtually every aspect
- There is evidence that eukaryotes evolved from prokaryotes
What chemicals can be produced by simulating conditions on the primitive Earth?
Chemicals and gases in the early atmosphere were collated.
- Amino Acids: Including all the biologically important ones.
- Purines/Pyrimidines: Including the four bases of RNA (adenine, cytosine, guanine and uracil, but NOT thymine, which replaces uracil in DNA) Important in RNA world theory
- Sugars
- Porphyrins (molecules which are the forerunners of important biological compounds like vitamin B12, chlorophylls, etc).
- Complex tar-like substances which defy analysis
Possible energy sources for the origin of life
Life most likely evolved through basic chemistry on planet Earth. We have seen that the necessary materials were available. There are a variety of possible environments in which life could have formed and a number of possible energy sources;
- Sun (large amount of energy, no O2 in the atmosphere so there’s no ozone to block sun’s rays).
- Radioactivity
- Electric discharges (e.g. lightning)
- Volcanic (hot springs, etc).
What came first, DNA or protein?
Proteins are the end product but are essential as enzymes in translating and copying information in DNA.
- A dramatic breakthrough occurred in 1980s when self splicing RNA was discovered. RNA was spliced off and rebuilt, replicating the genes. The ones that were better at replication, resulted in more being replicated. Eventually a bottleneck was reached, as certain chemicals began to run out. The ones that became better at gathering nutrients and replicating survived to reproduce.
- Shows Darwinian evolution at a chemical level; natural selection of RNA molecules.
Where did origin of life take place (possibly)?
- Oceans (nutritious broth); associated with oil slicks (protects from UV which breaks down DNA/RNA)?
- Hot springs with clay minerals, black smokers etc (chemical rich environments)?
Early Metabolic Pathways
Initially, early RNA-based life would have survived using the chemicals compromising the ‘primordial soup’. When this ran out they would have had to develop novel metabolic pathways;
-Chemoheterotrophs: C source and energy both from consuming organic molecules
-Chemoautotrophs: C source CO2; energy from oxidising inorganic substances, e.g. H2S, NH3, Fe2+ etc.
-Photoautotrophs: C source CO2, energy from light
-Photoheterotrophs: C source organic material, energy from light
This requires the synthesis of cytochromes (basis of oxygen metabolism) and porphyrins and related compounds that are the forerunners of photosynthetic pigments (the chlorophylls).
-Obligate anaerobes: poisoned by O2 and live exclusively by fermentation or anaerobic respiration where electrons are accepted by NO3-, or SO4-, rather than O2.
-Obligate aerobes: Use O2 for cellular respiration and cannot grow without it.
-Facultative aerobes: Use O2 if its present but can grow by fermentation in an anaerobic environment.
