The Rock Record • The rock record is meagre. • Oldest rocks are 4.0 billion years old from the Slave Province in Canada • Oldest zircons (derived from sedimentary precursors) are 4.2 billion years old. • Not much known, but assumed that an early atmosphere and ocean formed via outgassing and bombardment
The Moon • The interval of time is recorded on our moon; however.
Early Atmospheric Conditions • Early crust emitted halogen gasses, ammonia, hydrogen, carbon dioxide, methane, water vapor, and other gasses. • This atmosphere was quite similar to the atmosphere of Titan, one of the larger moons of Saturn. • The primordial atmosphere is believed to have reached a pressure of 250 atmospheres. • Little by little our planet assumed a more familiar look, with a dense gaseous cloud zone we could call an atmosphere, a liquid zone with oceans, lakes and rivers, or hydrosphere, and a solid zone, or lithosphere with the first outlines of what would one day become continents.
Hadean Atmospheric/Hydrologic system • At the same time, another important series of events began to unfold that led to the formation of sedimentary rocks through the processes of erosion, drift, and accumulation. • These processes began to occur as soon as the surface cooled enough to allow the water cycle to establish itself. • When temperatures finally cooled sufficiently, the clouds began to melt into rain, and the primordial atmosphere produced storms of unimaginable proportions. • Eventually, the water could accumulate in the depressed regions of the Earth's surface, forming the first oceans. • On the primordial continents, the first river networks were created, and they transported detritus torn from elevated regions and then deposited on the bottom of the primordial seas. • The metamorphism and remelting of the products of the erosion ultimately produced magmas and lava increasingly rich in silicates, and therefore of different composition from the mantle and the primitive crust.
Hadean Continents/Oceans • Faint young Sun Paradox • Stars get warmer as the mature, incoming solar radiation was reduced by 20-25% during the late Hadean interval. • Earth stayed ‘warm’ due to volcanic outgassing and high levels of CO2 • High UV influx?
Orthodox View on rise of Oxygen • Oxygen free atmosphere (anaerobic). • The notion is that the complex biomolecules responsible for self-replicating organisms could not exist in an aerobic environment. • Evidence for anaerobic (or low oxygen) environment?
Evidence for Anaerobic Conditions • Primary argument is based on Banded Iron Formations. FeO is dissolved in acidic waters and tends to precipitate in alkaline conditions. Iron in BIF’s is oxidized iron (either magnetite or hematite). Silica tends to precipitate in acid conditions and dissolve in acid conditions. Suggestion is that oxygen producing organisms led themselves to their own demise.
Other Evidence • Uraninite- Unoxidized uranium ore would seem to support the generally anaerobic conditions posited for the earliest time on earth.
The Alternative View • Did geologists ‘find’ what biologists wanted? • Biologists argued that once photosynthesis started, oxygen levels would rise quickly. • It may be that atmospheric oxygen levels reached 50% present-day levels by 2.0 Ga
The Formation of Life One of the most interesting problems facing science is to explain how the first replicating organisms came to be. Darwin proposed a ‘warm little pond’ where life may have first formed. “It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed”.
Oparin-Miller-Urey • In the 1920’s A.I. Oparin and Haldane each proposed that an earth with a reducing atmosphere and with methane (CH4) and ammonia (NH3) would have been the ideal ‘primordial soup’ • In 1953, Stanley Miller and Harold Urey tested the ‘primordial soup’ hypothesis. They created a chamber with methane, nitrogen, hydrogen and water vapor and electrical discharges (lightning). • After a week, the warm little pond turned brown and at least 4 amino acids (building blocks of life) were formed. • Later experiments produced 12 of the most common 20 amino acids.
Miller-Urey • Miller and Urey did not produce life, but only some simple amino acids necessary for life. Still, the amino acids that formed were far more complex than the system input. • Apparently, creating amino acids is not all that difficult. • Amino acids are found in meteorites and cometary material as well (panspermia)
Amino Acids Proteins Sugars
DNA- Double strand deoxyribonucleic acid RNA-Single strand ribonucleic acid • The molecules of Life! • Problems: • To produce RNA or DNA, a base is needed to polymerize the amino acids. • Amino acids are easily destroyed in open environments. • Even if RNA or DNA could form, they are highly susceptible to destruction. • So we also need some way to protect self-replicators
Some Working Hypotheses- No Answers YET! Simple cells have an out membrane formed of fatty acids Fatty acids are easily synthesized (found in some meteorites Combining alcohol with fatty acids produces lipids.
Lipid molecules are polar, with an alcohol head and a tail composed of fatty acids. The charged alcohol head is attracted to water and the tails are repelled by water. When fatty acids are dried and concentrated, then wetted again, they spontaneously condense into spherical balls that can trap any biopolymers inside (a protective membrane!).
Proteinoids & Coacervates Deamer found that when simple proteins were trapped inside the balls with DNA, they caused the amount of DNA to multiply 100-fold. Sidney Fox followed up with experiments similar to Deamers which spontaneously produced droplets of protein. Oparin found similar behavior that produced protein droplets called coacervates. The structures behave, in some ways, like bacteria. They maintain their organization under changing conditions, increase in size and bud spontaneously. They also absorb and release compounds in a manner analagous to excretement and feeding.
RNA, DNA and Proteins-Chicken or Egg? If life is going to form, it needs not only the protection for the complex proteins, it also needs to self-replicate. The ability to self-replicate is one of the hallmarks of life and thus protenoids and coacervates are not living entities. RNA and DNA carry the necessary proteins to communicate the information needed for replication. In essence, these two nucleic acids code for the proteins yet some proteins act as enzymes to catalyze the replication of DNA. Protein synthesis and DNA replication are both essential processes and each depends on the other. Which came first the chicken (proteins) or the egg (nucleic acids).
RNA WORLD OR PROTEIN WORLD? Fox and others argue proteins came first. Fox claims that protenoids exhibit many features of living systems. Yet, only nucleic acids can replicate (that we know of). If protenoids had developed a way to self-replicate, why turn over the job to nucleic acids? Others argue that Nucleic acids came first (RNA WORLD). RNA can self-replicate and can produce the proteins needed to catalyze further construction of nucleic acids. The idea is one of a ‘naked gene’. Nucleic acids are far harder to synthesize and only nucleic acids of a few dozen nucleotides have been created in the lab (hundreds are needed for true genetic code).
Are these the only possibilities? Graham Cairns-Smith claims that both scenarios are too complex and we should look for a simpler start. The key to replication is a template that could organize molecules in close order so that the could develop the proper length to self-organize. CLAY WORLD-Clays are natural catalysts with lots of space available for complex organic molecules. Imperefections in the clay structure would cause ‘mutations’ and life started out as a silicate form. Gunter Wachterhauser suggests that pyrite served as the original template for life. Fool’s Gold World- Pyrite has positively charged surfaces that could attract negatively charged particles such as phosphate backbones found in many organic molecules (including nucleic acids). If they all lined up against the pyrite, they could polymerize to form organic bonds and unzip themselves from the pyrite.
Origin of Life- Final Answer The final answer to origin of life questions is that we simply don’t know. We only know that life did form and it formed early in the history of life. As organisms became self-replicating, they made copying errors. In some cases, the copying errors led to increased survivability and adaptibility so the organisms adopted the copying errors as the standard. Evolution began once life began.
Early Forms of Life • Oldest known fossils are thought to be cyanobacteria (blue-green algae). • Prokaryotes- single cell with genetic material not inside a organized nucleus. The cyanobacteria are photosynthesizing organisms (autotrophs). Began to oxygenate the earth. CO2 + H2O + light CH2O + O2
Oldest Fossils • Oldest confirmed fossils are from the Warrawoona Complex in Australia (3.5 Ga). • Similar fossils in the Onverwacht and Fig Tree Groups in Transvaal, South Africa. • Some of these fossils have been questioned in the literature.
Stromatolites • Even if the ‘cyanobacteria’ in the groups at 3.5 Ga are found not to be organic, stromatolitic mounds have been found at many locations in Archean strata. Modern Stromatolites, Australia Fossil stromatolites, India
Prokaryotes • As far as we know, these simple forms dominated the planet for 1.5 billion years. • Very little or no evolution of bauplans. • Schopf argues that these organisms (that persist to the present day) are robust and reproduce asexually (no mixing of genetic material). • Significant evolution in biochemical diversity. Heterotrophs and autotrophs.
The First Significant Stressor on Biological Systems Between about 2.1-1.8 Ga there appears to be a major change in the Earth’s environment. The time period is of interest for several other reasons as well. Oxygen Holocaust- Oxygen levels increased slowly to about this time. One school of thought is that by 2.1 Ga oxygen levels were at about 1-2% of modern-day values. Anaerobes were driven into the subsurface by this poisonous environment. Some prokaryotes adapted to this new environment. Snowball Earth- Glacial deposits of 2.1 Ga have been found on many continents. Some yield information suggesting they were formed at low-latitudes and low elevation. The earth may have become totally glaciated at this time (a tremendous stress). First Large Supercontinent Formed- Called “Nuna” or “Columbia”, this was the first large agglomeration of landmasses. Global circulation patterns would change driving climatic change and opening up new environments.
Snowball Earth Columbia according to JJW Rogers and M. Santosh (2002)
Notable Changes at 2.1-1.8 Ga Banded Iron Formation stops- Replaced by the formation of redbeds (hematite-rich beds). Uraninite Formation ends Pyritic Rich Conglomerates end All signal (with caveats) a rise in oxygen levels on Earth Interestingly, a rise in oxygen levels occurs again at 1.3 Ga and again in the Late Neoproterozoic all times of supercontinent assembly or breakup.