4. Origin and development of life on Earth Some organic molecules arrived on Earth from space. How well could they also have formed on Earth itself? Miller s origin of life experiment (1950s) Atmosphere of methane, ammonia, and H 2 Energy from sparks (lightning) Boiling water as an ocean Synthesis of Organic Molecules on Earth Energy supply: mainly the Sun, but also locally from lightning, radio active decay, volcanoes (primordial heat), shock waves (entry of meteorites) Note that the rate of meteor impacts 4 Gyr ago was much higher. This provide organic molecules (which may vaporize on impact), but also energy. This could result in further complex organic molecules Experiment produced amino acids easily [Current knowledge of Earth s early atmosphere (CO 2 + N 2 ) suggests it is much more difficult.] Hints from chirality of amino acids 4.2 Making more complex molecules Some molecules can exist in right-handed and left-handed forms. This is called chirality 19 of the 20 amino acids can exist in left- and right-handed forms Chemical reactions that make amino acids generally create equal numbers of R and L handed isomers Mixing R and L isomers would hinder proteins from performing their biological functions Life on Earth only uses left-handed amino acids Early in its history, life must have begun with L-amino acids, locking biology in its preference Was this random? Maybe not! Meteorites have an excess of L-amino acids.prebiotic material may have come from meteores UV circular polarized light seen in star forming regions destroys one type of isomer easier than the other, which could have caused the bias The rate of chemical reactions increases with the concentration of the reactants How can organic matter be concentrated? Lagoons and tidal pools, with solutions evaporating out Freezing of the solution (water freezes first) Mineral surfaces can trap organic matter. Clays can incorporate molecules in their structure Creating polymers and macromolecules 4.3 Boundary layers and cell structure Important chemical reactions see section 2.1 The high level of order and complexity of macromolecules In living organisms need more sophisticated methods, making use of catalyzers (enzymes) At some point, the complex chemicals need to be kept together, for protection and confinement, to stop them from wondering about. How have these structures formed? 1
I Lipid monolayers Lipids are amphiphiles, molecules with Hydrophilics heads and hydrophobic tails Amphiphiles float on water, heads in water tails in the air, creating a single layer of molecules, a monolayer Shake the water, and they form spherical structures, called micells The double layer equivalent is called a bilayer vesicle. These structures are formed spontaneously, and can trap molecules Grouping together without cell structure When polymers are added to water, they Form droplets called coacervates. This has its origin in the polarity of molecules And the formation of H-bonds with water Sidney Fox experiment: polymerization of amino acids can occur by heating them Dissolving these in water they form small, double walled spheres They can absorb more protein material from the solution Deamer (1985): amphiphilic molecules are also present in the Murchison meteorite, forming boundary layers when added to water. The role of minerals To reach structure and complexity, minerals could play an important role Protection: from dispersal and distruction Support: structure for molecules to accumulate and interact Selection: crystal phases can select left and/or right handed amino acids. 4.5 The first biological systems As seen in the previous sections, many possible pieces of the jigsaw puzzle of how to form a living cell are proposed No one has yet synthesized a protocell using basic Components How a combination of organic molecules can form a Protocell is unknown. Catalysis: Nitrogen gas can flow over a metallic surface to form ammonia, becoming a valuable source for biological reactions. L F E There are many hypotheses Most important ones: RNA world hypothesis Metabolism first hypothesis The RNA world hypothesis It proposes that RNA was the first life form on Earth, later developing a cell membrane around itself, and becoming the first prokaryotic cell Why? RNA can store, transmit and duplicate information, But it can also act as enzyme, performing the tasks of both DNA and proteins. The nucleotides in RNA are more easily synthesized than those in DNA. One could imagin that DNA evolved from RNA, taking over its role due to its greater stability RNA are likely to have evolved before proteins. How could the latter otherwise replicate? 2
Evolution in the RNA world Free floating nucleotides regularly form bonds that break up easily Certain sequences are more stable - the longer the chain the more it attracts matching nucleotides. Natural selection makes the more efficient catalyzer and replicator to survive more easily and could evolve into modern RNA Further competition may have favored cooperation between chains, and giving advantage to those that could serve as ribozymes. Eventually DNA, lipids, and carbohydrates were recruited to form cells Retroviruses: RNA life is still present today! They consist of fragments of nucleic acid with a protein coating In the outside world they do not carry out functions of life Within the cell of an organism they take the cell s energy to make proteins and nucleic acids to copy them selves Retroviruses (e.g. HIV) reverse the cellular process by transcribing RNA into the cells DNA, to make more viral RNA Some problems with the RNA world theory H I V 1. Large RNA molecules are very fragile they strongly absorb UV radiation 2. Prebiotic simulations show that nucleotides can not be made at the same time as sugars. They must have been synthesized separately and brought together Metabolism first models A cycle of chemical reactions that produce energy that is used subsequently by other processes. Example: Iron-Sulfur world theory (J. Wachterhauser) Life occurred first on mineral surfaces near black smokers (high T and P) One black smoker would operate like one big cell. Primitive metabolic cycles produce more complex compounds (amino acids and peptides) Other hypotheses Lipid world hypothesis, clay theory, bubble theory, Auto-catalyses theory, deep hot biosphere model,... The temperature and chemical gradient around the smoker would play an important role How would it evolve into cellular life? 3
The top-down approach Life does not reject what evolution has created, but builds on what has gone before Phylogenetic tree Try to extrapolate back to the origin of life by using the Information contained in DNA in organisms All present-day life is related - last common ancestor seems to be a heat loving chemosynthetic organism that lived near hydrothermal vents Alternative hypothesis: Panspermia Can life itself survive in space, and could it Have arrived on Earth from a distant planet? Why not? Large dosis of fatal radiation(?) Alternatively, panspermia could have come on a meteor. May not be completely stupid... (see ALH 84001), although some versions of it are 4.7 Diversity of life in extreme environments To know under what extreme range of environments life can thrive on Earth, may help us to understand how widespread life me be in the universe Extremophiles: organisms that tolerate or thrive in extreme environments. Hyperthermophiles Psychrophiles Radiation extremophiles Vacuum tolerant Pressure extremophiles Salinity, ph, chemical extremes, etc... 5. Prerequisites for life elsewhere Life on earth can exist under extreme environments However, for more complex life (that uses oxygen) to develop, the conditions are more narrow. It would require an ocean, some dry land, O 2, low CO 2 To allow O 3, and a seasonable stable climate. The circum-stellar habitable zone Liquid water seems crucial for the development of life on Earth The circumstellar habitable zone: the range of distances to a star for which liquid water can exist on the planet s surface. For a planet like Earth this is between 273 and 373 K Effective Temperature of a planet, Teff: The average T of a planet determined by the balance of incoming solar radiation and the planet s outgoing thermal emission 4 T eff = (1-A) L star 2 16 π d σ 4
Time-dependent habitable zone The solar luminosity (or that of any star) is not constant in time. 4 Gyr ago the Sun was about 30% weaker than today, moving the habitable zone inwards H.Z. as function of stellar type The luminosity of a star is a strong function of its mass. Continuous habitable zone: The region in which a planet may reside and maintain liquid water throughout most of the star s lifetime. The greenhouse effect The surface temperature on a planet with an atmosphere can be significantly higher than T eff Teff (Earth, Venus) = 255 K, 2?? K. Tsur (Earth, Venus) = 288 K, 733 K. Atmosphere passes light from the Sun, but is optically thick for reradiated heat from the planet Acts as a warm blanket Prominent greenhouse gases are H 2 O and CO 2 Stability of greenhouse effect CO2 is a very important factor in the Earth s greenhouse effect. It is removed from the atmosphere through chemical weathering It is released in the atmosphere through volcanoes Life on Earth also removes CO 2 by photosynthesis A huge anount of carbon is stored in the Earth. On Venus this balance is almost completely shifted to one direction (96.5% of its atmosphere is CO 2 Was the young Earth a habitable planet? How could it be so warm on the young Earth? Oldest rocks on Earth are 4 Gyr old, which have been deposited in water. This means that there was liquid water, and that the temperature was about like now How is that possible - the sun was 25-30% less luminous? [T eff =-33 c] Plate tectonics was particularly violent on the young Earth. Energy from: 1. Primordial heat 2. Radio-active decay 3. Iron and Nickel sinking to the core The enhanced plate tectonics keep the CO 2 in the air. Where does the Earth s water come from? Comets? (but do have a different isotope ratio) 5
Other energy sources We assumed that the main heat source of a planet is its star Other potential energy sources: radio-active decay primordial heat/core formation tidal interactions This makes other planets outside the habitable zone still interesting for the search for life, such as The moons of Jupiter and Saturn 6