The Origin of Life on Earth

Objectives D.1.1 – Describe four processes needed for the spontan- eous origin of life. D.1.2 – Outline the experiments of Miller & Urey into the origin of organic compounds. D.1.3 – State that comets may have delivered organic compounds to Earth. D.1.4 – Discuss possible locations where conditions would have allowed synthesis of organic compounds. D.1.5 – Outline two properties of RNA that would have allowed it to play a role in the origin of life. D.1.6 – State that living cells may have been preceded by protobionts, with an internal chemical en- vironment different from their surroundings.

Objectives (cont’d.) D.1.7 – Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere. D.1.8 – Discuss the endosymbiont theory for the origin of eukaryotes.

The early Earth Life on Earth originated between 3.5 & 4.0 billion yrs ago. Earth formed about 4.5 billion years ago, but left-over rocks bombarded the surface for the first few 100 million yrs, making it unlikely life could survive. Oldest rocks found today are 4.2 billion years old. Oldest fossils are in rocks from western Australia from 3.8 billion yrs ago - resemble bacteria, sug- gesting that life originated much earlier, possibly as early as 3.9 billion yrs ago, when Earth began to cool below 100 o C.

Origin of life on Earth Recall: A hypothesis is a testable explanation for 1 observation; where- as a theory is a collection of related & tested hypotheses that can support a broad range of observations. One major theory of biology attempts to explain how life began. There are three possibilities: 1) Divine creation - life was put on Earth by divine forces. This idea can’t be tested by science; it requires religious faith.

Origin of life on Earth How did life develop on Earth? Three possibilities: 2) Extraterrestrial origin - life was carried to Earth upon a meteorite or asteroid. We can test this idea by collecting samples from meteorites, but so far this idea is not proven. We can also search for life on other planets - SETI project. Meteorite Crater, Arizona, USA

Origin of life on Earth How did life develop on Earth? Three possibilities: 3) Abiogenesis (Origin from nonliving matter) - random events over 100s of millions of years produced self-replicating molecules, and natural selection acted upon these to produce the first cell. This is a testable hypothesis. There are 4 steps in this hypothesis: a) the synthesis of simple organic molecules, b) the assembly of these molecules into polymers, c) the origin of self replicating molecules that made inheritance possible, and d) packaging of these molecules into membranes with an internal chemistry different than their surroundings.

Non-living synthesis of organic molecules Experiments of Miller and Urey, who created, in the laboratory, the atmospheric conditions that had been postulated for early Earth. They discharged electrical sparks in a simulated at- mosphere of H 2 O, H 2, CH 4, and NH 3. They produced a variety of amino acids and other organic molecules. (Repeatable by other scientists.) Miller-Urey apparatus (right): in vitro Water re-circulates many times

Locations of organic synthesis Where were these organic molecules first produced? Submerged volcanoes and deep-sea vents where hot water & minerals gush into the deep ocean from below ground.

Locations of organic synthesis Chemistry around oceanic volcanoes

Locations of organic synthesis Where were these organic molecules first produced? Extraterrestrial locations – brought within meteors from Mars, other planets from other solar systems Casings found within a meteor from Mars, announced in 1996, resemble bacteria.

Importance of RNA in the origin of life RNA may have been the first genetic material. Life is defined partly by inheritance. Many researchers have proposed that the first hereditary ma- terial was RNA, not DNA. Notice the RNA can bind with itself.

Importance of RNA in the origin of life RNA can function as an enzyme. In the lab, short polymers of RNA can be synthesized abiotically. Add these to a solution of ribonucleotide monomers, and sequences up to 10 bases long are copied from the template according to the base-pairing rules. If zinc is added, sequences may reach 40 nucleotides with less than 1% error.

Importance of RNA in the origin of life Lab experiments demonstrate that RNA sequences are self-replicating and can evolve in abiotic conditions. RNA molecules have a genotype (nucleotide sequence) and a phenotype (3-dimensional shape). In certain conditions, some RNA sequences are more stable and replicate faster and with fewer errors. Occasional copying errors create mutations, and natural selection screens these for stability or for best self-replication.

Protobionts preceded living cells Protobionts have some of the properties of life and can form by self-assembly. Protobionts... encapsulated chemical reactivity encapsulated chemical reactivity catalyze chemical reactions, aggregates of pre-biotic molecules, or macromolecules that acquire a boundary to maintain an interior chemical environment that is different from the “primordial soup”. Examples: Coacervates Proteinoid microspheres Liposomes

Protobionts preceded living cells Coacervates - droplets of charged organic material that form spontaneously in water; (1-100 μ m in diameter) They are surrounded by a film of bound water molecules and held together by hydrophobic forces. They possess osmotic properties, and They allow absorption of simple molecules. Can store enzymes

Protobionts preceded living cells Proteinoid microspheres - water filled vesicles surrounded by a protein boundary. If a dry mixture of amino acids is heated to o C they polymerize; If these are then cooled in water they form small spheres about 2 μ m in diameter. They have an osmotically active boundary which can be seen as a double layer.

Protobionts preceded living cells Liposomes – vesicles surrounded by lipid bilayers. These undergo osmotic swelling or shrinking in different salt concentrations. They also store energy as a voltage cross the surface. Liposomes grow by engulfing smaller liposomes or by “giving birth” to smaller liposomes. They can maintain an inter- nal chemical environment different from their sur- roundings and show prop- erties associated with life: metabolism, excitability.

Creation of an oxygen atmosphere Prokaryotes dominate from 3.5 to 2.0 billion yrs ago. For the first ¾ of evolutionary history, Earth's only organ- isms were microscopic and mostly unicellular (no nucleus) Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea. Rich sources for early prokaryote fossils are stromatolites (fossilized layered micro- bial mats) and sediment from ancient hydro- thermal vents.

Oxygen began accumulating in the atmosphere 2.7 bya. Cyanobacteria, photosynthetic organisms that split water & produce O 2 as a byproduct, evolved over 2.7 bya. This early O 2 initially reacted with dissolved iron to form Fe 2 O 3 (rust) as seen today in banded iron rock formations. Creation of an oxygen atmosphere

Atmospheric O 2 was slow to rise until 2.2 bya, when it shot up to 10% of current values. “Corrosive” O 2 doomed many many prokaryotes. Some survived in habitats that remained anaerobic. Others (cyanobacteria) evolved cellular respir- ation, using O 2 to help harvest the energy stored in organic molecules. Ozone developed from O 2 in the upper atmosphere, allowing life to come out onto the land ~500 mya. Algae & plants made more O 2.

Theory of endosymbiosis Some biologists believe that eukaryotes descended from prokaryotes. Evidence suggests that chloroplasts and mitochondria descended from free-living pro- karyotic cells. This theory was developed by D. Lynn Margulis and is called the endosymbiont theory.

Theory of endosymbiosis Mitochondria & chloroplasts came from free-living bacteria. Both have circular DNA similar to bacteria. Both have ribosomes similar to bacteria; make proteins. Both reproduce themselves, and sizes are similar.

History of life on Earth Green algae Plants