EVOLUTION “The Origins of Life”

Pre-biotic Earth Biochemical evolution: life appeared after a period of chemical reactions, according to physical and chemical laws.

Pre-biotic Earth The earliest sediments on Earth suggest that there was a reducing atmosphere on the primitive Earth. No free oxygen (O2). But free hydrogen (H2) and the elements C, O and N combined to form fully saturated hydrides (CH4, NH3 and H2O).

Pre-biotic Earth Energy for chemical reactions between these gases could come from electric discharge in storms or solar energy (UV light would penetrate the atmosphere more easily as there was no ozone layer) The Earth’s surface temperature was probably hotter than today.

Formation of Monomers This idea led to an experiment in 1953 (Miller and Urey) that aimed to recreate these conditions in vitro and find out what may be formed.

Formation of Monomers The water is heated and the mixture circulates for many days. After a week Miller and Urey isolated 15 amino acids in the mixture. Other biologically important molecules had been formed including ethanoic acid, lactic acid and urea.

Formation of Monomers Later similar experiments were done using CO2 that produced nucleotides. Even though these experiments cannot reproduce the exact conditions on the primitive Earth, it can be shown that the basic building blocks for the large macromolecules can be synthesized in vitro from inorganic compounds.

From Monomers to Polymers The combination of monomers, such as amino acids, into polymers, such as polypeptides, could have occurred when dry or highly concentrated monomers are heated. Condensation reactions take place forming peptide bonds between amino acids and phosphodiester bonds between nucleotides.

Clay Minerals Clay mineral particles may have also contributed to the process.  Molecules adsorb to the clay particles (stick to the surface). The adsorbed molecules become concentrated together. Clay particles may have been essential catalysts in the formation of polymers.

Clay Minerals Once formed polynucleotides show a tendency to copy themselves using complementary base pairing. This was probably catalysed by the presence of clay particles and metal ions. These single stranded polynucleotides would have been the equivalent of RNA.

The First Hereditary Information RNA was probably the first hereditary molecule having the ability to copy itself. RNA shows enzymic (catalytic) properties – called ribozymes. sRNPs that edit eukaryotic mRNA, cutting out the introns, are RNA based. The best-known ribozyme is the ribosome. The ribosome's active centre (where the amino acids are brought into place) is made entirely of RNA.

The First Hereditary Information Polynucleotides are very good molecules at storing and transmitting information but they lack the versatility for all the chemical functions of a cell. Polypeptides, which can form complex 3-dimensional structures (proteins), are much better at this. At some stage a partnership must have formed between the polynucleotides and the polypeptides where the polynucleotides directed the synthesis of the polypeptides.

The First Hereditary Information Today it is clear that information only flows from polynucleotides to polypeptides. Translation had started. Later the hereditary information was probably stored in the form of DNA which is more stable than RNA. The new partnership with proteins no doubt helped producing the necessary complex enzyme systems for transcription and replication. The passage of information from RNA to DNA is possible in nature. The reverse transcriptase enzyme of the retro viruses shows this.

The first membranes, the first cells Membranes defined the first cell. The phospholipids are amphipathic molecules – one end is strongly hydrophobic the other is strongly hydrophilic. This means they form lipid bilayers when they are surrounded by water (rather than lipid droplets).

The first membranes, the first cells All the components of a simple prokaryotic cell were now assembled. They diversified in their metabolism. By 2 billion years ago free oxygen was appearing in the atmosphere due to the activity of cyanobacteria and other photosynthetic bacteria. Chemosynthetic bacteria in what is today South Africa left gold deposits associated with organic carbon.

Endosymbiotic Theory Endosymbiosis – a large anaerobic cell teams up with an aerobic cell.  The aerobic prokaryote became a mitochondrion. Eukaryotic cells were formed, bigger and more complex, eventually forming multicellular organisms.

Endosymbiotic Theory The evidence for endosymbiosis is strong Certain eukaryotic organelles have their own DNA that is a single naked loop of DNA, like the prokaryotes. But the amount of hereditary information is a lot less than free-living prokaryotes.

Endosymbiotic Theory These organelles have their own ribosomes that are smaller (70S) than those in the cytoplasm (80S). They are the same size as those in prokaryotes. The protein synthesis of these organelles is semi-independent of that taking place in the cytoplasm and it is inhibited by the same antibiotic that affects prokaryotes (chloramphenicol).

Endosymbiotic Theory These organelles are found in membrane envelopes as though they were captured in a vacuole or vesicle by a larger cell. These organelles are about the same size as a prokaryotic cell.

Endosymbiotic Theory The idea is that mitochondria represent an aerobic prokaryote that took up residence in a larger cell. These are found in all the eukaryotic kingdoms (plants, animals, fungi and protoctista). Chloroplasts represent a cyanobacterium type of prokaryote that was trapped in ancestral plants and some protoctista.

Endosymbiotic Theory Some scientists think that bacteria called spirochetes are the ancestors of eukaryotic flagellae found in plant, animal and protoctista kingdoms. However, no trace of extra nuclear DNA has been found associated with flagellae