14-3: The First Life Forms

The Origin of Heredity ► DNARNAProteins ► Why is RNA so important in the process?  Its’ STRUCTURE ► Takes on a variety of shapes ► Questions + observations led to the speculation that some RNA molecules might actually behave like proteins and catalyze reactions

The Origin of Heredity ► The Roles of RNA  Thomas Cech found that a type of RNA in some unicellular eukaryotes is able to act like an enzyme ► Ribozyme - acts as an enzyme, catalyzes chemical reactions, + can self-replicate  Hypothesis made that indicated life may have started with self-replicating molecules of RNA  Model known as “RNA WORLD” ► RNA has heredity, responds to natural selection, and will evolve

The Origin of Heredity  Two other hypotheses about RNA or other simple self-replicating systems could have evolved into modern cellular life: Minerals formed template so organic molecules could form polymers Self-replicating RNA molecules started to evolve inside microspheres or coacervates

The First Cells ► Description?  The first organisms were small, anaerobic, prokaryotic heterotrophs ► Depended on organic molecules as food ► Competed against one another ► Environmental pressure to evolve into autotrophs  Did not depend on photosynthesis

Chemosynthesis ► Archaea  Unicellular organisms that can survive in harsh environmental conditions ► Hydrothermal + sulfurous hot springs, rims of volcanos  Thought to have populated Earth 4 billion years ago  Were autotrophic but not by photosynthesis ► Chemosynthesis  CO 2 serves as source to assembly organic molecules (carbs)  Energy obtained from inorganics – sulfur

Photosynthesis + Aerobic Respiration  Some life forms became photosynthetic 3 billion years ago ► Discovered fossils in Australia ► Related to modern day cyanobacteria  Group of photosynthetic unicellular prokaryotes  Oxygen was damaging to many unicellular organisms  It could destroy some coenzymes essential to cell function  Certain organisms would bond to oxygen, not allowing it to be harmful ► This was a first step towards aerobic respiration

Photosynthesis + Aerobic Respiration ► Oxygen eventually reached the upper layer of the atmosphere more than one billion years later ► Sunlight can split O 2 to form a highly reactive single O atom  These combine to form ozone, O 3  Ozone is poisonous to both plants and animals  Absorbs UV radiation from sun to prevent DNA mutations ► Without the ozone layer, life could not exist on land

The First Eukaryotes ► We learned in Ch. 4 that Prokaryotes are different than Eukaryotes...but how did these large cells evolve? ► Eukaryotic differences :  Cells are larger  DNA is organized in chromosomes  Have membrane-bound organelles ► First Invasion :  2 billion years ago, a small aerobic prokaryote invaded and live inside a large anaerobic eukaryote  The relationship was beneficial to both ► Known as endosymbiosis  small aerobic prokaryote = modern mitochondria ► Second Invasion :  Photosynthetic cyanobacteria into large anaerobic eukaryote ► Gave rise to chloroplasts

► Invasions (cont.)  Evidence to support eukaryotic evolution? ► Chloroplasts + mitochondria replicate independently ► Chloroplasts + mitochondria contain their own DNA (circular) ► Both are double- membrane bound structures The First Eukaryotes

A Endosymbiosis…