Evolutionary History Chapter 20

Before life…  Chemical evolution:  The formation of small organic molecules preceded larger ones  Larger, more complex molecules formed  The truth is – we just don’t know exactly how this occurred  After life started, we have the fossil record to help us out

Chemical Evolution  Earth’s age is estimated at 4.6 billion years  Early atmosphere very different than today:  Carbon dioxide, water vapor, carbon monoxide, hydrogen, nitrogen, ammonia, hydrogen sulfide, and methane  No free oxygen, lots of energy  Earliest traces of life are approximately 3.8 billion years old (we think)

Stage 1 – abiotic synthesis of organic monomers  Monomer examples: amino acids, nucleotides  Primordial soup hypothesis :  Miller- Urey experiment in 1953  amino acids produced  Iron-sulfur world hypothesis :  Thermal vents similar to what we have deep in the ocean provided energy and chemicals for synthesis  Extraterrestrial origins hypothesis :  Comets and meteorites provided needed chemicals

Stage 2 – evolution of polymers  Where did the enzymes come from to catalyze the reactions?  Iron-sulfur world hypothesis  We know that amino acids form peptide bonds under conditions found at thermal vents  Protein-first hypothesis  Maybe heat from the sun caused amino acids to bind together  RNA-first hypothesis  Some viruses have RNA as their genetic material

Stage 3 – evolution of protocells  Membrane-first hypothesis  In water fatty acids arrange themselves into spheres  The hydrophobic tails are to the inside  Perhaps these micelles organized themselves into a bilayer

Stage 4 – evolution of a self- replicating system  RNA-first hypothesis  RNA, then RNA reverse transcriptase, then DNA  Today’s process in most cells goes:  DNA  RNA  protein

The first cells  Microfossils: the oldest fossil cells that are widely accepted are 2 billion years old  Stomatolites – rock columns of prokaryotic cells  Earliest cells were prokaryotes, heterotrophs, and anaerobes  Later cells developed that were photosynthetic autotrophs  Photosynthetically produced oxygen was necessary for aerobic respiration to evolve

Eukaryotic Cells  Endosymbiont theory :  Organelles such as mitochondria and chloroplasts arose from symbiotic relationships between 2 prokaryotic cells  Now these organelles are obligated to remain within the cells  These organelles are close in size to bacteria, contain their own DNA, divide independently from the cells they inhabit, have folded internal membranes, and some protein production abilities