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