History of Life on Earth• Chemical Evolution (prebiotic evolution) – most

biologists believe that life developed from nonliving matter

• Alexander Oparin (Russian) and John B. S. Haldane (England) were the first scientists (independently) to advance the idea that simple organic molecules could form spontaneously from more simple raw materials (1920’s)

• they noted that the oxygen-rich atmosphere of today would not have permitted the spontaneous formation of organic molecules

• they speculated that the Earth’s early atmosphere was very low in oxygen and rich in hydrogen in the form of hydrogen gas (H2), methane (CH4), and ammonia (NH3) – also contained carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), and nitrogen (N2)

Conditions on primordial Earth • Earth is about 4.6 billion years old

• Earth was very hot when first formed

four requirements must have existed for chemical evolution:

1. little or no O2 – Earth’s early atmosphere was probably strongly reducing which would cause any free oxygen to react and form oxides and be removed from the atmosphere

2. a source of energy – early Earth was a place of high energy

• violent thunderstorms with torrential rainfall• widespread volcanic activity• bombardment from meteorites (caused cataclysmic

changes in crust, ocean, and atmosphere)• intense radiation (including UV radiation, since there

was no ozone layer and younger suns emit more UV light)

3. presence of chemical building blocks – water, dissolved inorganic minerals (present as ions), and the gases present in the early atmosphere

4. time for molecules to accumulate and react with one another – Earth is approximately 4.6 billion years old, the earliest traces of life are approx 3.8 billion years old

Oparin and Haldane’s hypothesis is tested by Stanley Miller and Harold Urey in the 1950’s

• they designed a closed apparatus that simulated conditions that presumably existed on early Earth

• they exposed an atmosphere rich in H2, CH4, H2O, and NH3 to an electrical discharge to simulated lightening

• analysis of the chemicals produced in a week revealed that amino acids and other organic molecules had formed

• recent evidence indicates that organic polymers may have formed and accumulated on rock or clay surfaces (rather than in a “primordial soup” in the sea)

• clay consists of microscopic particles of weathered rock and may have acted as a site for early polymerizations because it binds organic monomers and contains zinc and iron ions that might have served as catalysts

• lab experiments using clay have confirmed that organic polymers form spontaneously from monomers on hot rock or clay surfaces

• Protobionts – scientists have been able to synthesize several different protobionts (assemblages of abiotically produced organic polymers)

• exhibit many characteristics of living cells – division after growth, maintaining an internal environment different from the external fluids

Microspheres• protobionts formed by adding water to polypeptides• microspheres show an electrical potential, may

absorb materials from the surrounding environment• microspheres may give clues as to the evolution of

the cell membrane • membranes are made of phospholipid bilayers with

proteins • scientists have heated amino acids without water

and produced long protein chains – when water is added, stable microspheres (coacervates) are formed

• microspheres can accumulate compounds inside them and become more concentrated than outside, they also attracted lipids and formed a lipid-protein bilayer around them

Protobionts

Microsphere Liposome

The first cells probably assembled from organic molecules

• Cells were evident in microfossils 3.5 billion years old, perhaps even 3.8 billion years ago

• The first cells were prokaryotic• Stromatolites offer more fossil evidence –

rocklike columns composed of many minute layers of prokaryotic cells (usually cyanobacteria)

• living stromatolite reefs are still found in hot springs and in warm, shallow pools of fresh and salt water

Fossilized Stromatolites – 3.5 billion years old

Modern day stromatolites

• A crucial step in the origin of cells was molecular reproduction

• both DNA and RNA can form spontaneously on clay, so… which came first?

• RNA is self-catalytic and is believed to have appeared first (according to the proposed model of the “RNA World”)

• chemistry of prebiotic Earth gave rise to self-replicating RNA that functioned both as enzyme and substrates for their own replication

• RNA has catalytic properties – enzymatic RNAs are called ribozymes (in modern cells, ribozymes help catalyze the synthesis of RNA and process precursors into rRNA, tRNA, and mRNA)

• ribozymes may have catalyzed the synthesis of RNA, and processed RNA molecules

• RNA could also catalyze protein formation (catalyzes peptide bonds formation) – protein catalysis of RNA formation happen later

• DNA probably evolved after RNA – it’s a more stable molecule

• may have evolved from RNA making double stranded copies of itself

• stability of DNA provides advantages as the information storage molecule

• The first cells were probably heterotrophs

• fermented organic molecules from the aqueous environment – appeared 3.1 – 3.4 billion years ago

• first cells were anaerobes, free O2 not available

• as concentration of free organic molecules in environment declined, photosynthetic organisms had a selective advantage

• first photosynthetic organism were autotrophs which split H2S as a hydrogen donor (purple and green sulfur bacteria)

• the first photosynthetic organisms to use H2O as a hydrogen donor were the cyanobacteria (released O2 as by-product)

• source of the first free oxygen in aquatic environment and atmosphere – O2 existed in significant quantities by 2 billion years ago

• Aerobes appeared after oxygen increased in atmosphere

• aerobic respiration was “added” to glycolysis after free O2 became available

• aerobic organisms are much more efficient in converting glucose to ATP

• carbon dioxide produced helped to stabilize concentration of CO2 and O2 in atmosphere (by-product of each process – photosynthesis and aerobic respiration – are raw materials for other process)

• O3 begins to accumulate in upper atmosphere to form ozone (protection from UV radiation) – allows organisms to live in more shallow water and ultimately on land

Evolution of Eukaryotic cells • evolved from prokaryotes about 2 billion years

ago• Endosymbiont Theory – first proposed by Lynn

Margulis – suggests that mitochondria were originally independent prokaryotic aerobic organisms which developed a symbiotic relationship with another prokaryote

• aerobic prokaryote was engulfed by endocytosis but not digested

• aerobic prokaryote continued to function and formed a symbiotic relationship with host

• similar process occurred later with the host cell and photosynthetic prokaryotes (which became chloroplasts)

other evidence:• mitochondria and chloroplasts grow and divide

like cells• they have a naked loop of DNA like prokaryotes• they synthesize some of their own proteins using

70s ribosomes, like prokaryotes• they have double membrane as expected since

cells were taken into a vesicle by endocytosis• cristae are similar to mesosomes of prokaryotes• thylakoids are similar to structures containing

chlorophyll in photosynthetic prokaryotes