Reece et al 9 Edition
The History of Life on Earth
I. Milestones in the History of Life on Earth (Table 25.1)
• The earth is believed to be 4.6 billion years old
• All life can be traced back to primeval prokaryotes that lived 3.5 to 3.9 billion years ago.
Prokaryotes dominated earth from 3.5 to 2 billion years ago.
Oldest fossils = Stromatolites 3.5 billion years
• O 2ccumulation in the atmosphere - 2.7 billion years ago
Most atmospheric O is2of biological origin
• Eukaryotic life – oldest fossils are 2.1 billion years old
• Multicellular life - 1.2 billion years ago (common ancestor predicted to have arisen 1.5
billion years ago)
• Explosion of animal diversity - start of Paleozoic era starting 542 million years ago
• Plants, fungi and animals colonize land 500 million years ago
II. The Origin of Life
• Current theory ▯ all life on earth now arises from preexisting life - biogenesis - life from life
• There is no support for spontaneous generation of life.
• Pasteur's famous swan-neck flask experiment (1860's) discredited the idea of spontaneous
generation of microbial life.
A. How did life first originate on Earth?
• The most widely accepted hypothesis states that the first organisms on Earth originated from
abiotic factors by chemical evolution.
• Conditions on Earth 4 billion years ago were much different.
• There was little atmospheric oxygen on early earth. The exact atmospheric
composition is unknown. The atmosphere likely contained water vapour, CO , meth2ne,
ammonia, hydrogen, H S, 2nd nitrogen and its oxides. It may or may not have been
reducing and there most likely were small regions (e.g., around volcanic vents) that were
definitely reducing (favourable for synthesis of organic molecules).As the earth cooled
the water vapour condensed into oceans and much of the hydrogen escaped into space.
Experiments attempting to mimic conditions on primitive earth supported the formation
of complex organic molecules (e.g., amino acids, nitrogenous bases). Reece et al 9 Edition
• Energy for this process likely came from lightning and intense UV radiation.
Primitive earth is believed to have experienced more intense lightning, volcanic activity,
meteorite bombardment and UV radiation than modern earth.
Hypothesis ▯ the first living organisms were products of chemical evolution that occurred in four
1. Abiotic synthesis and accumulation of small organic molecules (e.g., nitrogenous bases and
amino acids – i.e. monomers of nucleic acids and proteins, respectively)
• Oparin-Haldane hypothesis: Earth’s early atmosphere was a reducing environment in which
organics compounds could have formed from simple molecules. Energy for the synthesis
could have come from lightning and intense UV radiation.
• Miller and Urey, 1953 tested the Oparin-Haldane hypothesis (Fig 4.2) – these types of
experiments have produced many organic molecules [amino acids and nitrogenous bases (but
not nucleotides) of DNA and RNA]
• Meteorites may also have been a source of organic compounds
2. Joining of organic monomers into polymers (e.g., proteins and nucleic acids) –Abiotic
synthesis of macromolecules.
• Dripping solutions of amino acids on hot sand, clay or rock has created amino acid polymers.
• Abiotic synthesis of RNA nucleotides
3. Formation of protocells or protobionts (pre-life) - aggregates of abiotically produced
molecules surrounded by a membrane or membrane like structure. Protobionts are able to
maintain an internal environment different from their surroundings and exhibited some life
properties such as simpl