Ch. 26: Early Earth & The Origin of Life

1.Prokaryotes dominated life on earth from BYA. During this time, two evolutionary branches diverged: Bacteria and Archaea. I.The Early History of Life A. Solar System: 12 BYA B. Earth: 4.5 BYA C. First life: BYA Fossilized ancient Bacterium (3.5 BYO) 2.Stromatolites: fossilized mats that contain ancient prokaryotes.

2.Cyanobacteria (photosynthetic prokaryotes) evolved about 3.5 BYA.  Helped generate O 2 in the atmosphere.  O2 levels in the atmosphere gradually rose from 2.6 to 2.2 BYA. 3.O 2 is “corrosive,” reacting with many compounds and was probably harmful to many prokaryotes. 4.Eukaryotes evolved 2.1 BYA. 5.Multicellular eukaryotes evolved 1.2 BYA. (Fossilized algae about 1.2 BYO) a. “snowball Earth” – MYA: multicellular diversity was low

6.Animal diversity exploded during the early Cambrian period, about 543 MYA. 7.Plants, fungi, and animals colonized the land about 500 MYA. a.Adaptations:-Prevent dehydration -Reproduce on land b.Fungi colonized land alongside plants. Fungi and plants have a symbiotic relationship: help plants absorb water and nutrients from the soil. c.Plants created new opportunities for all life, including herbivorous (plant-eating) animals and their predators. d.Arthropods, reptiles, and amphibians are the most diverse and widespread of animals. e. Primates appeared MYA. f. Homo sapiens appeared 5 MYA.

II.The Origin of Life A.Spontaneous generation v. biogenesis 1.Spontaneous generation: life emerges from inanimate material. 2.Biogenesis: “life from life” 3.Louis Pasteur’s famous experiment in 1862 proved biogenesis.

a.Pasteur sterilized beef broth b.Flasks left opened spoiled and micro- organisms grew. c.Flasks sealed did not spoil and no microorganisms grew. (critics argued no “life force.”) d.Flasks with swan necks trap dust and microorganisms from reaching the broth.

B.Biogenesis does not exist today but the evolution of life started out with the evolution of chemicals in four stages (all of this was allowed by the earth’s primordial conditions – energy sources from lightning, UV radiation, and volcanic activity and no oxygen in the atmosphere) : 1.The abiotic synthesis of small organic molecules 2.The synthesis of polymers 3.Origin of self-replicating molecules 4.Packaging of all these molecules into “protobionts”

C.In the 1920’s A.I. Oparin and J.B.S. Haldane independently hypothesized that organic molecules could be synthesized from inorganic molecules present in the early atmoshpere and seas. 1. Oxygen could not be present in the primitive atmosphere as oxygen attacks chemical bonds. 2. Energy from lightning and intense UV radiation provided energy for the synthesis of organic molecules. D.In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis. 1.They created an apparatus that mimicked the Earth’s early conditions. 2.Their “atmosphere” contained gases H 2 O, H 2, CH 4, and NH 3 (we now know that the gases that probably existed at that time were CO 2, CO, N 2, and H 2 O.

E.RNA was probably the first genetic material. 1.Because RNA can also function as an enzymes (Ch. 17), it helps resolve the paradox of which came first, genes or enzymes. 2.RNA polymers can be made in the laboratory: 3.RNA is “autocatalytic”: a. Discovered by Thomas Cech in the 1980’s b. RNA catalysts called ribozymes remove introns c. Ribozymes also catalyze the synthesis of new rRNA, tRNA, and mRNA.

F.Probionts are predecessors of prokaryotic cells. 1.They are aggregates of abiotically produced molecules. 2.They can form by self-assembly. 3.They cannot reproduce. 4.They can maintain an internal chemical environment. (Simple “Reproduction”: Liposome - lipids resemble cell membrane) (Simple metabolism)

G.Debate about the origin of life: 1.Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth. 2.Where did organic molecules or life come from? 3.Where on Earth did life first evolve?

III. The Major Lineages of Life