CHAPTER 16 The Origin and Evolution of Microbial Life: Prokaryotes and Protists. Modules 16.1 – 16.6. EARLY EARTH AND THE ORIGIN OF LIFE. 16.1 Life began on a young Earth. Planet Earth formed some 4.6 billion years ago - PowerPoint PPT Presentation
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• It is hypothesized that life may have developed from nonliving materials as early as 3.9 billion years ago.
• The earliest entities may have been aggregates of molecules which would have then evolved into aggregates of polymers with simple metabolism and self replication ability.
• How this occurred was hypothesized by the Russian biochemist, A.I. Oparin (1923), and the British geneticist, B.S. Haldane..
Figure 16.1C
= 500 million years ago
Earliest animals; diverse algae
Earliest multicellular eukaryotes?
Earliest eukaryotes
Accumulation of atmosphericO2 from photosyntheticcyanobacteria
• They hypothesized that small organic molecules must have appeared first
– This probably happened when inorganic chemicals were energized by lightning or UV radiation.
– The early atmosphere had no oxygen, a strong oxidizing agent that tends to disrupt chemical bonds by attracting electrons from them.
– Before the early prokaryotes added oxygen to the air, earth most likely had a reducing atmosphere, donating electrons, bringing simple molecules together to combine into more complex ones.
– Oparin and Haldane never tested their hypothesis , but in 1953, Stanley Miller and Harold Urey did at the University of Chicago.
The Present Controversy• Question: Was the early Earth atmosphere reducting
or oxidizing? In some books, it is said that reducting, in others the opposite. What is the latest scientists' opinion? Answer:
• The oxidizing or reducing state of the early atmosphere is not well understood. Fifty years ago it was thought to be reducing, with lots of hydrogen, methane and ammonia. Later studies tended toward mildly oxidizing, with carbon dioxide as the major gas. Recently both carbon dioxide and methane have been suggested as necessary to provide sufficient greenhouse effect to warm the Earth at a time when the Sun was about 30% dimmer than it is today. Thus the latest scientists' opinion is that the early atmosphere was probably neither strongly reducing nor strongly oxidizing,with carbon dioxide and nitrogen the main gases. This opinion may well change, however, as we learn more about the ancient environment of our planet.
• David MorrisonNAI Senior Scientist(NASA Astrobiology Institute) 30 May 2006
16.3 Talking About Science: Stanley Miller’s experiments showed that organic materials could have arisen on a lifeless earthhttp://www.accessexcellence.org/WN/NM/miller.html
• Surrounding membranes may have protected some of these polymers and macromolecules as they evolved rudimentary metabolism (protobiont).
• Stanley Fox (1912) did research on structures of this nature.
• These structures form spontaneously in the lab from solutions of organic molecules.– Microspheres composed of may protein molecules
are organized into a membrane like structure.– Coacervates are collections of droplets that are
composed of molecules of different types which may include: lipids, amino acids, sugars and nucleic acids.
Microspheres and coacervates have a number of lifelike qualities: take up substances from the surroundings, increase in size, form buds, fuse with similar structures.
This proves a membrane bound structure did not need genetic information to form. –The Debate is to the nature of the membrane sac!!! Protein or Lipid??????????????
The Evolution of prokaryotes• The early protobionts which relied on molecules
present in the primitive soup (primitive heterotrophs) were gradually replaced by organisms that could produce their own needed compounds (chemoautotrophs).
• The diversification of these autotrophs led to the emergence of true heterotrophs that relied on the autotrophs.
• These inhabitants (prokaryotes) were the earth’s sole inhabitants from 3.5 to about 2 billion years ago.
• They began to transform the atmosphere as atmospheric oxygen began to appear about 2.7 billion years ago as the result of prokaryotic photosynthetic autotrophs.