Chapter 19/20

Section 19-3: Earth’s Early History

The Mysteries of Life’s Origins Earth formed as pieces of cosmic debris

collided Young planet struck by one or more huge

objects and melted Elements redistributed by density Millions of years of violent volcanic activity,

comets/asteroids hitting surface About 4.2 bya surface cooled enough for solid

rocks to form, water to condense, permanent oceans form

The Mysteries of Life’s Origins Early atmosphere had little to no oxygen

Mostly carbon dioxide, water vapor, nitrogen Smaller amounts of carbon monoxide, hydrogen

sulfide, hydrogen cyanide Sky pinkish-orange Oceans brown with dissolved iron

The First Organic Molecules In 1953, chemists

Stanley Miller and Harold Urey tried recreating conditions on early Earth to see if organic molecules could be assembled under these conditions

The First Organic Molecules After a week they had produced 21 amino

acids Showed how mixtures of organic compounds

necessary for life could have arisen Idea of atmospheric composition incorrect

Formation of Microspheres Geological evidence shows that about 200-300

mya after Earth cooled enough to carry liquid water cells similar to bacteria were common

Large organic molecules form bubbles called proteinoid microspheres under certain condition

They are not cells, but have some living characteristics – selectively permeable membranes, means of storing/releasing energy

Thought to have acquired characteristics of living cells about 3.8 bya

Evolution of RNA and DNA Central dogma The “RNA World” hypothesis about the origin

of life suggests RNA evolved before DNA Simple RNA-based system underwent several

changes to DNA-directed protein synthesis Experiments show how small RNA sequences

could have formed from simpler molecules Under certain conditions, RNA sequences help

DNA replicate, process mRNA after transcription, catalyze chemical reactions

Some can even grow/replicate on their own

Production of Free Oxygen Microfossils,of prokaryotes that resemble

bacteria have been found in rocks more than 3.5 billion years old

Evolved in the absence of oxygen Photosynthetic bacteria became common and

producing oxygen by 2.2 bya Oxygen combined with iron in the oceans,

producing iron oxide which sank to ocean floor and formed great bands of iron that are the source of most iron ore mined today

Oceans changed blue-green

Production of Free Oxygen Next oxygen started accumulating in the

atmosphere Ozone layer formed, skies turned blue Early atmosphere thought to be similar to

volcanic gases

Production of Free Oxygen First cells evolved in absence of oxygen Deadly poison , many cells went extinct Some evolved metabolic pathways to use the

oxygen (cellular respiration) or ways to protect themselves from it

Evolution of Eukaryotic Cells It is believed that about 2 bya some ancient

prokaryotes began evolving internal membranes – ancestors of eukaryotes

According to endosymbiotic theory, prokaryotic cells entered and began living inside those ancestral eukaryotes

Over time, they developed symbiotic relationships

Evolution of Eukaryotic Cells Microscopists saw that the membranes of

mitochondria and chloroplasts resembled the cell membranes of free-living prokaryotes

Two related hypotheses: Mitochondria evolved from endosymbiotic

prokaryotes that were able to use oxygen to generate energy-rich ATP molecules (now could use oxygen)

Chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize

Modern Evidence During the 1960s, Lynn Margulis of Boston

University noted that mitochondria and chloroplasts contain DNA similar to bacterial DNA

Also have ribosomes that resemble those of bacteria

Mitochondria and chloroplasts, like bacteria, reproduce by binary fission

Significance of Sexual Reproduction During asexual reproduction (prokaryotes) ,

genetic variation is restricted to mutations in DNA

When eukaryotes reproduce sexually, offspring receive genetic material from two parents

Meiosis and fertilization shuffle genes, generating genetic diversity.

Offspring of sexually reproducing organisms are never identical to parents or siblings

Increases the likelihood of a population’s adapting to new or changing environmental conditions

Multicellularity Multicellular organisms evolved a few hundred

million years after the evolution of sexual reproduction

Even greater diversity