Chapter 17 Section 2
Formation of Earth
-Earth is believed to have been created about 4.6
billion years ago
-pieces of cosmic debris attracted to one another
over the course of 100 million years
-collisions with this mass produced enough heat
to melt the globe
-once Earth melted, its elements arranged
themselves according to density
-least dense elements formed first atmosphere
-made up of hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen,
hydrogen sulfide and water
-no oxygen, so no life existed
-about 4 billion years ago elements eventually
cooled
-volcanic eruptions and meteor showers brought
material to form solid rocks
-oceans did not exist because it was still very hot
-about 3.8 billion years ago Earth’s surface
cooled enough for water to remain a liquid
-thunderstorms drenched Earth for 1000’s of
years to create oceans
First Organic Molecules
-Stanley Miller and Harold Urey simulated
conditions of early Earth in a lab setting
-had a mixture of the gases present in sealed
tubes and added spark to simulate lightning
-result was amino acids formed
-these amino acids were what gave rise to life
forms
Life’s Origin
-about 200-300 million years after Earth cooled
enough to carry liquid water, bacteria was
common
How?
-proteinoid microspheres were produced
-not cells, but have some characteristics of
living systems
-scientists believe over time they acquired
more and more characteristics of living cells
Evolution of DNA and RNA
-they cannot exist without each other
Which came first?
-scientists are not sure
-it is thought that RNA was the first info storage
molecule and created DNA
-DNA is the main info molecule today
-Questions still exist
Free Oxygen
microfossils (fossils of microscopic singlecelled prokaryotic organisms) have been found
in rocks that are more than 3.5 billion years old
-must have evolved in absence of oxygen
anaerobes- organisms that can live without
oxygen
-photosynthetic bacteria evolved next
-started churning out oxygen
-ozone layer formed
-to the first cells oxygen was deadly
-many anaerobes died
-many new species tat require oxygen (aerobes)
evolved
-this set the stage for evolution of modern life
Origin of Eukaryotes
-prokaryotes began evolving internal cell
membranes
-result was eukaryotic cells
endosymbiotic theory- eukaryotic cells formed
from a symbiosis among several different
prokaryotic organisms
Sexual Reproduction and Multicellularity
-prokaryotic cells reproduce asexually which
produces an identical clone
-eukaryotes reproduce sexually which allows for
more genetic diversity
-evolution can take place at a rapid rate