Name: _________________________________________________________ Period: ___
The Age of the Earth, its environment, and the fossil record
The College Board wants you to know that, taken together, the evidence below provides a
plausible range of dates when the origin of life could have occurred.
(1) The Earth formed 4.6 billion years old (bya).
So far, scientists have not found a way to determine the exact age of the Earth directly from
Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate
tectonics. If there are any of Earth's primordial rocks left in their original state, they have not yet been
found. Nevertheless, scientists have been able to determine the probable age of the Solar System and
to calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in the
Solar System formed at the same time and are, therefore, of the same age.
The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived
radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with halflives of 700 million to more than 100 billion years to stable isotopes of other elements. These dating
techniques, which are firmly grounded in physics and are known collectively as radiometric dating,
are used to measure the last time that the rock being dated was either melted or disturbed sufficiently
to rehomogenize its radioactive elements.
In Western Australia, single zircon crystals found in younger sedimentary rocks have
radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be
found on Earth so far. The source rocks for these zircon crystals have not yet been found. The ages
measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.3 billion years in
age but do not reveal the exact age of Earth's formation.
From USGS Age of the Earth: http://web.archive.org/web/20051223072700/http://pubs.usgs.gov/gip/geotime/age.html
(2) The environment on Earth was too hostile for life until 3.9 bya.
Assuming that the gases we presently observe were also released by early volcanoes, the
atmosphere would be made of water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2),
hydrochloric acid (HCl), methane (CH4), ammonia (NH3), nitrogen (N2), & sulfur gases. The
atmosphere was reducing (no free oxygen). The present-day atmosphere is quite different: 78%
nitrogen, 21% oxygen, 1% argon, 0.03% carbon dioxide plus small amounts of water vapor
Early in Earth history, water vapor formed clouds, rain, and ultimately all of the surface water
(oceans, ground water, lakes, rivers, glaciers). The presence of the ancient oceans and lakes is
recorded by different types of sedimentary rocks.
Perhaps the best geologic evidence for the composition of the early atmosphere is the
presence and abundance of Banded Iron Formations. These rocks are made of layers of sulfide
minerals (evidence for a reducing environment) and chert or fine-grained quartz. These rocks are not
present in rocks younger than 1.8 - 2.5 billions of years ago, when oxygen starting becoming more
abundant.
FromVolcanoWorld, Oregon State: http://web.archive.org/web/20090314114237/http://volcano.oregonstate.edu/education/gases/origin.html
Name: _________________________________________________________ Period: ___
(3) The earliest fossil evidence for life dates back to 3.5 bya.
The oldest well dated and
well-preserved microsfossils
were described by J.W.
Schopf from the Apex Chert
at Marble Bar, Western
Australia. These are dated at
3.465 Ga. Microfossils from
Swaziland (South Africa)
have a similar age.
From NASA: http://web.archive.org/web/
20110720073327/
http://astrobiology.ucla.edu/
pages/res3b.html
Image from Schopf, Science (1993),
260:640-646.
Name: _________________________________________________________ Period: ___
Prebiotic Soup -- Revisiting the Miller Experiment
The College Board wants you to know that:
1. Primitive Earth provided inorganic precursors from which organic molecules could have been
synthesized due to the presence of available free energy and the absence of a significant quantity
of oxygen.
2. In turn, these molecules served as monomers or building blocks for the formation of more
complex molecules, including amino acids and nucleotides.
3. The joining of these monomers produced polymers with the ability to replicate, store and
transfer information.
4. These complex reaction sets could have occurred in solution (organic soup model) or as
reactions on solid reactive surfaces.
See http://astrobiology.berkeley.edu/PDFs_articles/Bada_Science2003.pdf and answer the following.
1) The Miller experiment -- from the first paragraph of this review article from Science, what did
Miller do and what were the results? Bullets OK for all of these!
2) Why did, at the time a “natural” origin of life was being discussed, the “assumption of an a
abiotic origin of organic compounds [rest] on firm ground?” Write a quick bullet point about two
major research breakthroughs.
3) How did Miller set up his experiment, with Urey’s help, to simulate primitive Earth?
4) Check out all the drama about publishing a paper. First two columns on the back are for
entertainment…but you should skip to the part in the second column that says “Miller’s paper
was published only a few weeks after Watson and Crick reported their DNA double-helix model
in Nature” and skim from there.