Download C14, History of Life, summary

Kristy A. Blankenship, Ph.D.
Boyd County High School Ashland, KY 41102
Modern Biology, Holt, Rinehart & Winston, © 2009 Chapter 14: History of Life, pp 279-290.
Biogenesis
- states all living things derive from other living things
- opposes common misconception of the 1700s, spontaneous generation, that claimed life could
arise from nonliving/ inanimate objects. Most believed this explained why raw meat gave rise to
maggots, fish/frogs/turtles arose from previously dried ponds, or mice appeared out of old rags.
- Scientists tried to answer this conflict using controlled experiments.
** Francesco Redi (1626-1697) observed the life cycle of flies and understood that in order for
maggots to appear on meat, adult flies first had to land on it. To demonstrate that flies gave rise
to maggots, and not merely the meat itself, Redi used controlled conditions of raw meat left in an
open jar vs. experimental conditions in which the jar containing the raw meat was covered with a
mesh cloth to prevent access by adult flies. In 1668 Redi demonstrated that flies were essential
for maggots to form and dealt the 1st significant blow to the theory of spontaneous generation.
**Microscopes revealed world of microorganisms. Many believed they arose spontaneously
from a “vital force” in the air.
{“Vitae” is Latin for “life”.}
**Lazzaro Spallanzani (1729-1799) believed in biogenesis and tried to show that
microorganisms did NOT arise spontaneously from the air. Spallanzani boiled broth in glass
flasks for both the experimental and the control groups to kill all residual microorganisms.
While flasks were still hot, he sealed the necks of flasks in the experimental group to prevent any
contamination from microorganisms in the air. Broth in the control flasks (open to the air)
became cloudy within days, but broth in the sealed flasks never supported microbial growth until
the necks of the flasks were broken to expose it to the air.
Supporters of spontaneous generation argued that Spallanzani boiled the broth so long that he
destroyed the “vital force” in the air inside the flasks, & this was the reason no growth occurred.
**The Paris Academy of Science in the mid-1800s offered a prize for conclusively settling the
Biogenesis vs. Spontaneous Generation dispute.
**Louis Pasteur (1822-1895) repeated Spallanzani’s experiment but modified the flask’s neck.
Instead of a normal straight neck, Pasteur used one with a curved neck that allowed air to access
the broth but prevented microorganisms from reaching it. Supporters of the “vital force” theory
could not refute his findings that no microorganisms could grow in this broth even after a year.
To prove that the broth itself COULD support growth, Pasteur broke off the curved neck and
showed microbial growth within 24 hrs. This won both the money and the argument.
Earth’s History
Early solar system: ~5 billion yrs ago a swirling mass of space gases, dust, & debris began
coming together by gravitational forces. Most of this formed the sun, and the rest revolved
around this young sun. Our planet formed within the next 4 x 108 yrs. from collisions &
compaction of debris. Scientists study Earth’s layers of sedimentary rock to determine geological
time. Nowadays, they also can use radiometric dating, a process that compares the % of a
radioactive isotope to the % of its stable, nonradioactive counterpart.
Using the known half-life of the isotope, which is itself a constant value, one can determine the #
of half-lives a particular isotope has gone through, and therefore, the age of that sample. A halflife is that amount of time in which half of the sample has decayed to a stable isotope. Different
isotopes have different half-lives. Some are very short, a few seconds, whereas others, like
Phosphorous-32 is a bit longer (14.7 days), Sulfur-35 (86.4 days), or Hydrogen-3 (12.32 yr).
Carbon-14 has a half-life of 5730 yrs, and therefore, can be used to radiometrically date human
Kristy A. Blankenship, Ph.D.
Boyd County High School Ashland, KY 41102
(Homo sapiens) remains. Older specimens such as dinosaurs that lived millions of years ago
could be radiometrically dated using Iodine-129 (17 million yrs), Plutonium-244 (82 million
yrs), Uranium-235 (704 million yrs). More ancient life would need to use Potassium-40 (1.25
billion yrs). To confirm the age of rocks formed as our planet coalesced, we would need to use
Uranium-238 (4.5 billion yrs) or some other isotope.
The figure above shows radioactive decay for all/ any radioisotope. Half-lives are added, so if
we calculate C-14’s decay, 1 ½-life = 5730 yr; 2 ½-lives = 5730 yr + 5730 yr = 11,460 yr; three
½-lives = 3 * 5730 yr = 17,190 yr, etc… Although the half-life of any given isotope is constant,
errors occur in sample handling, experimental procedures, and instrumentation. For this reason,
C-14 dating is only accurate to within 100 yrs.
First Organic Compounds
All the elements composing organic molecules (C, H, O, N) were present in the early Earth.
HOW these elements assembled into biomolecules, however, is unknown. Alexander Oparin
(1894-1980) & John Haldane (1892-1964) proposed that the early atmosphere contained NH3,
H2, H2O, & CH4. When the Earth cooled, these would deposit in liquid water. Time, heat, and
energy from lightning & UV radiation could promote chemical reactions that lead to the
formation of amino acids/ polypeptides/ proteins.
Stanley Miller (1930- ) & Harold Urey (1893-1981) developed the Miller-Urey apparatus to test
Oparin’s hypothesis. Steam interacted with a mixture of gases proposed to exist in our early
atmosphere which was subjected to electrical shocks (lightning simulation) and the resulting
organic molecules were collected as a condensate. They demonstrated these conditions could
produce various organic compounds, including amino acids, proteins, ATP, and nucleotides.
Although others propose the atmosphere could’ve contained CO2, H2O vapor, & N2 as the
carbon, nitrogen, & hydrogen [and even O2] source, they also agree that organic molecules
could’ve formed automatically in areas protected from the atmosphere. This protection was
necessary because CO2 & O2 interfered with organic compound synthesis.
Kristy A. Blankenship, Ph.D.
Boyd County High School Ashland, KY 41102
In 1970 organic compounds were also discovered in an uncontaminated meteorite,
opening up the possibility that space-formed organic compounds could’ve covered the surface of
the newly formed Earth.
The First Life-forms
Requirements for life: cellular organization, growth & development, metabolism, homeostasis,
respond to stimuli, reproduction, & adaptation/ evolution
DNA codes for proteins necessary for these characteristics/ functions. RNAs (mRNA, tRNA, &
rRNA) are the actual cellular molecules involved in protein synthesis. Some RNA can also
behave as enzymes (ribozymes) to catalyze biochemical reactions, including self-replication.
Thomas Cech (1947- ) proposed RNA could’ve been the initial genetic material for life on Earth.
One hypothesis is that RNAs competed for nucleotides & natural selection favored those
molecules that could acquire their monomers or those molecules that produced proteins to
respond faster/ better to environmental signals. Some propose that replicating RNA started to
evolve in microspheres or coacervates. If RNA could’ve directed the assembly of structures in
their coacervates, cells could arise.
***We do not know about the 1st cells or their actual origin. ***
Scientists propose the 1st cells were likely anaerobic (independent of O2), heterotrophic (fed on
others or nutrient broth of oceans), and prokaryotic (cells without membrane-bound organelles).
Modern archaeans thrive under extreme conditions that mirror early times on Earth. Many
archaeans are autotrophs, using chemosynthesis rather than photosynthesis to obtain nutrients
and are anaerobic. Chemosynthesis using CO2 as carbon source & oxidation of inorganic
compounds, such as sulfur, as their energy source to build food molecules. It is reasonable to
propose that early heterotrophs would eat up all the available freely made organic molecules and
then turn predatory. Newly evolved autotrophs would then have an evolutionary advantage.
Evidence to support the notion that early life was anaerobic, heterotrophic prokaryotes is that the
oldest fossils @ 3.5 billion yrs old resemble modern cyanobacteria, even including their
formation of colonies in layered structures called stromatolites. Cyanobacteria’s ancestors
probably were responsible for Earth’s early photosynthetic production of oxygen. It took an
estimated 1 billion yrs to create the O2-rich atmosphere we currently have. This oxygen rose to
the upper atmosphere where UV-radiation converted O2 into O3 (ozone) which became a blanket
to protect organisms from radiation damage to cells & genetic material. The formation of ozone
permitted life to advance from the seas onto land.
Because pure oxygen can be so damaging to cells, prokaryotes had to evolve some mechanism to
neutralize O2. Those that could bind O2 to another molecule could eventually evolve into
organisms capable of aerobic respiration, a process currently housed in mitochondria.
The theory of endosymbiosis proposes that primitive eukaryotes engulfed smaller prokaryotes
that had evolved mechanisms for autotrophy, such as photosynthesis, &/or mechanisms to
prevent oxidative damage, such as aerobic respiration. These smaller prokaryotes were protected
against predation and also against extreme fluctuations in their environment. The new
eukaryotes now had an inbuilt nutrient source, protection from O2, and incidentally, a way to
create and store surplus energy as ATP. These prokaryotes evolved into chloroplasts &
mitochondria. Several lines of evidence support this. (1) Both chloroplasts & mitochondria have
circular DNA like prokaryotes have. (2) Their DNA is unique & older than the eukaryotes’
nuclear DNA. (3) Both organelles replicate independently of cellular replication.