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Chapter 17: The History of Life – Notes
The fossil record provides evidence about the history of life on Earth. It also shows how different groups of
organisms, including species, have changed over time (more than 99% of all species that have ever lived on
Earth have become extinct.
Interpreting fossil Evidence
Paleontologists are scientists who collect and study fossils.
Age of fossils is determined using two techniques:
1. Relative dating: Comparing its placement with fossils in other layers of rock.
Oldest layers on the bottom, recent layers on top.
2. Radioactive dating : Some elements found in rocks are radioactive.
Radioactive elements break down, into non radioactive elements at a steady rate.
A half life is the length of time required for half of the radioactive atoms in a sample to
decay.
The age of a fossil is calculated based on the amount of remaining radioactive isotopes it
contains.
Q. The half-life of Carbon-14 is 5,730 years. How much of the original C-14 in an organism would
remain in a fossil that is 11,460 years old? _____________
Q. The half-life of the fictitious radioactive element X is 5,420 years.
1. How old is a fossil that has 1/8th the original amount of the element.
_____________________________years old
2. If I start out with 30 grams of element X, how much will I have after 3 half-lives?
_____________________grams
3. If a fossil is found that has 5 grams of element X in it and I know that it originally had 40 grams in
it, how old is that fossil? ___________________________________ years old
Geologic Time scale
Geologic time begins with Precambrian Time after which the basic divisions of the geologic time scale are
eras and periods.
Q.Eras are divided into? __________
Q. What is the name of the earliest era in the geologic time?
________________.
Q. What are the periods of the Paleozoic era (from oldest to
youngest)? ___________, ___________, ___________,
___________, _______________, _______________.
Q. When did cretaceous period end? _____ million years ago
Q The dinosaurs were dominant during the ___________ Era.
Q. Which era and the period do we live in? _________ Era
and ___________ period.
Earth’s early History
Earth is about 4.6 billion years old. Earth's early atmosphere probably contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.
The First Organic Molecules:
In the 1950s, Miller and Urey Simulated Earth's Early
Atmosphere in a laboratory setting.
By passing electric sparks through a mixture of hydrogen,
methane, ammonia, and water they produced amino acids, which
are needed to make proteins,
It suggested how mixtures of the organic compounds necessary
for life could have arisen from simpler compounds present on a
primitive Earth.
Q. Why did they use a mixture of hydrogen, methane, ammonia,
and water? __________________________________________
Q. Why did they used sterilized closed system? To prevent
_____ from entering and to avoid __________________ by bact.
Q.what was the purpose of electric sparks? To stimulate _______
and to provide energy for the chemical reactions
Life's Origin
Formation of Microspheres - Large organic molecules can form tiny bubbles called proteinoid micropheres.
Microspheres are not cells, but they have some characteristics of living systems (selectively permeable
membranes, store and release energy)
Evolution of RNA and DNA – an unanswered question!
Under the right conditions, some RNA sequences can help DNA replicate.
Some RNA sequences process messenger RNA after transcription & catalyze chemical reactions.
Some RNA molecules can even grow and duplicate themselves—suggesting that RNA might have existed
before DNA.
Hypothesis - The small sequences of RNA could have formed and replicated on their own. From this relatively
simple RNA-based form of life, several steps could have led to the system of DNA-directed protein synthesis
that exists now.
Free Oxygen
Microfossil - Microscopic fossils of single-celled prokaryotic organisms. The life forms must have evolved in
the absence of oxygen .
Over time, fossil evidence indicates that photosynthetic bacteria became common in the shallow seas. These
organisms produced oxygen, as an end product of photosynthesis.
Hence concentrations of methane and hydrogen sulfide began to decrease, the ozone layer began to form.
The rise of oxygen in the atmosphere drove some life forms to extinction. Some life forms evolved new, more
efficient metabolic pathways that used oxygen for respiration others evolved as anaerobic organisms.
Origin of Eukaryotic Cells (with nuclei)
The prokaryotic cells began evolving internal cell membranes. The result was the ancestor of eukaryotic cells.
According to endosymbiotic theory some prokaryotes entered this ancestral eukaryote & began living inside the
larger cell. Over time, a symbiotic relationship evolved.
The prokaryotes that had an ability to use oxygen to generate energy-rich molecules of ATP, evolved into the
mitochondria. The prokaryotes that carried out photosynthesis evolved into the chloroplasts.
Sexual Reproduction and Multicellularity
Most prokaryotes reproduce asexually (no genetic variation except if there is mutation).
Some time after eukaryotic cells arose, those cells began to reproduce sexually (shuffles genes in each
generation) It increases the chances of evolution due to natural selection.
Patterns of evolution
Macroevolution - large-scale evolutionary patterns and processes that occur over long periods of time.
1. Mass extinction – huge numbers of species along with ecological systems disappear, disrupts energy
flow, and food webs collapse.
2. Adaptive radiation - single species or a small group of species evolved, through natural selection into
diverse forms that live in different ways.
e.g. Darwin’s Finches
e.g. The first dinosaurs and the earliest mammals evolved at the same time. Dinosaurs and ancient
reptiles, underwent an adaptive radiation first and “ruled” Earth for about 150 million years. During that
time, mammals remained small and relatively scarce. The disappearance of the dinosaurs cleared the
way for the great adaptive radiation of mammals.
3. Convergent evolution - unrelated organisms resemble one another.
The organisms undergo adaptive radiation in different places or at different times but in similar
ecological environments/climate. The natural selection molds different body structures and them look
similar & function in the same way.
e.g. similar shapes of sharks and dolphins (streamlined bodies with parts that work like paddles).
e.g. structures such as a dolphin's flukes and a fish's tail fin (look and function similarly but are made up
of parts that do not share a common evolutionary history, are called analogous structures).
4. Coevolution – two species evolve in response to changes in each other over time.
Organisms that are closely connected to one another by ecological interactions evolve together.
e.g. flowering plants can reproduce only if the shape, color, and odor of their flowers attract a specific
type of pollinator.
An evolutionary change in one organism may be followed by a corresponding change in another
organism.
e.g. relationships between plants and plant-eating insects. Some plants have evolved poisonous
compounds that prevent insects from feeding on them. Natural selection in herbivorous insects began to
favor any variants that could alter, inactivate, or eliminate those poisons.
5. Gradualism versus Punctuated equilibrium – According to Darwin, biological change is slow and
steady, known as gradualism.
According to Stephen Jay Gould, the long and stable periods interrupted by brief periods of more rapid
change is called as Punctuated equilibrium. Rapid evolution may occur when a small population
becomes isolated and evolve rapidly or when a small group of organisms migrates to a new
environment.
6. Changes in developmental genes – The “master control genes,” called hox genes, guide development
of major body structures in animals.
Changes in the expression of developmental genes may explain how evolution occurred.