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Chapter 14: The History of Life

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Ch. 14.1 Fossil Evidence of Change

Fossils provide evidence of the change in organisms.

Earth's Early History

Land Environments

Scientists have concluded that Earth formed from molten elements about 4.6 billion years ago. Due to intense heat, life probably did not exist.


Due to gravitational field, Earth was able to maintain an atmosphere. Minerals in the oldest rocks suggest that the early atmosphere had little or no free oxygen.

Clues in rocks

The earliest clues about life on Earth date back to about 3.5 billion years ago.

The Fossil Record

Fossil- any preserved evidence of an organism. Possibly 99% of the species that ever lived are now extinct. Most organisms decompose before they can become a fossil. Only organisms that are buried rapidly in sediment are readily preserved; this occurs more frequently with organisms living in water.

Fossil Formation

Nearly all fossils are formed in sedimentary rock. Paleontologist- a scientist who studies fossils. Paleontologists use fossil evidence to infer the environment in which an organism lived.

When geologists began to study rock layers (strata), they noticed that layers of the same age tended to have the same kinds of fossils regardless of where they were found. This led to a relative age scale for rocks all over the world.

Dating Fossils

  1. Relative Dating- a method, based on the law of superposition, used to determine the age of rocks by comparing them with those in other layer.
  2. Law of Superposition- states that the younger layers of rock are deposited on top of older layers.
  3. Radiometric Dating- uses the decay of radioactive isotopes to measure the age of a rock.
    Half-life needs to be known, as well as the decay product. Half-life- the amount of time it takes for half of the original isotope to decay.

    Scientists calculate the ratio of the parent isotope to the daughter isotope. Uranium-238 decays to Lead-206. Radioactive isotopes are not found in sedimentary rock. Used for assigning relative dates to fossils. Carbon-14 used for organisms, good up to 60,000 yrs old.

The Geologic Time Scale

Geologic time scale- a record of Earth's history. The geologic time scale is divided into two segments: Precambrian time and Phanerozoic eon.

  • Eon- the longest unit of time in the geologic time scale (billions of years).
  • Era- consists of two or more periods (hundreds of millions of years).
  • Period- consists of two or more epochs (tens of millions of years).
  • Epochs- the smallest units of geologic time (several million years).


The first 4 billion years of Earth's history; nearly 90% of Earth's history. Time during which Earth formed and autotrophic prokaryotes, like cyanobacteria of stromatolites, flourished. Extensive glaciation marked the second half of the Precambrian and may have slowed evolution until the ice receded.

Paleozoic Era

Cambrian explosion occurred at the start of the Paleozoic. Cambrian explosion- a span of a few million years in which most major animal groups diversified. A mass extinction ended the Paleozoic era at the end of the Permian period.

Mass extinctions have occurred every several million years. Between 60-70% of the species alive went extinct in each of these events.

The Mesozoic Era

As life flourished and diversified a meteorite struck Earth about 65 million years ago. K-T boundary- layer of of sediment containing iridium that exists between the Cretaceous period and the Paleogene period. Iridium is rare on Earth but common in meteorites. Many scientists think this impact is related to the mass extinction at the end of the Mesozoic era.

Not all species were eliminated but scientists believe debris from the impact probably stayed in the atmosphere affecting global climate.

Scientists also think that the course of evolution in the Cenozoic era was shaped by geological changes.
Alfred Wegener presented the first evidence for continental drift in the 1920's which then became part of the theory of plate tectonics. Plate tectonics- the movement of several large plates that make up the surface of Earth.

The Cenozoic Era (recent life)

Mammals have becoe the dominant land animals. After the mass extinction at the end of the Mesozoic era mammals began to diversify into distinct groups. Humans appeared near the end of the geologic time scale, in the current Quaternary period.

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Ch. 14.2 The Origin of Life

Evidence indicates that a sequence of chemical events preceded the origin of life on Earth and that life has evolved continuously since that time.

Origins: Early Ideas

Spontaneous Generation- the idea that life arises from non-life.
One of the first recorded investigations of spontaneous generation came in 1668 by Francesco Redi. He hypothesized that flies, not meat, produced other flies. However, his experiments failed to convince everybody despite the increasing usage of microscopes. Some thought that microorganism must appear spontaneously even if flies didn't.

Theory of biogenisis- states that only living organisms can produce the living organism.
Louis Pasteur's experiment in the mid 1800's showed that biogenisis was true even for microorganisms. Using swan-necked flasks and nutrient broth Pasteur allowed only air to reach broth in one group, microorganisms and air could reach broth in the other group. Only the group in which microorganisms and air could reach broth resulted in the growth of microorganisms.

Origins: Modern Ideas

Most biologist agree that life originated through a series of chemical events in which complex organic molecules were generated from simpler ones.

Simple Organic Molecule Formation

In 1920's Oparin and Haldane suggested the primordial soup hypothesis. They thought certain gases in the atmosphere could react with the oceans of Earth to form organic molecules. UV light from Sun and electricity from lightning might have been the primary energy sources.

In 1953 Stanley Miller and Harold Urey showed that simple organic molecules could be made from inorganic compounds, as proposed by Oparin and Haldane. Miller and Urey: built a glass apparatus, filled the apparatus with water and gases they thought made up early atmosphere; water was boiled, electricity added. This created a variety of organic molecules, including amino acids.

More recent experiments have produced adenine from hydrogen cyanide. Further experiments have produced amino acids, sugars, and nucleotides.

Some scientists believe early organic reactions began in the hydrothermal volcanic vents of the deep seas where sulfur forms the base of the food chain. Others think that meteorites brought the first organic molecules to Earth.

Making Proteins

Amino acids alone are not sufficient for life. Life requires proteins, which are chains of amino acids. Early amino acids could bond but also separate just as quickly. Clay, which would have been a common sediment, could have provided a framework for the amino acids to bind.

Genetic Code

A coding system was also needed. RNA and DNA is that system in modern life. RNA is believed to be first coding system. Researchers have demonstrated that RNA systems are capable of evolution by natural selection. Some RNAs can behave like enzymes; called ribosymes, which could have carried out some early life processes.

Molecules to Cells

Another important step in the evolution of life was the formation of membranes. The connection between the various chemical events and the overall path from molecules to cells remains unresolved.

Cellular Evolution

Earliest fossilized microbes are 3.5 billion years old but chemical markings in rock go back to 3.8 billion years.

The First Cells

Many scientists think that modern prokaryotes called archaea are the closest relatives of Earth's first cell. These organisms live in harsh environments that may be similar to environments of early Earth.

Photosynthesizing Prokaryotes

Scientists think that oxygen was absent from Earth's earliest atmosphere until about 1.8 billion years ago. Any oxygen that appeared earlier likely bonded with free ions of iron as oxygen does today. Evidence of this is found in sedimentary formations. Scientists hypothesize that once the free iron was saturated, oxygen accumulated in the atmosphere.

Fossil evidence of cyanobacteria has been found in rocks as old as 3.5 billion years. These bacteria eventually produced enough oxygen to support formation of an ozone layer. Eukaryotic cells could then appear.

The Endosymbiont Theory

Eukaryotic cells appeared in the fossil record about 1.8 billion years ago. Mitochondria and chloroplasts have features similar to prokaryotes.

In 1966 Lynn Margulis proposed the endosymbiont theory. Endosymbiont theory- ancestors of eukaryotic cells lived in association with prokaryotic cells. Prokaryotes could have been undigested prey, or a parasite. The relationship was mutually beneficial so prokaryotic symbionts became organelles.

Evidence for the Endosymbiont Theory

  1. Like prokaryotes, mitochondria and chloroplast have their own DNA that is arranged in a circular pattern.
  2. Mitochondria and chloroplasts have ribosomes that are similar to prokaryotes.
  3. Mitochondria and chloroplast reproduce by binary fission, independent from the rest of the cell.

Scientists do not know the steps that led to early life but fossil, geologic, and biochemical evidence supports many of the proposed steps in life's subsequent evolution. Future discoveries might alter any or all of these steps.

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