Chapter 14

The History of Life

What was early Earth like? It likely formed 4.6 billion years ago from gravity

pulling in debris in space. Early Earth was likely very hot on the outside and

inside. There were probably many volcanoes. The early atmosphere probably contained no free

oxygen, but did contain water vapor, ammonia (NH3), methane (CH4), & hydrogen.

Many scientists believe that life began in the oceans 3.4-3.9 billion years ago.

How can we tell what Earth was like? There is no direct evidence of Earth’s earliest

years because rocks are constantly destroyed and formed, but there are other sources of information.

Fossils keep records of organisms that lived in the past.

By studying rocks & fossils, much can be told about Earth’s history.

How can we tell how old fossils are? There are 2 methods:

Relative dating: Determining the order in which events occurred, not the actual age Assumption: If undisturbed, the oldest layers are on the

bottom, youngest ones are on top, so the oldest organisms would be found the farthest down in the layers.

Radiometric dating: Determining the actual age of rocks by measuring the radioactive isotopes contained in the rocks. Scientists compare the amount of radioactive isotope to

the stable isotope to determine how old it is.

The Geologic Time Scale By using the information from relative and

radioactive dating of fossils and rocks, scientists have put together a timeline of Earth’s history, called the Geologic Time Scale.

Before we learn specifics of the Geologic Time Scale, you will create your own timeline.

Directions for Personal Timeline Create a timeline of the events of your life. Some things

to include are births, deaths, marriages, graduations, trips, moves, significant first, and/or religious events. Every event should have a direct relationship to you. You may include symbols, pictures, and/or illustrations.

Decide on a scale for your timeline, for example 1 inch : 2 years.

Label your timeline as “__________’s Personal Timeline.

Example Timeline

The History of LifeChapter 14.2

Spontaneous Generation What does it mean for something to be

spontaneous? It used to be thought that living things could

arise from nonliving things. This idea was called spontaneous

generation.

Experiments disproving spontaneous generation Redi—1668

He used a cover on one flask, mesh on another, and no cover on the last one.

He found that maggots didn’t appear on the meat that was covered and free of flies.

Therefore, S.G. was not true.

Experiments disproving spontaneous generation Spallanzani—1700s

Microorganisms were able to be seen with a microscope, were thought to arise from a “vital force” in the air.

He boiled two flasks of broth, one was left open, one was sealed.

The open flask became filled with microorganisms, the sealed one remained clear.

The debate? Broth was exposed to air.

Experiments disproving spontaneous generation Pasteur—mid 1800s

Boiled broth in curved neck flask to allow air to reach broth but not microorganisms

Broth only became cloudy when neck was broken off.

New Idea These experiments proved spontaneous

generation incorrect. The idea that living things come from other

living things is called biogenesis.

Earth’s History Evidence from computer models of the sun

shows that the early solar system was a swirling cloud of gas and dust about 5 billion years ago.

Most of this material collapsed to become the sun.

Earth formed when large pieces of debris in the early solar system collided over a period of 400 million years.

Earth’s History The estimated age of Earth is 4.6 billion

years, which was established using radiometric dating.

The time it takes for half of a sample on an isotope to decay is its half-life.

The age of a material can be determined by measuring the amount of a particular radioactive isotope that it contains.

Before life began, two things had to be true: 1) Simple organic molecules must have

formed. 2) These molecules must have become

organized into complex organic molecules (proteins, carbs, etc.)

Where did the Organic Compounds come from? It’s thought that all of the elements found in

organic compounds were present on Earth and in the solar system when Earth formed.

How did these organic compounds come to be?

Alexander Oparin developed a hypothesis called the primordial soup hypothesis.

Oparin’s Hypothesis He proposed that the early atmosphere

contained ammonia (NH3), hydrogen gas (H2), water vapor, and methane (CH4).

He thought that once the Earth cooled and the organic compounds collected in lakes & seas, energy supplied by lightning and ultraviolet (UV) radiation started complex chemical reactions that led to the formation of proteins and other organic compounds, the precursors to life.

Testing Oparin’s Hypothesis Oparin didn’t test his

hypothesis, but scientists Miller & Urey did.

They were able to produce organic compounds including amino acids.

From amino acids to… These amino acids, when bonded to clay,

may have formed the first proteins.

It is also thought that RNA was the first system for protein production In other words, RNA was the first genetic

material.

How did the first cells possibly form? Sidney Fox produced protocells from solutions of

amino acids by heating them. life-like properties: take up substances from the

surroundings, growth, surrounded by a membrane. Non-life-like-properties: no hereditary characteristics, can’t

respond to natural selection

The Evolution of Cells The first forms of life may have been

prokaryotic forms that evolved from a protocell. little or no oxygen gas in existence: anaerobic cells. They likely took in organic compounds from their

environment for food (heterotrophs) Chemosynthesis These were probably like present-day archaebacteria (or

archaea) that live in harsh environments Since these molecules probably became scarce, it

was necessary for autotrophs to evolve.

The Evolution of Cells Eventually, photosynthesizing prokaryotes

evolved. Increasing the amount of oxygen in the

atmosphere. (cyanobacteria) Lead to aerobic cells (with oxygen).

Finally, complex eukaryotic cells were probably able to evolve due to protection from ultraviolet radiation by the ozone layer.

How did eukaryotic cells come about? VIDEO Lynn Margulis (1966) has proposed that

eukaryotes arose from prokaryotes in endosymbiosis:: 1. A small aerobic prokaryote entered a large

anaerobic prokaryote and began to live and reproduce inside it.

2. Eventually this small prokaryote became what is now the mitochondria (sites of aerobic respiration). This may explain why mitochondria have their own DNA.

How did eukaryotic cells come about? 3. Sometime later, a small photosynthetic

prokaryote entered the large prokaryote 4. This photosynthetic prokaryote may have

given rise to the chloroplasts, the sites of photosynthesis. Chloroplasts also have their own DNA (circular,

like in prokaryotes)

What else shows that mitochondria and chloroplasts may have been their own organisms? They have their own ribosomes. They reproduce (by fission) independently of

the cells that they’re inside.Last point, “The evolution of life is better

understood than how the first life appeared. Fossil, geologic, and biochemical evidence support many of the proposed steps in life’s subsequent evolution.”