Chapter 10: The Origin and Diversification of Life on Earth (Part I)

Understanding biodiversity

Learning ObjectivesBe able to describe how: Life on earth most likely originated from nonliving

materials. Species are the basic units of biodiversity. Evolutionary trees help us conceptualize and categorize

biodiversity. An overview of the diversity of life on earth.

Phase 1: The Formation of Small Molecules Containing Carbon and Hydrogen

10.1 Complex organic molecules arise in non-living environments.

The Urey-Miller Experiments: The first demonstration that complex organic molecules could have arisen in earth’s early environment

Take-home message 10.1 Under conditions similar to those on early earth,

small organic molecules form which have some chemical properties of life.

10.2 Cells and self-replicating systems evolved together to create the first life.

Life on earth most likely originated from nonliving materials. Enzymes Required

Phase 2: The formation of self-replicating, information-containing molecules.

RNA appears on the scene. RNA can catalyze reactions necessary for replication.

The “RNA World” Hypothesis a self-replicating system a precursor to cellular life?!! RNA-based life and DNA-based life

What Is Life? Self-replicating molecules? How do we define life?!

Life Is Defined by Two Characteristics1) the ability to replicate

2) the ability to carry out some sort of metabolism

Phase 3: The Development of a Membrane, Enabling Metabolism, and Creating the First Cells

Membranes make numerous aspects of metabolism possible.

How Did the First Cells Appear? Spontaneously? Mixtures of phospholipids Microspheres Compartmentalization

within cells

Figure 10-4 Are microspheres a key stage in the origin of life?

Take-home message 10.2

The earliest life on earth appeared about 3.5 billion years ago, not long after earth was formed.

Self-replicating molecules—possibly RNA—may have formed in earth’s early environment and later acquired or developed membranes

Membranes enabled these self-replicating molecules to replicate and make metabolism possible, the two conditions that define life.

10.3 What is a species?

Biological Species ConceptSpecies are natural populations of organisms that:•interbreed with each other or could possibly interbreed

•cannot interbreed with organisms outside their own group (reproductive isolation)

Species: different kinds of organisms

Two Key Features of the Biological Species Concept:

1) actually interbreeding or could possibly interbreed

2) “natural” populations

Barriers to Reproduction1) Prezygotic barriers2) Postzygotic barriers

Prezygotic Barriers Make it impossible for individuals to mate with

each other or Make it impossible for the male’s reproductive

cell to fertilize the female’s reproductive cell

These barriers include:

Courtship rituals Physical differences Physical or biochemical factors

involving gametes

Postzygotic Barriers Occur after fertilization Generally prevent the production of fertile

offspring Hybrids Take-home message 10.3

Species are generally defined as:1) populations of individuals that interbreed with each

other or could possibly interbreed.

2) Species cannot interbreed with organisms outside their own group.

This concept can be applied easily to most plants and animals, but for many other organisms it cannot be applied.

10.4 How do we name species?

We need an organizational system!

Carolus Linnaeus and Systema Naturae

A scientific name consists of two parts:1) genus2) specific epithet

Hierarchical System

Inclusive categories at the top…

…leading to more and more exclusive categories below.

Take-home message 10.4 Each species on earth is given a unique name,

using a hierarchical system of classification. Every species on earth falls into one of three

domains.

10.5 Species are not always easily defined.

Difficulties in Classifying Asexual Species Doesn’t involve fertilization or even two

individuals Does not involve any interbreeding Reproductive isolation is not meaningful Evidence for reproductive isolation???

It may not be possible to identify an exact point at which the change occurred.

Difficulties in Determining When One Species Has Changed into Another

Chihuahuas and Great Danes generally can’t mate.

Does that mean they are different species?

Difficulties in Classifying Hybridizing Species

Hybridization• the interbreeding of closely related species

Have postzygotic barriers evolved? Are hybrids fertile?

Difficulties in Classifying Ring SpeciesExample: insect-eating songbirds called greenish warblersUnable to live at the higher elevations of the Tibetan mountain rangeLive in a ring around the mountain rangeWarblers interbreed at southern end of ring.The population splits as the warblers move north along either side of mountain.When the two “side” populations meet at northern end of ring, they can’t interbreed.What happened?! Gradual variation in the warblers on each side of the mountain range

has accumulated… …the two populations that meet have become reproductively

incompatible… …no exact point at which one species stops and the other begins

Morphological Species Concept Focus on aspects of organisms other than

reproductive isolation as defining features Characterizes species based on physical

features such as body size and shape Can be used effectively to classify asexual

species

Take-home message 10.5

The biological species concept is useful when describing most plants and animals.

It falls short of representing a universal and definitive way of distinguishing many life forms.

Difficulties arise when trying to classify asexual species, fossil species, speciation events that have occurred over long periods of time, ring species, and hybridizing species.

In these cases, alternative approaches to defining species can be used.

10.14 All living organisms are divided into one of three groups.

The two-kingdom systemAnimal and plant

The five-kingdom system

Classification Systems

Monera, plant, animal, fungi, and protists

Classification Takes a Leap ForwardCarl Woese, an American biologist, and his colleagues Examined nucleotide sequencesTracking changes

Woese’s approach is not

perfect.

Are viruses alive?

Take-home message 10.14 All life on earth can be divided into three domains—

bacteria, archaea, and eukarya—which reflect their evolutionary relatedness to each other.

Plants and animals are just two of the four kingdoms in the eukarya domain, encompassing only a small fraction of the domain’s diversity.

10.15 The bacteria domain has tremendous biological diversity. Why is morning breath so stinky?

MAPPING THE ORAL MICROBIOME: An analysis of the taxonomic distribution of bacteria present in human saliva as classified by Kraken shows that the majority of the reads fell into one of three genera: Streptococcus (30%), Haemophilus (17%), and Prevotella (13%).WOOD AND SALZBERG, GENOME BIOL, doi:10.1186/gb-2014-15-3-r46, 2014

Bacteria Are a Monophyletic GroupAll bacteria have a few features in common: single-celled organisms with no nucleus or

organelles one or more circular molecules of DNA several methods of exchanging genetic information asexual organisms

Take-home message 10.15

The bacteria all share a common ancestor and have a few features in common:• All are prokaryotic, asexual, single-celled organisms with

no nucleus or organelles.• All have one or more circular molecules of DNA as their

genetic material.• All have several methods of exchanging genetic

information.• Bacteria have evolved a broad diversity of metabolic and

reproductive abilities relative to Eukarya.

10.16 The archaea domain includes many species living in extreme environments.

Several Physical Features Distinguish Archeans from the Bacteria Archaeans’ cell walls contain polysaccharides not

found in either bacteria or eukaryotes. Archeans have cell membranes, ribosomes, and some

enzymes similar to those found in eukaryotes.

Archaea, many of which are adapted to life in extreme environments, physically resemble bacteria but are more closely related to eukarya.

Because they thrive in many habitats that humans have not yet studied well, including the deepest seas and oceans, they may turn out to be much more common than currently believed.

Take-home message 10.16

10.17 The eukarya domain consists of four kingdoms.

Protists, Plants, Fungi, and Animals,

Take-home message 10.17 All living organisms that you can see with the naked eye are

eukarya, including all plants, animals, fungi, and protists. The eukarya are unique among the three domains in that

they have cells with organelles.