Life: Early Cells, Classification of Life

• evolution of cells
• earliest cells
• prokaryotic cells
• eukaryotic cells
• classification of life

Evolution of early cells

• Theories about evolution of cells
• evolution of cells
• early organic molecules assembled into functional, independent units
  • cells are "bags of fluid"
  • contents differed from environment outside "cell"
  • interior had a higher concentration of specific organic molecules
• how did "bags of fluid" evolve from simple organic molecules?
• bubbles
  • spherical, hollow structures
  • molecules with hydrophobic regions spontaneously form bubbles in water
  • structure shields hydrophobic regions from contact with water

The Earliest Cells

• Earliest evidence of life appears in microfossils

• dating from ~3.5 billion years ago

• Characteristics of earliest life forms
• small (1-2 nanometers)
• single-celled
• no external appendages
• little internal structure
• no nucleus
• resembled today’s bacteria
• in group called prokaryotes ("before nucleus")

• Bacteria
• divided into two groups
• archaebacteria
• eubacteria

Prokaryotic versus eukaryotic cells

• All life
• two types of cells
• prokaryotic cells
• eukaryotic cells

Prokaryotic cells are small and structurally simple

• Prokaryotic cells
• first appeared ~ 3.5 billion years ago
• "before nucleus"
• small, ~1/10th size of eukaryotic cells
• lack true, membrane-bound nucleus
• surrounded by plasma membrane
• lack true, membrane-bound organelles
• less complex than eukaryotic
• contain a simple DNA molecule

The First Eukaryotic Cells

• Eukaryotic cells
• first appeared ~ 1.5 billion years ago
• "true nucleus"
• larger than prokaryotic
• rapidly evolved to produce diverse life forms that inhabit earth today
• complex interiors

• complex interior organization
• extensive compartmentalization
• many membrane-bound organelles, internal membranes
• true, membrane-bound nucleus
• complex DNA molecule
• contain vesicles and vacuoles which function in storage and transport

The eukaryotic cell probably originated as a community of prokaryotes

• fossil record indicates
• eukaryotes evolved from prokaryotes ~1.5 BYA
• how did eukaryoites arise?
• theory: through a combination of 2 processes
• membrane infolding
• endosymbiosis

• membrane infolding
• of plasma membrane of ancestral prokaryotic cells
• gave rise to endomembrane system of eukaryotic cells
• endosymbiosis
• thought to have generated first
• mitochondira
  • heterotrophic prokaryote came to reside in ancestral prokaryote
• chloroplast
  • photosynthetic prokaryote came to reside in ancestral prokaryote

• Endosymbiont theory
• critical stage in evolution of eukaryotic cells involved symbiotic relationships with prokaryotic organisms (bacteria)
• heterotrophic bacteria engulfed by larger bacteria - evolved into mitochondria
• photosynthetic bacteria engulfed by larger bacteria - evolved into chloroplasts
• Support for the endosymbiont theory
• existence of symbiotic relationships
• presence of DNA in organelles
• many organelles have their own DNA
  • mitochondria
  • chloroplasts
• organelle DNA is similar to bacterial DNA in size and character

Figure- A model of the origin of eukaryotes

Classification of Life

• diversity of life can be arranged into three domains
• how we classify life
• therefore, organisms
• To bring order to diversity of life, a system of classification exists
• Taxonomy
• science of classifying and naming organisms

• earliest classification schemes
• only two broad groups recognized (kingdoms)
• as knowledge increased about significant differences among living organisms
• classification system was developed that recognized a taxonomic level higher than kingdom
• domain
• All life can be classified into one of
• three domains
  • Archaea
  • Bacteria
  • Eukarya

• Domain Archaea
• single-celled, "ancient" bacteria
• Domain Bacteria
• single-celled, "true" bacteria
• Domain Eukarya
• single-celled protists, paramecia, single- and multi-cellular algae
• fungi
• plants
• animals

• All organisms are grouped into a few major categories
• earliest classification systems recognized 2 kingdoms of life
• animal kingdom
• plant kingdom
• kingdoms were added over time
• new organisms were discovered
• understanding of relationships/differences grew

• How many kingdoms?
• all "life" currently classified into 6 kingdoms
• Archaebacteria
• Eubacteria
• Protista
• Fungi
• Plantae
• Animalia

Six Kingdoms Relative to 3 Domains:

• Domain Archaea
• Kingdom Archaebcateria
• Domain Bacteria
• Kingdom Eubacteria
• Domain Eukarya
• Kingdom Protista
• Kingdom Fungi
• Kingdom Plantae
• Kingdom Animalia

Six Kingdoms Relative to Prokaryotic Versus Eukaryotic Cells:

• Prokaryotic kingdoms
• Archaebacteria
• Eubacteria
• Eukaryotic kingdoms
• Protista
• Fungi
• Plantae
• Animalia

Figure - Three domains of life

Classification of Life

• binomial system
• early
• developed by Swedish biologist, Carl Linnaeus (1707-1778)
  • gave two-part (binomial) name to each species
  • names eventually came to be written in Latin
• current
• unique 2-part name for each organism
• first part designates genus
  • capitalized
  • underlined or italicized in print
• second part designates species
  • not capitalized
  • underlined or italicized in print

• Examples
• Homo sapiens or H. sapiens (human)
• Quercus alba or Q. alba (white oak)

Taxonomic Classification is Hierarchical

• taxonomic heirarchy
• over time, genera were grouped into large, more inclusive categories known as families
• grouping intended to reflect relationships between genera included
• taxonomic system extended to include several, more inclusive units
• Species
• grouped to form a genus
• Genera (plural of genus)
• grouped together to form a family
• Families
• grouped to form orders
• Orders
• grouped to form classes
• Classes
• grouped to form divisions or phyla
• Phyla or Divisions
• grouped into kingdoms

Classification of the Human Being

• Domain: Eukarya
• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Primates
• Family: Hominidae
• Genus: Homo
• Species: sapiens

Figure - Classifying life