Back to Biology 1402 pageEarly Earth - Origin of Life
Chapter 26
Theories on How Life Originated
Chemical evolution - abiotic synthesis of organic monomers - formation of complex polymers - aggregates of abiotically-produced molecules (protobionts) developed ability to maintain an environment different from surroundings - developed ability to reproduce - led to the origin of life
Panspermia - organic matter reached earth via comets and meteors - evolved into living material
Theories on Where Life Originated
Traditional view is that life originated in shallow water environment or moist sediments
More recently, evidence suggests that life may have originated on ocean bottom near areas of volcanic activity (sea vents)
Matter has not been resolved - both ideas supported by credible evidence
Origin of Hereditary Information
Very significant development - made reproduction possible and allowed natural selection to operate - made Darwinian evolution possible
RNA was apparently the first genetic material to appear - this has been challenged by some who suggest a simpler hereditary mechanism (not involving nucleic acids)
Major Events in History of Life
Origin of life - about 4 billion years ago
Appearance of prokaryotes - 2.5 billion years ago
Photosynthetic prokaryotes released oxygen - changed nature of atmosphere
Appearance of eukaryotes - 2 billion years ago
Appearance of multicellular organisms - 1 billion years ago
Colonization of land by plants and fungi - 400 million years ago
Comparison of Cell Types
Prokaryotic
Few organelles - no endomembrane system
Chromosome circular - occurs in "nucleoid" region (no nucleus)
No cytoskeleton
No mitosis, meiosis, or sex in life cycle
Other differences
Eukaryotic
Numerous organelles - extensive endomembrane system
Chromosomes linear - located in membrane-bound nucleus
Cytoskeleton present
Life cycles include mitosis, meiosis, sex
Three Major Lineages of Life
Bacteria - prokaryotes - decendents include modern bacteria
Archaea - prokaryotes - decendents include organisms known as extreme thermophiles, extreme halophiles & methanogens
Eukarya - eukaryotic - decendents include all eukaryotes
Classification Systems
Two Kingdom - Plantae and Animalia
Five Kingdom - proposed by R. H. Whittaker in 1969 - lumps prokaryotes into Kingdom Monera - many of the primitive eukaryotes into Kingdom Protista - separated Fungi from Plantae
Eight-Kingdom - separated Monera into Bacteria and Archaea - further subdivided Protista
Three-Domain - most recent - established Domain concept (Bacteria, Archaea & Eukarya)