Early Earth and the Origin of Life Chapter 26

Evolution of Life on Earth Life on earth originated between 3.5 and 4.0 billion years ago. Earth formed about 4.5 billion years ago. The oldest fossils of prokaryotes are 3.6 billion years old.

First 3/4 of evolutionary history- organisms were microscopic* *based on molecular clocks.

Domination of Prokaryotes Prokaryotes dominated evolutionary history from 3.5 to 2.0 bya. The two domains of prokaryotes, Bacteria and Archaea, diversified as a variety of metabolic types living near hydrothermal vents and in shallow water communities that left fossils called stomalites.

Clock Analogy for Key Events in Evolutionary History

Introduction of Oxygen Oxygen began accumulating in the atmosphere about 2.5 bya. Oxygenic photosynthesis evolved in cyanobacteria. As O2 accumulated in the atmosphere, the reactive molecule posed an environmental challenge for life. Some species survived in habitats that remained anaerobic. Among other survivors, a diversity of adaptations to the changing atmosphere evolved (cellular respiration).

Evolution of Eukaryotic Life Eukaryotic life began by 2.1 bya. The oldest fossils of eukaryotes date back 2.1 billion years. The eukaryotic cell evolved from a prokaryotic ancestor that hosted smaller internal prokaryotes. Endosymbiotic Theory

Evolution of Multicellular Life Multicellular eukaryotes evolved by 1.2 bya. There are fossils of multicellular algae dating back 1.2 billion years. The oldest fossils of animals are about 600 million years old.

The Cambrian Explosion Animal diversity exploded during the early Cambrian period. Most phyla of animals make their first fossil appearance during a relatively brief span from about 540-520 million years ago. Plants, fungi, and animals colonized land about 500 million years ago. A symbiotic relationship of plants with fungi contributed to the move onto land. Herbivorous animals and their predators followed.

Figure 26.8 The Cambrian radiation of animals

The Origin of Life The first cells may have originated by chemical evolution on a young Earth. Though life today arises by biogenesis, the very first cells may have been products of a prebiotic chemistry. This idea of life emerging from inanimate material is called spontaneous generation.

The Origin of Life Although there is NO evidence that spontaneous generation occurs today, conditions on the early Earth were very different. Relatively little atmospheric oxygen to tear apart complex molecules Energy sources such as lightening, volcanic activity, and UV light were more intense

Four-Stage Hypothesis for the Origin of Life According to one hypothetical scenario, the first organisms were products of chemical evolution in four stages: Abiotic synthesis of small organic molecules, such as amino acids and nucleotides. Joining of small molecules (monomers) into polymers, including proteins and nucleic acids. Origin of self-replicating molecules that eventually made inheritance possible Packaging of these molecules into “protobionts” droplets with membranes that maintained an internal chemistry different from the surroundings.

BIOCHEMICAL EVOLUTION 1) The Earth and its atmosphere formed Gasses present when the atmosphere was first formed included CO, CO2, H2, N2, H2O, S, HCl, HCN (hydrogen cyanide), but little or no O2. A.I. Oparin and J.B.S. Haldane independently theorized that simple molecules were able to form only because oxygen was absent. WHAM prevalent in atmosphere… (water, hydrogen, ammonia, methane) As a very reactive molecule, oxygen, had it been present, would have prevented the formation of organic molecules by supplanting most reactants in chemical reactions.

BIOCHEMICAL EVOLUTION 2) The primordial seas formed. As the earth cooled, gases condensed to produce primordial seas consisting of water and minerals (beginning of hydrologic cycle). 3) Complex molecules were synthesized. Chemicals present in the ancient seas: Acetic acid, formaldehyde, and amino acids. These kinds of molecules would later serve as monomers, or unit building blocks, for the synthesis of polymers.

How were the first organic molecules created? Energy catalyzed the formation of organic molecules from inorganic molecules. An organic “soup” formed. NO ENZYMES WERE NEEDED. Energy was provided mostly by ultraviolet light (UV), but also lightening, radioactivity, and heat- hydrothermal vents (hot volcanic outlets in the deep-sea floor).

Figure 26.10x Lightning

Abiotic Synthesis is Testable Laboratory experiments performed under conditions simulating those of the primitive Earth have produced diverse organic molecules from inorganic precursors.

Figure 26.9 Pasteur and biogenesis of microorganisms (Layer 3)

Stanley Miller and Harold Urey Using an airtight apparatus, CH4 (methane), NH3 (ammonia), H2O, H2 and a high voltage discharge, they found that after one week the water contained various organic molecules including amino acids. (WHAM! Water, hydrogen, ammonia, methane) The amino acids synthesized are the building blocks of proteins for organisms. Proteinoids are abiotically produced polypeptides. They can be experimentally produced by allowing amino acids to dehydrate on hot, dry substrates. Adenine and other nucleotides are the building blocks of RNA (also- Adenine for ATP).

Figure 26. 10 The Miller-Urey experiment http://bcs. whfreeman

RNA – First Genetic Material? The “RNA world” preceded today’s “DNA world”. RNA may have been the first genetic material. The first genes may have been abiotically produced RNA, whose base sequences served as templates for both alignment of amino acids in polypeptide synthesis and alignment of complementary nucleotide bases in a primitive form of self-replication.

Figure 26.11 Abiotic replication of RNA

The First Heterotrophs Prokaryotic Heterotrophs feeding on organic molecules in the seas began to develop metabolism. The first form of metabolism (fermentation) using glycolysis most likely arose because the atmosphere lacked free oxygen: anaerobic

Autotrophic Evolution The first autotrophs were probably nonoxygenic photosynthesizers. They did not split water and liberate oxygen (cyclic only) The first organisms to use noncyclic photosynthesis or oxygenic photosynthesis (water-splitting enzyme) were probably cyanobacteria (blue-green algae)

Creating the Ozone A byproduct of oxygenic photosynthesis was oxygen and as it accumulated in the atmosphere (2.7-2.2 billion years ago), First dissolved into the surrounding water until the seas and lakes became saturated with oxygen. Additional oxygen would then react with dissolved iron and precipitate as iron oxide. Then additional oxygen finally began to “gas out” of the seas etc. and accumulate in the atmosphere. The ozone layer was created. As the ozone absorbed UV rays, the major source of energy for abiotic synthesis of organic molecules and primitive cells was terminated.

Effect of Oxygen on Earth The oxygen had a tremendous impact on Earth: Corrosive O2 attacks chemical bonds, doomed many prokaryotes. Some survived in anaerobic environments (obligate anaerobe survivors) Others adapted- cellular respiration.

The First Eukaryotes http://highered. mcgraw-hill Evolution of Eukaryotic organelles from prokaryotes occurred about 2.1 billion years ago. Mitochondria and Chloroplasts are descendents of “endosymbionts”- symbiotic cells living within larger host cells. Many eukaryotes may have evolved from prokaryotes enjoying a mutually beneficial relationship (symbiosis). Endosymbiotic theory- Margulis.

Endosymbiosis Theory (Lynn Margulis, 1970’s) Model for origin of eukaryotes (A) endomembrane system of eukaryotes may have evolved from specialized infoldings of plasma membrane of ancestral prokaryotes (B) chloroplasts are descendants of photosynthetic prokaryotes, probably cyanobacteria - proposed ancestors of mitochondria were endosymbiotic bacteria that were aerobic heterotrophs a. may have first gained entry into larger cells as undigested prey or internal parasites Be a good skeptic! Where’s the evidence?

Evidence for Endosymbiosis Mitochondria and chloroplasts resemble bacteria and cyanobacteria with respect to their DNA, RNA, and protein synthesis machinery. Mitochondria and chloroplasts reproduce independently of their eukaryotic host cell. Ribosomes of mitochondria and chloroplasts reproduce independently of their eukaryotic host cell. The thylakoid membranes of chloroplasts resemble the photosynthetic membranes of cyanobacteria.

Timeline of Classification 1. 384 – 322 B.C. Aristotle 2 Kingdom Broad Classification – Plants or Animals 2. 1735 - Carl Linnaeus 2 Kingdom Multi-Divisional Classification (Kingdom, Phylum, Class, Order, Family Genus, Species) 3. Evolutionary Classification – (After Darwin) Group By lines of Evolutionary Descent 4. Five Kingdom System – 1950s (Whittaker) – 1950s – Plantae, Fungi, Animalia, Protista, Monera 6. Three Domain System – late 1990s late 1990s – Bacteria, Archaea, Eukarya 31

Linnaeus System Evolves from TWO Kingdoms to FIVE As we learned more about different kinds of life, there needed to be more Kingdoms 1800’s – Added Kingdom Protista Amoeba, Slime Molds 1950’s – Added Fungi and Monera Fungi distinguished from Plants Prokaryotes (no nucleus) bacteria given category 1970’s – Split Kingdom Monera into 2 separate Kingdoms Eubacteria – bacteria with peptidoglycan Archaebacteria – bacteria without peptidoglycan 32

The Five-Kingdom System Reflected increased knowledge of life’s diversity Kingdom is highest – most inclusive taxonomic category Five Kingdoms include: Monera Protista Plantae Fungi Animalia Recognized 2 types of cells: prokaryotes & eukaryotes 33

The Five-Kingdom System Described classification as: Plantae, Fungi, Animalia, Protista, Monera recognizes only 2 types of cells: prokaryotic and eukaryotic sets all prokaryotes apart from eukaryotes prokaryotes are in their own kingdom (Monera) distinguished 3 kingdoms of eukaryotes based on mode of nutrition protista were all eukaryotes that did not fit the definition of plants, fungi, or animals

Figure 26.15 Whittaker’s five-kingdom system

Figure 26.16 Our changing view of biological diversity

The Three-Domain System http://bcs. whfreeman Molecular analyses have given rise to the most current classification system – the Three Domain System Domain is larger than kingdom (superkingdoms) The 3 Domain System is the most recent classification system and includes: Bacteria Archaea Eukarya 37

The Three Domain System Describes classification as: Not all prokaryotes are closely related (not monophyletic) Prokaryotes split early in the history of living things (not all in one lineage) Archaea are more closely related to Eukarya than to Bacteria Eukarya are not directly related to Eubacteria There was a common ancestor for all extant organisms (monophyletic) Eukaryotes are more closely related to each other (than prokaryotes are to each other)

Classification of Living Things Section 18-3 Classification of Living Things Go to Section: 39