Chapter 5: History of Life on Earth. Dates of origins 1. Everything (Universe):14 Bya 2. Solar...
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Transcript of Chapter 5: History of Life on Earth. Dates of origins 1. Everything (Universe):14 Bya 2. Solar...
Chapter 5: History of Life on Earth
• Dates of origins• 1. Everything (Universe): 14 Bya• 2. Solar System (Earth): 4.6 Bya
• Conservative estimates• 3. Prokaryotic cells: 3.5 Bya• 4. Eukaryotic cells: 1.5 Bya• 5. First hominins: 7 Mya• 6. Genus Homo: 2.4 Mya• 7. First anatomically modern human 170,000 Kya
Science October 200911 papers
• Evidence for time of cell origins
• Stromatolites
FossilsExtant: mats of cyanobacteria growing on a surface of accreted sediments andcalcium carbonate.
Modern distribution limited to a fewextreme environments.
1-2-By-old fossil stromatolites from Montana, USAOldest: Warrawoona, Australia: 3.5 Bya
© Marli Miller/Visuals Unlimited
Stromolite sections: lamellar growth
Extant
Fossil
Fossils (?):3.26 ByoSouth Africa
LivingbacteriaUndergoingdivision
Fossil eukaryotes (?)
590 MyCell wall or reproductive cyst(alga)Egg case of anearly consumer 250 μm
850-950 My40 μm
1.4-1.5 By60 μm
950 My
850 My
1.55 My
2 By
Extantcyanobacteria
Figure 2.1 The Tree of Life (Part 3)
• Origin of energy transforming organelles
• Lateral gene transfer (prokaryoteseukaryotes)
• 1. Endosymbiosis = a cell from one species begins living inside a host cell of a different species.
• e.g., bacteria associated with ancestors of present day eukaryotic cells
• A purple bacterium derivative mitochondria
• A cyanobacterium derivative chloroplasts
• b.c. bacteria have c. 5,000 genes, mitochondria and chloroplastids represent the largest lateral transfer events in the history of life.
• Evidence that mitochondria and chloroplasts originated from endosymbiotic bacteria.
• 1. Phylogenetic position.• 2. Size• 3. Bacteria-like circular chromosome• 4. A double membrane
• Differences• Fewer genes in mitochondria and chloroplastids
– Chloroplastid genes: for elements of certain photosynthesis reactions.– Mitochondrial genes: code for mitochondrial tRNAs, rRNAs,
ribosomal proteins, and certain proteins involved in cellular respiration.
• Fate of “lost information”
• A. Some genes were transferred to nuclear DNA.• e.g., there are c. 630 alpha-proteobacterial genes in
human and yeast genomes.
• B. Some were deleted in the fine-tuning of the mutualistic relationship.
• Evidence for organelle-to-nucleus transfer of DNA
• 1. Chloroplastid
– RuBPCase: enzyme: fixation of CO2 in photosynthesis.
– Enzyme is a dimer
– Larger subunit gene is in chloropastid DNA.
– Smaller subunit gene is in nuclear DNA.
• 2. Mitochondrial ribosomes
– rRNAs coded for by mtDNA
– Ribosomal proteins coded for by nuclear DNA
• 2. Lateral transfer of genes via secondary endosymbiosis
• Eukaryotic cell with a chloroplast is engulfed by a eukaryotic cell that lacks one.
• If the chloroplast is retained as a functional organelle, it has been acquired by secondary endosymbiosis.
The evidence
• An example• Cryptomonas F : a green alga• Chloroplastid is enclosed in four membranes• A typical circular DNA is located inside the plastid.• Between the inner and outer pairs of membrane is a nucleus-
like organelle – a nucleomorph • The nucleomorph has a DNA molecule• This DNA codes for a functional ribosome that remains
between the two pairs of membranes.• rRNAs of nucleomorph and nucleus reveal large differences in
sequences; i.e., they are not closely related. • Conclusion: The RNAs had different origins.
• All eukaryotes have mitochondia.
– Therefore, acquisition must have taken place early in eukaryotic evolution.
• Chloroplastid phylogeny
– Original endosymbiotic event took place in ancestor of today’s red algae, green algae, and land plants.
– All other photosynthetic eukaryotes acquired their plastids via secondary (and/or tertiary) endosymbiosis.