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 human170,000 Kya

Science October papers

Evidence for time of cell origins Stromatolites Fossils Extant: mats of cyanobacteria growing on a surface of accreted sediments and calcium carbonate. Modern distribution limited to a few extreme environments.

1-2-By-old fossil stromatolites from Montana, USA Oldest: Warrawoona, Australia: 3.5 Bya © Marli Miller/Visuals Unlimited

Stromolite sections: lamellar growth Extant Fossil

Fossils (?): 3.26 Byo South Africa Living bacteria Undergoing division

Fossil eukaryotes (?) 590 My Cell wall or reproductive cyst (alga) Egg case of an early consumer 250 μm My 40 μm By 60 μm

950 My 850 My 1.55 My 2 By Extant cyanobacteria

Figure 2.1 The Tree of Life (Part 3)

Origin of energy transforming organelles Lateral gene transfer (prokaryotes  eukaryotes) 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 CO 2 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.