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.