Lecture 10: Early life on Earth and Rise of Eukaryotes Flashcards
Four supergroups of Eukaryotes
Unikonta: Including fungi and animals
SAR: Stramenopiles, Alveolates, Rhizarians
Archaeplastida: Including plants
Excavata
Some relationships on the tree are unresolved.
Protist
a polyphyletic group of
eukaryotic organisms which are not
plants, fungi or animals.
what supergroups are protists observed to be part of
Observed in all four supergroups of
eukaryotes. Many protists are
chemoheterotrophs while others
are photoautotrophs.
earliest fossil of life
Stromatolites are the earliest fossils of life, observed from about 3.5 billion years ago. Some prokaryotes (mainly cyanobacteria) build thin, mineralized layers on top of another, which became stromatolites.
before the oxygen revolution, life on Earth was ___
anaerobic
what was Earth’s oxygen produced by?
Earth’s oxygen is produced mostly by biological activity via oxygenic photosynthesis, mainly by cyanobacteria 3.5 billion years ago to saturate water and atmosphere.
why was there a large spike in oxygen 2.7 billion years ago?
water was completely saturated with O2. Many organisms adapted and took advantage
of oxygen giving rise to aerobic respiration
Eukaryotes are thought to be descendants of an ___
ancestral, possibly anaerobic, Archean
How is the mitochondria believed to have evolved?
- The ancestor engulfed an aerobic bacterium which begun co-existing inside the cytoplasm of the host
- Mutualistic relationship where the symbiont provides the host access to aerobic respiration, and the host provides nutrients, safety, etc
- Both fused and the mitochondria is believed to a descendent
Mitochondria
Organelle for aerobic respiration
In eukaryotes, the Krebs cycle occurs in the mitochondrial cytosol, and the ETC is located in the mitochondrial inner membrane.
Mitochondria is very similar to a
Gram negative bacteria
Mitochondria and G-N bacteria share:
- Two membranes, inner and outer
- Circular genomes (bacterial chromosome vs. the mitochondrial DNA)
- Molecular machines inside the cytosol such as Ribosomes for protein translation
- Homologous proteins on the inner membrane
- When using molecular phylogeny, the eukaryotic mitochondrial DNA is placed within the Domain Bacteria
- Molecular analysis suggest that the mitochondria is very close to (or within) the clade alpha-proteobacteria
Primary endosymbiosis
ancestral, heterotrophic eukaryote engulfed a cyanobacteria as a symbiont, host gains oxygenic photosynthesis
The two species would fuse to form ___ as the cyanobacteria turned into ___ around 1-1.5 billion years ago
ancestral algae
chloroplast
This ancestral algae gave rise to
Archaeplastida: clade of land plants,
Red algae and Green algae
Evidence for the cyanobacterial origin of chloroplasts
Cyanobacteria and the chloroplast shares multiple morphological and genetic similarities, including:
* Photosynthetic pathways
* Systems for transcription and protein translation
Secondary endosymbiosis spread photosynthesis to other eukaryotes
Red algae and green algae has been engulfed by another eukaryote in multiple independent occasions, spreading oxygenic photosynthesis to other eukaryotic clades
Red algae engulfment gave photosynthesis to
- Stramenopiles (Diatoms and Brown algae)
- Alveolates (Dinoflagellates)
- Cryptophytes
- Haptophytes
Green algae engulfment gave photosynthesis to:
- Excavata (Euglenozoans)
- Rhizarians (Cercozoans)
Evidence for secondary endosymbiosis
- Paramecium are non-photosynthetic,
chemoheterotropic ciliates
(Alveolates, SAR supergroup) - Some members, such as Paramecium bursaria, are observed to host symbiont Green algae
Further evidence for secondary endosymbiosis
- Traces of different endosymbiotic events can be observed in
some algae such as the cryptophytes - Cryptophytes gained photosynthesis by engulfing Red algae
- Nucleomorph: Cryptophyte plastids have a ‘nucleus’ remaining
from the original red-algae endosymbiont - Nucleomorphs contain DNA surrounded by a nuclear membrane
- Chlorarachniophyte algae (Rhizaria, SAR supergroup) also has a
nucleomorph
Therefore, cryptomonads have four separate genomes
- Nuclear DNA (DNA of the host protist which engulfed the red algae)
- Nucleomorph DNA (Remains of nuclear DNA of the red algae)
- Plastid DNA (Remains of cyanobacteria DNA, originally held by the red algae)
- Mitochondrial DNA (Remains of the alpha-proteobacteria)
Mitochondrion endosymbiosis
Archean engulfed an alphaproteobacteria
Primary endosymbiosis
Ancestral Archaeplastida engulfed a
cyanobacteria
Secondary endosymbiosis
- Red algae engulfment gave rise to
photosynthetic Haptophytes, Cryptophytes, Stramenopiles, Alveolates - Green algae engulfment gave rise to photosynthetic Rhizarians and Excavates
Genomes humans have:
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
Genomes plants and green algae have:
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
- Plastid DNA (cyanobacteria origin)
Genomes Cryptophytes have:
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
- Plastid DNA (cyanobacteria origin)
- Nucleomorph DNA (green algae origin)
