lpetrich

This paper, <a href="http://www.biomedcentral.com/1471-2148/1/4" target="_blank">A genomic timescale for the origin of eukaryotes</a>, proposes a most interesting history.

About 4 billion years ago, the big prokaryote divergence happened, between Eubacteria and Archaea; not long afterward, the ancestor of the eukaryotes diverged from the Archaea.

About 2.7 billion years ago, the first eukaryote endosymbiosis took place, with some early Gram-negative eubacterium; only some genes remain.

About 2.5 billion years ago, O2-releasing cyanobacteria emerged, adding oxygen to the Earth's atmosphere (&lt;1% to &gt;15%), consuming carbon dioxide, and oxidizing methane -- the reduction in the latter two greenhouse gases caused a big ice age at around 2.2-2.4 billion years ago.

Around the time of that ice age, the ancestors of Giardia lamblia diverged. This protist has no mitochondria, but the discovery of mitochondrion-like genes has suggested the possibility of secondary loss -- some of its ancestors had them, then lost them. This paper proposes that these genes were instead from an earlier endosymbiosis, and that Giardia had never had mitochondria. Expect the "never had them" vs. "lost them" controversy to continue.

About 1.8 billion years ago, some protist acquired Rickettsia-like alpha-proteobacteria, with this endosymbiosis producing mitochondria. Rickettsia bacteria like to live inside of cells, suggesting that they are part of the way there. And mitochondrial genes can be distinguished as late arrivals that are close to Rickettsia, rather than early arrivals that branch off lower in the bacterial-relative family tree, as that paper shows.

Some later one acquired cyanobacteria, making chloroplasts and other plastids; this process would sometimes be repeated, with some protist turning a photosynthetic protist into an endosymbiont.

Late_Cretaceous

Sounds like an interesting paper, I am printing off a copy right now.

Dr.GH

I have also found the following intersting.

Brocks, Jochen J., Gram A. Logan, Roger Buick, Roger E. Summons
1999 Archaen Molecular Fossils and the Early Rise of Eukaryotes. Science 285
(5430):1033-1036

Dyall, Sabrina D., Patricia J. Johnson
2000 “Origins of hydrogenosomes and mitochondria: evolution and organelle biogensis.”
Current Opinion in Microbiology 3:404-411

Xiong, Jin, William M. Fischer, Kazuhito Inoue, Masaaki Nakahara, Carl E. Bauer.
2000 “Molecular Evidence for the Early Evolution of Photosynthesis” Science
298(5485): 1724

They are helpful in recognizing the chop-job that J. Wells pulled of in his Icons of Evolution, particularly the chapters on the Miller/Urey experiment, and on the" tree of life."

lpetrich

Basically, that discusses some molecules typical of cyanobacteria (hopanes) and eukaryotes (steranes) -- but the interesting question is whether some of their non-cyanobacterium and non-eukaryote ancestors had had these same molecules. Not many present-day anaerobic organisms have been tested for such molecules, as the article at the top of this thread points out.

I'd have to register to find out more about this article. But for those unfamiliar with the subject, hydrogenosomes appear to be degenerate mitochondria that have lost their genomes; they are composed of parts coded for by nuclear genes, which include only part of the energy-producing respiratory chain. In a full-scale mitochondrion, the chain would end in "cytochrome oxidase", which combines hydrogen equivalents (electrons) with oxygen to make water. However, in a hydrogenosome, that last step is missing, and some intermediate stage simply lets the hydrogen equivalents form hydrogen gas.

Works out the family tree of enzymes that do biosynthesis of photosynthetic pigments. The purple bacteria are the earliest branchers, followed by the green sulfur and nonsulfur bacteria as a group, then the heliobacteria, then the cyanobacteria (O2-releasers; some of them became chloroplasts).

However, that is only the family tree of those molecules; its deeper branchings disagree with that from ribosomal RNA's, suggesting that the genes involved in photosynthesis may have been transferred from microbe to microbe (lateral gene transfer). However, they would have been transferred as a group, since they have strongly coevolved.

Dr.GH

Howdy,

There are two issues that I find interesting here: 1) what was the timing of the origin of
oxygenic photosynthesis, and 2) what was the redox state of the Hadean and Archean .

The Hedges et al article cited at the start of the thread was interesting in that they have
apparently located specific cellular innovations that are in general agreement with some
geological data. Given that, their early age estimate for the divergence of archaebacteria
and eukaryote genes from archaebacteria (~ 4Ga) pushes past the geological data. The
earliest geochemical data of molecular signatures possibly related to life is about 3.9 Ga
(Mojzsis et al 2000, Mojzsis et al 2001, but see also Whitehouse 2000).

How might the tempo and discreet events of early evolution be related to geological, and
geochemical events is also significant as there was a clear shift from the Precambrian to
the Phanerozoic (ie Schopf 1994).


Mojzsis, Stephen J., T. Mark Harrison,
2000 “Vestiges of a Beginning: Clues to the Emergent Biosphere Recorded in the Oldest Known Sedimentary Rocks” GSA Today, April

MOJZSIS, STEPHEN J., T. MARK HARRISON, ROBERT T. PIDGEON
2001 ”Oxygen-isotope evidence from ancient zircons for liquid water at the Earth's surface 4,300 Myr ago” Nature 409, 178-181 (11 January )

Schopf, J. William
1994 “Disparate rates, differing fates: Tempo and modes of evolution changed from the Precabrian to the Phanerozoic” PNAC-USA v.91: 6735-6742

Whitehouse, Martin.
2000 “Time Constraints on When Life Began: The oldest Record of Life on Earth?” The Geochemical News #103, April.

Dr.GH

I should have mentioned that the Dyall & Johnson article persuasively argues that the
hydrogenosomes and mitochondria share a common ancestor. Hydrogenosomes are
found in eukaryotes that lack mitochondria and like mitochondria produce ATP.
However they use a very different fermentative pathway than the Krebs cycle, and
enzymes which have no counterpart in the mitochondria.

Most of my interest stems from the notion that the chemistry of the earliest forms of life
may reflect the chemistry of their earliest environments. Dyall & Johnson make the very
attractive hypothesis that the endosymbiotic events were related to the oxygen spike
recorded geologically at around 2.2 to 2.0 Ga (Holland 1999, and references in Dyall &
Johnson).


Holland, Heinrich D.
1999 “When did the Earth’s atmosphere become oxic? A Reply.” The Geochemical News #100: 20-22