Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

I still don't know if the mitochondrion or chloroplast was first? I've looked for it on the internet and in various books but haven't found anything. Does anyone have the answer and a theory which backs up this answer?

share|improve this question

3 Answers 3

up vote 11 down vote accepted

Mitochondria evolved before chloroplasts.

We know this because Mitochondria form a monophyletic group: e.g. all life with mitochondria traces back to a single common ancestor (source). Since the group with chloroplasts groups within this clade, it must be the case that either (a) chloroplasts were obtained by an organism that already had mitochondria or (b) chloroplasts were independently lost by multiple lineages within the Eukaryotic clade and then many of these lineages re-acquired chloroplasts by secondary endosymbiosis. Since (a) is a (much) more parsimonious explanation it is the one it makes sense to accept.

share|improve this answer
    
Straight and convincing. –  har-wradim Apr 2 at 14:39

I do not have a definitive answer but I can argue that mitochondria came into existence before chloroplasts despite the fact that, between their free living ancestors- $\alpha$-proteobacterium and Cyanobacteria, the latter seems to be older in evolution.

I have following points to support this argument:

  • The organisms that have chloroplasts also have mitochondria but the converse is not true.
  • There have been events of secondary endosymbiosis (especially in the Chromaveolata) in which organisms that already contained chloroplasts became endosymbiotic organelles in another host.
  • There is a very recent case of endosymbiosis of a chloroplast like organelle called spheroid bodies in the diatom Rhopallodia gibba which already has a secondary plastid. See this.

Usually a cell has many mitochondria but fewer chloroplasts. Though this doesn't say much about the evolutionary age but higher number allows more organelle to nuclear gene transfer, considering the limited transfer window hypothesis. More the transfer, more "dependent" will the organelle become on the host.

Even though I claim in the first paragraph that Cyanobacteria is older than $\alpha$-proteobacterium (In fact I dont have a strong reason to support that claim), it doesn't mean that photosynthesis has an older origin. Chemosynthetic pathway and membrane bioenergetics have evolved very early on. Coupling of light to assist these processes, most likely came later when organisms surfaced to the limnetic zone.

share|improve this answer
    
The use of molecular oxygen as an electron acceptor evolved some time after photosynthetic processes had made molecular oxygen abundant in the earth's atmosphere. So I think you can make the argument that photosynthesis has an older origin compared to the oxygen dependent metabolism that is performed by mitochondria. –  stords Apr 1 at 8:05
    
Point taken. But oxygen is not the only electron acceptor... Mitochondria which use $Fe^{3+}$ also exist (one of the $e^-$ acceptors other than Oxygen). So yes, photosynthesis is older than aerobic respiration but ATP-synthase is older than both :) –  WYSIWYG Apr 1 at 8:20
    
@WYSIWYG: your third argument doesn't seem very strong. –  har-wradim Apr 1 at 8:52
    
@har-wradim yes that's the weakest of all :) I mentioned it because it is likely that mitochondria are numerous because they became associated early on. Also considering the limited transfer window hypothesis, the chance of mitochondria to nuclear transfer of genes is easier because of its higher number. Although, one can argue that $\alpha$-proteobacterium was inherently fast dividing than Cyanobacteria and therefore the higher numbers in the cell. –  WYSIWYG Apr 1 at 9:18
    
I think you mean "descendants", not "ancestors", and even then, this is probably incorrect. The original endosymiont is probably in a sister clade to the extant bacteria. I also think the single origin of both endosymbionts is the strongest argument, anyway. –  Sparhawk Apr 2 at 5:06

According to my knowledge, this was the molecular and cellular evolutionary path of life.

  • Synthesis of essential building blocks (fatty acids, aminoacids, cofactors...)
  • Condensation of building blocks
  • First RNA replicase; RNA genomes (differentiation between genomic and
    functional RNA, primitive metabolism)
  • Ribosomes and first matrix dependent translational machinery
  • Copying RNA into DNK (rNDP reductase, reverse transcriptase)
  • PROGENOTES (DNA genome, introns in genes, slow growth, anaerobic heterotrophs)

From here you have two main branches, organisms with branched lipids, and with unbranched lipids (the first will become archaea, and the latter urkaryotes and Bacteria)

In the Urkaryotes, you have selection of complexity. Their growths is inefficient, and they tolerate extra DNA. Bacteria favored efficient growth, autotrophy and loss of extra DNA.

At this point the endosymbiosis occurred (presumably), and the bacteria became a very specialized parasite. Mitochondria as you probably know, have their own genome which replicates independently from the nuclear genome. Cells and mitochondria became permanently linked when the mitochondria "decided" to relocate parts of it's genome to the nucleus, incorporating it's genome in the host's.

Now we have the first Eukaryotic organisms that are similar to today's. They were heterotrophic, but one more event of endosymbiosis occurred. That was the formation of chloroplasts.

Many people think that autotrophy is more primitive compared to heterotrophy. That simply isn't true. The conditions when these organisms arose were quite different. Building blocks were readily available and plentiful, so the organisms were all "friends" with each other. It is when food became scarce that selective pressure started nudging species into different directions (autotrophy).

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.