Re: Re: Is the endosymbiont theory true?

You, as an undergraduate, should realise that, since it is a 'theory' and 
not a 'law', your question CANNOT have a simple answer.  The purpose of 
your education is to train you to analyse facts and draw your own 
conclusion.
So... what is the theory and what are the facts?
The theory suggests that chloroplasts and mitochondria began as free-
living organisms; that they became trapped by the earliest eukaryotic 
cells and that they then evolved together to become totally interdependent.

The evidence for this is considerable - and, always a good sign, the more 
we learn about the detailed biochemistry of the cell, the more it supports 
the theory.

Prokaryotes invariably have 70S ribosomes; eukaryotes 80S.  Both of these 
organelles have the prokaryotic form.  Their rate of mutation being known, 
studies have suggested that the origin of these organelles roughly 
coincided with the changes in the Earth's atmosphere and fossil record 
suggesting that life had 'moved up a gear'.  Oxygen being poisonous, the 
early cells probably benefitted more from the lowered oxygen tension than 
from the release of energy in the form of ATP - that came later.
Both organelles have closed circular coiled DNA - the form found in 
prokaryotes.  If these organelles were part of the usual pattern of cell 
organelles, synthesised in response to nuclear DNA, then it is unclear why 
extra-nuclear DNA would be required.  Furthermore, the organelle DNA codes 
for the proteins at the heart of the organelle - the inner membrane 
proteins and those of the Kreb's cycle and ox-phos pathway (mitos)and 
those of the light and light-independent reactions of photosynthesis 
(chloroplasts). In the case of mitos, this is about 50% of the total; in 
chloroplasts 
about 70%.
Both organelles have two membranes (some suggest the thylakoid membranes 
of chloroplasts are a third layer, but they are normally contiguous with 
the inner membrane, so I suspect not).  The outer membranes are 
synthesised by the host cell; the inner membrane substantially by the 
organelle DNA.  This is consistent with the idea that they were 
originally 'engulfed' by the host cell - as many Protists do to their prey 
today.
Its influence on cell biology is that it suggests a sequence for the 
evolution of life on Earth at a time when the fossil record is necessarily 
all but non-existent.  We have an insight into life during the first 25% 
of 
the development of modern life-forms.  It suggests that life continually 
adapts itself to exploit prevailing conditions and that co-operation is 
sometimes of benefit to both species - even if one eventually becomes 
dependent on the other.
Both organelles have a 'semi-autonomous' existence in which their numbers 
in any one cell appears to be outside the direct control of the nucleus.  
This means that they can be affected by certain antibiotics (such as 
chloramphenicol)which have no effect on the rest of the cell - they can be 
selectively 'poisoned'.  
It is to be hoped that the human genome project will enable us to see the 
genes that are influential in the regulation of these organelles and 
enable us to gain some insight into their origin.
Perhaps life is not, always, 'red in tooth and claw'!

Like all theories, it originated from Man's continual search to find 'the 
ultimate truth'.  In other words, scientists wanted an explanation for 
some unusual and apparently contardictory facts and the endosymbiont 
theory was devised as a possible explanation.

I am unaware of any serious evidence contradicting it - this has been 
'mainstream' theory for well over 30 years.