The Origin and Evolution of the Eukaryotic Cell
The Nucleus as Endosymbiont
Hartman, H.
IASB, 880 Spruce St, Berkeley, California 94707, USA
The earliest cell to dominate the biosphere was a cell based on RNA.
Surrounded by a membrane, it eked out its anaerobic existence. Its
genome was not united but dispersed throughout the cell attached to its
membrane. The cytoskeleton composed of proteins and RNA kept the cell in
expanded form and allowed it to move about. From this ancient monster
evolved the bacteria (eubacteria and archaebacteria) whose genome was
based on DNA. This ancient monster the Kronocyte ( named after the Greek
God Kronos) began to swallow its evolutionary progeny. First after many
endosymbiotic events, a nucleus was formed, then a mitochondrion and
finally a chloroplast.
"The prokaryotes and eukaryotes last shared a common ancestor about 2
billion years ago." (1) This claim based on the amino acid sequences of
57 different enzymes has been hotly debated as there is fossil evidence
that the prokaryotes go back to 3.5 billion years ago. The simplest
explanation for these results is that about 2 billion years ago there
was a massive intrusion of prokaryotes (eubacteria and archaebacteria)
into a cell which was not a prokaryote and which resulted in the
formation of the nucleus. This horizontal transfer of prokaryotic genes
into a non prokaryotic cell would reset the evolutionary clock for the
divergence of the eukaryotic cell from the prokaryotes.
What event in the biosphere set off this massive endosymbiosis? The
answer to this puzzle is oxygen. There are a number of other events which seem to be triggered off by
oxygen. The appearance of oxygen in the atmosphere seems not to have
arisen gradually but in a pulsed fashion. There is evidence that the
next pulse of oxygen occurred 1 billion years ago which can be then
correlated with the appearance of multicellular eukaryotes. The
mitochondria may have appeared at this time.
Finally the Cambrian explosion may have been set off by another and
final pulse of oxygen which can be correlated with the complex
multicellular bauplans. The chloroplasts may have appeared at this time.
1) Doolittle, R.F., Feng, D., Tsang, T.,Cho, G., and Little, E. (1996) Science, 271, 470-477 .
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Tuesday, April 7 |
05:20 pm |