Why do cells come together?

How did eukaryotic cells come into existence?

• It is said prokaryotic cells were the first cells on earth and the cell theory states that all cells come from pre-existing cells. Then how did eukaryotic cell come from a prokaryotic cell even though they don't have nucleus in them?

• Answer:

  I'll try to clear this out a bit, since it seems that what is really murky in this topic is the grasp of some concepts and the current available literature on the subject, although the classification and the generation of phylogenetic trees is indeed very problematic. The Endosymbiotic theory cited here before is about the symbiotic origins of organelles and not of the origins of eukaryotic life as we understand it today. However, the arise of mitochondria-containing cells is indeed a crucial evolutionary hallmark, and is involved in the main, most accepted model. Also, the notion that eukaryotic cell preceded prokaryotes or archeans is preposterous and dismissed by all current models regarding the origins of eukaryotic life. I would like to urge everyone interested in the subject to read the excellent paper by Thiergart and co-authors, which is accessible and not only will clarify the crucial points (the differences between autogenous and symbiogenic models of the eukaryotic emergence) it also offers significant support to the symbiogenic models, of which the eocyte hypothesis is the most well known. http://gbe.oxfordjournals.org/content/4/4/466.long As a preview, I'll quote a very elucidating portion of their text here: Although the evolutionary details of the prokaryote-to-eukaryote transition are still incompletely resolved the crucial role that mitochondria played in that transition is becoming increasingly evident. Presently, two main categories of competing hypotheses address the prokaryote-to-eukaryote transition: autogenous and symbiogenic. Autogenous models posit that eukaryotes arose from a single ancestral lineage via mutation in a gradualist type of evolutionary process. Symbiogenic models posit that eukaryotes arose via a symbiotic association of divergent prokaryotic cells, with symbiosis (and gene transfer from endosymbiont to host in some formulations) forging the prokaryote-to-eukaryote transition, with phases of evolutionary innovation marked by distinctly non-gradualist characteristics. Both the autogenous and the symbiogenic categories harbor a number of specific competing alternative hypotheses, respectively, each of which in turn generates testable predictions about the phylogenetic affinities of eukaryotic genes to prokaryotic homologues. (References omitted in this excerpt. Please refer to the original article linked above)