Origin of Eukaryotes
* The origin of eukaryotes is important to understand the origin of modern complex cells. There are three main separate theories that hypothesize the origins: the three-domain system, eocyte theory, and endosymbiosis. Each one have there own merits and evidence supporting.
These theories suggest the evolution of cells from the most primitive prokaryotes, unicellular organism having cells lacking membrane-bound nuclei, to the most complex eukaryotes, single or multicellular organisms with a membrane enclosed nucleolus and organelles.
The Three Domain Hypothesis refers to the proposal by Carl Woese in 1990 that; archaebacteria form a monophyletic group, this clade is sufficiently different from all other prokaryotes to deserve elevation to a separate Domain called Archaea (the other two Domains are Bacteria and Eukarya each arising from a progenote), eukaryotes are more closely related to archaebacteria than to other prokaryotes, and the root of the universal tree of life lies in the branch leading to Bacteria. The three-domain system met with some opposition on the differences between archaea and bacteria.
Research of large subunits of RNA polymerase, some aminoacyl-tRNA synthetases (aspartyl, leucyl, tryptophanyl, and tyrosyl), and outer membrane molecules distinctions indicated that Woese was right in the classification and that these organisms were so genetically distinct (in the 165rRNA genes and differences in cell structures) that they needed their own domains. * In the 1984 James Lake theorized eukaryotes evolved from a specific group of ancestrial archea, the eocyte. The idea that eukaryotes could have arisen from a lineage of prokaryotes, using expanded molecular sequence datasets and phylogenetic approaches.
Using a matrix of amino acid sites, traditional methods such as maximum parsimony resulted in the 3-domains topology, but an eocyte tree was obtained when maximum-likelihood and Bayesian analyses were performed. In sum this analyses provide support for the eocyte tree, rather than the 3-domains tree. This is supported by the concept that eukaryotic nucleo-cytoplasm evolved from within archaebacteria. Eukaryotes would have had to replace their old lipid synthesis with a eubacterial-type system since the operational genes of eukaryotes are primarily eubacterial, not archaebacterial (National Academy of Science of the United states 2008).
Eukaryotes are seen as an evolutionary marvel for they can pack hundreds of energy-generating mitochondria into a single cell. Hundreds of millions of years ago, eukaryotes formed permanent colonies in which certain cells dedicated themselves to different tasks, such as nutrition or excretion, and whose behavior was well coordinated. This specialization allows them to grow, and evolving into new elaborate purposes. These cells have a true nucleus, bound by a double membrane. Prokaryotic cells have no nucleus.
The purpose of the nucleus is to sequester the DNA- functions of the eukaryotic cell into chamber for increased efficiency. This function is unnecessary for the prokaryotic cell, because it is much smaller in size; materials within the cell are close together. There is an area of nuclear DNA unbound by a membrane called a nucleoid. Eukaryotic cells are larger, more advanced and have a higher output of energy in comparison to Prokaryotes. Lynn Margulis (1970) defined the hypothesis of Endosymbiosis as the engulfment of one cell by another larger cell, with the engulfed cell evolving into an organelle.
Margulis claimed that as a result of communal and parasitic lives, bacterial cells turned into plants and animals through endosymbiosis. In this theory, plant cells developed when a cyanobacteria (chloroplast) was swallowed by another bacterial cell and animal cells were formed through mitochondria being engulfed by host cell. Another example is between a termite and microorganisms in its gut. The termite consumes wood, but it cannot digest it, the protozoan’s in the termite’s gut break down the cellulose into simple sugars which both organisms can digest.
When the protozoa digest the wood cellulose, they release acetic acid and other acids that the host termite is able to metabolize. Thus, the termite and the protozoan uniquely supply food for each other (Applied and Environmental Microbiology 2005). The origin of the eukaryotic cell was important, since they include all complex cells and almost all multi-cellular organisms. The timing these events is hard to determine, each hypothesis have there own evidence that support itself. Until further evidence can be found scientists can only speculate on the origins of Eukaryotes.