3.5 billion to 700 million years ago

mitochondria endosymbiosis

Around 2 billion years ago, larger and more complex eukaryotic cells evolved. Some of the new features of eukaryotic cells resulted from an endosymbiotic relationship of prokaryotic cells living inside the proto-eukaryotic host cells.

From 2 to 1.5 billion years ago simple eukaryotic organisms (protists) evolved and diversified rapidly. The first eukaryotic fossils are the hundreds of specimens of the alga Grypania from 2.1 billion years ago that could measure .5 mm wide and up to .5 m long (Han, 1992). Eukaryotic cells were common by 1.7 billion years ago (Kerr, 1995). Microscopic fossils can be difficult to interpret given that it is difficult to distinguish between prokaryotic and small, unicellular eukaryotic cells (Yoon, 2004). Eukaryotic cells are larger, much more complex & have internal membrane-bound organelles (such as nuclei, mitochondria, chloroplasts). Not only do modern eukaryotic cells compose these protists (mostly unicellular organisms) but are also the type of cell which make up fungi, plants, and animals.

Endosymbiosis refers to a condition in which one cell lives inside another cell for the benefit of both. Is this possible? Yes. There are hundreds of known examples of endosymbionts such as bacteria living inside of protists and algae living inside corals, worms, clams, and even mollusks called nautiloids. There are hundreds of known examples of endosymbionts such as bacteria living inside of protists, algae living inside corals, worms, clams, even mollusks called nautiloids. For example, there are a large number of species in the protozoan family Trypanosomatidae, many of which cause human diseases such as Chaga's disease and African sleeping sickness. Some members of the family possess endosymbionts (such as Blastocrithidia cullicis, Crithidia deanei, C. desouzai, C. oncopelti, and Heretomonas roitmani (de Souza, 1999). Not only are endosymbionts known in modern organisms (including modern termites), they have been identified in fossils as well. Spirochete and protest symbionts of a termite in Miocene amber (Wier, 2002). It is common that genes which were originally present in the endosymbiont eventually are transferred to the host nucleus. The genome of the plastids of dinoflagellates has been greatly reduced, consisting of single-gene minicircles which encode about 15 proteins. Most of the genes for the photosystems have been translocated to the nucleus (Hackett, 2004).
There is a great deal of evidence that supports the theory that mitochondria and chloroplasts are actually the descendents of eubacteria that were engulfed by a proto-eukaryotic cell and survived as endosymbionts (discussed presently). It is possible that the eukaryotic nucleus also resulted from an endosymbiotic event. In such a model, an archaeal cell would have engulfed a eubacterial cell. It is also possible that ancient prokaryotic cells which encoded a plasma membrane, simply produced additional membrane which surrounded the bacterial chromosome. The DNA of the eubacterium Gemmata obscuriglobus is enclosed in two membranes. It is not known whether this membrane will provide insight into the evolution of the eukaryotic nuclear membrane and there is no evidence of nuclear pores or of a nucleolus (Fuerst, 1991).

Not only are there a number of endosymbionts which have evolved recently and still retain most of their ancestral nature, there is also evidence of more ancient endosymbiotic events, including those which resulted the mitochondria and chloroplasts of eukaryotic cells. Mitochondria and chloroplasts are eukaryotic organelles which have a number of features which suggest they are derived from eubacterial ancestors. They are similar in size to bacteria and they possess their own chromosomes which are circular, like those of bacteria. (As a result, it is incorrect to say that human cells have 46 chromosomes: the mitochondrial chromosome composes a 47th and it may be present in many copies in any given cell.) Mitochondria and chloroplasts are also similar to bacteria in their ribosomes, cytochrome c, genetic code, translation initiation, translation initiation factors, and internal structure. Both mitochondria and chloroplasts reproduce by fission as do bacteria and cannot be synthesized by the genes in the nucleus. If they are removed from a cell, the cell cannot replace them (Gray, 1992; Margulis, 1996).
Both mitochondria and chloroplasts are sensitive to antibiotics which affect bacteria such as streptomycin, spectinomycin, neomycin, & chloramphenicol while are unaffected by agents such as cyclohexamide that affect the cytoplasm. Many of these antibiotics act on bacterial ribosomes. However, eukaryotic mitochondria possess their own genes which contribute to ribosomes. There are two rRNAs encoded by the mitochondrial genome: MTRNR1 (nucleotides 648-1601) and MTRNR2 (nucleotides 1671-32229). Not only are high doses of certain antibiotics potentially dangerous to all humans (because they inhibit mitochondria in addition to inhibiting bacteria) some people possess variations in these mitochondrial rRNA genes which make their mitochondria more "bacteria-like" and thus can cause serious reactions if they take an antibiotic (OMIM).
Plasmids are small pieces of DNA which exist outside major chromosomes. Although virtually all plasmids are known from bacteria, some are known to exist in mitochondrial. A number of linear mitochondrial plasmids are known in fungi and higher plants, some of which require the presence of two plasmids in order to replicate (Chan, 1991). Plasmid-like DNAs are known from both fungal and plant mitochondria (Gray, 1992).

Both mitochondria and chloroplasts seem to have originated from the endosymbiosis of prokaryotic cells in early eukaryotic cells. The mitochondrial endosymbiotic event which led to mitochondria would have predated that which led to chloroplasts, given that virtually all eukaryotes possess mitochondria. Giardia, depicted below, is one of the most primitive eukaryotes which lacks mitochondria, although evidence suggests that their ancestors possessed these endosymbionts which were later lost.

Chloroplasts originated in a later endosymbiosis event which occurred in the lineage leading to algae and plants.
chloroplast endosymbiosis