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,
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,
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;
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.