Encyclopedia of Evolution.pdf - Online Reading Center

It is not clear what kind of eubacterium might have been
the original host. It may have resembled today’s myxobacteria,
which unlike other bacteria use molecules for intercellular
communication and can form multicellular structures, just
like eukaryotes.
It is also not clear where the membrane that surrounds
the nucleus came from. Lynn Margulis, who convinced most
evolutionary scientists of the symbiogenetic origin of chloroplasts
and mitochondria (see Margulis, Lynn), proposes that
the nuclear membrane began as a fusion of the membranes of
eubacterial and archaebacterial attachment structures.
The nucleus evolved from structures in bacterial ancestors.
Most biologists, biology teachers, and biology students
assume that eukaryotes have fully formed nuclei, and bacteria
have nothing of the sort. It turns out that an obscure group of
bacteria called planctomycetes have their DNA almost completely
enclosed in a membrane that looks like a nuclear membrane.
Another group of recently discovered bacteria, which
live only inside of sponges, also appear to have structures that
look like nuclei. Some researchers even claim that these bacteria
have nuclei, and that the eukaryotic nucleus may have
evolved from this bacterial nucleus. Critics point out that,
despite appearances, the eukaryotic nuclear membrane and the
membrane inside these bacteria are structurally very different.
The nucleus evolved from a virus. A few researchers point
out that nuclei, like viruses, are genetic material surrounded
by a protein coat. They maintain that an ancient virus invaded
an archaebacterium, and instead of killing the host, the virus
maintained a multigenerational mild infection inside of it. It
could have been a virus similar to modern mimiviruses, which
have up to a hundred times as many genes as the smallest
viruses. Gradually, horizontal gene transfer moved bacterial
genes into the virus, which became the nucleus. Their evidence
is that nuclei, like viruses, depend on the cell around
them for metabolism, and while bacterial DNA molecules are
circular, the DNA of viruses is linear, like that of eukaryotic
nuclei. Some viruses not only have a protein coat but have an
envelope, which they make by stealing a chunk of the host
cell membrane, just like the eukaryotic cell uses internal membranes
to make the nuclear membrane. Some large viruses
even have telomeres, nucleotide sequences at the ends of chromosomes
which, in eukaryotes, help keep the process of DNA
replication itself from breaking off chunks of essential genes.
These three possibilities are not mere guesses but can be
framed as scientific hypotheses (see scientific method). If
the first eukaryotic cells were mergers between archaebacteria
and eubacteria, then some eukaryotic genes should more
closely resemble those of archaebacteria, while others should
more closely resemble those of eubacteria (see DNA [evidence
for evolution]). This appears to be the case. DNA
sequences for the genes in eukaryotes that are involved in the
use of genetic information resemble archaebacterial genes.
This includes the production of histone proteins, known only
from eukaryotic and archaebacterial cells. DNA sequences
for the eukaryotic genes involved in metabolism (energy use
and construction of molecules) resemble eubacterial genes.
Cell biologists are pursuing the information necessary to test
eukaryotes, evolution of
other hypotheses. If the nucleus began as an extension of an
undulipodium, which began as a spirochete, there should
be significant similarities between the proteins that move
the chromosomes during cell division, the proteins of undulipodia,
and the proteins of spirochetes. If the first eukaryotic
cells evolved from planctomycete bacteria, these bacteria
should have a closer genetic resemblance to eukaryotes than
do other kinds of bacteria.
According to these views, the first nucleated cell was
a chimera, a fusion of two different life-forms. In Greek
mythology, a chimera was a combination of two different
animals. This would mean that the origin of many structures
found in the eukaryotic cell occurred through symbiogenesis.
If this is the case, the reason that scientists have not found
a gradual series of transitional forms in the evolution of
eukaryotic cell structures is that they never existed.
The tree of life constructed from DNA analysis shows
that the three main branches of life are the Archaea (archaebacteria),
Eubacteria, and Eukaryotes. The branches within
the eukaryotes cannot be clearly distinguished from one
another. Most of the diversity of eukaryotes exists among the
primarily single-celled organisms usually called protists (formerly
“algae” and “protozoans”). Being usually small, of relatively
simple structure, and living in places humans seldom
look (such as oceans, ponds, soil, and inside of animal guts),
protists have been less intensively studied than plants and animals.
Not only is less known about protists than about plants
and animals, but new groups of protists, including some as
small as bacteria, continue to be discovered. Each such discovery
has the possibility of changing scientific understanding
of eukaryotic evolutionary relationships. One recent
reconstruction of these relationships, given by biologist S. L.
Baldauf, divides eukaryotes into the following groups:
• amoebalike organisms mostly without hard outer coverings
• amoebalike organisms that usually have hard outer coverings
• ciliates, dinoflagellates, and their relatives
• euglenas and their relatives
• anaerobic single-celled organisms that lost their mitochondria
• brown algae, diatoms, and their relatives
• red algae, green algae, and plants
• fungi, animals, and their relatives
Many eukaryotes are multicellular. Multicellularity
evolved several times independently: in brown algae, in some
slime molds, in fungi, in plants, and in animals. The lineages
that contain multicellular organisms also contain singlecelled
organisms. The classification above also contains some
surprises. One surprise is that the evolution of chloroplasts
from cyanobacteria occurred only in the lineage leading to
red algae, green algae, and plants. There are other photosynthetic
eukaryotes, such as the brown algae and diatoms in
one lineage, some euglenas in another lineage, and dinoflagellates
in yet another linage. The chloroplasts in these last
three lineages did not evolve directly from cyanobacteria.
Instead, they are the evolutionary descendants of eukaryotic
algae (perhaps green algae). This means that eukaryotes such
as brown algae and dinoflagellates do not consist of cells