Evolution What the Fossils Say and Why it Matters

Life’s Origins 165

has the identical 9 + 2 fiber structure (nine sets of microtubule doublets surrounding a pair
of single microtubules in the center) as the prokaryotes known as spirochetes, which cause
syphilis. As each of these smaller prokaryotes came to live within a larger cell, they subli-
mated their functions to that of their host, so that the cyanobacteria became chloroplasts that
are now homes for photosynthesis, and the purple nonsulfur bacteria became mitochondria
and performed the role of the energy converter for the cell.
In addition to the detailed similarities of these prokaryotes to the organelles, Margulis
pointed to many other suggestive lines of evidence. Organelles are not usually enclosed
within the eukaryotic cell membrane but separated from the rest of the cell by their own
membranes, strongly suggesting that they are foreign bodies that have been partially incor-
porated within a larger cell. Mitochondria and chloroplasts also make proteins with their
own set of biochemical pathways, which are different from those used by the rest of the
cell. Chloroplasts and mitochondria are also susceptible to antibiotics like streptomycin and
tetracycline, which are good at killing bacteria and other prokaryotes, but the antibiotics
have no effect on the rest of the cell. Even more surprising, mitochondria and chloroplasts
can multiply only by dividing into daughter cells like prokaryotes and thus have their own

FIGURE 6.8. According to Lynn Margulis, complex eukaryotic cells arose from two or more prokaryotic cells
that combined to live symbiotically. Cyanobacteria are apparently the precursors of the photosynthetic
chloroplasts, which provide photosynthesis in plant cells. Purple nonsulfur bacteria have the same structure
and genetic code of the mitochondria, which provide energy in the cell. And the flagellum has the same
structure as the prokaryotes known as spirochetes, which are also responsible for causing syphilis.

Cyanobacteria

Paracocci

Thermoplasmas

Spirochetes

Motility

Fermentation

Respiration

Photosynthesis

Microtubular

symbiosis

Protoeukaryotic

cell

Mitochondrial

symbiosis

Protista

(algae, water molds,

slime molds, slime

nets, protozoa)

Fungi

Animalia

Plantae

Chloroplast

symbiosis

Monera
(Eubacteria,
Archaebacteria,
early eukaryotes)

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