166 Evolution? The Fossils Say YES!
independent reproductive mechanisms; they are not made by the cytoplasm of the cell. If a
cell loses its mitochondria or chloroplasts, it cannot make more.
When Margulis’s startling ideas were first proposed 50 years ago, they were met with
much resistance. But as biologists began to see more and more examples of symbiosis in
nature, the notion became more plausible. We humans have many symbiotic bacteria living
in and on us. Our intestines are full of the bacterium Escherischia coli (E. coli for short), famil-
iar from petri dishes and those news alerts about sewage spills or contaminated kitchens.
These bacteria actually do most of our digestion for us, breaking down food into nutrients
in exchange for a home in our guts. Most of our fecal matter is actually made of the dead
bacterial tissues after digestion, plus indigestible fiber and other material that we cannot
metabolize. There are many other examples of endosymbiosis in nature. Termites, sea tur-
tles, cattle, goats, and many other organisms have specialized gut bacteria that help break
down indigestible cellulose so these animals can eat plant matter wholesale. Tropical corals,
large foraminifera, and giant clams all house algae in their tissues that produce oxygen and
help secrete the minerals for their large skeletons.
The strongest evidence came when people started studying the organelles more closely
and found that not only did they have the right structure to have once been independent
prokaryotic cells, but they also have their own genetic code! Mitochondria and chloroplasts
both have their own DNA, which has a different sequence than the DNA in the cell nucleus.
This would make absolutely no sense unless mitochondria and chloroplasts had once been
free-living prokaryotes that reproduced independently. In fact, the mitochondrial DNA is
different enough and evolves at a different rate from nuclear DNA, so it can be used to solve
problems of evolution that the nuclear DNA cannot. This evidence would make no sense if
the eukaryotic cell had tried to generate the organelles from scratch (they would not have a
genetic code if that were true), and certainly it makes no sense in a creationist explanation of
the cell as having been created the way we see it. If so, then why did God give the organelles
their own DNA as if they had once been free-living prokaryotes? The creationists may fall
back on Gosse’s Omphalos idea again, but that’s not science.
The final clincher is that we have many living transitional forms that show that this pro-
cess is occurring right now! The simpler eukaryotes, such as the freshwater amoebae Pelo-
myxa and Giardia (famous for causing dysentery in hikers who drink contaminated water),
lack mitochondria but contain symbiotic bacteria that perform the same respiratory func-
tion. In the laboratory, scientists have observed amoebae that have incorporated certain bac-
teria in their tissues as endosymbionts. The parabasalids, which live in the guts of termites,
use spirochetes for a motility organ instead of a flagellum. Thus, from the wild speculation
of 1967, Margulis’s idea is now accepted as the best possible explanation of the origin of
eukaryotes and organelles. Lynn Margulis has even received the National Medal of Science
for her groundbreaking and daringly original ideas.
One final point: Margulis showed that the eukaryotic flagellum was derived from the
syphilis-causing spirochete prokaryotes. It so happens that the flagellum is one of the ID
creationists’ (e.g., Behe 1996) favorite examples of “irreducible complexity.” Behe argues that
the structure of the flagellum is too complex to explain by evolution. Apparently, he is com-
pletely unaware that this distinctive 9 + 2 structure of the flagellum already exists in nature
in the structure of the prokaryotic spirochete, a much simpler life form than the eukaryotic
cell. As Miller (2004) has shown, even the biochemical processes are the same. The basal
body of the flagellum has been found to be similar to the type III secretion system, which