Evolution What the Fossils Say and Why it Matters

164 Evolution? The Fossils Say YES!

Communal Living Builds Complex Cells

We are symbionts on a symbiotic planet, and if we care to, we can find symbiosis

everywhere. Physical contact is a nonnegotiable requisite for many differing forms

of life.

—Lynn Margulis, Symbiotic Planet

Creationists love to point to the complexity of the eukaryotic cell (fig. 6.7), with all its diverse
organelles (such as mitochondria, chloroplasts, flagella, and so on), and try to persuade
their nonscientific audiences that evolution could never construct such an amazing arrange-
ment. What they don’t mention is that the solution to how to make a complex eukaryotic
cell has been known for decades and that it doesn’t require anything more complex than
living together in peace and harmony. If we were to try to take a simple prokaryote like a
bacterium and develop all the organelles from scratch within it, such a task would seem
improbably difficult. But in 1967, Lynn Margulis proposed a radical idea that solved the
problem (independently and unknowingly reviving an obscure older idea suggested by
K. S. Merezhkovsky in 1905) and gave us a much simpler solution: endosymbiosis. Instead of
“inventing” mitochondria and chloroplasts and the rest from scratch, Margulis argued that
these organelles were originally independent prokaryotic cells that came to live within the
walls of a larger cell in exchange for food or protection (fig. 6.8). Chloroplasts apparently
started out as cyanobacteria, which are photosynthetic even though they are prokaryotes
without organelles. Purple nonsulfur bacteria have much the same structure and function
as mitochondria, and apparently that’s where these organelles came from. The flagellum

FIGURE 6.7. Prokaryotes, such as the Archaebacteria and true bacteria, are small cells only a few microns
in diameter. Their genetic material (DNA) is not enclosed within a nucleus but floats within the cell, and
they lack organelles. Eukaryotes (all other living organisms) have larger, more complex cells, with discrete
nuclei containing their DNA. They also may have a number of other organelles, including mitochondria,
chloroplasts, Golgi apparatus, endoplasmic reticulum, cilia, flagella, and other subcellular structures.

Cell wall

Flagellum

Chloroplast

Nucleolus

Nucleus

Endoplasmic reticulum

Golgi apparatus

Mitochondrion

Plasma membrane

DNA