Lecture 16: Life on Earth—Single-celled Organisms
creation of multi-cellular organisms, though in the case of eukaryotes all
the organisms coexisted within a single cell. Most eukaryotes are 10 to
1,000 times larger than prokaryotes; some can be seen just with the naked
eye. The DNA of eukaryotes is protected within a special container, the
nucleus, which limits the damage to genetic material and increases the
accuracy of reproduction.Many eukaryotic cells contain mitochondria, special “organelles” that
can generate energy from oxygen—a more powerful source of energy
than “fermentation,” the reaction used to generate energy in prokaryotes.
So eukaryotes À ourished in an oxygen-rich atmosphere. The appearance
of eukaryotes marks a signi¿ cant increase in the complexity of life. Lynn
Margulis and Dorion Sagan write, “The difference between the new cells
and the old prokaryotes in the fossil record looks as drastic as if the Wright
Brothers’ Kitty Hawk À ying machine had been followed a week later by the
Concorde jet” (Margulis and Sagan, Microcosmos, p. 115).The fourth crucial transition is the appearance of sexual reproduction about
1 billion years ago. Prokaryotes regularly exchange genetic material, but
they normally reproduce simply by splitting into two identical individuals
or “clones.” In most eukaryotes, two organisms exchange genetic material
before reproduction, so that offspring contain a mix of genetic material from
two parent individuals. As a result, offspring are no longer simply clones
of their parents. Sexual reproduction introduces greater variation between
individuals. As natural selection “selects” from such variations, the result of
sexual reproduction is to speed up the rate of evolution. This is why evolution
seems to have accelerated during the last 1 billion years.This lecture has surveyed the ¿ rst 3.5 billion years of the history of life on
Earth, during which all organisms were single-celled. How were multi-celled
organisms created, and how did they evolve over the last 600 million years?
That is the subject of the next lecture. Ŷ