Acquiring Genomes by Lynn Margulis and Dorion Sagan is a well-argued, lucid book about
evolutionary theory that is accessible to the non-specialist reader. Margulis
and Sagan offer a radical extension of evolutionary theory. They propose a mechanism for evolutionary
change that is largely outside the mainstream of evolutionary theory that
emphasizes mutation by random genetic mutation. The extension offered is the "symbiogenesis" of new
species. Symbiogenesis is the process of creating new species by the merging of
previously separate genomes that have come to have a symbiotic relationship to
each other.

The idea of symbiosis is central to Margulis and Sagan’s thesis.
This is a familiar idea — different species live together so closely that they
are mutually dependant on each other. 
For instance, we all have in our guts a particular kind of bacteria that
is essential for the digestion of food. 
The bacteria are essential to us because we would die of starvation
without them. We are essential for
those bacteria as well. The bacteria
and we live in a state of mutual dependence. 
Symbiosis can occur at a high level. For instance there are birds that
make their living by plucking parasites from the backs of rhinoceroses. The
birds and the rhinos are symbionts. Margulis and Sagan make this idea of
symbiosis a little more rigorous than usual. 
Since each kind of creature has a unique genome (that is, a unique
genetic structure), they define symbiosis as the "living together of
different genomes".

Under conditions of stress, the previously coexisting genomes
actually merge to become one genome. A new unified individual is created where
formerly there were two. This passage from page 90 describes the process.

"Under stress, different kinds of individuals, of very
different origins and abilities, often associate. With continued and predictable
stress, often cyclical and seasonal, these acquaintances become intimate and
extend beyond a single encounter. To become significant to the evolutionary
process, the former strangers must interact frequently enough to form stable,
unique relationships and form a permanent or deeply seasonal affair. Put succinctly, in the cases important in
evolution, associations lead to partnerships that lead to symbioses that leads
to new kinds of individuals formed by symbiogenesis."

Symbiogenensis is a difficult idea to accept from the perspective
that is natural for humans. For us,
significant creatures are other creatures our ancestors and we have encountered
in our daily lives. We relate to birds and mammals and fish and reptiles and
plants easily because all of these kinds of life are on a scale that is easy
for us to perceive. And it is hard to
imagine the rhinos and their symbiont birds merging to become a new kind of
creature.

The matter is easier to understand if we consider cows. Cows have
a special stomach, containing a special bacterium. This second stomach is one of the defining characteristics that
distinguishes cows from (say) antelopes. 
That second stomach is caused by the genome of the cow to provide an
environment for the genome of the bacteria that live in that stomach. It isn’t
so much of a jump from here to the idea that the cow is in fact a mixture of
two genomes.

Most life on the planet Earth is bacterial. Bacteria exist at the
top of the atmosphere and deep in the earth’s crust. Whereas there are many
metabolic systems among bacteria, all animals and plants share only one
metabolic process. Central to the
thesis of Acquiring Genomes is the realization that species do not exist
among bacteria. Bacterial genomes are so fluid and changeable that the concept
of species is meaningless when applied to them.

One of the central points of Acquiring Genomes is that
symbiotic associations of bacteria are the basis of all multicellular forms of
life. Traditionalists will object and
point out that bacteria are fundamentally different from the cells of which
multicellular life is constructed. The
cells of animals and plants have a well-defined nucleus that exists within its
own membrane (sort of a cell within a cell). Such cells are called
"eukaryotic cells". In
contrast, bacteria do not have a nucleus within a membrane — they are
"prokaryotic cells". Margulis
pointed out many years ago that eukaryotic cells are actually symbiotic
combinations of prokaryotic cells. DNA, located in the nucleus of eukaryotic
cells, came from one kind of bacterium and the mitochondria and the cellular
machinery outside the nucleus came from another. Therefore, the very existence of multicellular life is a result
of a symbiotic merging among bacterial predecessors and therefore the process
of symbiogenesis is fundamental to the very existence of species. To the question: "How did species
acquire their genomes?" Margulis and Sagan answer, "By the merging of
separately evolved bacterial genomes."

Acquiring Genomes introduces the reader to a very interesting idea of surprising
scope. As a bonus, the book is well
written and a pleasure to read and is chock full of interesting examples. For example — have you heard about the
photosynthetic slug? It incorporates algae into its tissues, is green, and
never eats – but basks in the sun in the shallow waters of the seashore. In the
mind of this reviewer Acquiring Genomes provides an excellent account of
where genomes come from; that is, from bacteria. The thesis though, that symbiogenesis is the main process driving
evolutionary change, is not so well established — just what causes a change
from an ancestor to a descendant "species" is still an open question.

© 2003
Martin Hunt

Martin Hunt is an artist living and working in Vancouver, Canada.
His work is inspired by math and science. Lately he’s been indulging an
interest in evolutionary theory and its relation to consciousness.

Categories: Genetics