Read an Excerpt
Chapter One
SYMBIOSIS EVERYWHERE
A Bee his burnished Carriage
Drove boldly to a Rose--
Combinedly alighting--
Himself-- (1339)
Symbiosis, the system in which members of different
species live in physical contact, strikes us as an arcane concept
and a specialized biological term. This is because of our
lack of awareness of its prevalence. Not only are our guts and
eyelashes festooned with bacterial and animal symbionts,
but if you look at your backyard or community park, symbionts
are not obvious but they are omnipresent. Clover and
vetch, common weeds, have little balls on their roots. These
are the nitrogen-fixing bacteria that are essential for healthy
growth in nitrogen-poor soil. Then take the trees, the maple,
oak, and hickory. As many as three hundred different fungal
symbionts, the mycorrhizae we notice as mushrooms, are
entwined in their roots. Or look at a dog, who usually fails to
notice the symbiotic worms in his gut. 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 kinds of life.
Practically everything I work on now was anticipated by
unknown scholars or naturalists. One of my most important
scientific predecessors thoroughly understood and explained
the role of symbiosis in evolution. The University of Colorado
anatomist Ivan E. Wallin (1883-1969) wrote a fine
book arguing that new species originate through symbiosis.
Symbiogenesis, an evolutionary term, refers to the origin of
new tissues, organs, organisms--even species--by establishment
of long-term permanent symbiosis. Wallin never used
the word symbiogenesis, but he entirely understood the idea.
He especially emphasized animal symbiosis with bacteria, a
process he called "the establishment of microsymbiotic complexes"
or "symbionticism." This is important. Although
Darwin entitled his magnum opus On the Origin of Species,
the appearance of new species is scarcely even discussed in
his book.
Symbiosis, and here I fully agree with Wallin, is crucial to
an understanding of evolutionary novelty and the origin of
species. Indeed, I believe the idea of species itself requires
symbiosis. Bacteria do not have species. No species existed
before bacteria merged to form larger cells including ancestors
to both plants and animals. In this book I will explain
how long-standing symbiosis led first to the evolution of
complex cells with nuclei and from there to other organisms
such as fungi, plants, and animals.
That animal and plant cells originated through symbiosis
is no longer controversial. Molecular biology, including gene
sequencing, has vindicated this aspect of my theory of cell
symbiosis. The permanent incorporation of bacteria inside
plant and animal cells as plastids and mitochondria is the
part of my serial endosymbiosis theory that now appears even
in high school textbooks. But the full impact of the symbiotic
view of evolution has yet to be felt. And the idea that new
species arise from symbiotic mergers among members of old
ones is still not even discussed in polite scientific society.
Here is an example. I once asked the eloquent and personable
paleontologist Niles Eldredge whether he knew of
any case in which the formation of a new species had been
documented. I told him I'd be satisfied if his example were
drawn from the laboratory, from the field, or from observations
from the fossil record. He could muster only one good
example: Theodosius Dobzhansky's experiments with
Drosophila, the fruit fly. In this fascinating experiment, populations
of fruit flies, bred at progressively hotter temperatures,
became genetically separated. After two years or so the
hot-bred ones could no longer produce fertile offspring with
their cold-breeding brethren. "But," Eldredge quickly added,
"that turned out to have something to do with a parasite!"
Indeed, it was later discovered that the hot-breeding flies
lacked an intracellular symbiotic bacterium found in the
cold breeders. Eldredge dismissed this case as an observation
of speciation because it entailed a microbial symbiosis! He
had been taught, as we all have, that microbes are germs, and
when you have germs, you have a disease, not a new species.
And he had been taught that evolution through natural selection
occurs by the gradual accumulation, over eons, of single
gene mutations.
Ironically, Niles Eldredge is author with Stephen Jay
Gould of the theory of "punctuated equilibrium." Eldredge
and Gould argue that the fossil record shows evolution to be
static most of the time and to proceed suddenly: rapid
change in fossil populations occurs over brief time spans;
stasis then prevails for extended periods. From the long view
of geological time, symbioses are like flashes of evolutionary
lightning. To me symbiosis as a source of evolutionary novelty
helps explain the observation of "punctuated equilibrium,"
of discontinuities in the fossil record.
Among the only other organisms besides fruit flies in
which species have been seen to originate in the laboratory
are members of the genus Amoeba and symbiosis was
involved. Symbiosis is a kind, but not the notorious kind, of
Lamarckianism. "Lamarckianism," named for Jean Baptiste
Lamarck, who the French claim was the first evolutionist, is
often dismissed as "inheritance of acquired characteristics."
In simple Lamarckianism, organisms inherit traits induced
in their parents by environmental conditions, whereas
through symbiogenesis, organisms acquire not traits but
entire other organisms, and of course, their entire sets of
genes! I could say, as my French colleagues often have, that
symbiogenesis is a form of neo-Larmarckianism. Symbiogenesis
is evolutionary change by the inheritance of acquired
gene sets.
Living beings defy neat definition. They fight, they feed,
they dance, they mate, they die. At the base of the creativity
of all large familiar forms of life, symbiosis generates novelty.
It brings together different life-forms, always for a reason.
Often, hunger unites the predator with the prey or the
mouth with the photosynthetic bacterium or algal victim.
Symbiogenesis brings together unlike individuals to make
large, more complex entities. Symbiogenetic life-forms are
even more unlike than their unlikely "parents." "Individuals"
permanently merge and regulate their reproduction.
They generate new populations that become multiunit symbiotic
new individuals. These become "new individuals" at
larger, more inclusive levels of integration. Symbiosis is not
a marginal or rare phenomenon. It is natural and common.
We abide in a symbiotic world.
In Brittany, on the northwest coast of France, and along
beaches bordering the English Channel is found a strange
sort of "seaweed" that is not seaweed at all. From a distance
it is a bright green patch on the sand. The patches slosh
around, shimmering in shallow puddles. When you pick up
the green water and let it slip through your fingers you
notice gooey ribbons much like seaweed. A small hand lens
or low-power microscope reveals that what looked like seaweed
are really green worms. These masses of sunbathing
green worms, unlike any seaweed, burrow into the sand and
effectively disappear. They were first described in the 1920s
by an Englishman, J. Keeble, who spent his summers at
Roscoff. Keeble called them "plant-animals" and diagrammed
them splendidly in the color frontispiece of his
book, Plant-Animals. The flatworms of the species Convoluta
roscoffensis are all green because their tissues are
packed with Platymonas cells; as the worms are translucent,
the green color of Platymonas, photosynthesizing algae,
shows through. Although lovely, the green algae are not
merely decorative: they live and grow, die and reproduce,
inside the bodies of the worms. Indeed they produce the
food that the worms "eat". The mouths of the worms become
superfluous and do not function after the worm larvae hatch.
Sunlight reaches the algae inside their mobile greenhouses
and allows them to grow and feed themselves as they leak
photosynthestic products and feed their hosts from the
inside. The symbiotic algae even do the worm a waste management
favor: they recycle the worm's uric acid waste into
nutrients for themselves. Algae and worm make a miniature
ecosystem swimming in the sun. Indeed, these two beings
are so intimate that it is difficult, without very high-power
microscopy, to say where the animal ends and the algae
begin.
Such partnerships abound. Bodies of Plachobranchus,
snails, harbor green symbionts growing in such even rows
they appear to have been planted. Giant clams act as living
gardens, in which their bodies hold algae toward the light.
Mastigias is a man-of-war type of medusoid that swims in
the Pacific Ocean. Like myriad small green umbrellas, Mastigias
medusoids float through the light beams near the
water's surface by the thousands.
Similarly, freshwater tentacled hydras may be white or
green, depending on whether or not their bodies are packed
with green photosynthetic partners. Are hydras animals or
plants? When a green hydra is permanently inhabited by its
food producing partners (called Chlorella), it is hard to tell.
Hydras, if green, are symbionts. They are capable of photosynthesizing,
of swimming, of moving, and of staying put.
They have remained in the game of life because they become
individuals by incorporation.
We animals, all thirty million species of us, emanate from
the microcosm. The microbial world, the source and wellspring
of soil and air, informs our own survival. A major
theme of the microbial drama is the emergence of individuality
from the community interactions of once-independent
actors.
I love to gaze on the daily life struggles of our nonhuman
planetmates. For many years Lorraine Olendzenski, my former
student, now at the University of Connecticut, and I
have videographed life in the microcosm. More recently we
have worked with Lois Byrnes, the vivacious former associate
director of the New England Science Center in Worcester,
Massachusetts. Together we and a fine group of U MASS students
make films and videos that introduce people to our
microbial acquaintances.
Ophrydium, a pond water scum that, upon close inspection,
seems to be countable green "jelly ball" bodies is an
example of emergent individuality that we recently discovered
in Massachusetts and redescribed. Our films show these
water balls with exquisite clarity. The larger "individual"
green jelly ball is composed of smaller cone-shaped actively
contractile "individuals." These in turn are composite: green
Chlorella dwell inside ciliates, all packed into rows. Inside
each upside-down cone are hundreds of spherical symbionts,
cells of Chlorella. Chlorella is a common green alga;
those of Ophrydium are trapped into service for the jelly ball
community. Each "individual organism" in this "species" is
really a group, a membrane-bounded packet of microbes that
looks like and acts as a single individual.
A nutritious drink called kefir consumed in the Caucasus
Mountains is also a symbiotic complex. Kefir contains grainy
curds the Georgians call "Mohammed pellets." The curd is
an integrated packet of more than twenty-five different kinds
of yeast and bacteria. Millions of individuals make up each
curd. From such interactive bodies of fused organisms new
beings sometimes emerge. The tendency of "independent"
life is to bind together and reemerge in a new wholeness at a
higher, larger level of organization. I suspect that the near
future of Homo sapiens as a species requires our reorientation
toward the fusions and mergers of the planetmates that
have preceded us in the microcosm. One of my ambitions is
to coax some great director into producing evolutionary history
as the microcosmic image in IMAX or OMNIMAX,
showing spectacular living relationships as they form and
dissolve.
Now and throughout Earth's history, symbioses, both stable
and ephemeral, have prevailed. Stories about this kind of
evolution deserve broadcasting.