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A diner sits down to a salad containing mushrooms and lettuce.
In the universal genealogy of life, the mushrooms are more closely related to the lettuce than to the diner, right?
Wrong.
As part of an outpouring of research that
is revolutionizing notions about
the genetic, biochemical, structural
and evolutionary relationships among living things,
fungi like mushrooms
have now been revealed as being closer to animals like humans than to
plants like lettuce.
Scientists are
consequently having to rethink some long-held ideas about
evolution.
The tree of life, which essentially maps
the path of evolution, has
simple single-celled organisms at its base and plants,
animals and fungi
-- and a newly delineated
kingdom that includes kelp -- at its crown.
For most of the history of science, experts
have drawn the tree by
comparing the gross
surface features of various creatures.
Fungi,
for example, were lumped with plants
because they look much like them
and grow
in one spot.
But now, by comparing organisms' genetic
material and microscopic internal structures, scientists are drawing the
tree all over again.
It used to be, for instance, that higher
organisms were divided into two kingdoms,
plants and animals.
The
revised crown of
the tree of life now includes five kingdoms:
red
algae, green plants, animals, fungi and a
more recently discovered group
called stramenopiles, most of which look like plants
but are not,
because they do not perform
photosynthesis.
"Brown" water plants
like
tiny diatoms and giant kelp share so many
characteristics with
stramenopiles that
they have been lumped with them.
All five of
these kingdoms have lineages that trace
back to different one-celled
organisms.
Among this grouping of kingdoms, the
green plants have
been the most intensively
reanalyzed.
They had been divided simply
into green algae and land plants; now they
are seen to be a far more
various but closely
related set of groupings, from fresh-water
algae on
one end of the evolutionary scale to
flowering plants on the other.
Moreover, some kinds of organisms are
being reassigned from one kingdom
to another.
Some slime molds, for instance, have
long been
classified with fungi, but now are
considered stramenopiles.
As the
tree of life has been re-drawn,
scientists have also had to revise some
of
their ideas about how evolution has proceeded.
It now appears,
for instance, that plants
first colonized the land not from the oceans
but from fresh water -- with some of the
land plants recolonizing the
ocean as green
seaweed.
And as the revision proceeds, scientists
are moving tantalizingly close to
identifying the first land plant and
the first
flowering plant.
The revision is also providing road
maps
that could enable scientists to speed up the
search for naturally
occurring medicines
and the development of new strains of
drought-resistant crops. But it is primarily
the sheer intellectual
excitement of figuring
out one of nature's grand designs that has
lately galvanized the efforts of hundreds of
scientists.
"It is
at least as exciting as exploring the
cosmos," says Dr. Brent Mishler, a
biologist
and green-plant specialist at the University
of California at
Berkeley, who is a leader in
the effort.
The reanalysis accepts and
builds on a
recent, fundamental realignment of all living things into
three basic groups: simple
bacteria, archaea and eukarya (or
eukaryotes).
Simple bacteria have no cell nuclei.
Archaea are
bacteria of ancient origin that
today often live in hostile environments
like
hot springs and are believed by many scientists to have represented
the first life on
earth.
Eukarya -- the group that includes people
-- are characterized by complex cells that
have nuclei and specialized
internal structures to process energy. They are the basis
of all higher
life; the cells of plants, animals,
fungi and stramenopiles are
eukaryotes.
The first simple bacteria emerged at
least as far back
as 3.5 billion years ago,
about a billion years after the earth's
formation. Eukarotic cells had materialized by
at least 2 billion years
ago, maybe earlier.
They evolved, scientists believe, by ingesting
other species of cells and then, instead of digesting them, adopting them
as
permanent, genetically reproducible parts
of themselves -- like the
chloroplasts that
enable plants to convert solar radiation into
chemical energy and the mitochondria that
process that energy in both
plant and animal cells. Because plants give off oxygen in
photosynthesis, they also produced an oxygen-rich atmosphere, eventually
enabling
oxygen-breathing animals like people to
evolve.
The
higher groupings of life -- what some
scientists call the eukaryote
crown groups
-- may have appeared in something of an
evolutionary "big
bang" about a billion
years ago.
In this efflorescence of life,
some eukaryotic cells combined to form multicelled organisms and then
began to diversify dramatically. Although their flowering has
been
spectacular, they constitute only a few
small branches of the entire
tree of life.
Microbes of immense variety still rule the
planet, and
compared to them the crown
eukaryotes are "almost evolutionary
afterthoughts," said Dr. Mitchell Sogin, an evolutionary biologist at the
Marine Biological
Laboratory at Woods Hole, Mass.
Nevertheless,
recent research on the
higher forms, particularly plants, has been
especially revealing.
This was clear from a
flood of evidence
presented this month at
the 16th International Botanical Congress in
St. Louis, a worldwide gathering of botanists
which takes place every
six years.
There, a
team of 200 scientists from 12 countries
reported the first results of a five-year,
Federally supported research
effort on the
green-plant portion of the tree of life.
The result
of the project, called Deep
Green, is said to be the most complete
reconstruction of any part of the tree so far.
While much uncertainty
over details remains, the scientists said, the main outlines
of
green-plant genealogy and evolution now
are clear.
Green plants,
in the form of algae, first
appeared in the cavalcade of evolution
about the same time as the explosion of
multicelled life a billion or so
years ago.
For
perhaps 500 million years, the algae were
confined
to the water.
Without water, they
could not reproduce; the only
way sperm
cells could reach egg cells was to swim or
float to them.
But eventually, according to
the new thinking, some of these algal
species found their way from the ocean to fresh
water, where they
inhabited shoreline environments that were sometimes dry and
sometimes
wet.
Special genetic adaptations to dryness
and to damaging
ultraviolet radiation from
the sun enabled these pioneer algae to
survive when water receded, according to this
idea, and they were able
to reproduce when
the water rose.
Mosses and ferns fit this
category even today -- they grow in spots
that are sometimes wet -- and
they were the
first land plants.
The next landmark in plant
evolution was
the appearance of seed plants, called gymnosperms, about
425 million years ago.
This
eliminated the need for sperm cells to
swim
free in the environment; now the male
reproductive cells were
contained in pollen.
The innovation proved much more effective
a means
of reproduction, and with it, the
first big plants like cone-bearing
trees developed, and green plants for the first time
became ecologically
dominant on land. Conifers are today's descendants of these
plants.
The latest of these fundamental branchings was the appearance of
flowering plants
about 150 million years ago.
These plants,
called angiosperms, have an advantage in
that their seeds are protected
by the fleshy
bodies of their fruits.
The lineages of many
angiosperms run in a virtually straight line
from then till now:
several groups of modern angiosperms, like sycamores, walnuts,
oaks and
dogwoods, also graced the countryside in the era of the dinosaurs more
than 65
million years ago.
"If you had been wandering around with
those dinosaurs, the landscape wouldn't
have looked that unfamiliar to
you," Dr.
Peter R. Crane, the director of Britain's
Royal Botanic
Gardens at Kew, said in St.
Louis.
There would be some
differences, he
and others said; most of the flowers of that
era were
smaller and less colorful than
modern ones, and neither big flowers like
sunflowers nor grasses had yet evolved.
Scientists have not yet
identified the original single-celled organism from which all
plants
evolved, or the first land plant, or the
first flowering plant. But they
are on the
scent.
In St. Louis, an early alga called
mesostigma, still growing on the earth, was presented as a "living
fossil" believed to have
evolved just before the first land plants.
Two other freshwater algae are seen as
promising candidates as closest
living relatives of the land plants.
Similarly, a small
cream-colored flower
called amborella, found only on the island of
New
Caledonia in the South Pacific, was
revealed in St. Louis as the oldest
known
living flowering plant.
While it is not the
"Eve" of
angiosperms, Dr. Mishler said, it
is close to that original ancestor
and thus
provides a model for what it might have
been like.
At
each stage of plant evolution, scientists
now believe, only one family
lineage was
able to surmount the considerable environmental obstacles to
its emergence.
For instance, just one among many groups of
algae
succeeded in making the transition to
land.
The result is that
all plants are more
closely related than had once been thought.
This emerges quite clearly in the case of
red, green and brown
chloroplasts, the engines of photosynthesis and the structures
that
define plants.
For a long time, scientists
thought that red, green
and brown plants
acquired different types of chloroplasts at
different
times.
But now it appears that all
chloroplasts stem from one
group of blue-green bacteria and are in fact more closely
related to one
another than are the plants
themselves.
For that matter, it now
appears that all
forms of higher life, for all their divisions,
are
more closely related than is commonly
believed; people are closer to
green plants,
for example, than to the E. coli bacteria that
inhabit
the human intestinal tract, and closer still to fungi.
One
consequence of the
latter relationship, experts say, is that treating
fungal infections in people is an exacting
business; the wrong treatment
might harm
the person as well as the fungus.
Humans are brothers
and sisters not just
to each other, it seems, but to the rest of life
as well.
Copyright 1999 The New York
Times Company
Genetic comparisons place fungi closer to
man than to plants.
The revelation is just one of many emerging new
ideas about family ties in the upper
reaches of the earth's
3.5-billion-year-old
tree of life, in particular ties among plants
and
between them and other higher forms of
existence.