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.
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.
diner sits down to a salad containing
mushrooms and lettuce.