The surface of the earth is molten rock. The oceans are steam or superheated water. Every so often a wandering asteroid slams in
with such energy that any incipient crust of hardened rock is melted again and the oceans are reboiled to an incandescent mist.
    Welcome to Hades, or at least to what geologists call the Hadean interval of earth's history. It is reckoned to have lasted from the
planet's formation 4.6 billion years ago until 3.8 billion years ago, when the rain of oceanboiling asteroids ended.
     The Isua greenstone belt of western Greenland, one of the oldest known rocks, was formed as the Hadean interval ended. And amazingly,
to judge by chemical traces in the Isuan rocks, life on earth was already old.
    Everything about the origin of life on earth is a mystery, and it seems the more that is known, the more acute the puzzles get.
    The dates have become increasingly awkward. Instead of there being a billion or so years for the first cells to emerge from a warm broth of
chemicals, life seems to pop up almost instantly after the last of the titanic asteroid impacts that routinely sterilized the infant planet.
Last week, researchers reported discovering microbes that lived near volcanic vents formed 3.2 billion years ago, confirming that
heatloving organisms were among earth's earliest inhabitants.
    The chemistry of the first life is a nightmare to explain. No one  has yet devised a plausible explanation to show how the earliest chemicals
of life  thought to be RNA, or ribonucleic acid, a close relative of DNA  might have constructed themselves from the inorganic chemicals
likely to have been around on the early earth. The spontaneous assembly of small RNA molecules on the primitive earth "would have
been a near miracle," two experts in the subject helpfully declared last year.
    A third line of inquiry into the beginnings of life has now also hit an unexpected roadblock. This is phylogeny, or the drawing of family
trees of the various genes found in presentday forms of life. The idea is to run each gene tree backward to the ancestral gene at the root of
the tree. The collection of all these ancestral genes should define the nature of the assumed universal ancestor, the living cell from
which all the planet's life is descended. The universal ancestor would lie some distance away from life's origin from chemicals, but might at
least give clues to how that process started.
    The phylogenetic approach worked beautifully when first applied in 1981 by Dr. Carl Woese of the University of Illinois to a single gene.
Dr. Woese chose a gene that makes an essential component of the cell's machinery for synthesizing proteins.
    The tree derived by analyzing the versions of this gene found in many different species showed an orderly branching into the three primal
kingdoms of life known as the bacteria, the archaea and the eukarya.
The archaea are singledcelled organisms often found in hot places like scalding springs and deep oil wells; the eukarya include all
multicellular forms of life like plants and animals.
    But the picture has become much less clear now that some 30 genomes from species in the three kingdoms have been decoded. For one thing,
all of these genomes have turned out to contain far more novel genes than had been expected. And if all of these genes had forebears in the
last ancestor, that primeval cell would have been implausibly complex.
    For another, family trees drawn on the basis of other genes showed a quite different pattern to that of Dr. Woese's proteinmaking gene.
Biologists have not despaired of restoring the universal ancestor with phylogenetic trees, but the unveiling will not take place nearly so
soon as expected.
    The puzzle that different genes yield different family trees, even though there can only be one family tree of evolution, is easily
explained in principle: some genes must have been transmitted horizontally instead of vertically.
    In other words, instead of being inherited by one generation from another, certain genes must have been exchanged between lineages of
organisms, just as living species of bacteria pass around among each other the genes that confer resistance to antibiotics.
    The horizontal exchange of genes seems to have started before the three kingdoms of life diverged from each other and the universal
ancestor. Indeed, it was so pervasive, Dr. Woese suggested recently, that the universal ancestor was probably not a singlecelled organism
but a commune  a loosely knit conglomerate of diverse cells that exchanged genetic information.
    These pieces of the genetic information would have been short modules  carrying several related genes, not the long chromosomes carrying
thousands of genes that are seen in most living organisms.
    Also, in Dr. Woese's view, they would have had a primitive and rather sloppy system for copying their genetic material, not the highly
accurate, proofread mechanism of DNA replication enjoyed by living cells today.
    But at some point, in Dr. Woese's reconstruction, the mechanism for translating genetic information into proteins would have become more
accurate and powerful, and the members of this ancestral community would have evolved to a stage at which it was difficult to incorporate
new material into their genomes. The commune members would have started to evolve independently. This would have been the moment when
the family tree of the bacteria, archaea and eukarya began.
    The ancestral commune theory explains why the three kingdoms seem to have a largely common set of proteinmaking genes, as reflected in Dr.
Woese's original tree, but a smorgasbord of other gene categories.
    Dr. Eugene V. Koonin, a computational biologist at the National Center for Biotechnological Information, agreed that Dr. Woese's idea was a
useful framework and that the horizontal transfer of genes was probably more common in life's early days than now.
    "It is not so preposterous anymore to think of the common ancestor as a  sort of Noah's ark, where pretty much every protein domain has been
represented," Dr. Koonin said. The proteins of living organisms are composed of mixandmatch functional units known as domains.
    Still, Dr. Woese's idea is a disturbing concept. Evolutionists are accustomed to portraying the evolutionary process in terms of neatly
branching trees, not Noah's arks.
    The horizontal transfer thesis has been taken even further by Dr. W. Ford Doolittle, an evolutionary biologist at Dalhousie University in
Nova Scotia. In a February article in Scientific American, titled "Uprooting the Tree of Life," Dr. Doolittle argued that extensive
horizontal transfers of genes occurred even after the emergence of the three kingdoms, making the origin of life look more like a forkful of
spaghetti than a tree.
    Genebased trees drawn for living animals can usually be dated by estimating the rate of DNA change and anchoring at least one branch of
the tree to a fossil of known age.
    But the rate of DNA change has probably not been constant throughout evolution, especially in its early days, making it hard to known if
genebased trees like Dr. Woese's do indeed extend to the last common ancestor as they seem to on paper.
    Dr. Doolittle believes the trees may reach back only a billion years or so, not to the fourbillionyear point when life began. "So many people
wanted to believe we can run the clock right back to the beginning," he said. But Dr. Koonin thinks the trees hold very ancient
information, even if their dates are not certain.
    "We can see very far," he said. "We can see beyond the last common ancestor." He cites the fact that certain genes, like those for the
proteins known as helicases and amino acid synthetases, are duplicated in all three kingdoms, and that these duplications must have occurred
in the common ancestor, before the kingdoms split.
    Several of the earliest branches on Dr. Woese's original tree lead to presentday bacteria or archaea that live in extremely hot places.
Since the early earth also was hot, it is tempting to think that the earliest forms of life may have emerged in places like the volcanic
vents that pierce the ocean bed. Last week, Dr. Birger Rasmussen, a geologist at the University of Western Australia, reported in Nature
that he had discovered the "probable fossil remains" of microbes that lived in volcanic vent deposits laid down 3.235 billion years ago.
    These are by far the oldest known ventassociated microbes,
although the oldest fossils of any kind are of bacteria that lived 3.5 billion years ago. Dr. Rasmussen found these microscopic filaments of life in
the Pilbara Craton of northwestern Australia. This and a formation in South Africa are the only two known Archaean age rocks in which
fossils have survived. All other rocks of the Archaean age, which lasted from 3.8 billion to 2.5 billion years ago, have been so heated
and reworked that any fossils have perished.
    In part because life must have originated well before these oldest known fossils, many biologists accept as the earliest evidence for
life the traces of possibly biologically processed carbon in the Isuan rocks of Greenland.
    But at least one expert, Dr. J. William Shopf of the University of California at Los Angeles, is doubtful. The traces "could equally well
be charred dregs of primordial soup, the remains of nonbiologic organic matter formed on the early earth or brought in with meteorites
or comets," he writes in "The Cradle of Life" (Princeton University Press, April 2000).
    Though there are several lines of evidence about life's origins, none yet provides a clear view of the critical events.
    The fossil evidence fades out at 3.5 billion years ago. The phylogenetic evidence is for the moment blurred by horizontal gene
transfer. The best efforts of chemists to reconstruct molecules typical of life in the laboratory have shown only that it is a problem
of fiendish difficulty. The genesis of life on earth, some time in the fiery last days of the Hadean, remains an unyielding problem.

    http://www.nytimes.com

GRAPHIC: Photo: Dr. Birger Rasmussen, a geologist at the University of
Western Australia, has reported finding the "probable fossil remains"
of microbes that lived in volcanic vent deposits 3.2 billion years ago
in northwestern Australia. (Louis Bucci)(pg. F2); (Fossil picture
courtesy of Birger Rasmussen)(pg. F1) Diagrams: Deep sea vent picture
by A. Ballard,Woods Hole Oceanographic Institution; Juan Velasco and
Steve Duenes /The New York Times Chart: "A Warm, Watery Party at
Life's Start" The origin of life on Earth remains cloudy, but current
research in the field points toward a warm habitat deep in the ocean
where life sprang from a community of primitive cells rather than from
a solitary unit. Rethinking the Universal Ancestor: A New 'Tree of
Life' The consensus tree of life has long been a clean progression of
dividing branches tracing back to a single point of origin, but as DNA
evidence is analyzed, the tree begins to look more like a web.

THE OLD TREE: In this theory, the three branches of life grew from a
single ancestral cell, and eukarya (including people and plants) was
the last group to arrive, evolving from archaea, but also taking genes
from bacteria. THE NEW TREE: Though the different branches still
separate as the tree matures, early evolution was characterized by the
swapping of genes across species lines so that the bottom of the tree
is a group of cells evolving at the same time. WHEN LIFE TOOK ROOT:
THE HADEAN ERA For its first 800 million years, the young Earth was
pounded by meteorites and the oceans existed only as a giant vapor
cloud. The surface was a molten stew, and the place was hardly
hospitable to life. The Archaean Era As the bombardment slowed and the
Earth cooled, a heavy rain flooded the surface and deep oceans formed
as a suitable cradle for life to take root. NEW FOSSILS Providing
evidence that early life developed near a deep sea volcanic vent,
scientists are reporting the discovery of fossil remains of threadlike
organisms (left) in rocks that are close to 3.2 billion years ago.The
rocks were once near a deep sea vent in Western Australia.
 From the deep sea vent, life diversified and spread to different
regions in the archaean landscape.

A Hot Habitat

The study of ribosomal RNA, genetic material that exists in all forms
of life, suggests that the earliest forms of life were microorganisms
living in very high temperatures, and a probable habitat for such a
creature is a hot volume of water or the neighborhood near a volcanic
vent on the ocean floor.  (Sources: Dr. Euan Nisbet; Dr. Carl R.
Woese, University of Illinois; Dr. W. Ford Doolittle, Dalhousie
University.) (pg. F1)