[Paleopsych] Science Week: Evolution: On Life on Early Earth

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Evolution: On Life on Early Earth

    The following points are made by Frances Westall (Science 2005
    1) Fifty years after the discovery of fossil microorganisms in
    2-billion-year-old rocks from the Gunflint Formation in Ontario [1],
    Canada, there is renewed interest in the history of life on the early
    Earth. This resurgence of interest is in part due to the burgeoning
    field of astrobiology and the recognition of the importance of
    studying Earth's early environment and seeking life on other planets.
    Such renewed interest necessitates a better understanding of the
    problems surrounding the identification of very ancient traces of life
    and the development of more sophisticated methods of investigation.
    Resolution of such problems is crucial if we are to obtain reliable
    evidence for traces of life on other planets, such as Mars, with any
    reasonable degree of certainty.
    2) When searching for evidence of past life, sedimentary environments
    are considered the most suitable because they are often formed in
    association with water, a fundamental requirement for life. There are
    only three known locations that host exposures of ancient sediments:
    Isua and Akilia in southwest Greenland, which are 3.8 to 3.7 billion
    years old (Ga), the Pilbara in northwestern Australia (3.5 to 3.3 Ga),
    and Barberton in eastern South Africa (3.5 to 3.3 Ga). These
    sediments, however, formed almost 1 billion years after the formation
    of the Earth (4.56 Ga). Any older sedimentary deposits, and with them
    any potential information on the origin of life and its initial
    evolution, have been destroyed by tectonic activity. Of the existing
    three exposures of ancient sediments, the Isua and Akilia rocks have
    been so altered by metamorphic changes over the past 3.8 billion years
    that they are no longer useful for microfossil studies. In contrast,
    large parts of the Pilbara and Barberton ancient terrains are
    exquisitely preserved, representing veritable goldmines for
    microfossil hunters.
    3) Until recently, investigations of early life on Earth concentrated
    on the search for fossils of cyanobacteria [2]. These relatively large
    microorganisms (from a few to tens of micrometers in size) evolved a
    sophisticated metabolism for obtaining energy from sunlight
    (photosynthesis), producing oxygen as a by-product (oxygenic
    photosynthesis). The attention lavished on these microorganisms stems
    from early discoveries of fossil cyanobacteria [1], but since then the
    study of early life has moved into a more contentious, if more
    realistic, sphere. New questions are being raised: (i) What
    characteristics of life (structural and biogeochemical) also are
    produced by abiogenic processes and, consequently, how can we
    distinguish between signatures of past life and signatures of nonlife?
    (ii) What is the nature of the earliest preserved microorganisms, and
    (iii) what environments did they inhabit? The first question is a
    particularly thorny one -- and is especially pertinent to the search
    for life on other planets -- because we have no examples of the
    transition from nonlife to life. The life forms preserved in the
    oldest terrestrial sediments were already highly evolved compared with
    the earliest cell and with LUCA (last universal common ancestor).
    4) Owing to the difficulties in distinguishing between life and
    nonlife, no one signature of life -- for example, the fractionated
    isotopic ratio, the molecular carbon composition, or an isolated
    microfossil -- should be considered unequivocal evidence for traces of
    past life. Hence, the most realistic approach to identifying evidence
    of past life is a global strategy that includes relevant environmental
    (habitat), structural (morphology), and biogeochemical (chemical
    composition, isotopic fractionation) information. Analyzing the
    geological context of rocks that potentially contain microfossils is
    crucial. Also needed is a better understanding of the differences in
    the spatial and temporal scale of interactions between microbes and
    their environment, and geological processes. For example, some
    microenvironments (such as volcanic rocks or hydrothermal veins)
    provide habitats suitable for microbes even though the overall
    environment may be inhospitable to life. [3-5]
    References (abridged):
    1. S. A. Tyler, E. S. Barghoorn, Science 119, 606 (1954)
    2. J. W. Schopf, Science 260, 640 (1993)
    3. M. Brasier et al., Nature 416, 76 (2002)
    4. J. M. Garcia-Ruiz et al., Science 302, [1194] (2003)
    5. M. Van Zuilen, A. Lepland, G. Arrhenius, Nature 418, 627 (2002)
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by Eoers Szathmary (Nature 2005
    1) In investigating the origin of life and the simplest possible life
    forms, one needs to enquire about the composition and working of a
    minimal cell that has some form of metabolism, genetic replication
    from a template, and boundary (membrane) production.
    2) Identifying the necessary and sufficient features of life has a
    long tradition in theoretical biology. But living systems are products
    of evolution, and an answer in very general terms, even if possible,
    is likely to remain purely phenomenological. Going deeper into
    mechanisms means having to account for the organization of various
    processes, and such organization has been realized in several
    different ways by evolution. Eukaryotic cells (such as those from
    which we are made) are much more complicated than prokaryotes (such as
    bacteria), and eukaryotes harbor organelles that were once free-living
    bacteria. A further complication is that multicellular organisms
    consist of building blocks -- cells -- that are also alive. So aiming
    for a general model of all kinds of living beings would be fruitless;
    instead, such models have to be tied to particular levels of
    biological organization.
    3) Basically, there are two approaches to the "minimal cell": the
    top-down and the bottom-up. The top-down approach aims at simplifying
    existing small organisms, possibly arriving at a minimal genome. Some
    research to this end takes Buchnera, a symbiotic bacterium that lives
    inside aphids, as a rewarding example. This analysis is complemented
    by an investigation of the duplication and divergence of genes.
    Remarkably, these approaches converged on the conclusion that genes
    dealing with RNA biosynthesis are absolutely indispensable in this
    framework. This may be linked to the idea of life's origins in an "RNA
    world", although such an inference is far from immediate.
    4) Top-down approaches seem to point to a minimum genome size of
    slightly more than 200 genes. Care should be taken, however, in
    blindly accepting such a figure. For example, although some gene set A
    and gene set B may not be common to all bacteria, that does not mean
    that (A and B) are dispensable. It may well mean that A or B is
    essential, because the cell has to solve a problem by using either A
    or B. Only experiments can have the final word on these issues.
    5) A top-down approach will not take us quite to the bottom, to the
    minimal possible cells in chemical terms. All putative cells, however
    small, will have a genetic code and a means of transcribing and
    translating that code. Given the complexity of this system, it is
    difficult to believe, either logically or historically, that the
    simplest living chemical system could have had these components.
    6) The bottom-up approach aims at constructing artificial chemical
    supersystems that could be considered alive. No such experimental
    system exists yet; at least one component is always missing.
    Metabolism seems to be the stepchild in the family: what most
    researchers in the field used to call metabolism is usually a trivial
    outcome of the fact that both template replication and membrane growth
    need some material input. This input is usually simplified to a
    conversion reaction from precursors to products.
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by S. Rasmussen et al (Science 2004
    1) All life forms are composed of molecules that are not themselves
    alive. But in what ways do living and nonliving matter differ? How
    could a primitive life form arise from a collection of nonliving
    molecules? The transition from nonliving to living matter is usually
    raised in the context of the origin of life. But some researchers(1)
    have recently taken a broader view and asked how simple life forms
    could be synthesized in the laboratory. The resulting artificial cells
    (sometimes called protocells) might be quite different from any extant
    or extinct form of life, perhaps orders of magnitude smaller than the
    smallest bacterium, and their synthesis need not recapitulate life's
    actual origins. A number of complementary studies have been steadily
    progressing toward the chemical construction of artificial cells
    2) There are two approaches to synthesizing artificial cells. The
    top-down approach aims to create them by simplifying and genetically
    reprogramming existing cells with simple genomes. The more general and
    more challenging bottom-up approach aims to assemble artificial cells
    from scratch using nonliving organic and inorganic materials.
    3) Although the definition of life is notoriously controversial, there
    is general agreement that a localized molecular assemblage should be
    considered alive if it continually regenerates itself, replicates
    itself, and is capable of evolving. Regeneration and replication
    involve transforming molecules and energy from the environment into
    cellular aggregations, and evolution requires heritable variation in
    cellular processes. The current consensus is that the simplest way to
    achieve these characteristics is to house informational polymers (such
    as DNA and RNA) and a metabolic system that chemically regulates and
    regenerates cellular components within a physical container (such as a
    lipid vesicle).
    4) Two recent workshops(1) reviewed the state of the art in artificial
    cell research, much of which focuses on self-replicating lipid
    vesicles. David Deamer (Univ. of California, Santa Cruz) and Pier
    Luigi Luisi (ETH Zurich) each described the production of lipids using
    light energy, and the template-directed self-replication of RNA within
    a lipid vesicle. In addition, Luisi demonstrated the polymerization of
    amino acids into proteins on the vesicle surface, which acts as a
    catalyst for the polymerization process. The principal hurdle remains
    the synthesis of efficient RNA replicases and related enzymes entirely
    within an artificial cell. Martin Hanczyc (Harvard Univ.) showed how
    the formation of lipid vesicles can be catalyzed by encapsulated clay
    particles with RNA adsorbed on their surfaces. This suggests that
    encapsulated clay could catalyze both the formation of lipid vesicles
    and the polymerization of RNA.
    References (abridged):
    1. http://www.ees.lanl.gov/protocells
    2. C. Hutchinson et al., Science 286, 2165 (1999)
    3. M. Bedau et al., Artif. Life 6, 363 (2000)
    4. J. Szostak et al., Nature 409, 387 (2001)
    5. A. Pohorille, D. Deamer, Trends Biotechnol. 20, 123 (2002)
    Science http://www.sciencemag.org

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