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Eugene V Koonin
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Title: The
Biological Big Bang model for the major transitions in
evolution (2007).
by Eugene V
Koonin*.
Address: National Center for Biotechnology
Information, National Library of Medicine, National Institutes of
Health, Bethesda, MD 20894, USA
* Corresponding
author
Biology Direct 2007, 2:21
doi:10.1186/1745-6150-2-21
This article is available from: http://www.biology-direct.com/content/2/1/21
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Abstract:
Background:
Major transitions in biological evolution show the same pattern of
sudden emergence of diverse forms at a new level of complexity. The
relationships between major groups within an emergent new class of
biological entities are hard to decipher and do not seem to fit the
tree pattern that, following Darwin's original proposal, remains the
dominant description of biological evolution. The cases in point
include the origin of complex RNA molecules and protein folds; major
groups of viruses; archaea and bacteria, and the principal lineages
within each of these prokaryotic domains; eukaryotic supergroups; and
animal phyla. In each of these pivotal nexuses in life's history, the
principal "types" seem to appear rapidly and fully equipped with the
signature features of the respective new level of biological
organization. No intermediate "grades" or intermediate forms between
different types are detectable. Usually, this pattern is attributed to
cladogenesis compressed in time, combined with the inevitable erosion
of the phylogenetic signal.
Hypothesis: I propose that most
or all
major evolutionary transitions that show the "explosive" pattern of
emergence of new types of biological entities correspond to a boundary
between two qualitatively distinct evolutionary phases. The first,
inflationary phase is characterized by extremely rapid evolution driven
by various processes of genetic information exchange, such as
horizontal gene transfer, recombination, fusion, fission, and spread of
mobile elements. These processes give rise to a vast diversity of forms
from which the main classes of entities at the new level of complexity
emerge independently, through a sampling process. In the second phase,
evolution dramatically slows down, the respective process of genetic
information exchange tapers off, and multiple lineages of the new type
of entities emerge, each of them evolving in a tree-like fashion from
that point on. This biphasic model of evolution incorporates the
previously developed concepts of the emergence of protein folds by
recombination of small structural units and origin of viruses and cells
from a pre-cellular compartmentalized pool of recombining genetic
elements. The model is extended to encompass other major transitions.
It is proposed that bacterial and archaeal phyla emerged independently
from two distinct populations of primordial cells that, originally,
possessed leaky membranes, which made the cells prone to rampant gene
exchange; and that the eukaryotic supergroups emerged through distinct,
secondary endosymbiotic events (as opposed to the primary,
mitochondrial endosymbiosis). This biphasic model of evolution is
substantially analogous to the scenario of the origin of universes in
the eternal inflation version of modern cosmology. Under this model,
universes like ours emerge in the infinite multiverse when the eternal
process of exponential expansion, known as inflation, ceases in a
particular region as a result of false vacuum decay, a first order
phase transition process. The result is the nucleation of a new
universe, which is raditionally denoted Big Bang, although this
scenario is radically different from the Big Bang of the traditional
model of an expanding universe. Hence I denote the phase transitions at
the end of each inflationary epoch in the history of life Biological
Big Bangs (BBB).
Conclusion: A Biological Big
Bang (BBB)
model is proposed for the major transitions in life's evolution.
According to this model, each transition is a BBB such that new classes
of biological entities emerge at the end of a rapid phase of evolution
(inflation) that is characterized by extensive exchange of genetic
information which takes distinct forms for different BBBs. The major
types of new forms emerge independently, via a sampling process, from
the pool of recombining entities of the preceding generation. This
process is envisaged as being qualitatively different from treepattern
cladogenesis.
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