The Biological Big Bang model for the major transitions in evolution

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 traditionally 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 tree-pattern cladogenesis.

Figure 1

A Bush of Life: a typical tree with unresolved deep branches. The tree was generated from simulated data using the TreeView program [114].

Figure 2

Biological Big Bangs and the emerging pattern of tree-like evolution: transitions between rapid and slow phases of evolution in the history of life. The transition between the rapid, inflationary, and slow, tree-like, phases of evolution is shown by the red line and denoted BBB. The similarity to the depiction of Big Bang events in the evolution of the multiverse in Fig. 3 is deliberate. I. The pre-cellular BBBs. The squiggles of different colors denote genetic elements in the primordial gene pool, and arrows denote recombination/fusion processes. The emerging trees are those of individual genes and virus-like agents. The emergence of the proto-archaeal (A) and proto-bacterial (B) cells is shown as well. II. Origin of the major bacterial lineages. The rounded rectangles show proto-bacterial cells with leaky membranes (broken lines). The arrows denote extensive horizontal gene transfer. The colored shapes denote emerging bacterial lineages (trees) with tighter membranes (solid lines). A similar schematic for the origin of archaeal lineages is not shown. III. Origin of the eukaryotic supergroups. The irregular shapes show proto-eukaryotic cells that already harbored mitochondria derived from α-proteobacteria (dark green shapes inside – shown to derive from one of the bacterial trees supposed to correspond to the proteobacterial lineage) and have evolved nuclei (red spheroids, so colored to emphasize the archaeal connection). The archaeal ancestry of eukaryotes is denoted by a broken arrow (the intermediate, inflationary phase is omitted). Arrows show secondary symbioses with other bacteria or primitive eukaryotes (colored shapes) that are postulated to give rise to the eukaryotic supergroups (trees).

Figure 3

The cosmological model of eternal inflation. A. Emergence of universes as nucleating bubbles of low-energy vacuum in the eternally inflating sea of false vacuum. B. A Big Bang event at the origin of a universe: views from the inside of the emerging universe and from the outside.